DEMOGRAPHY: INFLUENCES ON POPULATION SIZE AND COMPETITION FOR LIMITED RESOURCES
After returning to England and while developing his ideas on natural selection, Darwin read the works of all the great scientists of the time. Probably the most important influence on his ideas was An Essay on the Principle of Population, by the English political economist Thomas Malthus (1766–1834). First published in 1798, Malthus’s book made the case that an abundance of food— enough to feed anyone born— would allow the human population to increase geometrically and indefinitely. In reality, the Essay argued, there simply is not enough food for everyone born, so population is limited by food supply (Figure 2.10). Who survives to reproductive age? Those who can successfully compete for food. Whose children thrive? Those of survivors who manage to feed their offspring. Applying Malthus’sdemographic ideas to human and nonhuman animals, Darwin concluded that some members of any species successfully compete for food because they have some special attribute or attributes. That an individual characteristic could facilitate survival was a revelation!
Darwin Borrows from Malthus
Five of Malthus’s observations inspired Darwin’s principle of natural selection.
OBSERVATION 1
For most organisms, every pair of parents produces multiple (sometimes many) offspring.
OBSERVATION 2
For most organisms, the population size remains the same. No increase occurs over time.
OBSERVATION 3
Population is limited by the food supply.
OBSERVATION 4
Members of populations compete for access to food.
OBSERVATION 5
No two members of a species are alike in their physical attributes— variation exists.
THEORY: EVOLUTION BY MEANS OF NATURAL SELECTION
Individuals having variation that is advantageous for survival to reproductive age produce more offspring (and more offspring that survive) than do individuals lacking this variation.
In presenting his theory of evolution by natural selection as laid out in On the Origin of Species, Darwin explained his three observations and two deductions:
Observation 1. All organisms have the potential for explosive population growth that would outstrip their food supply. A female bullfrog, for example, may lay 100,000 eggs every spring. Even humans, with their very low reproductive potential compared with most animals, can undergo exponential population growth, as evidenced by the global population explosion. Darwin took this idea directly from Malthus, who had been concerned with human population growth.
Observation 2. But when we look at nature, we see populations that are roughly stable.
Deduction 1. Therefore, there must be a struggle for existence. That is, the bullfrog’s 100,000 eggs may yield no more just one adult frog. This Darwin labeled natural selection to parallel the term artificial selection in use by animal breeders of the period.
Observation 3. Nature is full of variation. Even in one animal group, every individual is slightly different from every other individual. If you look closely enough, even a basketful of uniform-looking bullfrogs will resolve into myriad small differences in size, shape, color, and other features.
Deduction 2. Therefore, some of these variations must be favored, and others must be disfavored, in a process, we can call natural selection.
Natural selection is a filtering process in which unfavorable traits lose the race to more favorable traits.
As Darwin saw it, natural selection is all about reproductive success. The time-honored definition of natural selection as “survival of the fittest,” a phrase coined by social theorist Herbert Spencer, is misleading. It is much more about the number of offspring that survive to reproductive age, which is a measure we call fitness, a biological measure of reproductive success (not a reference to physical fitness).
Natural selection can therefore be defined as differential reproductive success across multiple generations and among the individuals of a given population of animals or plants.
For natural selection to work, three preconditions must be met (Figure 2.19):
1. The trait in question must be inherited.
2. 2. The trait in question must show variation between individuals.
3. 3. The filter between the organism and its genetic makeup is the environment, which must exert some pressure in order for natural selection to act.