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. While we tend to differentiate species based on their looks, 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.
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)
Species are a group of reproductively isolated individuals, meaning that they can exchange genes between individuals of the same group, but not with individuals of another group.
How do species appear ?
Most of the time speciation occurs when a geographic barrier appears, thereby separating a population in two. Then, because the species live in different environments, they will develop unique characteristics, until they are too different from one another to exchange genes, for several possible reasons. Speciation can also occur in species of the same area, either by allopolyploid or autopolyploid speciation.
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.
They affect speciation because the species continues to evolve and to develop specific characteristics, if they are split they will certainly develop different characteristics and since they can't interbreed they won't "share" these new characteristics and will eventually develop in different species.
What are the other ways that speciation occurs ?
OlivierM 15 mars 2012 à 08:45 (CET) --RaphaelB 9 mars 2012 à 16:36 (CET)
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