Nov 17 2022

New Method of Speciation

Published by under Evolution
Comments: 0

Evolution requires that speciation events occur – events in which one species becomes two. All that is required for a speciation event to occur is that two populations of the same species stop interbreeding. There are two basic types of speciation: allopatric, where the populations are physically separated by geography, and sympatric, where they live in overlapping ranges but either can’t or don’t interbreed. For the purpose of speciation, interbreeding means producing fertile young.

Allopatric speciation is easy to understand. Most species have a large enough range that they are spread out into definable populations. They may even develop definable characteristics. Populations on the edge of a range, say a prairie species pushing into the desert, will likely develop some adaptions not possessed by the main population. At some point these adaptation may push the population into a range that does not overlap with the parent population. It also may happen that environmental change may doom the parent population to extinction, but the subpopulation’s adaptations allow them to survive as a new species. Sometimes geography simply changes, physically separating species (canyons open up, mountains rise, rivers change their course, land masses move).¬† Sometimes physical separation may be abrupt, such as when plants and animals find their way to islands and set up a new population, adapting to the new environment (like the Galapagos).

Sympatric speciation has been trickier to understand. Pollen will spread, animals will interbreed. It’s what they do. Research has focused on genetic events that make two populations unable to interbreed, because their offspring would be infertile. This will happen after species diverge sufficiently, but how will they diverge in the first place if they are exchanging genetic material? There must have been some genetic event, even in an individual, that instantly created genetic incompatibility. In plants this is commonly autopolyploid speciation, where the chromosome number is accidentally doubled during reproduction. The offspring cannot interbreed with the parent species because of chromosome number incompatibility. This is why some plants, like potatoes, can have very high numbers of chromosomes.

There is also allopolyploid speciation, where two closely related species breed creating a hybrid offspring that cannot breed with either parent species. This is also far more common in plants, but can happen rarely in animals. How can this happen if the new individual cannot breed with either species? Most of the time, the hybrid individual is an evolutionary dead-end, unable to produce fertile young. But given the number of individual animals and millions of years, extremely rare events can happen. For example, multiple siblings of a hybrid event can breed with each other. Or a rare but successful backcrossing event with one of the parent species occurs. You only need a few compatible individuals to have a chance of kickstarting a new species.

This brings us to the news item – can we have sympatric speciation without genetic incompatibility? This would have to be entirely behavior based. This is called sympatric homoploid speciation, because the new species has the same number of chromosomes as the parent species. There are rare cases of this that are known, mostly in insects. Researchers now report a case of sympatric homoploid speciation in Midas cichlid fish in Crater Lake Xilo√°. This was apparently caused by a hybrid (but still homoploid – so both species have the same number of chromosomes) event – two cichlid species interbred and created fertile offspring. The offspring likely backcrossed with one of the parent species. But the result was a new population of cichlid fish that does not interbreed with either parent species.

How does this happen? Mate selection in these fish is determined by coloring and other physical characteristics. The original event was likely a case of “mating error” but generally these fish will stick with their physical kind. The resulting hybrid looked very different from either parent species, and so once stabilized as a new population breeds only within its own population – therefore being a new species. Further, the new species has unique genetic characteristic, not present in either parent species. The new species was therefore reinforced by some new mutations.

Further, the physical characteristics of the new hybrid species allows it to occupy a niche different than either parent species. This is a small lake, about a kilometer across, so competition is fierce. But the new species has a tail morphology that allows it to swim faster than either parent species. This allows it to pursue food sources that the parent species do not – therefore they do not compete with each other.

The situation is a perfect storm for homoploid sympatric speciation – the species are close enough to produce fertile young but are separated by color markings and behavior that keep their populations mostly separate. A mating error, however, was coupled with a hybrid with unique coloring, novel genetics, and behavioral differences that allow it to occupy a new niche in the lake. Now we have a new species.

No responses yet