WEIT Chap7
The origin of species
In order to understand evolution, we have to look at the origin of species and what kind of mechanism they have developed in order to achieve speciation.
What is a species?
"Species are a group of interbreeding natural population that are reproductively isolated from other such groups". This is Mayr's definition but in other words we can define a species as a population or group of populations whose members have the potential to interbreed with one another to produce fertile offspring but who can't interbreed with members of other species. reproductively isolated means that members of different species have differences in appearance, behavior or physiology that prevent them from successfully interbreeding while members of the same species can interbreed readily.
What is speciation?
Speciation is the differentiation of one species from it's parent. Causes for this mechanism are mainly geographic.
There are two different types of speciation:
- Allopatric speciation is a physical isolation from one species to another. Geographic changes like a mountain or a creeping glacier ,for example, may isolate a species from the precedent generation.The smaller and the more isolated the species is, the more likely this species is to have it's gene pool changed by genetic drifts and natural selection. However even if a small, well isolated species survives to this allopatric speciation it does not necessaraly evolve into a new species, it can adapt to the new conditions. So, if speciation is due to geographic separation, the new species will not breed with it's ancestral population even if the two species come back into contact. One example of allopatric speciation is the different species of squirrels that live at the two opposite parts of grand canyon.
- Sympatric speciation is due to genetic changes during mitosis. Sympatric speciation can occur in a single generation if a genetic change produces a reproductive barrier between mutants and the parent population.it is hard to observe it in the nature because it's difficult to split one gene pool in two while the two members remain in the same area. One way to observe sympatric speciation is to look at habitat island. The land is too small to generate geographical barriers so any speciation mechanism is sympatric.
An example of sympatric speciation is a fish who lives in small isolated lakes in Cameroon in each lake there are miniradiations of species who are descending from a commun ancestor.
What is biodiversity?
Reproductive barriers
As discussed in class, there are two different kinds of barriers that can prevent interbreeding and maintain reproductive isolation. Those barriers are called isolation mechanisms. There are two different kind of mechanism.
- premating isolation mechanism (prezygotic barriers) due to geographical, ecological, temporal, behavioral isolation or mechanical incompability. for example: some species can't mate with others because they have different matings rituals. Or Some species can't mate because they don't breed at the same times. Also, some species can't mate because they don't occupy the same habitats.
- postmating isolation (postzygotic barriers) due to gametic infetility, hybrid inviability or hybrid infertility.for example : an hybrid can fail to survive to maturity or for gametic incompability : some species have in their sperm a protein that allows them to bind ONLY with an egg of the same species.
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