MadSci Network: Evolution

Re: Could Neutral genes be involved in Speciation?

Date: Wed Jan 26 11:35:13 2000
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Roche Molecular Systems
Area of science: Evolution
ID: 947948017.Ev

Well Lucy, this isn't an easy question to answer. It is complicated by the lack of much information about about what goes on at the genetic level during speciation. I think that the answer is "yes", but I wish you had elaborated on your question a bit, so I knew a bit better what you were asking. All the same, I'll give it my best shot.

The concepts, of neutrality and speciation are both important for a clear understanding of evolution. In answering your question, I'll briefly descibe and define what these terms mean, so please bear with me if you already know some of this.


Although your question asked about neutral genes, we really talk about neutral alleles, or neutral mutations. What we call a "gene" is most often a DNA sequence that encodes information used to direct the synthesis of a particular protein. A given gene can have many sequence variants, called alleles, which arise via the process of mutation. When an organism enjoys an increased or decreased degree of reproductive success because of the alleles it has inherited, natural selection is said to be operating on that organism.

Many mutations have negligible effect on an organism's ability to reproduce, and are not affected by natural selection. These are neutral mutations, and they are found on neutral alleles. This was first suggested by Kimura in 1968, and is part of what is called the Neutral Theory of Evolution.

Other mutations do affect an organism's reproductive ability, and natural selection operates on these deleterious or advantageous mutations and the alleles in which they are found.


Speciation happens when two closely related populations of organisms become incapable of inter-breeding. There are many different reasons why this might occur, but basically, when two populations are no longer capable of exchanging genes, they can be said to be separate species.

There are two ways that reproductive isolation and speciation can come about. The more common way, called allopatric speciation, ocurrs when a population is separated into two geographically isolated areas. Once separated, the gene pool of each population is free to change independently of the other. When the gene pools have changed to the point where interbreeding is impossible, the two populations are said to have speciated.

The other way that speciation comes about is called sympatric speciation, and occurs when two populations become reproductively isolated without necessarily being geographically isolated. Sometimes this happens when a subset of organisms change some aspect of their behavior, such as feeding at the bottom of a lake instead of at the top, or laying eggs on a new sort of plant. Becuase this form of speciation consists of physical separation on a very small level, it is also called micro-allopatric speciation. As was the case with allopatric speciation, once the two populations are separate, their gene pools are free to change independently of each other.

When we talk about gene pools changing, we are referring to the actions of natural selection (which affects the levels of advantageous and deleterious alleles), and genetic drift (which can affect the levels of all alleles). Genetic drift is a random process that can result in the relative levels of alleles going up or down, and has a much larger affect in small populations. If a sub-population undergoes a large degree of genetic drift, due to its small size (a population bottleneck) or close relatedness of the members of that sub-population (a founder effect), the relative proportions of many alleles can change dramatically.

What does it all mean?

So, my interpretation of your question, "can neutral genes be involved in speciation", is that you are asking if individual alleles that are not under the influence of natural selection can play a vital role in preventing the exchange of genes between populations.

I think that this could be true under certain circumstances. In particular, I think that you could have a sympatric speciation if a particular behavior resulted from the expression of a neutral allele, and if genetic drift was on your side. Imagine a species of fish with a range of sizes, and that a fish's size is controlled by a gene with a set of neutral alleles (mind you, I am just making these hypothetical fish up for the sake of argument); large fish have one set of alleles, small fish have another, and the intermediate sized fish have some of each set. Lets say the size of the fish determines where they swim and what they eat. Big fish eat large bugs on the surface, medium sized fish eat things swimming in the water, and the smallest fish eat tiny creatures that live in the mud at the bottom of the lake. Accept that there is plenty of food to go around, and that size doesn't matter as far as reproducing goes.

Now, suppose we have a drought, most of our lake dries up, and two groups of fish survive in two isolated pools. The survivors in one pool are all small, and the survivors in another pool are all large. Eventually, the lake fills again and the pools are re-united, but there are no intermediate sized fish with a combination of the two sets of alleles, so the descendants of one group of survivors lives at the top, and the descendants of the other group lives at the bottom, and they never inter-breed. Eventually, these two populations could diversify to the point where inter- breeding is impossible, but the initial barrier to reproduction would have been due to the effect of dramatic genetic drift on a neutral allele. Note that had things happened differently, and there had been just one group of survivors from the middle level of the lake, there would not have been a separation after the lake filled in. The middle level survivors would have had alleles from both the large and small set, and a few generations would have seen the regeneration of fish of all sizes.

That is a hypothetical model that I just made up while thinking about your question, but there are probably examples of phenomena like this in the real world. I hope this helps clarify some of the issues involved -- random chance (manifesting as genetic drift) often has as much to do with evolution as a more mechanistic-seeming force like natural selection.

Some References

Kimura, M. Evolutionary rate at the molecular level. 1968. Nature : 217 : 624-626.

Kimura, M. The neutral theory of molecular evolution. 1983. Cambridge University Press, Cambridge.

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