Driving Forces in Evolution: Mutation and Selection
The basic mechanisms in evolution based on mutation and selection (generated by factors in ecology as well as within the species itself) as well as coincidence – as such mutation itself has to be regarded – are well researched and to a high degree understood. Progress in genetics of recent are breathe taking.
Today the mechanisms of “normal” mutation are a relatively simple event to science and well understood. A mutation concerning one ore several genes is either of advantage, of disadvantage or neutral. An advantageous mutation will spread successively because mutants and non mutants can normally cross without problem. Thus a single event – mutation as such always being a single event – becomes an event concerning a species as such. But there is one kind of mutation which is problematic in as far organisms reproducing sexually are concerned. This is that kind of mutation either leading to fission ore fusion of chromosomes. However “species” is defined – and this definition is not totally general in Biology – a difference in the chromosomal number is by and large a massive hindrance to hybrid offspring.
The dead branch
Thus such a mutation is a dead branch as long as not “by incident” the same mutation is to be found among potential mates in the surrounding. But even with a pair of mates the beginning of this new species would mean an extreme “bottle neck”. Adding is the fact that mating behavior wouldn´t be limited to the same kind of mutant. As often to be found in techniques of the “war against insects” the letting free of infertile mating partners in order to bring down or reduce an insect population also in this case the mating of the specific mutants is rather unlikely, thus rendering a spreading of the mutation even unlikely with quite advantageous effects of the mutation. Nevertheless the change of chromosomal numbers is widely to be found in evolution. Some families of organisms are tending to fusion others to fission. The described paradox of the dead branch has to be dissolved by plausible scenarios.
Mutation “en masse”
This paradox can only be solved assuming the widespread happening of the same mutation. Only then the coming about of a stable population with the new chromosomal organization could be imagined. That would mean a preparation by another extended happening incidents bringing about a molecular situation favoring fusion respective fission. For these two variants could be imagined both extending the “paradigm of branches” by the “paradigm of roots”.
It is known that certain viruses are able to get incorporated into the genetic material of a cell. Does this happen with germ cells in great numbers at the same place with the same chromosomes and would this addition extend the probability of the respective chromosomal mutation then the probability of mutants meeting each other would significantly increase. Thus the splitting in two different species would be prepared by the fusion of genetic materials of very different sources thus displaying a “paradigm of roots”.
Variant II, Crossing of Distant Sub Species
The second scenario is based on ecological and geographic changes bringing two sub species again into broader contact which have been isolated for a longer time. They might have developed far going genetic differences. By a broad hybridization incidentally a tendency for this mutation may emerge or the Hybrids might directly display this mutation. Possible might also be that this mutation comes into existence by re crossing the hybrids.
In connection with this scenario there are thoughts to be found by an evolutionary “outsider” (because of his highly questionable “river ape theory”) leading to this direction (1).
Probably both scenarios have played their role in evolution bringing different numbers in chromosomes. Phenotypic effects might be little in a number of cases but a look on “Trisomie 21” throws light on possible consequences if nothing has changed but the number of chromosomes.
This approach could nourish the search for an echo of such incidents in evolution since both variants could have left detectible genetic traces. Facing the fact that Man differs by chromosomal fusion from all great Apes, Darwin´s famous word could again get a new accent:
„Light will be thrown on the origin of man“
In this context also results of genetic researches could be seen which suggest a longer ongoing phase of separation between the ancestors of Man and Chimps (2) thus being understood as the reflex of hybridization between subspecies of early Homo. A later fusion of chromosomes would not explain these results.