At the level of the individual, the study of genetics shows how traits are transmitted from parent to offspring, enabling a prediction about the chances that any given individual will display some phenotypic characteristic. At the level of the group, the study of genetics takes on additional significance, revealing how evolutionary processes account for the diversity of life on earth.
A key concept in genetics is that of the population, or a group of individuals within which breeding takes place. Gene pool refers to all the genetic variants possessed by members of a population. It is within populations that natural selection takes place, as some members contribute a disproportionate share of the next generation. Over generations, the relative proportions of alleles in a population change (biological evolution) according to the varying reproductive success of individuals within that population. In other words, at the level of population genetics, evolution can be defined as changes in allele frequencies in populations. This is also known as microevolution. Four evolutionary forces—mutation, gene flow, genetic drift, and natural selection—are responsible for the genetic changes that underlie the biological variation present in species today. As we shall see, variation is at the heart of evolution. These evolutionary forces create and pattern diversity.
In theory, the characteristics of any given population should remain stable. For example, generation after generation, the bullfrogs in a farm pond look much alike, have the same calls, and exhibit the same behavior when breeding. The gene pool of the population—the genetic variation available to that population—appears to remain stable over time.
Although some alleles may be dominant over others, recessive alleles are not just lost or destroyed. Statistically, an individual who is heterozygous for a particular gene with one dominant (A) and one recessive (a) allele has a 50 percent chance of passing on the dominant allele and a 50 percent chance of passing on the recessive allele. Even if another dominant allele masks the presence of the recessive allele in the next generation, the recessive allele nonetheless will continue to be a part of the gene pool.
Because alleles are not “lost” in the process of reproduction, the frequency of the different alleles within a population should remain exactly the same from one generation to the next in the absence of evolution. In 1908, the English mathematician G. H. Hardy (1877-1947) and the German obstetrician W. Weinberg (1862-1937) worked this idea into a mathematical formula called the Hardy-Weinberg principle. The principle algebraically demonstrates that the percentages of individuals homozygous for the dominant allele, homozygous for the recessive allele, and heterozygous will remain the same from one generation to the next provided that certain specified conditions are met. Among the conditions are these: Mating is entirely random; the population is sufficiently
Population In biology, a group of similar individuals that can and do interbreed.
Gene pool Ah the genetic variants possessed by members of a population.
Evolution Changes in allele frequencies in populations; also known as microevolution.
Hardy-Weinberg principle Demonstrates algebraically that the percentages of individuals that are homozygous for the dominant allele, homozygous for the recessive allele, and heterozygous should remain constant from one generation to the next, provided that certain specified conditions are met.
In evolutionary terms, mutations serve as the ultimate source of all new genetic variation. A generally positive force, most mutations have minimal effect or are neutral. Nevertheless, human-produced mutagens—such as pollutants, preservatives, cigarette smoke, radiation, and even some medicines—increasingly threaten people in industrial societies. While the negative effects of mutation are evident in the clear link between cigarette smoke and cancer, the positive side of mutation has been fictionalized in the special talents of the X-Men.
Large for a statistical average to express itself; no new variants will be introduced into the population’s gene pool; and all individuals are equally successful at surviving and reproducing. These four conditions are equivalent to the absence of evolution. Geographic, physiologic, and social factors may favor mating between certain individuals over others.
Thus changes in the gene pools of populations, without which there could be no evolution, can and do take place. The mechanisms by which these changes might lead to the formation of new species will be discussed in detail in Chapter 6.
World History
