Episode 78 – Exaptation – COMMON DESCENT

Listen to Episode 78 on PodBean, Spotify, YouTube, or anywhere podcasts can be found!

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.

Feathers are the classic example of an exaptation. Fossil evidence shows feathers were not originally evolved as tools for flight. Most likely the first feathers were for insulation or display, then later aspects of feathers turned out to be useful for catching the air and gave rise to flight in birds. Image by Yoky and Roundhere44 from Wikimedia.

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.

The suture lines between skull bones in humans have been cited as an example of exaptation. Originating as a side effect of the fusion of multiple bones into a solid skull, human fetuses now use those sutures as points of flexibility to allow a baby’s big head to fit through the birth canal. Image by OpenStax College and Anatomist90 from Wikimedia.

Examples

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.

The term has also be adopted by other fields, to describe changing uses of human technologies or the way words in different languages can take on new meaning.

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.

Tetrapod limbs also show exaptation. The fish that would lead to the earliest tetrapods did not have limbs selected for the function of living on land. These early limbs were evolved in the water and very useful there, and were later co-opted for land locomotion. Image by Dmitry Bogdanov and Conty from Wikimedia.

The Debate

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!

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Brett Jones January 20, 2020 / 11:54 am

At some point I think it’d be awesome if you guys did an episode on Gorgonopsians! I honestly don’t know anyone that knows much about them.

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- commondescentpc January 22, 2020 / 9:56 am
  
  We don’t know much about them either! But we’ll put them on the list!
  
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