Operating on the products of the combination of opportunities is natural selection. Natural selection determines the viability of any species by selecting which combination of genes will contribute most to the next generation. Thus, selection causes a drift in the proportion of gene alleles in a population toward dominance or loss. Even a very small advantage will result in major changes. Pauling (1968) calculated that a mutation rate as small as one in 20,000 per gene generation coupled with a minute advantage like 0.01% more progeny for the mutant would result in replacement of the standard by the mutant within one million years, for many primate species.
The conditions for evolution are defined by the Hardy-Weinberg law, summarized next in Table 1-2. The law states the circumstances which, if met, would preclude evolution. Any isolated population meeting the requirements of the law, without immigration or emigration, would not change because the effects of natural selection are negated.
Table 1-2 Summary of the Hardy-Weinberg Law.
| Evolution will not occur in an isolated population with no immigration or emigration if: 1. There is an infinite population, 2. There is no or an equilibrium of mutation, 3. There is random reproduction, which requires, Equal survival Equal fecundity Random mating |
The first condition required for no evolution is an infinite population. Since any change is finite, an infinite would be unaffected by any amount of finite change. Clearly, there are no infinite populations, but the first condition also indicates large populations would be more resistant to change. Thus, a large population could be threatened with extinction by not being able to change quickly and could be as endangered as a group of rare organisms.
The second postulate needed to prevent evolution is no mutation or the equivalent, an equilibrium of mutation. In an equilibrium, any mutation in one direction is balanced by back mutations by other genes. Without mutation, no original genetic material can influence a population. However, no population is without mutations and an enormous amount of mutation has already occurred in all populations.
The third requirement that would prevent evolution is random reproduction. Random reproduction requires three elements. One is equal survival of individuals through their reproductive period of life. Otherwise, unequal survival would produce differential reproduction. Unequal reproduction is how survival effects evolution, not individual struggle. A second element is equal fecundity of all individuals, because unequal ability to reproduce in the population would not be random. Finally, completely random mating is necessary for random reproduction. Nonrandom mating introduces a direction to reproduction.
Any living population necessarily violates these conditions and is, therefore, under selection pressure to change, resulting in evolution. The more a population deviates from the conditions of the Hardy-Weinberg law, the faster that population is evolving.
Survival is a consideration only in the final requirement of the law and then strictly as survival to reproduce, not as individual survival per se, like the “survival-of-the fittest” idea. Just as in dice tossing, laws apply to the population not the individual. Evolution is differential reproduction of the population, not individual survival. The most fit to survive individual, without reproduction, has no direct influence on evolution.
The fact that evolution is population drift towards the composition of the reproducers is not an argument for uncontrolled, or even any, mating or for large families. Some organisms, like many species of fishes, do rely on sheer fecundity, by leaving thousands of fertilized eggs behind, to insure some adults for the next generation. Natural factors like temperature change and predation keep those populations in check. Other species, however, have evolved a parental care system, in which, one or both parents (or substitute adults) remain with the young, protecting them until adulthood. A parental care mechanism requires small families to avoid dissipating protection by covering too many young. Childhood accidents remain a major danger of elimination in human families, indicating the need for the parental care mechanism. In addition, an individual may indirectly influence the success of the species by contributing to the accumulated cultural heritage, whether or not, the individual participates directly in determining the next generation through reproducing. Many species, like ants and humans, are heavily dependent on non-reproducing individuals.
An understanding of evolution as differential reproduction permits consideration of some prevalent ideas. Viewing the distribution of people throughout the world, the fact becomes apparent that the vast majority of humanity is lower class. Thus, the lower class is the main source of people for the next generations. With an expanding population, each generation is composed of more and more lower class. From these facts the conclusion can be drawn that human beings, as a species, are undergoing diminished adaptive abilities. There are two aspects to such a conclusion, the qualitative and the quantitative. Qualitatively, evolutionary principles can not justify the idea of less adaptive abilities in the lower class, because class distinctions are based on economic, social, and political values, not on genetics. Genetically, there can be no difference which class is the main producer of the next generation, unless class distinctions are genetically based. Quantitatively, however, the conclusion has merit because the lower class does not limit family size. The result of lower class reproduction has been a booming human population. The increasing numbers invoke the concept of the first condition of the Hardy-Weinberg law and illustrate how large populations can become endangered.
Another common idea is that there is a genetic basis for the incest taboo in human societies. However, the incest taboo is not defined genetically. Some societies allow uncle-niece but not aunt-nephew marriage. Genetically, these marriages are the same. The definition of incest also varies. Some states permit first cousin marriages, but others do not. Incest taboos existed long before humans had any significant knowledge of genetics. The taboos really reflect the role of marriage in society. Marriage was an alliance of families. If a son and a daughter in the same family married each other, that would prevent two alliances with other families and weaken their own family. Only if the family were already powerful, like royalty, could intra-marriage be useful in retaining, rather than dissipating, the power of the family. Thus, the rules for royalty were to marry their relatives, namely, other royalty.
The practice of intra-marriage among royalty is taken as proof of the genetic basis for the incest taboo, because the Royal Family of England, and its relatives, experienced repeated occurrences of hemophilia in its lineage. However, if a family has no genes for hemophilia, incest will not produce it. The consequences of incest depend on the genes of the family. Positive results of incest are responsible for the development of crops and breeding of animals, like dogs and horses.
Incest is assumed to be genetically detrimental, because mating with close relatives causes more pairing of recessive alleles than would a random match from the general population. However, human mating is not random. We tend to pick mates who are like us and, therefore, are related or at least share many genetic characteristics. Each person has two parents, four grandparents, eight great grandparents, etc. The expansion of ancestors doubles for each past generation. However, in a few generations the number of possible ancestors (2n) exceeds the population of the Earth in the past. Thus, we all must share common ancestors and be related. Most importantly, the idea that pairing recessives is bad comes from the idea that recessive is evil and dominant is good. Dominant and recessive are merely terms expressing which gene will evidence in the phenotype, when they occur in combination. Dominant and recessive do not mean good and evil, because the mutations that produced the genes were random and without regard for the needs of the organism. Natural selection determines the adaptive value of the alleles, not human social values. Positive, negative, or neutral consequences from incest would depend on the genetic lineage of the mates. A genetic pairing with detrimental recessives would tend to evidence those characteristics in incest. But a positive lineage would evidence those positive characteristics. Thus, the incest taboo has no genetic basis, but was produced by the important social values of human mating.
Significance of Evolution for Behavior
Hardy-Weinberg also explained the normal distribution of structural characteristics, like height and weight, in terms of the