Example: 1) molecule Rhodopsin in halobacteria for producing energy from light. 2) molecule Rhodopsin for vision in human. These are said to be of different lineages and their high similarity are due to convergent evolution. Obviously then over long lapses of geologic and pre-geologic time, as the saying goes, things may be evolving to a best or most efficient at least in the particular case. So I'm wondering, much is made of the similarities of genomes of high and low Species to "prove" man is descended from the lower species. But could not the acceptable concept of convergent evolution, in general, be used to explain the similarity of say the genome of an ape and that of a human?

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    $\begingroup$ For each of 20,000 genes arranged in the same way? $\endgroup$
    – David
    Dec 21 '19 at 14:27
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    $\begingroup$ there are no higher or lower species, gorilla have several advantages over humans, like a much higher genetic diversity. $\endgroup$
    – John
    Dec 21 '19 at 18:16

Because there are usually many different possible molecules that can perform nearly identical functions, it is possible but unlikely that two independent evolutionary lineages will end up with exactly the same molecule to perform the same function. If molecule A1 and A2 perform the function FA in lineages 1 and 2 respectively, and molecules B1 and B2 perform the function FB in lineages 1 and 2 respectively; and if the probability that A1 is identical to A2 is PA, and the probability that B1 is identical to B2 is PB, then the probability that both lineages use the same molecules respectively to perform function FA and FB is (PA)(PB): the product of the two probabilities.

Let's say the probability PA is 0.01 and the probability PB is also 0.01. Then the probability that the two lineages will have identical molecules respectively for each of the two functions is (PA)(PB) = 0.0001. Go through the same exercise for just ten such functions, and the result is the product of all ten probabilities: (PA)(PB)(PC)...(PJ) = $10^{-20}$, a VERY small number. So the likelihood of convergent evolution occurring on the molecular scale over a significant portion of the genome is negligible.

On the level of phenotype, though, convergent evolution is much more likely. Different molecules and different development trajectories can produce phenotypic structures that perform closely similar functions and can even resemble each other closely. Consider for example the wings of bats, birds, and insects. The phenotypes can be similar, but at the genomic level there is very little similarity.

The conclusion to be drawn is that IF there is close genomic similarity between two lineages over more than a few genetic loci, then the lineages are closely related and diverged from a common ancestor.

  • $\begingroup$ The probability you calculate over prolonged lapse of time T is extremely small. But the probability that the genome resulted from evolution from an abiotic state is probably as small. $\endgroup$
    – user56930
    Dec 21 '19 at 15:50
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    $\begingroup$ @user56930 not it's really not, convergent evolution has to be probabilistic at the genetic level, while genome complexity is a much simpler directional selection, the latter can take advantage of reinforcement effects like muller's ratchet and normal natural selection. $\endgroup$
    – John
    Dec 21 '19 at 18:22
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    $\begingroup$ @user56930 — This answer was an honest response to your question as to whether convergent evolution could explain the relationship between man and e.g. chimpanzees. Your question said nothing whatsoever about the abiogenesis, nor does this answer. It is thus quite unjustified to use the comment section of his answer, not to query it, but to introduce the topic of the probability of abiogenesis. I suggest you withdraw your comment. $\endgroup$
    – David
    Dec 24 '19 at 15:54

You are getting confused by terms.

there is a difference between convergent evolution at the genetic level and the functional level.

Rhodopsin is a group of similar protein molecules with similar structure some of which have a similar function. It is not a single identical molecule. Humans and halobacteria do not have the same rhodopsin molecule nor rhodopsin genes.

The rhodopsin molecular structure may be similar but it is not the same AND the genetics and process of its production are drastically different. halobacteria and animals have very very different rhodopsin genes, and even in animals rhodopsins genes var a great deal, but vary in a predictable pattern which mirrors their evolutionary relationships.

This is convergent evolution at a functional level, stumbling across similar configurations that produce similar results, even if the coding for that structure is drastically different. think of the similarities between insect and hummingbird wings. These are not similar genetics but similar molecules or structures.

Convergent evolution at the genetic level is extremely rare and is mostly due to the law of large number than anything else. They are always small similarities in the middle of vast seas of difference. the equivalent of two books by two different authors happening to contain the same sentence even if the entire rest of the book is different. And even then it tends to only occur in closely related species where enough of the genetics are similar that the chances of getting the same mutation twice is statistically possible.

sources: https://pubs.acs.org/doi/abs/10.1021/bi00136a001





No. First, the idea of "lower" and "higher" species is one of those "not even wrong" beliefs that we are burdened with for reasons better not discussed here. An ape, or a monkey or starfish, is no better or worse suited to its environment than a human. And we do have a good bit of evidence, such as the fossil record, connecting apes & humans independently of the genomic evidence. (Much of which was known decades before the discovery of DNA.)

@S. McGrew's answer discusses the statistical likelihood of similar genomes coming about as a result of convergent evolution. But let's approach the question from the other direction, and look at a couple of examples of actual convergent evolution.

First, the ichthyosaurs https://en.wikipedia.org/wiki/Ichthyosaur and the cetaceans https://en.wikipedia.org/wiki/Cetacea Shaped by the pressures of their marine environment, they look enough alike that a casual observer might have trouble deciding which group a particular species belonged to. Yet one group are reptiles, the other mammals, separated by hundreds of millions of years from their last common ancestor. Their bodily form is similar, but their genomes are very different.

Another, and even more widely separated, example are the hummingbirds https://en.wikipedia.org/wiki/Hummingbird and the hummingbird moths https://www.fs.fed.us/wildflowers/pollinators/pollinator-of-the-month/hummingbird_moth.shtml They look enough alike that they're hard for the casual observer to distinguish, they occupy the same habitat (I see both in my summer garden), feed in much the same way - yet one's a bird, the other an insect. Again, the similarities of appearance & behavior are the result of evolutionary pressures acting on very different genomes.

There are many other examples of convergent evolution. In most cases, if not all, the similarities have come to be as the result of similar environmental pressures acting on genomes, NOT as the result of genomes evolving to be nearly similar. That is, if you compared the genomes of two such convergent species, they would have no more in common than two non-convergent species from the same groups.


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