Disclaimer: I am not a biologist. I'm interested in this topic but I haven't studied it that much. These are my random thoughts on how a species can form:

It seems to me that we only “need” new species when things are going badly. In these stressful situations, the population of an existing species will decline. A small population is more likely to lead to inbreeding/incest. What happens with inbreeding? We get increased mutations. The history of royalty in Europe has some examples (hemophilia, six fingered folk, etc.). It's exactly when things are going badly with a species that a new species has a chance, and I think it's no coincidence that mutation rates are higher then.

More specifically, I think susceptibility to mutation is an evolved characteristic. Species that “fixed” the problem with genetic errors would not evolve, and would eventually be wiped out. Only the species that had mutations would survive. Secondly, I think the mutation rate varies, and it responds to stress and inbreeding, not by accident, but because a variable mutation rate evolved as well. When a population no longer fits well into the environment, it needs to increase the mutation rate so that it can turn itself into a new species.

A consequence of this line of thinking is that when populations are large, we should rarely see new species form. We shouldn't see many new species forming until the environment changes drastically.

I also think in extreme cases, a very small population might lead to asexual reproduction with a high mutation rate. Species that allowed for asexual reproduction in rare cases are more likely to survive.

If small populations lead to new species, what would we observe?

  • If there are several populations of a species, and one of them mutates into a new species, we will see a new species and call it a “branch” on the tree of life.
  • If on the other hand the small population is the only surviving instance of a species, then as it turns into a new species, the old species will be wiped out. There may be no record of it. In the tree of life, we would only see two distinct species if the creatures are physically different and if both populations left fossils.

I think most new species are of the latter form, and never show up in the fossil record. How could I call this a new species then? If we had a time machine and brought a creature of the first form forwards in time, and it tried to breed with a creature of the second form, we'd be able to decide whether the old and new creatures are genetically compatible. If they are, I'd say they are the same species. But in a lot of cases, they won't be able to interbreed, and we have a new species. We can't really test this without a time machine.

To summarize: I think that variable-rate mutation is an evolved behavior that shows itself when populations are small and stressed, and that there have been a lot more species than the ones we see in the fossil record.

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3 comments:

bob wrote at Wednesday, March 21, 2007 at 2:28:00 PM PDT

Hello, I'm sorry to say this but I think you might be mistaken: AFAIK inbreeding does NOT produce increased mutations.
The increased probability of malformed children as in the examples you mention are not due to new mutations but rather to the following phenomenon:
Hemophilia, six fingered folk, etc. are all genetic problems that are caused by recessive genes. This means that the gene can be carried in the DNA without expressing itself or having any effect over many generations. Recessive genes express themselves only when they are passed down by BOTH parents.
This means that recessive genes can be passed down a family for a very long time without ever causing any problem.
An example of a common recessive gene is the gene for red headedness, this gene is quite common since it does not cause any malformations.
"Bad" genes, however, are very rare. The probability of two individuals from different families passing down the same recessive genes to their children is extremely low (although both are likely to carry different ones).
The problem is that if you take two individuals from the SAME family, then they are likely to both carry the same gene and thus pass it to any children they would have.

There is no magical control system in the body which is able to detect if you mated with a sibling or anything.
However, your theory is right on some points: High levels of stress, for example, are known to increase mutation levels. This has been theorized to be an evolved characteristic for the reasons you mentioned.

Amit wrote at Wednesday, March 21, 2007 at 8:20:00 PM PDT

Yes, you're right in that it's not mutation per se that shows up during inbreeding. The rate of change is higher though, because of all the recessive genes suddenly expressing themselves. I didn't know that bad genes were rare, although I wonder if the noncoding DNA contains "bad" genes that can express themselves in cases of extreme inbreeding.

Thanks for the comment!

Ben Darnell wrote at Wednesday, April 18, 2007 at 9:57:00 PM PDT

Amit, if you haven't read The Mote in God's Eye by Larry Niven and Jerry Pournelle, you should check it out - one of its main themes is the impact of small and stressed populations on evolution.