The newfound respect for hybridization and introgression in mammals

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An article in Quanta magazine in August by Jordana Cepelewicz is a very readable account of scientists’ newfound respect for hybridization and introgression in many lineages of mammals and insects: “Interspecies Hybrids Play a Vital Role in Evolution”.

I’m really proud to have been among the first to bring the term “introgression” into the conversation about human evolution. The evidence from hominins like Neandertals and Denisovans was part of the forefront of this new respect for hybridization, because of the investment in the Human Genome Project and development of ancient DNA in hominins first.

What has been happening over the last five years is a huge increase in genomic data from other mammalian families, where–surprise!–introgression is just as widespread as in hominins.

For example, Cepelewicz writes about new research on the phylogeny of the big cats and the evidence for introgression in jaguar evolutionary history:

Some of these adaptations, however, may not have originated in the jaguar lineage at all. Eizirik’s team found evidence of many crossings between the different Panthera species. In one case, two genes found in the jaguar pointed to a past hybridization with the lion, which would have occurred after their phylogenetic paths had forked. Both genes turned out to be involved in optic nerve formation; Eizirik speculated that the genes encoded an improvement in vision the jaguars needed or could exploit. For whatever reasons, natural selection favored the lion’s genes, which took the place of those the jaguar originally had for that trait.
Such hybridization illustrates why the Eizirik group’s delineation of the Panthera evolutionary tree is so noteworthy. “The bottom line is that this has all become more complex,” Eizirik said. “Species eventually do become separated, but it’s not as immediate as people would frequently say.” He added, “The genomes we studied reflected this mosaic of histories.”

The mosaic of histories is a nice way of expressing what in hominins we’ve been calling the “braided stream”. The fact is that different parts of our genomes have different histories. Some of our genes spent time in very different lineages of ancestral hominins.

Braided stream
A braided stream. Photo Credit: Sam Beebe, Ecotrust via Compfight cc

The same is true in elephants, in cats, in cattle, and dogs, and bears, and basically every familiar mammalian family with enough living members to compare genomes.

Related: “What is the ‘braided stream’ analogy for human evolution?”