EVOLUTION AND POPULATION GENETICS Now that you understand the basic principles of Mendelian genetics involving the transmission of traits from parents to offspring, we can examine the effect of this on the population as a whole and provide a formal definition of evolution. The product of sexual reproduction as we have just seen is individual genetic variation. The extent of this variation in terms of different genotypes (g) is a function of the number of different alleles per locus (r) and the number of loci (n) examined, as we have previously discussed. Individual variation, however, is best studied at the population level of organization, not simply by comparing parents and offspring. Population genetics is an extension of Mendelian genetics concerned with the level of variation existing in a community of reproducing individuals, variously called a deme, local population or Mendelian community. The basic unit studied in population genetics is the gene pool, defined as the sum total of genes (both loci and alleles) found in the entire population. Each individual is a temporary vessel housing a small fraction of the gene pool's variation so the concept of a gene pool is an abstract pooling of the genes of all individuals in the population - it does not exist apart from the individuals themselves. If mating is random, the population is said to be panmictic so that the gametes of each male have an equal chance of fusing with the gametes of each female. Due to panmixis, the genes are pooled and the offspring of the next generation are produced by randomly selecting from this pool pairs of alleles for each locus. Some marine invertebrates conform to this simple model as they simply release their gametes into the water. The theory of population genetics discussed below assumes diploid species in which genes are unlinked and so has been called "beanbag genetics." One Gene Model of Population Genetics Each species has a set number of loci which influence the development of particular aspects of the phenotype. A species may have hundreds of loci and each locus may possess several allelic variants. Although an individual in a diploid species can only carry two different alleles for each locus (e.g., a heterozygote), there may be several different alleles for some loci in the population as a whole allowing a large number of genetically different heterozygotes. As you can easily imagine the total amount of genetic variation in terms of both loci and alleles in a single population is immense and impossible to analyze in its entirety. For this reason population geneticists use a simple model to analyze the genetic nature of a population and, like any model, their model oversimplifies reality but enables one to grasp the basic principles of genetics at the population level. This model is called the one gene model of population genetics since it assumes that individuals contain only a single locus; it further assumes that this locus has only two allelic expressions. Thus, according to this model there is only locus A with alleles a1 & a2. Allelic or gene frequency The structure of a population is described in terms of the relative frequency of its alleles. Even with so simple a situation as that described by the one gene model, it is possible to have variable populations. Let the letter "p" represent the frequency of a1 and the letter "q" represent the frequency of a2. The total frequency of locus A in the population has to equal 1.0 and so p + q = 1.0. If there are equal numbers of the two alleles in the population, then p = q = 0.5. This means that 50% of the alleles for locus A are represented by a1 and the other 50% by a2. Calculate allelic frequencies just as you would percentages except that the total or whole equals 1.0, not 100. Not all populations, however, will have equal frequencies of the two alleles so that different populations can vary in allelic frequency. Consider a population in which p = 0.7. What does q or the frequency of a2 equal? The answer must be 0.3 because q = 1 - p (and p = 1 - q) since p + q = 1.0. The Hardy-Weinberg Law In 1908 two theorists independently discovered that allelic and genotypic frequencies which characterize a population will not change from generation to generation due simply to the process of sexual reproduction. This discovery, known as the Hardy-Weinberg law after its co-discoverers - a British mathmatician named Hardy and a German physician named Weinberg, provides the basis for population stability and continuity over time. Thus, allelic and genotypic frequencies in a population will be faithfully replicated when the next generation is produced by sexual reproduction. Given the allelic frequencies p and q, the H-W law predicts the following genotypic frequencies in a population: p2 = the frequency of the genotype a1a1; q2 = the frequency of the genotype a2a2, and 2pq = the frequency of the genotype a1a2. Just as p + q = 1, so too p2 + 2pq + q2 will also equal 1 (the sum total of genotypes for locus A in the population). Population stability (equilibrium) according to the Hardy-Weinburg law will be maintained each generation. Assumptions Underlying the Hardy-Weinberg Law A population will remain stable in Hardy-Weinberg equilibrium if the following conditions are met: the alleles are stable and neutral and the population is closed and infinitely large. These conditions constitute the assumptions underlying the Hardy-Weinberg law. If any of these four assumptions are violated, then both allelic and genotypic frequencies will change from parent to offspring generation. The Hardy-Weinberg equilibrium is an unstable equilibrium, so that if upset, the population will not return to the original allelic and genotypic frequencies, but rather will equilibrate at the new frequencies. Let's examine each of these assumptions in more detail because the factors which make these assumptions invalid are the factors known to cause evolution. Stable alleles The Hardy-Weinberg law assumes that an individual's alleles are passed on intact during gamete formation. During the S stage of the first interphase of meiosis when DNA is replicated, it is possible for a copy mistake to occur and so produce a new allele. Thus, an a1 allele could be changed during DNA replication to a2 or to a completely new allele, e.g., a3. Such a mistake in DNA replication is called a mutation. Mutations are the ultimate source of genetic variation because they produce new alleles which are then shuffled into new genotypes by sexual reproduction. Neutral alleles Neutral alleles are alleles which are passed on to offspring in the same proportion as they exist in adults because they are neutral with respect to survival and reproduction. This assumption is violated when one allele has a higher probability of being represented in the next generation because the individual possessing it has a higher probability of surviving to reproduce. The process of natural selection results in a violation of this assumption because it confers an adaptive (=fitness) advantage on one allele over another. Closed population The Hardy-Weinberg law assumes that populations (demes) are isolated so that individuals neither leave (emigration) nor enter from another deme (immigration). In an open population genes flow from one deme to another an so will disrupt the Hardy-Weinberg equilibrium. Thus, migration or gene flow will upset the H-W equilibrium and cause evolution. Consider an extreme case in which deme #1 contains only individuals homozygous for a1 and deme #2 contains only individuals homozygous for a2. In this example p = 1.0, q = 0.0 in deme #1 and p = 0.0, q = 1.0 in deme #2. If just one individual from deme #1 migrates to deme #2 and reproduces, then in the next generation p will be greater than 0.0 and q will be less than 1.0, thus upsetting the Hardy-Weinberg equilibrium. Infinitely large population The Hardy-Weinberg law, like Mendel's laws, is a statistical law which requires an infinitely large population. Small populations, like small samples, are subject to chance deviations from the expected. The smaller the population, the greater is the probability that the parental gene frequency will not be faithfully reproduced in the offspring generation. Any deviation in gene frequency between these generations due to sampling error is called genetic drift, which as the name implies is nondirectional. This contrasts with natural selection wherein the change in gene frequency is directional due to an adaptive advantage of one allele over another. Consider a population with only two individuals (a male and a female) both heterozygous for the two allelic expressions of locus A. In this population p = q = 0.5. Assume further that this pair produces only four offspring. The Hardy-Weinberg law maintains that the offspring generation will also be characterized by p = q = 0.5. According to Mendel's law of segregation, a monohybrid cross should produce three genotypes in the following ratio: 1 a1a1 : 2 a1a2 : 1 a2a2. If this expectation is realized then the offspring population would have p = q = 0.5. How much money would you be willing to bet that the four offspring would follow this expected result? Since gamete fusion in syngamy is random, it is possible for all four offspring to be homozygous for the same allele, thus eliminating one allele from the population. Should this occur, then the change in gene frequency between generations would be simply due to chance. In addition to chance gamete fusion in syngamy, chance deviation in gene frequency between generations which results in genetic drift could also result from accidental death, chance meeting of mates with different genotypes during the breeding season and chance variation in number of offspring produced by different individuals in the population. Consequently, there are many specific factors which can result in genetic drift, just as there are many specific environmental factors which act as agents of natural selection to exert a selective pressure on alleles. Microevolution Evolution refers to a genetic change over time, but what kind of a genetic change? It certainly does not refer to a change in genes themselves because this type of genetic change is called a mutation. Nor does it refer to a change in an individual's genotype because this does not change over the lifetime of the individual (any somatic mutation - one occurring in a non-reproductive cell - would change a cell's genotype but would not be passed on to offspring). Individuals do change during their ontogeny (life history) but this is called development, not evolution. Species also change over time and if such change is a consequence of a change in their genes, then species change constitutes evolution. But species evolution, also called macroevolution, takes thousands of years and is studied by paleontologists, not geneticists. Definition of microevolution From a genetic standpoint, evolution is a change in allelic frequency in a single population between successive generations. This is an important definition because it describes what evolution means to a biologist and emphasizes that the unit of evolution is the population or gene pool. The term "evolutionary significance" which I will use often in this course should be interpreted within the context of the definition of evolution given above and so refers to the significance of some phenomenon in changing allelic frequency in a population. Macroevolution is simply a consequence of microevolution occurring over many generations until a single species changes so dramatically in phenotype that it is classified as a new species. Evolution results whenever the Hardy-Weinberg law is upset by violating its four assumptions due to (1) mutation, (2) migration, (3) genetic drift, and (4) natural selection. These four factors result in evolution and so are the causes of evolutionary change. The study of population genetics establishes evolution as essentially a genetic phenomenon described as a change in allelic frequencies in a gene pool between successive generations. The Hardy-Weinberg law is the foundation of population genetics and establishes the population as the unit of evolution. This law states that populations are genetically stable over time despite the continual production of individual variation by sexual reproduction. Only when this genetic equilibrium is disturbed will evolution occur, and there are only four known factors which will upset the Hardy-Weinberg equilibrium: mutation, migration (gene flow), genetic drift and natural selection. Population genetics is essentially the study of how these four factors interact to produce microevolution. The following is a summary of the basic concepts related to the field of population genetics. 1. Evolution is a genetic phenomenon defined as "a change in allelic frequencies in a single popu- lation between successive generations." 2. The unit of evolution (microevolution) is the population or gene pool. 3. The Hardy-Weinberg law is the foundation of pop- ulation genetics and establishes gene pool stab- ility over time despite the continuous genera- tion of individual variation (both phenotypic and genotypic) by sexual reproduction. 4. According to the Hardy-Weinberg law, allelic and genotypic frequencies will not change from generation to generation provided the following assumptions are met: genes are stable and neutral, the population is infinitely large and closed. 5. Violation of these four assumptions will result in evolution due to mutation, natural selection, genetic drift and migration, respectively. The reason is that each of these factors causes a change (either increase or decrease) in the frequency of alleles in the gene pool. 6. The ultimate cause of genetic variation is mutation - the only process whereby new alleles can be formed. 7. Sexual reproduction by itself will not result in evolution, i.e., a change in gene frequency, but it is important in evolution as the major and proximate source of individual variation upon which natural selection acts to produce directional change through adaptation and genetic drift acts to produce random or nondirected change. 8. The one gene model used to explain population genetics is a gross oversimplification of reality in that it ignores gene interaction in producing phenotypes. It does, however, provide a basis for defining and examining evolution.