equivalent organisms like the conjugation of similar individual Paramecium. Specialization of sexual organs in multicellular species led to hermaphroditism. Interchange of genetic material was still between equivalent individuals employing self fertilization to adjust evolutionary rates, like many species of worms today. Sexual specialization of individuals into separate males and females then occurred through secondary loss of the organs of the opposite sex by hermaphrodites. Genetic interchange can be accomplished effectively by either hermaphrodites or separate sexes. Separate sexes trade away redundancy to gain efficient reproductive specialization in individuals, but do not fundamentally alter the sexual mechanism. In this view, abandonment of the sexual mechanism has not happened, except partially, in some species like those in worm phyla which have returned to or retained asexual fission. Asexual fission by-passes the sexual functions of hermaphroditic reproduction and thereby adjusts the rate of genetic exchange.
In either theory, sexual forms were generated by having one switch with two positions. Thus, in humans, the switch is the hormone testosterone. Testosterone presence in early development means male, while testosterone absence means female. Humans are not basically female with males needing an additional hormone, rather one hormone switch simply determines development. Two switches, one for each sex, are not required. Natural selection has a tendency to be parsimonious.
Modern Evolution
Modern life science recognizes the fact that evolution depends on genetic, phenotypic (constitutional), and ecological opportunity. But the combination of opportunities in the right place and time is dependent upon probabilistic chance. Therefore, evolution is a matter of probability. The probabilistic nature of evolution makes it a one-way sequence and precludes an exact repetition of the process. If everything could be restarted, the same opportunities would not be likely to recur, especially in the same sequence.
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,
