20.1 Evolution by Natural Selection

The modern theory of evolution is so completely identified with Darwin’s name that many people think Darwin himself first proposed the concept that organisms have evolved, but that is not the case. The idea that life changed over time was circulating in scientific circles for many decades before Darwin’s historic voyage. The great question was, How did life change? For some, the explanation was a series of special creations by God. To others, such as Jean-Baptiste Lamarck (1744–1829), change was caused by the environment acting directly on the organism, and those changes acquired in an organism’s lifetime were passed on to its offspring.

What Darwin provided was a detailed explanation of the mechanism of the evolutionary process that correctly incorporated the role of inheritance. Darwin’s theory of evolution by natural selection begins with the variation that exists among organisms within a species. Individuals of one generation are qualitatively different from one another. Evolution of the species as a whole results from the fact that the various types differ in their rates of survival and reproduction. Better-adapted types leave more offspring, and so the relative frequencies of the types change over time. Thus, the three critical ingredients to evolutionary change Darwin put forth were variation, selection, and time:

765

Can it, then, be thought improbable…that variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? …Can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favorable variations and the rejection of injurious variations I call Natural Selection. (On the Origin of Species, Chapter IV)3

Darwin’s writings and ideas are well known, and justifiably so, but it is very important to note that he was not alone in arriving at this concept of natural selection. Alfred Russel Wallace (1823–1913), a fellow Englishman who explored the jungles of the Amazon and the Malay Archipelago for a total of 12 years, reached a very similar conclusion in a paper that was co-published with an excerpt from Darwin in 1858:

The life of wild animals is a struggle for existence. …Perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals. …It is also evident that most changes would affect, either favourably or adversely, the powers of prolonging existence. …If, on the other hand, any species should produce a variety having slightly increased powers of preserving existence, that variety must inevitably in time acquire a superiority in numbers.4

While today Darwin’s name tends to be exclusively linked to evolution by natural selection, in their day, the theory was recognized as the Darwin-Wallace theory. Perhaps the current perception is at least in part due to Wallace himself, who was always deferential to Darwin and referred to the emergent theory of evolution as “Darwinism.”

KEY CONCEPT

Darwin and Wallace proposed a new explanation to account for the phenomenon of evolution. They understood that the population of a given species at a given time includes individuals of varying characteristics. They realized that the population of succeeding generations will contain a higher frequency of those types that most successfully survive and reproduce under the existing environmental conditions. Thus, the frequencies of various types within the species will change over time.

There is an obvious similarity between the process of evolution as Darwin and Wallace described it and the process by which the plant or animal breeder improves a domestic stock. The plant breeder selects the highest-yielding plants from the current population and uses them as the parents of the next generation. If the characteristics causing the higher yield are heritable, then the next generation should produce a higher yield. It was no accident that Darwin chose the term natural selection to describe his model of evolution through differences in the rates of reproduction shown by different variants in the population. As a model for this evolutionary process in the wild, he had in mind the selection that breeders exercise on successive generations of domestic plants and animals.

766

We can summarize the theory of evolution by natural selection in three principles:

  1. Principle of variation. Among individuals within any population, there is variation in morphology, physiology, and behavior.

  2. Principle of heredity. Offspring resemble their parents more than they resemble unrelated individuals.

  3. Principle of selection. Some forms are more successful at surviving and reproducing than other forms in a given environment.

Figure 20-2: The interplay of evolutionary forces influences variation
Figure 20-2: The effects on allele frequency of various forces of evolution. The blue arrows show a tendency toward increased variation within the population; the red arrows, decreased variation.

A selective process can produce change in the population composition only if there are some variations among which to select. If all individuals are identical, no differences in the reproductive rates of individuals, no matter how extreme, will alter the composition of the population. Furthermore, the variation must be in some part heritable if these differences in reproductive rates are to alter the population’s genetic composition. If large animals within a population have more offspring than do small ones but their offspring are no larger on average than those of small animals, then there will be no change in population composition from one generation to another. Finally, if all variant types leave, on average, the same number of offspring, then we can expect the population to remain unchanged.

KEY CONCEPT

The principles of variation, heredity, and selection must all apply for evolution to take place through a variational mechanism.

Heritable variation provides the raw material for successive changes within a species and for the multiplication of new species. The basic mechanisms of those changes (as discussed in Chapter 18) are the origin of new genetic variation by mutation, the change in frequency of alleles within populations by selective and random processes, the divergence of different populations because the selective forces are different or because of random drift, and the reduction of variation between populations by migration (Figure 20-2). From those basic mechanisms, a set of principles governing changes in the genetic composition of populations can be derived. The application of these principles of population genetics provides a genetic theory of evolution.

KEY CONCEPT

Evolution, the change in populations or species over time, is the conversion of heritable variation between individuals within populations into heritable differences between populations in time and in space by population genetic mechanisms.