Seminar One
Biology, culture and psychology
Natural selection and crypsis

Scorpionfish on a Philippine coral reef
Scorpionfish, lying in wait for a small passing fish which it will suck into its mouth, is hard to distinguish from the colorful reef encrustations all around it, about sixty feet down on a Philippine coral reef. This photo was taken after nightfall, when the camouflage may not be adaptive, as it is during the day.
Photography by Greg and Mary Beth Dimijian

Natural selection provides the best explanation of these cryptic traits that we have. As Darwin and Wallace said, natural selection is nothing mysterious, but just the survival of genes and traits which contribute more than other genes and traits do to survival and reproduction.

You can imagine that if mutations occurred in genes which brought about a variety of cryptic features, some of those genes might confer survival value on their host better than others, in a world where predators abound. It only takes a few extra survivors to make a difference in gene representation in new generations. It is said that a trait may be highly “visible” to natural selection.

A Lonomia moth on the forest floor of the Monteverde Cloud Forest Reserve in Costa Rica
A Lonomia moth lies motionless on the forest floor of the Monteverde Cloud Forest Reserve in Costa Rica. Its wings, with a rib running down the middle (resembling a leaf midrib), are hard to tell from the dead leaves around it, and its head—almost invisible—is at left.
Photography by Greg and Mary Beth Dimijian

Natural selection can occur only if there are variants of genes in a population. These variants, or alleles, code for variations of a trait (anatomical or behavioral), and one variant may work better in the prevailing environment than another. In a sense, they are competing with each other. As generations ensue, changes in the environment will lead to different selective pressures, and different gene variants will so-called win out over others, and traits will change. Over time, organisms will evolve.

Natural selection is just as straightforward as that. And it is not the same as chance. Although mutations of genes may be mostly by chance (they occur as a result of cosmic rays, mutagenic chemicals, and copy errors when DNA is replicated), natural selection is anything but chance. This is a simplistic description of natural selection, but it serves our purposes.

Natural selection is not alone in fashioning the traits of organisms, but shares the stage with other causes of change, which include sexual selection and chance, both of which we will look at carefully soon.

A leaf-tailed gecko in Madagascar
A leaf-tailed gecko in Madagascar is active at night and scurries about low branches in search of food.
Photography by Greg and Mary Beth Dimijian
Two captive Twig Mimic snakes in Madagascar
Two captive Twig Mimic snakes in Madagascar, female above and smaller male below. When they are in trees and remain motionless, they resemble dead twigs.
Photography by Greg and Mary Beth Dimijian
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