Nests, Neurons, and the Evolution of Behavior. How and Why Do Brain Cells Die?Biology 

Nests, Neurons, and the Evolution of Behavior

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By Emily Willoughby

@eawilloughby

Evolution is a beautiful thing. We tend to appreciate its elegance and creative productivity most when looking backward into deep time: in our distant ancestors, in dinosaurs, and in entire strange lineages that rose and fell before humanity. To most of us, evolution is a thing that happened so long ago that it has no practical relevance to our lives today. We see its results in the bodies of living things and the relationships among them—and these results, in all their wonder, seem static, like final products at the end of an eons-long assembly line.

But evolution is still happening. It is working its magic all around us: in the emergence of hundreds of domestic subspecies from the wolf, in the sprawling diversity of tropical warblers, and in the uncanny wonder of gorillas and chimpanzees that now suffer under competition from another ape. And it is working even closer to home as well, within our own species. It has given rise to the unique sets of instructions for building and regulating the proteins that lie hidden within our cells, and to the phenotypes these proteins produce. Selection, whether natural or otherwise, requires variation to work. Whenever the cold mechanism of selection favors genetic variants for their causal effect on fitness, evolution is happening. 

Just as this process has produced a docile dog from a wild wolf, in humans too it cannot have stopped below the brain. If we assume that the present time is an unexceptional one in the genetic history of our species, individual differences in behavior must exist because they are the raw material on which natural selection works. And in a species whose success is dictated more by behavior than by canine teeth or body size, it should come as no surprise that our actions, choices, and abilities are affected by heritable variation. This vast reservoir of genetic diversity is in turn the result of millions of years of mutation and natural selection acting on our planet’s dominant species, a snapshot in geologic time of a frothing, active gene pool.

Evolution, Nests and Neurons: Variation in shape and structure of the delicate, intricate nest of the Taveta golden weaver is likely to be influenced by heritable genetic variants. Published on Wikimedia Commons by Robert Lawton under a Creative Commons Attribution-Share Alike 2.5 Generic license.
Variation in shape and structure of the delicate, intricate nest of the Taveta golden weaver is likely to be influenced by heritable genetic variants. Published on Wikimedia Commons by Robert Lawton under a Creative Commons Attribution-Share Alike 2.5 Generic license.
Understanding Behavior in the Genomic Age

In 1982, the evolutionary biologist Richard Dawkins suggested in his book The Extended Phenotype that its titular concept, normally defined as the totality of an organism’s observable traits, should be extended to all the effects its genome has on its environment. Limiting the influence of genes to an animal’s anatomy is arbitrary, Dawkins claimed. “I find that my kind of paradigm examples are things like beaver dams and birds’ nests, where I’m trying to shake people into realising that you could have a ‘gene for’ a certain shape of birds’ nest, just as surely as you could have a certain shape of beak. You could selectively breed for nest shape. You could do a Mendelian experiment. You could do an artificial selection experiment. You could take a hundred generations to breed weaver birds that make nests of a different shape.”

There is surely no single gene “for” nest-building, just as there is no gene “for” any complex human behavior. But consider what is meant by the idea that the shape of a bird’s nest is affected by its genes. The act of building is influenced by a great many things: the builder’s ability to notice and find certain materials, the sensitivity to the weight and balance of how they are laid, a tradeoff between spending less time and energy or having greater protection for the eggs, and so on. In reality a number of different genes influence each of these qualities, and together they allow a bird to form a nest whose shape is heritable in the same way that beak shape is heritable.

Evolution, Nests and Neurons
Studies of identical twins have been a cornerstone of behavior genetic research for decades, but may eventually be replaced by GWA studies. Photo published on Wikimedia Commons by user Narek75 under a Creative Commons Attribution-Share Alike 4.0 International license.

A combination of twin studies and psychometric testing has repeatedly shown us that most measurable human behaviors have some kind of heritable component as well. Of particular interest has been intelligence, as measured by an IQ score or standardized tests. Over a century’s worth of data has converged on a few key points: Intelligence is highly heritable; it is expressed via a great many genetic variants; and it is predictive of certain life outcomes. Taken together, these suggest that our evolutionary history has generated a set of genetic variants that can have relevant influence over our environment—the extended phenotype come to life.

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