let us start wit Darwinian RMNS in speciation first, igoring others
Hi Jianyi,
After posting my last thread, I took a look at your website to examine the evidence you have against the involvement of RMNS in speciation.
First, you say that
Assuming speciation did occur by natural selection from unknown mechanism, the new species needs to find the same species at the same time in a very close geographical location along the evolutionary pathway across a few million years. If one fly were generated somewhere along the pathway, then it had to mate with another fly in the same species to have offspring. If the second fly were generated a few miles away, the chances of meeting would be dismal. If the second were generated in the following year, then they would die without reproduction. Had the speciation occurred according to Darwin’s theory, it would be mandatory to randomly generate a group of organisms with same genetic structure simultaneous in a very close geographical location. The probability for such thing is zero.
your mistake here is to think that speciation occurs in individual organisms. This is a significant misunderstanding of the conemporary view of speciation, in which speciation occurs between divergent
populations of organisms. all of the references I provided in the article by Wu and Ting take this idea for granted.
let me try to explain the Xiphophorus example in a more conceptual manner. I am going to depart slightly from the description given by Wu and Ting, and give an imaginary example based on the idea of a melanin oncogene. hopefully this will make the idea clearer.
Imagine you have a population of fish which use black coloration to attract mates. The gene causing black coloration has to be expressed at a fairly precise level. Not enough black coloration leads to an absence of sexual marking, and a failure to reproduce. Overexpression of black coloration leads to melanoma, and death.
In order to control the expression of the gene that produces black coloration, the fish have two regulatory genes. One is a promoter (which encourages expression of the gene) and one is an inhibitor (which discourages expression of the gene).
Now, imagine that the population of fish becomes geographically isolated. Perhaps one group of fish swim upstream, into a lake, while another group swims downstream, into some ditches. It is very rare for a fish from the lake to accidentally find itself in the ditches, and it is very rare for a fish from the ditches to swim all the way up to the lake.
In the lake population, random mutations in the promoter gene cause
slight overexpression of the gene controlling black coloration. This slightly increases the prevalence of melanoma in the population, but not to the extent that they all die of cancer. Nevertheless, any mutation causing
slight overexpression of the inhibitor gene are favoured by natural selection.
In the ditch population, random mutations cause the opposite to happen. Mutations in the promoter gene reduce its biological activity, and cause slight underexpression of the gene controlling black coloration. Any mutation causing underexpression of the inhibitor gene is consequently favoured by natural selection.
Over many generations, the two populations get quite different regulatory systems operating on the same gene. One population has an extremely strong promoter and an extremely strong inhibitor. The other population has an extremely weak promoter and an extremely weak inhibitor. Both populations express the correct amount of black coloration, because the strength of promoters and inhibitors balance each other out.
Now imagine that the two populations come back into contact. When they hybridize, you end up with two unfit hybrids.
1. An offspring has an extremely strong promoter, but an extremely weak inhibitor.
2. An offspring has an extremely strong inhibitor, but an extremely weak promoter.
The first type of offspring produce too much melanin, get cancer, and die.
The second type of offspring produce not enough melanin, are unable to find mates, and do not reproduce.
As a consequence, the two populations are reproductively isolated from each other. Only if they mate "with their own kind" are they successful in producing viable young.
This is ONE idea of how speciation might occur by RMNS. It depends upon selection upon alleles in populations, not individuals.
There is not one single mechanism of how speciation might occur by RMNS. The example I have described clearly only applies for species with an oncogene that is involved in reproductive activity. But, in principle, any gene involved in reproduction and regulated by a number of loci might undergo a similar process.
Mick
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