Ch. 18 – Sympatric Speciation and Darwin’s Finches
All allopatric speciation invokes an imporobable number of range splitting and rejoinings – this leads some to consider parapatric and sympatric speciation. Sympatric speciation is a good way to account for presence of large number of sister species in environment with many niches
1st Step: A stable polymorphism can exist in heterogeneous environments with two niches, even with adults forming a single randomly mating population
2nd Step: Evolution of reproductive isolation between populations in the two niches. May involve habitat selection, female choice, or a large number of other variables.
It’s unclear how likely sympatric speciation is to occur in nature. even well investigated models can be explained with allopatric speciation. It’s nearly impossible to reject one explanation in favor of the other as more likely.
Sympatric speciation is best investigated with polymorphic species that show current signs of splitting. However, these signs can be temporary – proceed with caution. Cactus finches appeared to be in the early stages of subdivision, but this was caused by a food shortage and intensive disruptive selection. Mating remained random, so the population could not subdivide further.
- Why Sympatric Speciation Couldn’t Occur
- niches were not different enough
- extreme fluctuations in environment prevent prolonged divergence. need temporal stability of environment for further divergence.
- random mating
- mate choice was not genetically correlated with the individual’s niche
- difference between male beak size was about 6%, which is not large enough to foster mating discrimination. 15% or so is required for coexistence
The likelihood of a subdivision like this going completely to speciation appears to be very low.
The most important factor so far appears to be temporal stability of a heterogeneous environment. Niches must be different enough to support sustained disruptive selection. This probably doesn’t occur often for vertebrates because they’re generally behaviorally flexible organisms. The between niche variation must be greater than the within niche variation.
- How it might happen in vertebrates:
- genetic drift might do what selection can’t. more things are possible with a smaller population.
- There are some genetic systems that make it more possible to split under the right conditions: species with two morphs could work, if the morphs are gene-based and they mate assortively.
