Response

Posted by Carlos on August 04, 1998 at 10:58:51:

In Reply to: Some questions about evolution posted by Linjun Xu on August 04, 1998 at 09:41:53:

Linjun Xu said:

We know that Human has 46 chromosomes, and other animals have like 48, 50, 34 ... All animals are different in Chromosome number. So during evolution, how many chromosomes do human ancestors have? I mean those apes :), not stone age human. And how many chromosomes do those ape's ancestors have? Do we keep 46 chromosomes thoughtout the whole evolution, starting from a single cell living? If our number of chromosomes has changed during evolution, then it sounds very strange to me. Imaging that two apes with 100 chromosomes, their sperms and eggs with 50 each. Then their children suddenly have 46 chromosomes and that is human cell. So now if Human being needs to evolve some more, how many chromosomes should we evolve into this time? If we will never evolve again, please tell me why not? Those questions must be stupid to experts, but I am not clear about that.

This is a very good question; I don't presume to be an expert, but I searched the Talk Origins web site, and found the following answer to a question similar to yours.

"A change in karyotype(number of chromosomes) is usually sufficient to produce either reduced fertility or complete sterility in crosses between the original population and the population with the new karyotype. There are many different ways that the karyotype can change. The most drastic is polyploidy, where the entire genomic complement gets duplicated two or more times. Within that, there is allopolyploidy, which happens in crosses between organisms of different karyotype, and autopolyploidy, which happens in crosses within the species. Wait a minute, you might say, didn't I just get through saying that different karyotypes were usually a bar to reproduction? Yes, the "usually" does not include allopolyploidy. Other means of changing karyotype include fusion (the merging of two or more chromosomes into one), fission, where one chromosome is divided to from two chromosomes, and replication of particular chromosomes (Trisomy-21 is the name for the retention of an extra copy of the 21st human chromosome, which causes "Down's Syndrome)."

"Certain lizard species display a rather odd arrangement. In a normally diploid (2n) species, there exists a population of female lizards that are all triploid (3n). Both the diploid and triploid females mate with diploid males. Also, the tree-frog Hyla versicolor appears to be a polyphyletic lineage (the species contains members derived from *different* ancestors) of tetraploid (4n) animals, where the tetraploidy appears to have arisen separately four times in the parent species. Strickberger mentions that Israeli mole rats contain within the population karyotypes of between 52 and 60 chromosomes. Unfortunately, there is a lot more. Reproductive incompatibility can arise due to small changes in genetics. Certain pseudo-scorpions seem to have speciated on the basis of about eleven alleles difference from the parent species. Reproductive incompatiblity may not be established even given large-scale changes in the genetics (as in the polyploid examples above). Remember the Harvard Law: Under the most carefully controlled conditions of temperature, pressure, light, diet, water, and habitat, the organism will do as it damn well pleases."

"For vertebrate animals, you are correct that speciation is normally assumed to be due to changes that accumulate within a population, and not due to multiple individuals having fortuituously undergone the same drastic mutation conferring reproductive incompatibility. However, we do have examples to the contrary (Hyla versicolor mentioned earlier). Biology doesn't lend itself to hard-and-fast rules. The flexibility of the genetic system underlying life precludes being able to neatly pigeonhole or predict (yet!) the consequences of certain crosses of species."