A physical difference in organs is evidence but not a proof of selection pressure on that specific organ. Harmless mutations and path dependency could cause divergence. Occasionally genes are responsible for several things at once.
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Replying to @CovfefeAnon @ImmuneHack
No. Many genes are pleiotropic. As a consequence, a change can in sttucture could carry no effect because it is a side effect of selection acting on a completely different function. On top of that, there is drift https://www.nature.com/scitable/topicpage/pleiotropy-one-gene-can-affect-multiple-traits-569/ …
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Replying to @LoCtrl @ImmuneHack
Right, you're insane. "No, you see brain structure and size differences by human species are caused by differential selective pressure in other areas that cause them but those differences then have no functional results."
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Replying to @CovfefeAnon @ImmuneHack
Look I was an evolutionary biologist in my former life. One of the first things you learn in evolutionary biology is to be careful about inferring specific selection pressure from a single difference in morphology.
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Regarding brain size — let me show specifically what it means to be careful. Men have larger brains than women. Does it mean men were naturally selected to be smart, unlike women? (This is not a question whether they are actually smarter). The answer is “no.” Why?
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In most species of mammals (just look at your cat), males have larger skulls. They need large skulls for biting and even simply intimidating opponents because they get into more fights. Let’s say there was a gene that determined “growth rate in head area, including brain.”
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That gene would be selected for and would drive larger skull size *and* larger brain size. But then an Internet amateur RW sleuth steps in and says: “Large brain = more smarts. Obviously male brains are bigger because they were selected for IQ. How could you not possibly see it”
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Replying to @LoCtrl @ImmuneHack
Larger brains are correlated with more intelligence. Replicated in tons of studies, makes complete evolutionary sense because brain tissue is metabolically expensive. Men are larger than women but still have larger brains per body weight and are a bit more intelligent on average
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Replying to @CovfefeAnon @ImmuneHack
As I said directly above, this is not a question of who is more intelligent. This is a refutation of your claim that appearance of some trait is always a direct result of natural selection *for* that specific trait.
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Much more complicated talking about the differences between men and women because they share a lineage. Far easier to evaluate causes of a difference across species.
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Replying to @CovfefeAnon @ImmuneHack
My example trivially translates to differences between species.
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