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I'm reading this web page which talks about phylogenies and DNA sequencing:

https://bioinf.comav.upv.es/courses/biotech3/theory/phylogeny.html

A certain sentence says, "If we consider dolphins and sharks to be closely related because they share a similar shape we would be mistaken."

If we were to take the DNA sequence of the dolphins and sharks and compare the genes encoding for shape, wouldn't they be very similar? Wouldn't we then be able to conclude that dolphins and sharks are similar?

Or is the workaround that we would examine the entire genome, i.e. the entire DNA sequence, to see if that's the only similarity they have?

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Genes reflect a phylogeny of their own. So no, the genes for "shape" (if they existed) would be very different between dolphins and sharks. Every single gene in a dolphin is more similar to its homologous counterpart in a cow than to any gene in a shark. Gene by gene, or with the entire genome, it would not matter. The similarities between sharks and dolphins are due to convergence. Convergence make sense in that there is selective pressure to have streamlines bodies to swim through the water. It is possible for genes to converge as well, but the nucleotides would differ, just as the fins on sharks and dolphins differ.

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There really aren't "genes encoding for shape". If you looked at genes encoding important metabolic pathways, it would be clear that dolphins are mammals, and sharks are not.

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  • $\begingroup$ Ok, I see. But for example if we took really related organisms, let's say marsupial wolves and placental wolves, which are two completely different species that are unrelated. Couldn't we mistakenly conclude that they're extremely closely related even if we were to sequence their genomes? For example, we'd see that they have perhaps similar teeth, or fur, etc. $\endgroup$ – DeepLearner Dec 7 '17 at 0:06
  • $\begingroup$ If you looked at all the coding genes, and the patterns of synteny, you would see that marsupial wolves and placental wolves looked like they belongs to different subclasses. External similarities mask differences present at the DNA level. $\endgroup$ – swbarnes2 Dec 7 '17 at 0:32
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    $\begingroup$ There are no unrelated organisms. Wolves are related to ferns, which are related to bacteria, because life is monophyletic. Marsupial wolves and placental wolves look alike because of convergence on an efficient phenotype. However every single gene in a marsupial wolf is closer to those of Koalas than it is to a placental wolf. $\endgroup$ – Karl Kjer Dec 7 '17 at 1:36

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