a. Natural populations are polymorphic, and mutations are always occurring in them.

b. Variation is the raw material for natural selection in a population.

c. Sidebar 16.1 reviews environment and heredity.

d. Populations can retain genetic diversity as natural selection stabilizes allele frequencies.

e. A species and its members can be geographically unified or isolated to various degrees (Figure 23.1).

f. Gene flow is the term for the inflow or outflow of genes with regard to a population; it's also known as gene mixing.

g. Population genetics theory is based on an idealized situation: a large population of sexually reproducing diploids that is isolated and exhibits random mating (Figure 23.2).

h. Assortative mating (Figure 23.2) occurs when individuals demonstrate a preference for their mates; it is the opposite of random mating, where any male and any female can mate.

i. A population of mice (Figure 23.3) is used to illustrate some of the mathematics that apply to populations where individuals are mating randomly.

a. Members of a Mendelian population share a gene pool, a theoretical collection of all their genes, from which all gametes are drawn, as any sperm can fertilize any egg, in a random mating scenario.

b. G.H. Hardy and Willhelm Weinberg independently discovered that an idealized Mendelian population will come to an equilibrium for each allele under the following conditions:

c. Notice that such a population is not evolving.

d. The Hardy—Weinberg Principle was used to discover that the ABO blood type system, which exhibits codominance in humans, is a system of alleles for the same gene, rather than one produced by different genes.

e. Hardy—Weinberg studies are commonly used to analyze populations mathematically and discover why they are changing (also, refer to Section 19.9, Linkage disequilibrium and mapping human genes).

f. The Hardy—Weinberg principle is illustrated for the MN human blood types, a codominant system with two alleles, M and N, for a single gene.

g. Allelic frequency denotes the rate of occurrence of a given allele in a population, and can vary from 0 to 1.

h. In the MN system, if the frequency of M is denoted as p, then the frequency of N is denoted as q, and p + q = 1, since they are the only two alleles for this gene.

i. In a system displaying simple dominance, p denotes the frequency of the dominant allele, by convention, and q denotes the frequency of the recessive allele.

j. The number of genes in a population is equal to twice the number of individuals.

k. In a population with nT individuals, three genotypes are possible in the MN system:

m. Breeding in this population is assumed to be random (Figure 23.4), and the probability of choosing M is p, while the probability of choosing N is q.

n. Therefore:

o. The Hardy-Weinberg principle states that, in a population that meets all the criteria set forth, the p2 : 2pq : q2 ratio will be attained and will not change from one generation to the next.

a. Since q2 is the frequency of the homozygous recessive individuals in a simple dominance system, the square root of q2, or q, can be determined if that frequency is known.

b. This principle has been applied in cases where the homozygous recessive frequency can be determined, and it has been used to derive values for p, and frequencies for the other genotypes.

a. The fitness of a genotype is a relative measure of its contribution to the gene pool of the next generation, in comparison with other genotypes.

b. Fitness measurements thus range from 0 to 1.

c. Homozygous recessive individuals are usually at a selective disadvantage and have a lower fitness (Figure 23.5).

d. When this happens, the recessive individuals will occur at a lower frequency of q2(1-s) in the next generation, where s is the coefficient of selection against the recessive allele.

e. Fitness is thus measured by comparing allele frequencies from one generation to the next.

f. Though some people think of fitness in dramatic terms, studies of finches, for example, have shown that extremely subtle changes in phenotype can result in large changes in fitness (Figure 23.6).

a. The mutation rate of a gene is the probability per individual per generation that the gene locus will mutate.

b. Mutation rates tend to be very small, in the range of 10-5—10-8.

c. Different genes mutate at different rates, especially due to their various sizes.

d. Mutations in body cells do not result in changes to the gene pool, since they are not passed on to future generations.

e. Mutations can reverse themselves: a back-mutation or reversion to the original state can occur, but minimally.

f. An allele may be increasing in frequency and decreasing at the same time, due to both selection and mutation; a graph of this would show the allele reaching equilibrium where the two lines of change crossed.

g. Mutation is the ultimate source of all genetic variation and therefore the foundation for evolution.

h. Sexual reproduction and recombination (Chapter 15) also contribute genetic variability to a population (Figure 23.7).

i. Concepts 23.1 addresses gene duplication and evolution.

a. In a Hardy-Weinberg population, which is large, allele frequencies remain constant, as would the frequencies of heads and tails in a lengthy coin flipping experiment.

b. In a small population, each generation is made by only a few "flips" of a "genetic coin," and frequencies can change rapidly from one generation to the next.

c. Computer simulations show this genetic drift, or change in allele frequencies, over a few generations (Figure 23.8).

a. Hybrid vigor, or heterosis, is the term for the increased general vigor of a hybrid made between two true-breeding strains.

b. Inbred populations can become homozygous for certain alleles, but when individuals from these are crossed, hybrid vigor can be observed.

c. Enzyme variants called isoenzymes are sometimes at the root of hybrid vigor, as they give a hybrid biochemical versatility, and it can survive in various environments.

d. Sickle-cell anemia (Section 5.12) is a classic example of heterozygote superiority; the heterozygote in this case has a selective advantage against the malarial parasite, and is more fit in tropical regions (Figure 23.9).

a. Recessive allele frequencies, contrary to some popular notions, do not change very rapidly in large populations.

b. For example, even for a fatal childhood disease that prevents homozygotes from reproducing, it would take 900 generations to change the frequency from 0.01 to 0.001.

c. The reason for this slow rate of change is that heterozygotes are carriers for these alleles, and pass them on regularly.

d. Even if an allele frequency were to fall extremely low, the mutation rate would probably equal it and maintain the allele in the population.

e. Perhaps the best way to change a deleterious allele's frequency in a population is to identify the carriers and somehow prevent them from reproducing.

a. Discussing inheritance in populations by focusing on one gene at a time is misleading, since each individual is a complex of different genotypes.

b. Though the interactions of genes are not well understood, it is known that they exist (e.g. in metabolic pathways).

c. Genes do work together to make a functional whole individual, as illustrated in the text example of a hypothetical plant that can make large or small leaves and thick or thin stems.

d. Genes that function together and that are brought together on the same chromosome, with tight linkage, are called supergenes (Figure 23.10).

a. Chromosomal duplications (Concepts 23.1), translocations, and inversions, all create novel genetic combinations.

b. A translocation occurs when part of a chromosome breaks off and attaches to a nonhomologous chromosome; the offspring are usually inviable, due to incorrect numbers of certain genes.

c. An inversion occurs when a chromosome breaks and reattaches in the opposite orientation; the gametes resulting from this are also generally not viable.

d. Inversions tend to inhibit crossing over, though the reasons for this are not well-known.

e. Inversions and translocations tend to keep together novel combinations of genes once a certain combination has been made.

a. Fitness must necessarily be measured as relative to particular environmental conditions.

b. A population that shows greater polymorphism would be expected to greater success in various environmental conditions.

c. A population shows balanced polymorphism when it maintains the gene for two or more forms because selection favors each form in a different situation.

d. The British land snail, Cepaea nemoralis, illustrates balanced polymorphism (Figure 23.11), as its different forms are successful at different times of the year, due to changes in the environment and the visibility of the snails to predators.

e. This snail pays a price for this ; this is known as the genetic load that the population bears as it produces individuals with the wrong patterns in each habitat.

f. Variation in populations is often selectively neutral, conferring no particular advantage or disadvantage.

g. The human MN blood groups appear to be selectively neutral, though the ABO types may not be.

h. An advantage of diploidy is that a population can have recessive mutations without harm, though it occasionally suffers when two deleterious alleles come together in a homozygote.

i. For example, the Curly wing and plum eye Drosophila mutants are both lethal, but together, in the same individual, they are not (Figure 23.12).

a. The features required for an organism to survive in a particular niche may shift over the species' geographic range, due to changes in environmental factors such as temperature and humidity.

b. Theodosius Dobzhansky studied genetic polymorphisms in Drosophila, and found a host of inversion rearrangements (Figure 23.13) that changed gradually as the critical environmental variables changed.

c. Recall from Chapter 18 that the expression of a gene may be influenced by its position.

d. The geographic distribution of inversions in Drosophila shows that each chromosomal difference can make a difference.

e. Each chromosome is a type of gene complex; the fitness of different gene complexes can change as the environment changes.

f. The relative fitness of each genotype clearly varies with time, and the whole population survives by maintaining several forms that are adapted to different conditions.

g. John Moore showed that leopard frogs were adapted to the temperature where they breed; these differences must reflect distinct gene complexes.

h. Distinct features therefore vary along different geographic gradients.