This understanding of the changing rules of mass extinction also impacts how we look at the process of evolution in general. Stereotypically, one imagines natural selection slowly improving species — weeding out organisms with traits not quite suited to a particular environment, favoring those with traits slightly better for promoting survival and reproduction. Over long periods of geologic time, one might imagine this process building some sort of super-organism — the “best of the best” for last billion years. However, David’s work argues against equating evolution with progress: “A lot of the factors that are honed by natural selection during the normal times do not promote survival during mass extinctions. And so that implies that all of the exquisite adaptations that we see around us are really not that likely to promote survival [in the geologic long term]. You can imagine that a lot of beautiful adaptations will be lost, not because they are poorly adapted to the vast bulk of evolutionary time, but because they happen not to be linked to the kind of factors that promote survival during those short-lived but intense mass extinctions.” So when one takes the long view (over, say, hundreds of millions of years), evolution is not particularly progressive.
On the flipside, many of the traits that make it through mass extinctions do so, not because they are particularly useful for surviving mass extinctions, but because they are found in organisms with a broad geographic range. In our example at right, spotted shells become common after a mass extinction, not because they are advantageous, but because they happened to be found on one of the few species that made it through the event. David terms this process “hitchhiking.” To see how it works, consider a genus of widely distributed, shrew-like mammals that survived the KT extinction and that soon after diverged into a variety of mammalian lineages. As paleontologists looking back after the fact, we might be tempted to chalk this group’s success up to some key mammalian characteristic — having hair, perhaps, or bearing live young. However, David’s hypothesis suggests another, less progressive possibility: the mammal genus survived, not because their special mammalian features were adaptive, but because the genus was widespread. Hair, bearing live young, and all those other mammalian traits may have simply hitchhiked through the mass extinction on the back of the genus’ geographic range. Or perhaps one trait, like having fur, helped with achieving that broad range by providing insulation against extreme climates, and the other traits (i.e., live young, milk production, complex molars, etc.) were swept along with the one trait conferring a broad geographic range. After the mass extinction, mammals would have been in the right place at the right time: niches and resources had been freed up by the extinction, and the mammals were the ones left standing — one of the few lineages still around to diversify into that space. The investigations of David and his colleagues suggest that we reexamine our assumptions about why certain groups survive and prosper after mass extinctions: is it because they were better adapted or because they “got lucky” when disaster struck?
David's research also has important implications for how natural selection might differentiate between species and clades. To learn more, visit The hierarchy of selection.
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