Re: Science has a news article up about "living fossils"

On 12/03/2024 16:04, erik simpson wrote:

On 3/12/24 6:44 AM, John Harshman wrote:

On 3/12/24 3:50 AM, Ernest Major wrote:

On 11/03/2024 23:28, John Harshman wrote:

On 3/11/24 4:17 PM, RonO wrote:

https://www.science.org/content/article/these-gars-are-ultimate-living-fossils
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Open access article:
https://academic.oup.com/evolut/advance-article/doi/10.1093/evolut/qpae028/7615529?login=false
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These researchers looked at Gar, but it also applies to sturgeons. These two bony fish lineages seem to have a very slow rate of molecular evolution.  The changes in their DNA accumulate so slowly that two lineages separated for over 100 million years can still form fertile hybrids.  3 million years is pushing it for species like lions and tigers that can still form hybrids, but the hybrids are sterile. Bonobos and chimps are around 3 million years divergent and can still form fertile hybrids, but the claim is that these fish evolve orders of magnitude more slowly than mammals.
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The Science news article claims that mammals accumulate 0.02 mutations per site per million years, while these fish averaged only 0.00009 mutations per million years.  For the 1100 coding exons that they looked at for this study these fish evolve much more slowly than mammals.
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The news article notes that other "living fossils" such as coelacanths (0.0005) evolve faster, but slower than amphibians (0.007).  It sounds like terrestrial animals evolve faster than fish.

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If it's repair mechanisms they hypothesize as the cause of slow evolution, they really should be looking at junk sequences rather than just 4-fold degenerate sites. I suggest introns. And if the introns aren't alignable, well, that kills the theory right there.
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Tree species thought to be separated by tens of millions of years are known to hybridise. For example Platanus orientalis and Platanus occidentalis, and also with Tilia, Quercus and Aesculus. In the case of Tilia I suspect that multiple rounds of introgression has served to limit the amount of divergence between species. However Tilia does appear as a short branch in cladograms, supporting the hypothesis that forest trees have a lower rate of evolution.
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Then again, ducks that are thought to be separated by tens of millions of years are also known to hybridize, and their rate of evolution isn't particularly slow.
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All sorts of seemingly long-separated species (both plant and animal) are observed.  What determines whether the hybrid offspring are fertile, infertile or sterile?  I found an article on Big Think https://bigthink.com/the-past/soviet-human-ape-super-warriors-humanzee-ivanov/ describing an unsuccessful attempt to produce a "humanzee".  Fortunately it didn't work.  The chromosome count is different in humans and chimpanzee, but does this imply that it's essentially impossible?
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 No. Hybrids between species with different chromosome counts are common. (The mule is the classic example.) They're commonly, but not universally sterile. There are even hybrids between Indian (2n=6/7) and Chinese (2n=
 There are even hybrids between Indian (2n=6/7) and Chinese (2n=46) muntjacs.
 Beyond that there are species which are polymorphic for chromosome count. (Even humans, where there are a few individuals with chromosomal fusions. They result in reduced fertility with individuals with the normal karyotype, but they're not completely sterile.)
 I studied the literature on chromosomal race some decades back. My conclusion was that most chromosomal races are cryptic species, but some represent genuine polytypic species. (That was before the latest period of ascendancy of taxonomic splitters.) The classical cases are Mus musculus and Nannospalax ehrenbergi.
 Plant high polyploids often have variable chromosome counts. One could ascribe this to inaccuracy in counting the chromosomes, or explain it as the genome being tolerant of aneusomy (not caring whether there are 8, 9 or 10 copies of a gene).
 In the case of a chromosomal fusion, during meiosis the fused chromosome will pair up with the two unfused chromosomes. If the right two chromosomes go the same way as the chromosomes separate then meiosis is successful. The tolerance of deviations from the regular diploid state during meiosis seems to vary between taxa - there are even plant species that manage to separate triploid genomes into diploid and haploid sets.