How And When Can Two Species Merge Into One? – Forbes


Could two species merge into one species through convergent evolution? originally appeared on Quora: the knowledge sharing network where compelling questions are answered by people with unique insights.

Answer by Suzanne Sadedin, evolutionary biologist, on Quora:

Two species can merge through hybridization, or through obligate endosymbiosis. It’s unlikely this would happen through convergent evolution.

Convergent evolution happens when different lineages independently acquire the same adaptations. Importantly, this convergence usually happens at the phenotypic level, not at the genetic level. That is, the genes underlying the convergent adaptation are different, even though the result appears the same for the organism. Convergence makes them superficially similar, but at the genetic level, they are generally less similar than before.

Even when convergent evolution happens within one species, the underlying genes are often different. For example, the gene for lactase, the enzyme that allows humans to digest lactose, is normally deactivated in adults, but populations that have relied heavily on milk often have mutations that cause lactase to persist into adulthood. One such mutation arose <7000 years ago and is now shared by most Europeans. But some African populations, who independently domesticated cattle, share a different set of lactase persistance mutations.

In general, organisms are classed as different species when they are separated by some form of reproductive barrier; they either cannot produce fertile offspring or do not choose to interbreed (sometimes simply because they choose different environments, like hawthorn/apple maggot flies). If these reproductive barriers are based in genetics (which is usually the case) then convergent evolution won’t generally* remove them.

Convergent evolution might also create new barriers. When you crossbreed two lineages that have the same adaptation with a different genetic basis, the results will be unpredictable. For instance, if the lineages both have a different gene that switches on the same enzyme via different pathways, their offspring could have double the optimal amount of enzyme. They won’t necessarily simply average their parents’ traits; they may have too much of some things and too little of others, and some things that simply don’t work. In other words, when two species that have convergent adaptations interbreed, there will be a lot of unpredictable interactions between their genes, even between the genes creating their convergent adaptations. Within species, this isn’t generally a problem because continual gene flow prevents any population acquiring adaptations that are incompatible with those of other populations. But when we are talking about hybridizing different species, convergent evolution is most likely actually divergent evolution at the genetic level, which if anything would decrease their compatibility.

Another thing to think about is what’s likely ecologically. For the question to make sense, you have to start with two species that are genetically similar enough that they can potentially hybridize. That means they are very closely related, and therefore probably ecologically similar. If they are evolving convergent adaptations, then they are becoming more phenotypically similar. Which means they are probably evolving in the direction of increased competition with the other species. And to hybridize, they must be living in the same environment. So this looks like a very unstable scenario; any inequality is likely to result in extinction of one parent species through ecological competition, rather than merging. At most, the extinct population might leave some genetic signature in the surviving one, which may be what happened to our fellow hominids. Alternatively, the parent species might diverge again, so that each specializes in a different niche, which could ultimately allow them to coexist in peace.

There are cases where populations have merged through hybridization. Some of these, such as the red wolves, have been considered separate species, though that’s often dubious. However, I can’t think of any offhand where convergent evolution is thought to have been involved. Even if they did converge, it seems unlikely convergence would play a positive role in the merging — it might well hinder it.

*One theoretical exception could be when the convergence directly refers to traits used in mating discrimination. For instance, Heliconius butterflies choose mates based on their wing patterns. These wing patterns are based on a small number of homologous loci in different species, and groups of different species often converge upon the wing patterns of certain distasteful species via Batesian mimicry. At least one species, H. heurippa, is thought to have arisen by hybridization between two other Heliconius species. However, in the case of H. heurippa. the parent species are not actually convergent, so whilst it makes the idea seem more conceptually plausible, this is not an actual example.

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