lostaquarium
Quite flawed
Thanks, that was very helpful! That question's been bothering me for ages. You answered it exactlyYour last sentence is wrong. An animal in which two chromosomes have fused can mate successfully with members of the same species with unfused genes. This kind of fusion happens rather frequently (look up Robertsonian translocation). Exactly what happens when they mate depends on the chromosomes involved and the species. The worst, and most common, case is that there will be a substantial reduction in fertility, which will be enough to keep the fused chromosome from spreading in the population (i.e., natural selection will prevent its spread). Note that even in this case, such a mating will produce normal offspring, some with and some without the fused chromosome. (You can be normal with a fused chromosome provided you have exactly one copy of the two chromosomes that make up the fused chromosome.) In other cases, there may be little or no loss of fertility, and the fused chromosome can spread in the population, either decreasing or increasing randomly in frequency. In still other cases, the fused chromosome can provide a selective advantage, and therefore increase in frequency, or can increase in frequency because of something called meiotic drive.
These are not just speculations. There are species in which chromosome number varies, sometimes by a lot, within members of the species, and scientists have studied what happens when different sets of chromosomes are combined. European shrews, for example, form a wild collection of races with different chromosome counts, and wild mice have several known Robertsonian fusions propagating successfully. In neither case does there seem to be any difficulty with mating between individuals with different chromosome counts.
I knowI should also point out that scientists are not (by and large) extremely stupid. If there were really such an obvious and trivial problem with evolution, the theory would have been rejected long ago.
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