WEIT 2012 Chap3

Vestiges

What are vestiges ?

Vestiges are degenerate or imperfectly developed organs or structures that have little or no utility, but that in an earlier stage of the individual or in preceding evolutionary forms of the organism performed a useful function.

Some examples:

Flightless birds

The wings of ostriches are good examples of vestiges. Like all flightless birds, ostriches are descended from flying ancestors and we know it from both fossil evidence and from the pattern of ancestry that flightless birds carry in their DNA , but they actually can't fly despite the fact that they still use their wings. Yet the wings are not useless, they've evolved new functions. They help the ostrich to maintain balance, mate , and threaten its enemies. We say then that the wings of ostriches are a vestigial trait : a feature of a species that was a useful in its ancestors, but that has lost partially or completely its usefulness , as in the ostrich , has evolved for new uses.

There are many other different flightless organisms as for examples the New Zealand kiwi , the kakapos , the ducks , and of course the penguins. The wings of some of these species evolved new functions as for example the penguin , which ancestral wings have evolved into flippers , allowing the bird to swim underwater with amazing speed. But in some , like the kiwi , the wings are so small that they don't seem to have any function. They're just remnants.

Human's coccyx

--AlexanderE 9 mars 2012 à 17:59 (CET)

What does the evolutionary theory say about vestiges ?

The opponents of evolution say that a trait can't be vestigial if it still has a function , or a function yet to be found. But in fact they missed the point. The evolutionary theory doesn't say that vestigial characteristics have no function. A trait can be vestigial and functional at the same time. It's vestigial because it no longer performs the function for which it evolved , and not because it's functionless.

--AlexanderE 9 mars 2012 à 18:57 (CET)

Why did some species lose their ability to fly ?

We're not absolutely sure , but we have some ideas. Most of the birds that evolved flightlessness live on islands - the extinct dodo on Mauritius , the Hawaiian rail, the kakapo and kiwi in Zealand , and the many other flightless birds named after the island they inhabit. As we'll see in the chapter " The geogrpahy of life " , one of the notable features of isolated islands is the small amount of mammals and reptiles - species that are dangerous predators for birds. The species that live on continents and not on islands , as ostriches , all evolved in the Southern Hemisphere, where there were far fewer mammalian predators than in the north.

Let say it easier : flight is metabolically expensive , using a lot of energy that could otherwise be diverted to reproduction. Birds mainly fly to stay away from predators , but on islands predators are often missing and food is obtained on the ground, so there is actually no real need having fully functioning wings.

--AlexanderE 9 mars 2012 à 20:40 (CET)

Atavisms

What are atavisms?

Atavisms are remnants of what were former features of the species. They don't appear regularly but are occasionally present in individuals.

Why aren't atavisms recurrent in a species?

Atavisms are due to the sporadic reactivation of genes that are no longer needed by the species and have thus gradually disappeared because of natural selection.

How does natural selection make traits disappear?

What is natural selection?

It is the process during which, in a given environment, favored species survive and unfavored ones die. The survivors reproduce much more and transmit their genome to their offspring. Because of the environment in which a species evolves, some traits show more useful than others: useful to survive from predators, to find food, or more efficient regarding energy loss.(1)

Do traits only disappear?

No. If some traits appear, other ones disappear; it all depends on the trait, case by case. However, only disappeared traits are important for atavisms, since they are reappearances.

How is it then possible that some traits appear again?

If the phenotype (the appearance) doesn't show a certain trait, it is still there in the genotype (the information); the genes related to the trait were deactivated: they "sleep" but don't disappear completely. Rarely, such genes "wake up", and an atavism is generated.

(1): http://www.biology-online.org/dictionary/Natural_selection

JohannesJR 9 mars 2012 à 16:40 (CET)

Dead Genes

What are dead genes ?

Dead gene is a gene which doesn't function. It is also called a pseudogene. Dead genes are the consequence of a trait, which becomes reduced or useless, like atavisms and vestigial traits. The genes that produce it stay in the genome, despite the fact they stop their action,in other terms their function. Normal function of a gene is to make a protein -a protein whose sequence of amino acids is determined by the sequence of nucleotide bases that make up the DNA. However if the DNA sequence of a given gene expressed anormally, it makes a unfunctionnal protein. So we called it "dead" gene.