Speciation.

Biological Species Concept:
A species is a set of interbreeding populations that are reproductively isolated from other such populations. This reproductive isolation must persist even if the populations are brought into contact.

This definition focuses on processes going on within the species that allow it to evolve differently from other species. No reference is made to diagnosable characters that a taxonomist would use to build a classification.

Speciation bas been observed, both in the wild and in laboratory populations

Examples:

Six individuals collected from a harbor in southern California in 1964.
These were used to found a laboratory population of >1000.
By 1991, the lab population could no longer interbreed with the wild population.
There was some premating isolation, and complete postmating isolation.
Chromosomal differences were also observed between the lab and wild populations.

Speciation in Mosquitoes.

Reproductive Isolation

• Prezygotic - Fertilization never occurs.
• Postzygotic - Fertilization occurs but hybrids have low survivorship or fertility.

Note that we are talking here about organisms that, at least occasionally, reproduce sexually.

Prezygotic isolation may result from:

• No mating
• No fertilization

Postzygotic isolation may involve:

• Hybrids do not develop or survive well
• Hybrids are sterile

Patterns of speciation:
Allopatric
        Populations initially isolated by geography

Parapatric
        Populations have ranges that are largely separate but with some overlap.

Sympatric
        Populations have greatly overlapping ranges, or one is nested within the other.

Allopatric Speciation:
Reproductive isolation initiated by geographic separation.

There are many cases where this has clearly happened.
For example: There are many pairs of species of marine snail species that have the property that:
- Each member of the pair is more closely related to the other than to any other species.
- One member of the pair is on the Atlantic side of the Isthmus of Panama while the other is on the Pacific side.

Subsequent evolution may make it so that populations do not interbreed even if brought back together.
Selection and drift may both contribute to this.

Buildup of reproductive isolation in Allopatry
We can look at populations that have been isolated for different amounts of time (and thus show different degrees of genetic differentiation), and cross these in the lab to determine where in the reproductive cycle reproductive isolation builds up first.

In Drosophila and the fish that have been studied, hybrid viability (survivorship of the offspring of the cross) drops off first.
In Rotifers, fertilization success drops off first, followed by propensity to mate (which starts to decline only when fertilization success has become very low).

In all cases studied, reproductive isolation increases gradually with genetic distance.

This suggests that it is the accumulation of many small changes, rather than one big change, that completes the process of speciation.

Haldane's Rule
For organisms with chromosomal sex determination, Haldane's rule states that as isolated populations diverge, post zygotic isolation (low hybrid survivorship or low hybrid fertility) appears first in the heterogametic sex.

This pattern is seen in ~95% of organisms that have been studied.
It applies both in cases where males are heterogametic, such as Mammals and Drosophila, and in cases where females are heterogametic, such as Birds and Lepidoptera.
[Heterogametic means having different sex chromosomes. In Mammals, males are heterogametic, since they are 'XY' while females ('XX') are homogametic. By contrast, in Birds males are 'ZZ' (homogametic) while females are 'ZW' (heterogametic)].

Explanation of Haldane's rule: Most mutations that create incompatibilities between diverged populations are recessive; they are thus initially masked in the homogametic sex. In the heterogametic sex, though, such mutations are expressed as soon as they appear, since there is no masking allele on another chromosome.

Bateson-Dobzhansky-Muller (BDM) model
It is possible for populations to drift to a point at which they are reproductively isolated without either one having to go through a fitness valley.
this requires epistasis, meaning non additive interactions between different genes.

For example, consider two loci, A and B, such that allele A₁ is compatible with both B₁ and B₂, and B₁ is compatible with both A₁ and A₂, but A₂ is incompatible with B₂ - meaning that an individual with both an A₂ and a B₂ has very low fitness.

Now imagine an initial population fixed for A₁ and B₁.
If this population is divided in two, A₂ could appear and go to fixation (due to drift or selection) in one population, and B₂ could go to fixation in the other. If the descendant populations are brought back into contact, some hybrids would have both A₂ and B₂, causing reduced fitness.

As more and more such pairs of incompatible alleles get fixed in the populations, the ability to interbreed when the populations are reunited declines.

Reinforcement

Even if hybrids show some viability and fertility, there is selection to avoid hybridization since it leads to low fitness offspring. This sort of selection to avoid interbreeding because of reduced hybrid fitness is called "reinforcement".

Evidence for reinforcement includes the observation that, for pairs of closely related species whos ranges show partial overlap, individuals in the region of overlap between the species are much more discerning in their mate preferences than are those who do not normally encounter the other species.