WEIT 2012 Chap7

THE ORIGIN OF SPECIES

A species ? What's that ?!

First of all, we will define a species as a group of population whose members have the potential to reproduce with one another in nature, and produce fertile offspring (this can be illustrate by the famous byword that says that cats don't generate dogs ! Meaning that cats as they reproduce their offspring will inherit half of each of their parents genetic informations. Thus when both parents are cats, generating dogs is IMPOSSIBLE !

We tend to differentiate species based on their looks, however, a woman coming from Japan can marry a French man and they can have children together even if visually the two parents don't look the same. And as mentioned previously, defining species by their appearance is possible, but there exists some animals such as the eastern meadowlark and the western one (two species of birds living in North America) are extremely similar in appearance, however they do not interbreed (which means reproduce with one another)! And conversely, sexually dimorphic species, species whose male and female look different, but still reproduce together, such as the painted bunding. (see appendix 1)

To sum up the definition of species we'll say that species are a group of reproductively isolated individuals ; meaning individuals that can exchange genes only between individuals of the same group.

How do species appear ?

After defining what was a species, we must explain how they appeared. The process of speciation is due to two main kind of barriers : the reproductive barriers and the geographic barriers.

Most of the time speciation occurs when a geographic barrier appears Speciation can also occur in species of the same area, either by allopolyploid or autopolyploid speciation, which will be discussed later.

What are geographic barriers and how do they affect speciation ?

Geographic barriers are natural elements such as mountains or seas which split one or several species in different places. For example, a population of a certain species of plant could be split in two if an island split in two because of erosion, or if a mountain appeared because of an earthquake. Geographic barriers affect speciation because, they separate a population in two. Then, because the populations live in different environments, they will develop unique characteristics. These characteristics won't be shared between the two populations, since they are not in the same place, and sooner or later, they will be too different from one another to exchange genes, even if they encounter each other again. For example...

This also means that man does not come from monkeys in the way we usually mean. Current monkeys are not our ancestors, but rather we come from a common ancestor, who the went to form separate species of men and monkeys.

What about hybrids ?

--RaphaelB 9 mars 2012 à 16:36 (CET)

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