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From: Ernest Major <{$to$}@meden.demon.co.uk>
Newsgroups: talk.origins
Subject: Re: Science has a news article up about "living fossils"
Date: Tue, 12 Mar 2024 19:41:53 +0000
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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
>>>>>
>>>>> Open access article:
>>>>> https://academic.oup.com/evolut/advance-article/doi/10.1093/evolut/qpae028/7615529?login=false
>>>>>
>>>>> 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.
>>>>>
>>>>> 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.
>>>>>
>>>>> 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.
>>>>
>>>> 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.
>>>>
>>>
>>> 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.
>>>
>> 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.
>>
> 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?
> 

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.

-- 
alias Ernest Major