Speciation

∞ generated and posted on 2016.12.22 ∞

Genetic partitioning of one gene pool into two.

As Speciation is the division of one species into more than one or at least a different species, it is important to understand what makes a species a species, with the answer being that a species displays at best only low levels of gene exchange with other species; Speciation thus is the blocking, particularly by active, genetically based means, of gene exchange between populations that formerly together made up only a single species.

This genetic partitioning of one gene pool into two – versus physical or temporal partitioning – is particularly where the two resulting populations continue to overlap in range. Speciation nevertheless is all about reproductive isolation and reproductive isolation means that the potential for members of two different populations to mate or subsequently produce successful offspring must be small.

Figure legend: Allopatric speciation. First there is a geographical barrier that splits one population into two or more. If two resulting subpopulations come back into contact, then there can can be costs to hybrization, and collectively these are described as prezygotic barriers. Prezygotic barriers then select for postzygotic barriers, and the result is speciation, i.e., the separation of one gene pool into more or less two.

The following video is confusing in spots, though note that this is a very complex subject; lots of nice examples are presented:

The following video covers the subject reasonably well, and particularly so in terms of sympatric speciation. (Caution, though, as there in fact are no puppies and the language can be a bit harsh.):

Various scenarios exist for how speciation can occur, especially as considered within an ecological context. These include:

Note also that reproductive isolation can occur by a variety of mechanisms that can be described as:

Geographical isolation does not directly involve genetic components but instead means simply that two individuals don't mate because they can't physically reach each other (kind of like a long-distance relationship, only worse). Postzygotic means that gene pool partitioning occurs despite the occurrence of conception (i.e., fertilization) while prezygotic means that gene pool partitioning occurs because conception is explicitly avoided by one or more of the parties involved.

Figure legend: The process of speciation taking into account particularly the impact of genetic drift. Note how the process typically is expected to be derailed, resulting in most cases in the extinction of especially small diverging populations.

Speciation, in a nutshell, can involve geographical isolation that gives rise, over time, to postzygotic isolation mechanisms which, in turn, then actively select for prezygotic isolation mechanisms. Two populations that display little mixing of genes due to robust prezygotic barriers or postzygotic barriers to conception – i.e., little mating success – typically will be described as 'good' biological species, with substantial testing of prezygotic isolation mechanisms potentially leading to the establishment of more effective prezygotic isolation mechanisms (what is known as reinforcement). This scenario basically is a description of what more formally can be described as allopatric speciation or peripatric speciation.

See also species concepts as well as introgression.

The following video looks at salamanders in California that are well on their way to speciation:


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