Natural Selection

The characteristics of birds result from evolutionary processes, the most important process being natural selection. It was Charles Darwin who first pointed out that just as stockmen shaped their herds by selecting which animals would be allowed to breed, so too nature shaped all organisms by "selecting" the progenitors of the next generation. Darwin's thinking had been influenced by the great economist Thomas Malthus, who emphasized the capacity of people and other organisms to multiply their numbers much more rapidly than their means of subsistence. Darwin realized, therefore, that most individuals born of any species could not have survived long enough to reproduce. He concluded that those that had been able to survive and reproduce had not been a random sample of those born, but rather variants especially suited to their environments.

Darwin knew nothing about genetics; the work of Gregor Mendel remained undiscovered until early in this century -- almost 50 years after the publication of Origin of Species. We now know that variation among individuals is due to both environmental and hereditary factors. The latter result from the joint action of mutation (changes in the genes themselves) and, in birds and all other sexually reproducing organisms, recombination. Basically, recombination is the reshuffling of genes that occurs during the process of sperm and egg production. Because of mutation and recombination, each individual bird is genetically unique -- that is, each has a unique "genotype." Geneticists typically examine only a small portion of the genetic endowment of an individual, such as the two pairs of genes (out of many thousands) that cause a cock's comb to be single and large or pea-shaped and small. Thus one might speak of the "single-comb" and "pea-comb" genotypes.

In modem evolutionary genetics, natural selection is defined as the differential reproduction of genotypes (individuals of some genotypes have more offspring than those of others). Natural selection would be occurring if, in a population of jungle fowl (the wild progenitors of chickens), single-comb genotypes were more reproductively successful than pea-comb genotypes. Note that the emphasis is not on survival (as it was in Herbert Spencer's famous phrase "survival of the fittest") but on reproduction. Thus while selection can occur because some individuals do not survive long enough to reproduce, sterile individuals also lack "fitness" in an evolutionary sense, as do individuals unable to find mates. We emphasize that fitness here refers only to the reproductive success of a kind of individual -- if big, handsome, male grouse madly displaying on a lek turn out to have fewer offspring than smaller, drab males that skulk in the bushes and waylay females, it is the wimpy males that are more fit.

Natural selection provides a context in which to view the physical and behavioral characteristics of birds. Whether it is the large size of a female Harris' Hawk in comparison with the male, the territorial behavior of a sandpiper, the bill shape of a Clark's Nutcracker, or the coloniality of a Common Murre, a key question to ask is "how did natural selection manage that?"

SEE: Sexual Selection; Coevolution; Size and Sex in Raptors.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.