Thread by @HeatherEHeying

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold I don’t tend to do remedial evolution on Twitter. But since you say you are an evolutionary geneticist, I’ll make an exception. I’ll go slow, since you seem to have missed a lot already. Consider it a public service. [thread 1/14]

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold This is an evolutionary tree. Some branches display parts of history that we never inhabited. One of those branches on this tree, for instance, terminates in the taxon called “orangutan”. We were never orangutans. But we were—and still are—apes. As are orangutans. 2/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold (Tree thinking—really grasping what phylogenetic trees are conveying, and what they are not—takes some time, but turns out to be necessary if you are to grok lineage level thinking, and macroevolutionary concepts like synapomorphy, homology, monophyly, etc.) 3/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold We are also fish. This does not mean that we are clownfish or eels or trout, or that we ever have been any of those things. Here’s another graphic that you may find helpful: 4/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold We are also reptiles. This does not mean that we are Komodo dragons or skinks or T. rexes, or that we ever have been any of those things. 5/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold Traits that are new and shared at the level of a clade (“synapomorphies”—e.g. vertebrae in vertebrates, mammary glands in mammals) are different from traits found in just a few members of a clade (e.g. sequential hermaphroditism in clownfish). 6/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold Conflating the individual with the whole is easily revealed as an error. Take facultative parthenogenesis (“virgin birth”) in Komodo dragons. If Komodo dragons can do it, then all reptiles can! And if all reptiles can, all animals can! And if all animals, all life can! 7/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold Reversals do happen, of course. For instance, snakes have lost their legs. Whales have returned to the sea, from whence they once came. Ostriches have lost flight. 8/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold Some traits, though, never, or nearly never, reverse. We have no evidence of any vertebrate species that, as an adult, has no vertebrae. Nor of any species of mammal that has females without mammary glands. Nor of any squamate male that lacks paired hemipenes (look it up). 9/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold Similarly, the *lineage* to which we belong is one long, uninterrupted string of sexual reproduction for at least the last 500 million years. Since we were indeed early fish, we have no evidence of anyone *in our particular lineage* going asexual. 10/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold The fact that, off on a couple different branches of the vertebrate tree *on which we never traveled,* there is some hermaphroditism, or parthenogenesis, does not mean that we have ever been a hermaphroditic, or parthenogenetic, species. 11/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold Furthermore, that number—that 500 million years of uninterrupted sexual reproduction? It’s conservative. In fact, our direct ancestors have probably been sexually reproducing, without pause, for somewhere between one and two *billion* years. 12/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold But evidence is extremely strong for 500 million years of uninterrupted sexual reproduction in our particular lineage, so I’m willing to forego the additional strength of those half a billion to one a half billion years. Just to be on the side of caution. 13/

@GodflyThe @JenelopeJohnson @maxrenke @SadLittleKobold To summarize: the human lineage has been sexually reproducing for at least 500 million years.

Males produce tiny zippy gametes (sperm). Females produce large, sessile gametes (eggs). And it’s been that way for a very long time. /end