This episode is about the complex and surprising ways that evolution can change course under new pressures, and about how scientists argue over what to call this phenomenon! It starts as far back as Darwin, and continues in debates to this day. Let’s talk about Exaptation.
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Exaptation: A History
Exaptation as a concept describes the evolutionary re-purposing of an adaptation for a new function – a trait naturally selected toward one function turns out to be beneficial for a whole other reason. This isn’t a new idea; Darwin mentioned it in On the Origin of Species, but he never gave the phenomenon a name.
The term “preadaptation” became popular to describe such features that seem to have had potential for new functions. But some argue that this word is misleading since it suggests planning or foresight in evolution.
Since then the term preadaptation has become popular to describe features that seemed to have already had potential for other functions that were eventually evolved. So, in 1982, Stephen Jay Gould and Elisabeth Vrba proposed a new term to take the place of preadaptation: exaptation
Their hope was that the term would be a handy reminder that a feature can be under different selective pressures throughout evolutionary history. Just because feathers are selected for flight today, for example, doesn’t mean they always have been. The word has been adopted by many scientists, but not everyone likes it.
For those that support the term, there are countless examples of exaptation in the natural world:
Turtle shells are important for defense, but appear to have originated with a different purpose.
Viral DNA left inside other organisms can be repurposed to benefit the host.
Sound production in some fish may have started out as a side effect of other behaviors.
The human brain surely didn’t evolve under pressure to use complex math, writing, and language skill, and yet we’ve co-opted it to do so.
Possible examples in evolution can be found across all groups of life, but it’s difficult to study the process directly, though some studies have been able to simulate exaptation using digital models.
This brings us to the reasons some scientists don’t like the term. Many consider the term too vaguely defined, too similar to regular adaptation. Some have pointed out that most, if not all, adaptations could be argued to be exaptations if you go back far enough, and thus all adaptations could be considered exaptations. Others argue that, since adaptations are often under multiple selective pressures, it’s difficult and perhaps impossible to know for sure what pressures led a trait to develop.
Others argue that the distinction is important, that while an adaptation is under pressure to help a species’ fitness, an exaptation is specifically a feature that becomes adaptive after evolving for a separate purpose. Some also argue that the term is an important reminder to consider complicated solutions to evolutionary questions.
Still others have suggested redefining the term to something more agreeable to everyone. And the debate continues today!