What form of reproduction did the first land animals use?

What form of reproduction did the first animals on land use*? Were they hermaphrodites, or did they have male and female sexes? [Is there a proper term for sexual separation in a species?]

Were any land species believed to have evolved from hermaphrodite ocean species (that crossed to land on roughly the same geological time periods that land creatures first evolved)?

*I know that 'first on land' is a complicated concept itself, since it could mean first creatures be able to survive on land at all, first to freely travel between land and water, or first to only live on land, or anywhere in between. I assume all three categories were sexual though?

Animal Reproduction - Fertilization

Okay, so now we know how gametes are made AND how mates are selected. Now, we can finally move on to the union of the gametes. First, we will discuss two different modes of introducing the sperm and egg: external fertilization and internal fertilization. Then, we will discuss what happens at the big moment when the sperm and the egg actually meet (other than fireworks).

External Fertilization

Animals that live in the water usually release their gametes directly into the water—plop, splash. The union takes place outside of the body, and is called external fertilization. Many fish, amphibians, and other sexual aquatic animals use external fertilization (the exceptions are aquatic mammals, sharks, and some other special types of fish).

External fertilization is beneficial because the mother does not have to physically carry the developing offspring. She can also lay a brood of many eggs at one time (we are talking hundreds to millions of eggs). On the other hand, it is also more dangerous because the developing offspring are at a risk to predators gobbling them up for breakfast or second breakfast. Humans aren’t the only ones who like caviar. Eggs are also not as protected from environmental changes (such as pH or temperature) as they would be inside their cozy mother.

It is important that both the eggs and sperm are released in synchronization. Courtship rituals are important to ensure that both parties are ready. Sometimes there is a single pair of fish, but since the eggs and sperm are just "out there," it is also common for multiple males or multiple females to spawn at the same time. There are even sometimes sneaky undesirable fish that hope to get in on the action. They release their gametes secretly near a better male's lady friend since they can't get one of their own.

Gametes can be released in various situations:

No Guarding, Predator Risk

  • Released into the open water where they are very vulnerable.
  • Buried in holes or hidden in other places. Bitterling fish even deposit their eggs into the gill filaments of a clam so they do not get eaten before they hatch.

Guarding, Requires More From Parents

  • Released into a nest, cave, or on a plant where they can guard them.
  • Released onto a parent where they attach to their body externally.
  • The eggs are collected after spawning and carried internally. This video shows a female cichlid fish laying eggs and quickly collecting them into her mouth where she holds them carefully until they are ready to hatch.

Internal Fertilization

In mammals, reptiles, birds, and some types of fish the gametes meet inside of the female's body. This is called internal fertilization. All land dwellers need to fertilize this way because sperm still prefer swimming to walking. If given the chance, we think sperm could probably bounce on their tails a la Tigger style. But even if they could bounce, they still need to stay in a wet environment.

The benefit is that it provides a safer environment for fertilization. In some species, the females can even safely store the sperm until they are ready to use it. The downsides are that internal fertilization requires special body parts. It also requires more energy than external fertilization.

In some species the males directly insert sperm into the female by using important external sex organs. In other species, the males release sperm by abutting against the female (in the case of chickens) (and crossing their fingers that their sperm make it inside). In another indirect type of internal fertilization, salamanders and scorpions deposit jelly-like sperm packets into the environment rather than into/onto the female. She then straddles these packets to allow sperm to get inside to her eggs.

Male Sex Organs

The primary male sex organ is the penis. The function of the penis is to enter into a female's orifice and deposit sperm. The penis typically swells with blood during copulation which makes it sturdier and easier to insert into the female.

Some lizards and snakes have a pair of hemipenes instead of a single penis. The hemipenis turns inside out like a finger on a glove to extend outside the body. There are also some types of spiders and squids that deposit sperm outside of their body and use their antennaes and tentacles to pick it up and place it inside the female. Talk about getting "handsy."

This guy is packing double trouble in the form of hemepenes. Image from here.

Female Sex Organs

The primary female orifice is either a vagina or a cloaca. A vagina is found in mammals and is specifically used for reproduction. The cloaca is found in birds, reptiles, amphibians, and monotremes. It is used for reproduction as well as excretion of waste. In some birds, both the male and female each have a cloaca which they press against each other for transfer of sperm. Presto.

If the cloacas are rocking don’t come a bawking. Image from here.


When these accessory organs are joined it is called copulationor coitus. There is usually a puzzle-piece fit between a male's and a female's sex organs. The penis is inserted into the vagina and the sperm are ejaculated inside.

The puzzle-piece exception is the banana slug which has a penis that is sometimes the entire length of his body. If he cannot find a mate large enough to accommodate it, then she will chew off the end. Chomp chomp.

Even worse are bedbugs they fertilize the female by stabbing a hole right in her abdomen. The sperm are injected directly into the body cavity. No correct fit needed.

Since copulation is not always easy, both females and males may try to escape before completion. To increase success, many species have built-in ways to prevent early separation.

In many reptiles and fish the male may have hooks or barbs on or near his penis (or hemipenis) that attach to the female during copulation. Sharks have been known to severely bite females to hold them during copulation (and these are not even vampire sharks). Dog penises also swell during copulation and are difficult to remove until completion.

Special Copulation Situations

Remember, there are also some animals that reproduce via parthenogenesis. Their offspring develop from an unfertilized egg (essentially, a virgin birth). For example, whiptail lizards are all female. However, they still like to perform copulation to stimulate egg development. They climb on each other and pretend they are copulating although no coitus actually takes place.

Honeybees also sometimes practice parthenogenesis. The queen bee can produce male honeybees (called drones) without fertilization. Female bees are all diploid and are produced through fertilization. Only the males are produced without fertilization.

Hermaphrodites also have to come together to trade gametes even though each possesses male and female parts. Sometimes there is a cross fertilization and both partners are fertilized. Other times, one partner takes the male role and the other takes the female role (depending on who remembered their wig).

Flatworms have a special way to determine who the "female" is. They duel with their erect penises until one gets stabbed. The loser is now the "female," but she shouldn’t worry. She is getting inseminated with genes which are stronger than her own. There are actually plenty of miraculous virgin births in the world if you look around.

En garde! 2 flatworms prepare for a penis battle. Image from here.

Sperm Meets Egg

Once a sperm is released it sometimes has a long swim before it actually reaches the egg. Human sperm ejaculations can have up to 300 million sperm, and there can be only one winner. For the lucky first one there, there is a sequence of basic events that generally must happen during fertilization.

1) Sperm must get through any protective layer that the egg has.

2) The egg and sperm must pass the "same-species" test. This is extra important for external fertilization since many types of sperm and eggs could be in the same area.

3) There are also some defensive moves to prevent other sperm from entering, called blocks to polyspermy.

4) The nuclei fuse to create a diploid zygote.

Detailed Fertilization Process

Sea Urchins

The process of fertilization has been well studied in sea urchins. When a sperm reaches the egg, the acrosome releases its enzymes and they eat through the outer egg layer. Once through, the head of the sperm elongates like a skinny finger pointing towards the egg. Next, a special species-specific protein on the acrosome tries to match with a receptor on the egg. If they match, then the sperm is engulfed into the egg for nuclei fusion.

The human fertilization process is similar. As the sperm travel, they are activated by the female reproductive tract and swim through an outer egg layer called the cumulus cells. Next, they interact with the internal protein membrane called the zona pellucida and the acrosome enzyme reaction begins to clear the way.

Just like in the sea urchin, there are special species proteins on the zona pellucida that must match the sperm in order for it to continue. If they aren’t the same species, it’s a no-go. If they are the same, then the egg then proceeds through its second division and the nuclei of the sperm and the egg fuse. Hello zygote.

Blocks of Polyspermy

Polyspermy means more than one sperm, and a block to polyspermy prevents the egg from being fertilized by more than one sperm. If two get in, it will become a triploid cell and will not be viable. In sea urchins, there is a fast electrical block to polyspermy that occurs immediately when the sperm enters the egg.

Sea urchins and humans also have a slow block that occurs a minute later. A release of calcium causes a physical barrier to appear or the zona pellucida to disappear. Both blocks prevent more sperm from entering.

Similar to a polyspermy block, males of some species have developed "otherspermy" blocks. These tactics prevent other suitors from copulating after they have finished. In mice, the excess sperm develops into a hard crusty plug that physically blocks other males from copulating. Trust us, it is as icky as it sounds. Some male insects even try to thwart previous suitors. They have special penises that are shaped to scoop any other sperm out of the female to better their own chances.

Multiple Offspring

In animals with external fertilization, multiple eggs are released, which can be fertilized by one or several different males. In internal fertilization, there can be multiple offspring if multiple eggs were fertilized. These offspring have the same father or they can have different fathers if the mother mated with more than one male during estrus. For example, a litter of puppies could be fathered by one or more fathers.

We all know this is what you came here for. For the uninitiated, these are puppies. Image from here.

In humans, it is normal to only have one offspring. A single egg is released at a time. Sometimes, two eggs are released at once and are fertilized by separate sperm. This causes fraternal twins, who are basically siblings that are carried in the same uterus at the same time. If the egg splits into two after fertilization, then identical twins will result. Sometimes eggs can split more than once and identical triplets, quadruplets, etc. can occur.

Brain Snack

The zona pellucida is important for the "same species" test. Scientists were able to remove a zona pellucida from a hamster egg and use a human sperm to fertilize the egg. This hamsthuman embryo does not develop due to other problems, but this experiment is helpful to determine if a sperm is capable of getting inside and fertilizing an egg. Fertility centers sometimes perform this test to pinpoint the cause of male infertility.

Details Of Evolutionary Transition From Fish To Land Animals Revealed

New research has provided the first detailed look at the internal head skeleton of Tiktaalik roseae, the 375-million-year-old fossil animal that represents an important intermediate step in the evolutionary transition from fish to animals that walked on land.

A predator, up to nine feet long, with sharp teeth, a crocodile-like head and a flattened body, Tiktaalik's anatomy and way of life straddle the divide between fish and land-living animals. First described in 2006, and quickly dubbed the "fishapod," it had fish-like features such as a primitive jaw, fins and scales, as well as a skull, neck, ribs and parts of the limbs that are similar to tetrapods, four-legged animals.

The initial 2006 report did not describe the internal anatomy of the head, because those parts of the fossil were buried in rock. In the October 16, 2008, issue of Nature, the researchers describe this region and show how Tiktaalik was gaining structures that could allow it to support itself on solid ground and breathe air.

"We used to think of this transition of the neck and skull as a rapid event," said study author Neil Shubin, PhD, of the University of Chicago and Field Museum and co leader of the project, "largely because we lacked information about the intermediate animals. Tiktaalik neatly fills this morphological gap. It lets us see many of the individual steps and resolve the relative timing of this complex transition."

"The braincase, palate, and gill arch skeleton of Tiktaalik have been revealed in great detail by recent fossil preparation of several specimens," said Jason Downs, PhD, a postdoctoral research fellow at the Academy of Natural Sciences and lead author on the new study. "By revealing new details on the pattern of change in this part of the skeleton, we see that cranial features once associated with land-living animals were first adaptations for life in shallow water."

"The new study reminds us that the gradual transition from aquatic to terrestrial lifestyles required much more than the evolution of limbs," said Ted Daeschler, PhD, of the Academy of Natural Sciences and co-leader of the team that discovered Tiktaalik. "Our work demonstrates that, across this transition, the head of these animals was becoming more solidly constructed and, at the same time, more mobile with respect to the body." These changes are intimately associated with the change in environment.

Fish in deep water move and feed in three-dimensional space and can easily orient their body in the direction of their prey. A neck, seen for the first time in the fossil record in Tiktaalik, is advantageous in settings where the body is relatively fixed, as is the case in shallow water and on land where the body is supported by appendages planted against a substrate.

Another important component of this transition was the gradual reduction of the hyomandibula, a bony element that, in fish, coordinates the cranial motions associated with underwater feeding and respiration. In the transition to life on land, the hyomandibula loses these functions and the bone becomes available for an eventual role in hearing.

In humans, as in other mammals, the hyomandibula, or stapes, is one of the tiny bones in the middle ear. "The bony part of Tiktaalik's hyomandibula is greatly reduced from the primitive condition," said Downs, "and this could indicate that these animals, in shallow water settings, were already beginning to rely less on gill respiration."

The discoveries were made possible by laboratory preparators Fred Mullison and Bob Masek, who prepared the underside of the skull of specimens collected in 2004. This painstaking process took several years. This work showed the underside of the skull and gill bones "beautifully preserved," said Shubin, "to a degree unlike any creature of its kind at this transition."

Having multiple Tiktaalik specimens enabled the researchers to prepare the fossils in ways that showed the bones of the head in "exceptional detail," Downs said.

The team discovered Tiktaalik roseae on Ellesmere Island, in the Nunavut Territory of Canada, 600 miles north of the Arctic Circle. Though this region of Nunavut is now a harsh Arctic ecosystem, at the time that Tiktaalik lived, the area was much further south and was a subtropical floodplain ecosystem.

The formal scientific name for the new species, "Tiktaalik" (tic-TAH-lick), was derived by the Elders Council of Nunavut, the Inuit Qaujimajatuqangit. The Inuktikuk word means "a large, shallow-water fish." The paleontology team works in Nunavut with authorization from the Department of Culture, Language, Elders and Youth. All fossils are the property of the people of Nunavut and will be returned to Canada after they are studied.

The fossil research in Nunavut is carried out with authorization from the Department of Culture, Language, Elders and Youth, Government of Nunavut. All fossils are the property of the people of Nunavut and will be returned to Canada after they are studied.

A cast of Tiktaalik, along with a fleshed-out model of the animal, are on display in the Evolving Planet exhibition at Chicago's Field Museum, where Shubin serves as Provost.

The research was supported by private donors, the Academy of Natural Sciences, the Putnam Expeditionary Fund (Harvard University), the University of Chicago, the National Science Foundation, and the National Geographic Society Committee for Research and Exploration.


Aspidella discs on the Fermeuse Formation near Ferryland, Newfoundland. Credit: Martin Smith

When in 1868 the Scottish geologist Alexander Murray discovered in Newfoundland, Canada some small disc-shaped markings in the rock, he could not imagine that he was opening an unknown primal chapter of the history of life on Earth. However, four years later, the palaeontologist Elkanah Billings proposed the idea that the strange circles were actually fossils, a suggestion that was dismissed at the time since they had been found in Precambrian rocks. Instead, it was argued that they had been formed by chemical deposits or gas bubbles.

Almost a century would pass before there was a general recognition of the existence of macroscopic life prior to the Cambrian. Murray’s discs, together with others in different places, were assigned to the species Aspidella terranovica, but this case illustrates the current confusion about the reality of the Ediacaran biota: fossils initially identified as cnidarians (jellyfish) and classified as Aspidella and other genera, were probably not disc-shaped organisms, but rather the imprints of rhizoids—protuberances by which different creatures with the appearance of fronds are anchored to the substrate. On the other hand, fossil prints similar to those left by anemones have also been found with Aspidella, which would support their classification as cnidarians. In short, uncertainty abounds.

The first chicken eggs

But wait—weren’t there some scientists who claimed that, in fact, the chicken came first?

This claim came from some researchers studying how chicken eggshells form. Eggshell is mostly made from calcium carbonate (CaCO₃). Hens get their supply of calcium for eggshell production from dietary sources (calcium-rich seafood shells, such as oyster or prawn shells, are a popular snack for backyard hens for this reason).

To form a shell, the calcium needs to be deposited in the form of CaCO₃ crystals, and hens rely on specific proteins that enable this process. One such protein, called ovocleidin-17 (or OC-17 for short), is only found in the ovary of a chicken, leading to the suggestion that the chicken must have come before the chicken egg, since without OC-17, there can be no chicken egg formation. (Interestingly, it seems that this protein is responsible for speeding up the rate of eggshell formation, enabling hens to build an egg from scratch and lay it within a 24-hour timeframe.)

Domestic chickens are extremely efficient egg layers, capable of producing a fresh egg roughly every 24 hours. Image adapted from: Australian Academy of Science

So, can we lay this age-old conundrum to rest? Or are scientists and philosophers still scrambling to find an answer?

At the end of the day, the question is something of a false dichotomy. Eggs certainly came before chickens, but chicken eggs did not—you can’t have one without the other. However, if we absolutely had to pick a side, based on the evolutionary evidence, we’re on Team Egg.

###some figure caption ##Image adapted from: #Name CC0# --> ##some citation here with a #link# --> This article was written by Emma Berthold, Content Producer, Australian Academy of Science and has been reviewed by the following experts: Professor Rick Shine AM FAA Professor of Evolutionary Biology, School of Biological Sciences, University of Sydney Associate Professor Trevor Worthy Vertebrate palaeontologist and Vice-Chancellor's Postdoctoral Research Fellow, College of Science and Engineering, Flinders University Dr Walter Boles Senior Fellow, Ornithology Section, Australian Museum


The evidence for evolution is given in a number of books. [9] [10] [11] [12] Some of this evidence is discussed here.

Fossils show that change has occurred Edit

The realization that some rocks contain fossils was a very important event in natural history. There are three parts to this story:

  1. Realizing that things in rocks which looked organic actually were the altered remains of living things. This was settled in the 16th and 17th centuries by Conrad Gessner, Nicolaus Steno, Robert Hooke and others. [13][14]
  2. Realizing that many fossils represented species which do not exist today. It was Georges Cuvier, the comparative anatomist, who proved that extinction occurred, and that different strata contained different fossils. [15] p108
  3. Realizing that early fossils were simpler organisms than later fossils. Also, the later the rocks, the more like the present day are the fossils. [16]

The most convincing evidence for the occurrence of evolution is the discovery of extinct organisms in older geological strata. The older the strata are. the more different the fossil will be from living representatives. that is to be expected if the fauna and flora of the earlier strata had gradually evolved into their descendants.

Geographical distribution Edit

This is a topic which fascinated both Charles Darwin and Alfred Russel Wallace. [17] [18] [19] When new species occur, usually by the splitting of older species, this takes place in one place in the world. Once it is established, a new species may spread to some places and not others.

Australasia Edit

Australasia has been separated from other continents for many millions of years. In the main part of the continent, Australia, 83% of mammals, 89% of reptiles, 90% of fish and insects and 93% of amphibians are endemic. [20] Its native mammals are mostly marsupials like kangaroos, bandicoots, and quolls. [21] By contrast, marsupials are today totally absent from Africa and form a small portion of the mammalian fauna of South America, where opossums, shrew opossums, and the monito del monte occur (see the Great American Interchange).

The only living representatives of primitive egg-laying mammals (monotremes) are the echidnas and the platypus. They are only found in Australasia, which includes Tasmania, New Guinea, and Kangaroo Island. These monotremes are totally absent in the rest of the world. [22] On the other hand, Australia is missing many groups of placental mammals that are common on other continents (carnivora, artiodactyls, shrews, squirrels, lagomorphs), although it does have indigenous bats and rodents, which arrived later. [23]

The evolutionary story is that placental mammals evolved in Eurasia, and wiped out the marsupials and monotremes wherever they spread. They did not reach Australasia until more recently. That is the simple reason why Australia has most of the world's marsupials and all the world's monotremes.

Evolution of horses Edit

The evolution of the horse family (Equidae) is a good example of the way that evolution works. The oldest fossil of a horse is about 52 million years old. It was a small animal with five toes on the front feet and four on the hind feet. At that time, there were more forests in the world than today. This horse lived in woodland, eating leaves, nuts and fruit with its simple teeth. It was only about as big as a fox. [24]

About 30 million years ago the world started to become cooler and drier. Forests shrank grassland expanded, and horses changed. They ate grass, they grew larger, and they ran faster because they had to escape faster predators. Because grass wears teeth out, horses with longer-lasting teeth had an advantage.

For most of this long period of time, there were a number of horse types (genera). Now, however, only one genus exists: the modern horse, Equus. It has teeth which grow all its life, hooves on single toes, great long legs for running, and the animal is big and strong enough to survive in the open plain. [24] Horses lived in western Canada until 12,000 years ago, [25] but all horses in North America became extinct about 11,000 years ago. The causes of this extinction are not yet clear. Climate change and over-hunting by humans are suggested.

So, scientists can see that changes have happened. They have happened slowly over a long time. How these changes have come about is explained by the theory of evolution.

Hawaiian Drosophila (fruit flies) Edit

In about 6,500 sq mi (17,000 km 2 ), the Hawaiian Islands have the most diverse collection of Drosophila flies in the world, living from rainforests to mountain meadows. About 800 Hawaiian fruit fly species are known.

Genetic evidence shows that all the native fruit fly species in Hawai ʻ i have descended from a single ancestral species that came to the islands, about 20 million years ago. Later adaptive radiation was caused by a lack of competition and a wide variety of vacant niches. Although it would be possible for a single pregnant female to colonise an island, it is more likely to have been a group from the same species. [26] [27] [28] [29]

Distribution of Glossopteris Edit

The combination of continental drift and evolution can explain what is found in the fossil record. Glossopteris is an extinct species of seed fern plants from the Permian period on the ancient supercontinent of Gondwana. [30]

Glossopteris fossils are found in Permian strata in southeast South America, southeast Africa, all of Madagascar, northern India, all of Australia, all of New Zealand, and scattered on the southern and northern edges of Antarctica.

During the Permian, these continents were connected as Gondwana. This is known from magnetic striping in the rocks, other fossil distributions, and glacial scratches pointing away from the temperate climate of the South Pole during the Permian. [11] p103 [31]

Common descent Edit

When biologists look at living things, they see that animals and plants belong to groups which have something in common. Charles Darwin explained that this followed naturally if "we admit the common parentage of allied forms, together with their modification through variation and natural selection". [17] p402 [9] p456

For example, all insects are related. They share a basic body plan, whose development is controlled by master regulatory genes. [32] They have six legs they have hard parts on the outside of the body (an exoskeleton) they have eyes formed of many separate chambers, and so on. Biologists explain this with evolution. All insects are the descendants of a group of animals who lived a long time ago. They still keep the basic plan (six legs and so on) but the details change. They look different now because they changed in different ways: this is evolution. [33]

It was Darwin who first suggested that all life on Earth had a single origin, and from that beginning "endless forms most beautiful and most wonderful have been, and are being, evolved". [9] p490 [17] Evidence from molecular biology in recent years has supported the idea that all life is related by common descent. [34]

Vestigial structures Edit

Strong evidence for common descent comes from vestigial structures. [17] p397 The useless wings of flightless beetles are sealed under fused wing covers. This can be simply explained by their descent from ancestral beetles which had wings that worked. [12] p49

Rudimentary body parts, those that are smaller and simpler in structure than corresponding parts in ancestral species, are called vestigial organs. Those organs are functional in the ancestral species but are now either nonfunctional or re-adapted to a new function. Examples are the pelvic girdles of whales, halteres (hind wings) of flies, wings of flightless birds, and the leaves of some xerophytes (e.g. cactus) and parasitic plants (e.g. dodder).

However, vestigial structures may have their original function replaced with another. For example, the halteres in flies help balance the insect while in flight, and the wings of ostriches are used in mating rituals, and in aggressive display. The ear ossicles in mammals are former bones of the lower jaw.

"Rudimentary organs plainly declare their origin and meaning. " (p262). "Rudimentary organs. are the record of a former state of things, and have been retained solely though the powers of inheritance. far from being a difficulty, as they assuredly do on the old doctrine of creation, might even have been anticipated in accordance with the views here explained" (p402). Charles Darwin. [17]

In 1893, Robert Wiedersheim published a book on human anatomy and its relevance to man's evolutionary history. This book contained a list of 86 human organs that he considered vestigial. [35] This list included examples such as the appendix and the 3rd molar teeth (wisdom teeth).

The strong grip of a baby is another example. [36] It is a vestigial reflex, a remnant of the past when pre-human babies clung to their mothers' hair as the mothers swung through the trees. This is borne out by the babies' feet, which curl up when it is sitting down (primate babies grip with the feet as well). All primates except modern man have thick body hair to which an infant can cling, unlike modern humans. The grasp reflex allows the mother to escape danger by climbing a tree using both hands and feet. [11] [37]

Vestigial organs often have some selection against them. The original organs took resources, sometimes huge resources. If they no longer have a function, reducing their size improves fitness. And there is direct evidence of selection. Some cave crustacea reproduce more successfully with smaller eyes than do those with larger eyes. This may be because the nervous tissue dealing with sight now becomes available to handle other sensory input. [38] p310

Embryology Edit

From the eighteenth century it was known that embryos of different species were much more similar than the adults. In particular, some parts of embryos reflect their evolutionary past. For example, the embryos of land vertebrates develop gill slits like fish embryos. Of course, this is only a temporary stage, which gives rise to many structures in the neck of reptiles, birds and mammals. The proto-gill slits are part of a complicated system of development: that is why they persisted. [32]

Another example are the embryonic teeth of baleen whales. [39] They are later lost. The baleen filter is developed from different tissue, called keratin. Early fossil baleen whales did actually have teeth as well as the baleen. [40]

A good example is the barnacle. It took many centuries before natural historians discovered that barnacles were crustacea. Their adults look so unlike other crustacea, but their larvae are very similar to those of other crustacea. [41]

Artificial selection Edit

Charles Darwin lived in a world where animal husbandry and domesticated crops were vitally important. In both cases farmers selected for breeding individuals with special properties, and prevented the breeding of individuals with less desirable characteristics. The eighteenth and early nineteenth century saw a growth in scientific agriculture, and artificial breeding was part of this.

Darwin discussed artificial selection as a model for natural selection in the 1859 first edition of his work On the Origin of Species, in Chapter IV: Natural selection:

"Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change. which may be effected in the long course of time by nature's power of selection". [9] p109 [42]

Nikolai Vavilov showed that rye, originally a weed, came to be a crop plant by unintentional selection. Rye is a tougher plant than wheat: it survives in harsher conditions. Having become a crop like the wheat, rye was able to become a crop plant in harsh areas, such as hills and mountains. [43] [44]

There is no real difference in the genetic processes underlying artificial and natural selection, and the concept of artificial selection was used by Charles Darwin as an illustration of the wider process of natural selection. There are practical differences. Experimental studies of artificial selection show that "the rate of evolution in selection experiments is at least two orders of magnitude (that is 100 times) greater than any rate seen in nature or the fossil record". [45] p157

Artificial new species Edit

Some have thought that artificial selection could not produce new species. It now seems that it can.

New species have been created by domesticated animal husbandry, but the details are not known or not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with Ovis orientalis, one species from which they are descended. [46] Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, gaur, yak, etc., as they readily produce fertile offspring with them. [47]

The best-documented new species came from laboratory experiments in the late 1980s. William Rice and G.W. Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was put into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups.

After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas. [48] [49]

Diane Dodd was also able to show how reproductive isolation can develop from mating preferences in Drosophila pseudoobscura fruit flies after only eight generations using different food types, starch and maltose. [50]

Dodd's experiment has been easy for others to repeat. It has also been done with other fruit flies and foods. [51]

Observable changes Edit

Some biologists say that evolution has happened when a trait that is caused by genetics becomes more or less common in a group of organisms. [52] Others call it evolution when new species appear.

Changes can happen quickly in the smaller, simpler organisms. For example, many bacteria that cause disease can no longer be killed with some of the antibiotic medicines. These medicines have only been in use about eighty years, and at first worked extremely well. The bacteria have evolved so that they are no longer affected by antibiotics anymore. [53] The drugs killed off all the bacteria except a few which had some resistance. These few resistant bacteria produced the next generation.

The Colorado beetle is famous for its ability to resist pesticides. Over the last 50 years it has become resistant to 52 chemical compounds used in insecticides, including cyanide. [54] This is natural selection speeded up by the artificial conditions. However, not every population is resistant to every chemical. [55] The populations only become resistant to chemicals used in their area.

Although there were a number of natural historians in the 18th century who had some idea of evolution, the first well-formed ideas came in the 19th century. Three biologists are most important.

Lamarck Edit

Jean-Baptiste de Lamarck (1744–1829), a French biologist, claimed that animals changed according to natural laws. He said that animals could pass on traits they had acquired during their lifetime to their offspring, using inheritance. Today, his theory is known as Lamarckism. Its main purpose is to explain adaptations by natural means. [56] He proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus use and disuse.

Lamarck's idea was that a giraffe's neck grew longer because it tried to reach higher up. This idea failed because it conflicts with heredity (Mendel's work). Mendel made his discoveries about half a century after Lamarck's work.

Darwin Edit

Charles Darwin (1809–1882) wrote his On the Origin of Species in 1859. In this book, he put forward much evidence that evolution had occurred. He also proposed natural selection as the way evolution had taken place. But Darwin did not understand about genetics and how traits were actually passed on. He could not accurately explain what made children look like their parents.

Nevertheless, Darwin's explanation of evolution was fundamentally correct. In contrast to Lamarck, Darwin's idea was that the giraffe's neck became longer because those with longer necks survived better. [17] p177/9 These survivors passed their genes on, and in time the whole species got longer necks.

Wallace Edit

Alfred Russel Wallace OM FRS (1823–1913) was a British naturalist, explorer, biologist and social activist. He proposed a theory of natural selection at about the same time as Darwin. His idea was published in 1858 together with Charles Darwin's idea.

Mendel Edit

An Austrian monk called Gregor Mendel (1822–1884) bred plants. In the mid-19th century, he discovered how traits were passed on from one generation to the next.

He used peas for his experiments: some peas have white flowers and others have red ones. Some peas have green seeds and others have yellow seeds. Mendel used artificial pollination to breed the peas. His results are discussed further in Mendelian inheritance. Darwin thought that the inheritance from both parents blended together. Mendel proved that the genes from the two parents stay separate, and may be passed on unchanged to later generations.

Mendel published his results in a journal that was not well-known, and his discoveries were overlooked. Around 1900, his work was rediscovered. [57] [58] Genes are bits of information made of DNA which work like a set of instructions. A set of genes are in every living cell. Together, genes organise the way an egg develops into an adult. With mammals, and many other living things, a copy of each gene comes from the father and another copy from the mother. Some living organisms, including some plants, only have one parent, so get all their genes from them. These genes produce the genetic differences which evolution acts on.

Darwin's On the Origin of Species has two themes: the evidence for evolution, and his ideas on how evolution took place. This section deals with the second issue.

Variation Edit

The first two chapters of the Origin deal with variation in domesticated plants and animals, and variation in nature.

All living things show variation. Every population which has been studied shows that animal and plants vary as much as humans do. [59] [60] p90 This is a great fact of nature, and without it evolution would not occur. Darwin said that, just as man selects what he wants in his farm animals, so in nature the variations allow natural selection to work. [61]

The features of an individual are influenced by two things, heredity and environment. First, development is controlled by genes inherited from the parents. Second, living brings its own influences. Some things are entirely inherited, others partly, and some not inherited at all.

The colour of eyes is entirely inherited they are a genetic trait. Height or weight is only partly inherited, and the language is not at all inherited. Just to be clear: the fact that humans can speak is inherited, but what language is spoken depends on where a person lives and what they are taught. Another example: a person inherits a brain of somewhat variable capacity. What happens after birth depends on many things such as home environment, education and other experiences. When a person is adult, their brain is what their inheritance and life experience have made it.

Evolution only concerns the traits which can be inherited, wholly or partly. The hereditary traits are passed on from one generation to the next through the genes. A person's genes contain all the traits which they inherit from their parents. The accidents of life are not passed on. Also, of course, each person lives a somewhat different life: that increases the differences.

Organisms in any population vary in reproductive success. [62] p81 From the point of view of evolution, 'reproductive success' means the total number of offspring which live to breed and leave offspring themselves.

Inherited variation Edit

Variation can only affect future generations if it is inherited. Because of the work of Gregor Mendel, we know that much variation is inherited. Mendel's 'factors' are now called genes. Research has shown that almost every individual in a sexually reproducing species is genetically unique. [63] p204

Genetic variation is increased by gene mutations. DNA does not always reproduce exactly. Rare changes occur, and these changes can be inherited. Many changes in DNA cause faults some are neutral or even advantageous. This gives rise to genetic variation, which is the seed-corn of evolution. Sexual reproduction, by the crossing over of chromosomes during meiosis, spreads variation through the population. Other events, like natural selection and drift, reduce variation. So a population in the wild always has variation, but the details are always changing. [60] p90

Natural selection Edit

Evolution mainly works by natural selection. What does this mean? Animals and plants which are best suited to their environment will, on average, survive better. There is a struggle for existence. Those who survive will produce the next generation. Their genes will be passed on, and the genes of those who did not reproduce will not. This is the basic mechanism which changes a population and causes evolution.

Natural selection explains why living organisms change over time to have the anatomy, the functions and behaviour that they have. It works like this:

  1. All living things have such fertility that their population size could increase rapidly for ever.
  2. We see that the size of populations does not increase to this extent. Mostly, numbers remain about the same.
  3. The food and other resources are limited. Therefore, there is competition for food and resources.
  4. No two individuals are alike. Therefore, they will not have the same chances to live and reproduce.
  5. Much of this variation can be inherited. Parents pass such traits to the children through their genes.
  6. The next generation can only come from those that survive and reproduce. After many generations of this, the population will have more helpful genetic differences, and fewer harmful ones. [64]Natural selection is really a process of elimination. [1] p117 The elimination is being caused by the relative fit between individuals and the environment they live in.

Selection in natural populations Edit

There are now many cases where natural selection has been proved to occur in wild populations. [4] [65] [66] Almost every case investigated of camouflage, mimicry and polymorphism has shown strong effects of selection. [67]

The force of selection can be much stronger than was thought by the early population geneticists. The resistance to pesticides has grown quickly. Resistance to warfarin in Norway rats (Rattus norvegicus) grew rapidly because those that survived made up more and more of the population. Research showed that, in the absence of warfarin, the resistant homozygote was at a 54% disadvantage to the normal wild type homozygote. [60] p182 [68] This great disadvantage was quickly overcome by the selection for warfarin resistance.

Mammals normally cannot drink milk as adults, but humans are an exception. Milk is digested by the enzyme lactase, which switches off as mammals stop taking milk from their mothers. The human ability to drink milk during adult life is supported by a lactase mutation which prevents this switch-off. Human populations have a high proportion of this mutation wherever milk is important in the diet. The spread of this 'milk tolerance' is promoted by natural selection, because it helps people survive where milk is available. Genetic studies suggest that the oldest mutations causing lactase persistence only reached high levels in human populations in the last ten thousand years. [69] [70] Therefore, lactase persistence is often cited as an example of recent human evolution. [71] [72] As lactase persistence is genetic, but animal husbandry a cultural trait, this is gene–culture coevolution. [73]

Adaptation Edit

Adaptation is one of the basic phenomena of biology. [74] Through the process of adaptation, an organism becomes better suited to its habitat. [75]

Adaptation is one of the two main processes that explain the diverse species we see in biology. The other is speciation (species-splitting or cladogenesis). [76] [77] A favourite example used today to study the interplay of adaptation and speciation is the evolution of cichlid fish in African rivers and lakes. [78] [79]

When people speak about adaptation they often mean something which helps an animal or plant survive. One of the most widespread adaptations in animals is the evolution of the eye. Another example is the adaptation of horses' teeth to grinding grass. Camouflage is another adaptation so is mimicry. The better adapted animals are the most likely to survive, and to reproduce successfully (natural selection).

An internal parasite (such as a fluke) is a good example: it has a very simple bodily structure, but still the organism is highly adapted to its particular environment. From this we see that adaptation is not just a matter of visible traits: in such parasites critical adaptations take place in the life cycle, which is often quite complex. [80]

Limitations Edit

Not all features of an organism are adaptations. [60] p251 Adaptations tend to reflect the past life of a species. If a species has recently changed its life style, a once valuable adaptation may become useless, and eventually become a dwindling vestige.

Adaptations are never perfect. There are always tradeoffs between the various functions and structures in a body. It is the organism as a whole which lives and reproduces, therefore it is the complete set of adaptations which gets passed on to future generations.

Genetic drift and its effect Edit

In populations, there are forces which add variation to the population (such as mutation), and forces which remove it. Genetic drift is the name given to random changes which remove variation from a population. Genetic drift gets rid of variation at the rate of 1/(2N) where N = population size. [45] p29 It is therefore "a very weak evolutionary force in large populations". [45] p55

Genetic drift explains how random chance can affect evolution in surprisingly big ways, but only when populations are quite small. Overall, its action is to make the individuals more similar to each other, and hence more vulnerable to disease or to chance events in their environment.

  1. Drift reduces genetic variation in populations, potentially reducing a population’s ability to survive new selective pressures.
  2. Genetic drift acts faster and has more drastic results in smaller populations. Small populations usually become extinct.
  3. Genetic drift may contribute to speciation, if the small group does survive.
  4. Bottleneck events: when a large population is suddenly and drastically reduced in size by some event, the genetic variety will be very much reduced. Infections and extreme climate events are frequent causes. Occasionally, invasions by more competitive species can be devastating. [81]
    ♦ In the 1880/90s, hunting reduced the Northern elephant seal to only about 20 individuals. Although the population has rebounded, its genetic variability is much less than that of the Southern elephant seal.
    ♦ Cheetahs have very little variation. We think the species was reduced to a small number at some recent time. Because it lacks genetic variation, it is in danger from infectious diseases. [82]
  5. Founder events: these occur when a small group buds off from a larger population. The small group then lives separately from the main population. The human species is often quoted as having been through such stages. For example, when groups left Africa to set up elsewhere (see human evolution). Apparently, we have less variation than would be expected from our worldwide distribution.
    Groups that arrive on islands far from the mainland are also good examples. These groups, by virtue of their small size, cannot carry the full range of alleles to be found in the parent population. [83][84]

Species Edit

How species form is a major part of evolutionary biology. Darwin interpreted 'evolution' (a word he did not use at first) as being about speciation. That is why he called his famous book On the Origin of Species.

Darwin thought most species arose directly from pre-existing species. This is called anagenesis: new species by older species changing. Now we think most species arise by previous species splitting: cladogenesis. [85] [86]

Species splitting Edit

Two groups that start the same can also become very different if they live in different places. When a species gets split into two geographical regions, a process starts. Each adapts to its own situation. After a while, individuals from one group can no longer reproduce with the other group. Two good species have evolved from one.

A German explorer, Moritz Wagner, during his three years in Algeria in the 1830s, studied flightless beetles. Each species is confined to a stretch of the north coast between rivers which descend from the Atlas mountains to the Mediterranean. As soon as one crosses a river, a different but closely related species appears. [87] He wrote later:

". a [new] species will only [arise] when a few individuals [cross] the limiting borders of their range. the formation of a new race will never succeed. without a long continued separation of the colonists from the other members of their species". [88]

This was an early account of the importance of geographical separation. Another biologist who thought geographical separation was critical was Ernst Mayr. [89]

One example of natural speciation is the three-spined stickleback, a sea fish that, after the last ice age, invaded freshwater, and set up colonies in isolated lakes and streams. Over about 10,000 generations, the sticklebacks show great differences, including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences. [90]

The wombats of Australia fall into two main groups, common wombats and hairy-nosed wombats. The two types look very similar, apart from the hairiness of their noses. However, they are adapted to different environments. Common wombats live in forested areas and eat mostly green food with lots of moisture. They often feed in the daytime. Hairy-nosed wombats live on hot dry plains where they eat dry grass with very little water or nutrition in it. Their metabolic rate is slow and they sleep most of the day underground.

When two groups that started the same become different enough, then they become two different species. Part of the theory of evolution is that all living things started the same, but then split into different groups over billions of years. [91]

This was an important movement in evolutionary biology, which started in the 1930s and finished in the 1950s. [92] [93] It has been updated regularly ever since. The synthesis explains how the ideas of Charles Darwin fit with the discoveries of Gregor Mendel, who found out how we inherit our genes. The modern synthesis brought Darwin's idea up to date. It bridged the gap between different types of biologists: geneticists, naturalists, and palaeontologists.

When the theory of evolution was developed, it was not clear that natural selection and genetics worked together. But Ronald Fisher showed that natural selection would work to change species. [94] Sewall Wright explained genetic drift in 1931. [95]

  • Evolution and genetics: evolution can be explained by what we know about genetics, and what we see of animals and plants living in the wild. [92][93]
  • Thinking in terms of populations, rather than individuals, is important. The genetic variety existing in natural populations is a key factor in evolution. [96]
  • Evolution and fossils: the same factors which act today also acted in the past. [97]
  • Gradualism: evolution is gradual, and usually takes place by small steps. There are some exceptions to this, notably polyploidy, especially in plants. [98][99]
  • Natural selection: the struggle for existence of animals and plant in the wild causes natural selection. The strength of natural selection in the wild was greater than even Darwin expected. [66]
  • Genetic drift can be important in small populations. [45]
  • The rate of evolution can vary. There is very good evidence from fossils that different groups can evolve at different rates, and that different parts of an animal can evolve at different rates. [60] p292, 397

Co-evolution Edit

Co-evolution is where the existence of one species is tightly bound up with the life of one or more other species.

New or 'improved' adaptations which occur in one species are often followed by the appearance and spread of related features in the other species. The life and death of living things is intimately connected, not just with the physical environment, but with the life of other species.

These relationships may continue for millions of years, as it has in the pollination of flowering plants by insects. [100] [101] The gut contents, wing structures, and mouthparts of fossilized beetles and flies suggest that they acted as early pollinators. The association between beetles and angiosperms during the Lower Cretaceous period led to parallel radiations of angiosperms and insects into the late Cretaceous. The evolution of nectaries in Upper Cretaceous flowers signals the beginning of the mutualism between hymenoptera and angiosperms. [102]

Tree of life Edit

Charles Darwin was the first to use this metaphor in biology. The evolutionary tree shows the relationships among various biological groups. It includes data from DNA, RNA and protein analysis. Tree of life work is a product of traditional comparative anatomy, and modern molecular evolution and molecular clock research.

The major figure in this work is Carl Woese, who defined the Archaea, the third domain (or kingdom) of life. [103] Below is a simplified version of present-day understanding. [104]

Macroevolution Edit

Macroevolution: the study of changes above the species level, and how they take place. The basic data for such a study are fossils (palaeontology) and the reconstruction of ancient environments. Some subjects whose study falls within the realm of macroevolution:

  • Adaptive radiation, such as the Cambrian Explosion.
  • Changes in biodiversity through time.
  • Mass extinctions. and extinction rates.
  • The debate between punctuated equilibrium and gradualism.
  • The role of development in shaping evolution: heterochrony hox genes.
  • Origin of major categories: cleidoic egg origin of birds.

It is a term of convenience: for most biologists it does not suggest any change in the process of evolution. [4] [105] [106] p87 For some palaeontologists, what they see in the fossil record cannot be explained just by the gradualist evolutionary synthesis. [107] They are in the minority.

Altruism and group selection Edit

Altruism – the willingness of some to sacrifice themselves for others – is widespread in social animals. As explained above, the next generation can only come from those who survive and reproduce. Some biologists have thought that this meant altruism could not evolve by the normal process of selection. Instead a process called "group selection" was proposed. [108] [109] Group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group.

For several decades, critiques cast serious doubt on group selection as a major mechanism of evolution. [110] [111] [112] [113]

In simple cases it can be seen at once that traditional selection suffices. For example, if one sibling sacrifices itself for three siblings, the genetic disposition for the act will be increased. This is because siblings share on average 50% of their genetic inheritance, and the sacrificial act has led to greater representation of the genes in the next generation.

Altruism is now generally seen as emerging from standard selection. [114] [115] [116] [117] [118] The warning note from Ernst Mayr, and the work of William Hamilton are both important to this discussion. [119] [120]

Hamilton's equation Edit

Hamilton's equation describes whether or not a gene for altruistic behaviour will spread in a population. The gene will spread if rxb is greater than c:

Sexual reproduction Edit

At first, sexual reproduction might seem to be at a disadvantage compared with asexual reproduction. In order to be advantageous, sexual reproduction (cross-fertilisation) has to overcome a two-fold disadvantage (takes two to reproduce) plus the difficulty of finding a mate. Why, then, is sex so nearly universal among eukaryotes? This is one of the oldest questions in biology. [121]

The answer has been given since Darwin's time: because the sexual populations adapt better to changing circumstances. A recent laboratory experiment suggests this is indeed the correct explanation. [122] [123]

"When populations are outcrossed [124] genetic recombination occurs between different parental genomes. This allows beneficial mutations to escape deleterious alleles on its original background, and to combine with other beneficial alleles that arise elsewhere in the population. In selfing [125] populations, individuals are largely homozygous and recombination has no effect". [122]

In the main experiment, nematode worms were divided into two groups. One group was entirely outcrossing, the other was entirely selfing. The groups were subjected to a rugged terrain and repeatedly subjected to a mutagen. [126] After 50 generations, the selfing population showed a substantial decline in fitness (= survival), whereas the outcrossing population showed no decline. This is one of a number of studies that show sexuality to have real advantages over non-sexual types of reproduction. [127]

An important activity is artificial selection for domestication. This is when people choose which animals to breed from, based on their traits. Humans have used this for thousands of years to domesticate plants and animals. [128]

More recently, it has become possible to use genetic engineering. New techniques such as 'gene targeting' are now available. The purpose of this is to insert new genes or knock out old genes from the genome of a plant or animal. A number of Nobel Prizes have already been awarded for this work.

However, the real purpose of studying evolution is to explain and help our understanding of biology. After all, it is the first good explanation of how living things came to be the way they are. That is a big achievement. The practical things come mostly from genetics, the science started by Gregor Mendel, and from molecular and cell biology.

In 2010 the journal Nature selected 15 topics as 'Evolution gems'. These were:

Gems from the fossil record Edit

  1. Land-living ancestors of whales
  2. From water to land (see tetrapod)
  3. The origin of feathers (see origin of birds)
  4. The evolutionary history of teeth
  5. The origin of vertebrateskeleton

Gems from habitats Edit

    in speciation
  1. Natural selection in lizards
  2. A case of co-adaptation
  3. Differential dispersal in wild birds
  4. Selective survival in wild guppies
  5. Evolutionary history matters

Gems from molecular processes Edit

  1. Darwin's Galapagos finches meets macroevolution resistance in snakes and clams versus stability
  • Nature is the oldest scientific weekly journal. The link downloads as a free text file, complete with references. The idea is to make the information available to teachers. [129]

Debates about the fact of evolution Edit

The idea that all life evolved had been proposed before Charles Darwin published On the Origin of species. Even today, some people still discuss the concept of evolution and what it means to them, their philosophy, and their religion. Evolution does explain some things about our human nature. [131] People also talk about the social implications of evolution, for example in sociobiology.

Some people have the religious belief that life on Earth was created by a god. In order to fit in the idea of evolution with that belief, people have used ideas like guided evolution or theistic evolution. They say that evolution is real, but is being guided in some way. [15] [132] [133] [134]

There are many different concepts of theistic evolution. Many creationists believe that the creation myth found in their religion goes against the idea of evolution. [135] As Darwin realised, the most controversial part of the evolutionary thought is what it means for human origins.

In some countries, especially in the United States, there is tension between people who accept the idea of evolution and those who do not accept it. The debate is mostly about whether evolution should be taught in schools, and in what way this should be done. [136]

Other fields, like cosmology [137] and earth science [138] also do not match with the original writings of many religious texts. These ideas were once also fiercely opposed. Death for heresy was threatened to those who wrote against the idea that Earth was the center of the universe.

Evolutionary biology is a more recent idea. Certain religious groups oppose the idea of evolution more than other religious groups do. For instance, the Roman Catholic Church now has the following position on evolution: Pope Pius XII said in his encyclical Humani Generis published in the 1950s:

"The Church does not forbid that (. ) research and discussions (..) take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter," Pope Pius XII Humani Generis [139]

Pope John Paul II updated this position in 1996. He said that Evolution was "more than a hypothesis":

"In his encyclical Humani Generis, my predecessor Pius XII has already [said] that there is no conflict between evolution and the doctrine of the faith regarding man and his vocation. (. ) Today, more than a half-century after (..) that encyclical, some new findings lead us toward the recognition of evolution as more than an hypothesis. In fact it is remarkable that this theory has had progressively greater influence on the spirit of researchers, following a series of discoveries in different scholarly disciplines," Pope John Paul II speaking to the Pontifical Academy of Science [140]

The Anglican Communion also does not oppose the scientific account of evolution.

Using evolution for other purposes Edit

Many of those who accepted evolution were not much interested in biology. They were interested in using the theory to support their own ideas on society.

Racism Edit

Some people have tried to use evolution to support racism. People wanting to justify racism claimed that certain groups, such as black people, were inferior. In nature, some animals do survive better than others, and it does lead to animals better adapted to their circumstances. With humans groups from different parts of the world, all evolution can say is that each group is probably well suited to its original situation. Evolution makes no judgements about better or worse. It does not say that any human group is superior to any other. [141]

Eugenics Edit

The idea of eugenics was rather different. Two things had been noticed as far back as the 18th century. One was the great success of farmers in breeding cattle and crop plants. They did this by selecting which animals or plants would produce the next generation (artificial selection). The other observation was that lower class people had more children than upper-class people. If (and it's a big if) the higher classes were there on merit, then their lack of children was the exact reverse of what should be happening. Faster breeding in the lower classes would lead to the society getting worse.

The idea to improve the human species by selective breeding is called eugenics. The name was proposed by Francis Galton, a bright scientist who meant to do good. [142] He said that the human stock (gene pool) should be improved by selective breeding policies. This would mean that those who were considered "good stock" would receive a reward if they reproduced. However, other people suggested that those considered "bad stock" would need to undergo compulsory sterilization, prenatal testing and birth control. The German Nazi government (1933–1945) used eugenics as a cover for their extreme racial policies, with dreadful results. [143]

The problem with Galton's idea is how to decide which features to select. There are so many different skills people could have, you could not agree who was "good stock" and who was "bad stock". There was rather more agreement on who should not be breeding. Several countries passed laws for the compulsory sterilisation of unwelcome groups. [144] Most of these laws were passed between 1900 and 1940. After World War II, disgust at what the Nazis had done squashed any more attempts at eugenics.

Algorithm design Edit

Some equations can be solved using algorithms that simulate evolution. Evolutionary algorithms work like that.

Social darwinism Edit

Another example of using ideas about evolution to support social action is social darwinism. Social darwinism is a term given to the ideas of the 19th century social philosopher Herbert Spencer. Spencer believed the survival of the fittest could and should be applied to commerce and human societies as a whole.

Again, some people used these ideas to claim that racism, and ruthless economic policies were justified. [145] Today, most biologists and philosophers say that the theory of evolution should not be applied to social policy. [146] [147]

Controversy Edit

Some people disagree with the idea of evolution. They disagree with it for a number of reasons. Most often these reasons are influenced by or based on their religious beliefs. People who do not agree with evolution usually believe in creationism or intelligent design.

Despite this, evolution is one of the most successful theories in science. People have discovered it to be useful for different kinds of research. None of the other suggestions explain things, such as fossil records, as well. So, for almost all scientists, evolution is not in doubt. [148] [149] [150] [151]

Sexual reproduction in plants

Just like animals, plants also have two types of reproduction, asexual and sexual reproduction. In their sexual reproduction, plants fuse two gametes together to create an offspring. However, unlike animals, plants are immobile and therefore reproduce sexually in a different way than animals. We will take a deeper look into the different ways animals do this later on in this article.

We also encourage you to check out this video about sexual reproduction in plants!

The Embryo Project Encyclopedia

Embryonic differentiation is the process of development during which embryonic cells specialize and diverse tissue structures arise. Animals are made up of many different cell types, each with specific functions in the body. However, during early embryonic development, the embryo does not yet possess these varied cells this is where embryonic differentiation comes into play. The differentiation of cells during embryogenesis is the key to cell, tissue, organ, and organism identity.

Once an egg is fertilized by a sperm, a zygote is formed. The zygote divides into multiple cells in a process known as cleavage, triggering the beginning of embryonic differentiation. During cleavage, the zygote divides but maintains its size in the process. This zygotic division produces blastomeres which later make up the hollow sphere known as the blastula. Cells migrate within the blastula to locations that will later define the structure of the embryo and consequent organism. In this process, called gastrulation, three germ layers arise: the endoderm, mesoderm, and ectoderm. Cells in these three layers will give rise to different parts of the organism. The endoderm eventually becomes the gut. The mesoderm develops into muscle, the skeletal system, some organs, and connective tissue. The ectoderm differentiates into the nervous system and skin.

As the embryo continues to develop, individual cells continue to differentiate. These differentiated cell types are made from what were initially the same types of pluripotent embryonic stem cells. An assortment of physiological mechanisms guides certain cells towards particular developmental pathways, creating varying cell types. This is made possible by the cell’s inherent ability to control what genes are expressed and translated into proteins. Every cell contains DNA within the nucleus, containing the blueprint to build many different proteins in the cell. Different signals can cause embryonic cells to select specific parts of the DNA which can then be used to synthesize proteins, eventually building different cell types.

Differentiation of cells in the embryo is brought about by both internal cellular factors as well as extracellular factors that act on the cell from the outside. Much remains to be understood about the exact molecular interactions that govern cellular differentiation. It is understood, however, diversifying the ratio of and types of internal and external influences on certain cells, allows many divergent cell types to arise.

There are two main types of cellular development that pertain to embryos: mosaic development or regulative development. In mosaic development (which is not characteristic of mammals, but of organisms such as annelids) differentiation occurs in steps that are set in order and progression, without input occurring between neighboring cells. On the other hand, regulative development involves the interaction of adjacent cells, within what is known as embryonic fields. The advantage of regulative development is the flexibility that it confers to differentiation. For example, a cell’s pathway may change depending on the cellular environment in which it is placed, not merely by its internal mechanisms.

The process of embryonic differentiation is crucial to proper animal development. The processes involved in embryonic differentiation continue to be explored and have relevance to studies involving embryonic stem cells and in vitro cell differentiation. As scientists continue to study the physiological mechanisms of embryonic development the process of embryonic differentiation should continue to be understood in greater and greater detail.

Who Were The First Organisms To Live On Land?

Most scientists give the credit to a group of mossy swamp-dwellers that originated underwater, but new evidence points to a mysterious life-form that lived--and died--millions of years earlier.

A new paper out in the journal Nature this week has stirred up an old debate among geologists about when, exactly, life on Earth first colonized dry land.

The conventional viewpoint is that the first terrestrial life migrated out of the water about 430 million years ago, in the midst of a period known as the “Cambrian Explosion of Life”–an evolutionary heyday when favorable conditions allowed life to swell and branch into most of the major forms in existence today. During this time, the theory goes, a group of freshwater plants inched their way onto muddy shores and into swamps and watery lowlands, and true land plants evolved from there. Before that, there was nothing living on the land.

But that’s not what Gregory Retallack, a geologist at the University of Oregon, thinks. Instead, Retallack argues that that the very first landlubbers belonged to an extinct group of organisms called Ediacara, which last lived on Earth some hundred million years before the rise of amphibious plants.

The Ediacara’s lack of living ancestors makes it a hard group of organisms to learn about. The clues that do exist come from the physical traces the Ediacara left behind. Those fossils suggest that they were small tubular or frond-shaped creatures, and that they first evolved some 630 million years ago at the end of an extraordinarily cold ice age, and that they disappeared about 90 million years later, right as the Cambrian period and its explosion were getting started.

Ediacaran Fossil: Marine Worm Or Land-Living Lichen?

But Ediacara’s faint remains also leave a lot of room for interpretation, as evidenced by the current controversy. When Retallack’s colleagues examine the fossils and their surrounding sediment, they see the outlines of marine animals articulated in hardened mud from the ocean floor. But when Retallack looks at Ediacara fossils, he sees traces of lichens, surrounded by the kind of rock that forms, not in the ocean, but on dry land.

Retallack’s Ediacara-first hypothesis is definitely the minority viewpoint, but–to quote EO Wilson paraphrasing Schopenhauer, “All truths–if that’s what you’re dealing with–are met first with ridicule, then with outrage, and finally with acceptance, saying ‘Well, it’s essentially what we knew all along.'”

Speaker Bio

Neil Shubin

Dr. Neil Shubin is a Professor in the Department of Organismal Biology and Anatomy and the Committee on Evolutionary Biology at the University of Chicago. Shubin’s research focuses on understanding the evolutionary origins of new anatomical features such as limbs. Shubin is well known for his discovery of Tiktaalik roseae,the 375 million year old fossil… Continue Reading

Watch the video: Λιοντάρια O μαγικός κόσμος των ζώων (January 2022).