Spandrels: This is NOT an evolutionary adaptation #Evolution #Adaptation #Biology

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Here I share with you a really neat example of a non-adaptive trait I found in giant, wild agave plants of Ecuador. Normally when I teach evolution, I focus on adaptations because they're so fun to think about, but many traits that organisms have are not directly selected for. They're often called "spandrels". In many cases, it's hard to tell if a trait is an adaptation or a spandrel, but in the case I show here in the agave plant, things are pretty clear.

Here's a bit of background:
"Adaptive evolution", or what Charles Darwin called "evolution by natural selection", happens when some mechanism (usually a mutation) generates a heritable trait that happens to help an organism survive better and/or reproduce more effectively than its neighbors, causing the trait to become more common in the population over generations.

"Evolutionary drift" (often called "genetic drift") is when a mutation changes an old trait in a neutral way or produces a new trait of no particular value. Sometimes neutral mutations become more common in a population by chance, despite their lack of value. This happens most often in small populations.

"Evolutionary Spandrels" are features that weren't directly shaped by natural selection; they just happen as side-effects of development or of the evolution of linked traits. If you do the math, you see that when a new spandrel pops up, if it is to stick around, it can't have a negative effect on those who inherit it, or if it does, that negative effect can't outweigh the beneficial effects of traits that were packaged with the spandrel. Your bellybutton, for example, might not be your prettiest feature, but having an umbilical chord was a huge plus when you were a fetus!

Genetic drift and spandrels show that sometimes, evolution can be random. There is plenty of functionless noise in our phenotypes.

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