19.1. Evolution in a Genetic Context (p. 304)
A. What Causes Variations?
1. A population is a group of organisms of the same species occupying a certain area.
2. The members of a population vary from one another.
3. Variation is the raw material for evolutionary change.
B. Gene Mutations
1. Gene mutations provide new alleles, and therefore are the ultimate source of variation.
2. A gene mutation is an alteration in the DNA (deoxyribonucleic acid) nucleotide sequence of an allele.
3. Mutations occur at random.
4. Mutations can be beneficial, neutral, or harmful.
C. Chromosomal Mutations
1. Some chromosomal mutations are alterations in the number of chromosomes inherited.
2. Others are alterations in arrangement of alleles on chromosomes due to inversions and translocations.
D. Recombination
1. In sexually reproducing organisms, genetic recombination is recombination of alleles and chromosomes.
2. Recombination results from crossing-over during meiosis, the random segregation of chromosomes to gametes during meiotic division, and the random combination of gametes during fertilization.
3. The entire genotype is subject to natural selection; new combinations of alleles may have selective value.
4. For polygenic traits, the most favorable combination may occur when the right alleles group by recombination.
E. How to Detect Evolution
1. Not only are variations created, they are also preserved and passed on from one generation to the next.
2. The gene pool is the total of all the alleles in a population, described in terms of gene frequencies.
3. Neither dominance nor sexual reproduction will change allele frequencies. (Fig. 19.1)
4. The Hardy-Weinberg Law
a. This law states an equilibrium of allele frequencies in a gene pool (using a formula p2 + 2pq + q2) remains in effect in each succeeding generation of a sexually reproducing population if five conditions are met.
1) No mutation: no allelic changes occur.
2) No gene flow: migration of alleles into or out of the population does not occur.
3) Random mating: individuals pair by chance and not according to their genotypes or phenotypes.
4) No genetic drift: the population is large so changes in allele frequencies due to chance are insignificant.
5) No selection: no selective force favors one genotype over another.
b. In reality, the conditions of the Hardy-Weinberg law are rarely, if ever, met, and allele frequencies in the gene pool of a population do change from one generation to the next, resulting in evolution.
c. Any change of allele frequencies in a gene pool of a population signifies that evolution has occurred.
d. The Hardy-Weinberg law tells us what factors cause evolution---those that violate the conditions listed.
e. A Hardy-Weinberg equilibrium provides a baseline by which to judge whether evolution has occurred.
f. Hardy-Weinberg equilibrium is a constancy of gene pool frequencies that remains across generations.
5. Microevolution is the accumulation of small changes in a gene pool over a relatively short period.
(Fig. 19.2)
F. What Causes Evolution?
1. Gene Mutation
a. Gene mutations provide new alleles and, therefore, are a source of variation in populations.
b. Gene mutations underlie all the other mechanisms that provide variation.
c. Due to DNA replication and DNA repair mechanisms, mutation rates of individual genes are low (about 105 per gene per cell cycle); but each organism has many genes, and a population has many individuals.
d. Therefore, mutations are relatively common, and the mutation rate is an adequate source of new alleles.
e. Studies reveal fruit flies are polymorphic at over 30% of all gene loci and heterozygous at 12% of loci.
f. High levels of molecular variation are the rule in natural populations; many mutations are hidden.
G. Gene Flow Brings New Genes
1. Gene flow moves alleles among populations through interbreeding, by migration of breeding individuals.
2. Gene flow increases variation within a population by introducing novel alleles produced in another population.
3. Continued gene flow tends to decrease the diversity among populations, causing gene pools to become similar.
4. Gene flow among populations can prevent speciation from occurring.
H. Mating Is Not Always Random
1. Random mating involves individuals pairing by chance, not according to their genotypes or phenotypes.
2. Nonrandom mating involves individuals inbreeding and assortative mating.
3. Inbreeding is mating between relatives to a greater extent than by chance; inbreeding can occur if dispersal is so low that mates are likely to be related and does not change allele frequencies, but it does decrease the proportion of heterozygotes and increase the proportions of both homozygotes at all gene loci.
4. Assortative mating occurs when individuals tend to mate with those that have the same phenotype.
1) Assortative mating divides a population into two phenotypic classes with reduced gene exchange.
2) Homozygotes for gene loci that control a trait increase, and heterozygotes for these loci decrease.
3) Other loci remain in Hardy-Weinberg equilibrium, except for those linked to the loci governing the trait.
I. Genetic Drift Promotes Changes
1. Genetic drift refers to changes in allele frequencies of a gene pool due to chance (random) events.
2. Genetic drift occurs in both large and small populations; a larger population suffers less sampling error.
3. Genetic drift causes gene pools of two isolated populations to become dissimilar as some alleles are lost and other are fixed. (Fig. 19.4)
4. Genetic drift occurs when founders start a new population, or after a genetic bottleneck with interbreeding.
a. Founder effect is a case of genetic drift in which rare alleles, or combinations of alleles, occur in higher frequency in a population isolated from the general population. (Fig. 19.5)
1) This is due to founding individuals containing a fraction of total genetic diversity of original gene pool.
2) Which particular alleles are carried by the founders is dictated by chance alone.
3) Dwarfism is much higher in a Pennsylvania Amish community due to a few German founders.
b. Bottleneck effect is genetic drift in which a severe reduction in population size due to natural disaster, predation, or habitat reduction, causes severe reduction in total genetic diversity of the original gene pool.
1) The cheetah bottleneck causes relative infertility because of the intense inbreeding.
2) The bottleneck effect prevents most genotypes from participating in production of next generation.
19.2. Adaptation Occurs Naturally (p. 310)
A. Natural Selection: Populations Adapt to Their Environment.
B. Natural Selection Requires
1. variation (i.e., the members of a population differ from one another),
2. inheritance (i.e., many of the differences between individuals in a population are heritable genetic differences),
3. differential adaptedness (i.e., some differences affect how well an organism is adapted to its environment), and
4. differential reproduction (i.e., better adapted individuals are more likely to reproduce).
C. Fitness
1. Fitness is the extent to which an individual contributes fertile offspring to the next generation.
2. Fitness is relative; it is measured against the reproductive success of other genotypes in the same environment.
D. Types of Selection.
1. Directional selection occurs when extreme phenotype is favored; the distribution curve shifts that direction.
a. The gradual increase in the size of the modern horse, Equus, correlated with a change in the environment from forestlike conditions to grassland conditions. (Fig. 19.6)
b. A shift of dark-colored peppered moths from light-colored correlated with increasing pollution. (Fig. 19.2)
c. Increases in antibiotic-resistant bacteria and insecticide-resistant insects correlate with the indiscriminate use of antibiotics and insecticides.
2. Stabilizing selection occurs when extreme phenotypes are eliminated and the phenotype is favored; for example, average human birth weight is near optimum birth weight for survival. (Fig. 19.7) [transp. 115]
3. Disruptive selection occurs when extreme phenotypes are favored and can lead to more than one distinct form; British snails (Cepaea nemoralis) vary because a wide range causes natural selection to vary. (Fig. 19.8)
E. Variations Are Maintained
1. Despite constant natural selection, the following forces promote genetic variation:
a. Mutation and genetic recombination still occur.
b. Gene flow among small populations can introduce new alleles.
c. Natural selection itself sometimes results in variation.
F. Diploidy and the Heterozygote
1. In sexually reproducing diploid organisms, a heterozygote is a repository of rare recessive alleles.
2. In sickle-cell disease, heterozygotes are more fit in malaria areas and homozygotes are maintained in the population. (Fig. 19.9)
19.3. Considering Speciation (p. 314)
A. Speciation is the splitting of one species into two or more species or the transformation of one species into a new species over time; speciation is the final result of changes in gene pool allelic and genotypic frequencies.
B. A biological species is a category whose members are reproductively isolated from all other such groups.
1. Reproductive isolation occurs when members of one species can only breed successfully with each other.
2. Linnaeus separated species based on morphology.
3. Modern biochemical genetics uses DNA hybridization techniques to determine the relatedness of organisms.
C. Reproductive Isolating Mechanisms
1. A reproductive isolating mechanism is any structural, functional, or behavioral characteristic that prevents successful reproduction from occurring. (Table 19.1)
2. Premating isolating mechanisms are anatomical or behavioral differences between the members of two species that prevent the possibility of mating.
a. Habitat isolation occurs when two species occupy different habitats, even within the same geographic range, so that they are less likely to meet and to attempt to reproduce. (Fig. 19.10)
b. Temporal isolation occurs when two species live in the same location, but each reproduces at a different time of year, and so they do not attempt to mate. (Fig. 19.11)
c. Behavioral isolation results from differences in mating behavior between two species.
d. Mechanical isolation is the result of differences between two species in reproductive structures or other body parts, so that mating is prevented.
3. Postmating isolating mechanisms are the result of developmental or physiological differences between the members of two species that prevent successful reproduction after mating has taken place.
a. Gamete isolation is physical or chemical incompatibility of gametes of two different species so that they cannot fuse to form a zygote; an egg may have receptors only for the sperm of its own species.
b. Zygote mortality is when hybrids (offspring of parents of two different species) do not live to reproduce.
c. Hybrid sterility occurs when the hybrid offspring are sterile (e.g., mules).
d. In F2 fitness, the offspring are fertile but the F2 generation is sterile.
D. How Do Species Arise?
1. Allopatric speciation occurs when new species result from populations being separated by a geographical barrier that prevents their members from reproducing with each other. (Fig. 19.12a) [transp. 116]
a. First proposed by Ernst Mayr of Harvard University.
b. While geographically isolated, variations accumulate until the populations are reproductively isolated.
2. Sympatric speciation would occur when members of a single population develop a genetic difference (e.g., chromosome number) that prevents them from reproducing with the parent type. (Fig. 19.12b) [transp. 116]
a. Main example of sympatric speciation is in plants, where failure to reduce chromosome number produces polyploid plants that reproduce successfully only with polyploids; backcrosses with diploids are sterile.
E. Adaptive Radiation Produces Many Species
1. Adaptive radiation is a rapid development from a single ancestral species of many new species.
2. The case of Darwin's finches illustrates the adaptive radiation of 13 species from one founder mainland finch.
3. On Hawaiian Islands, a wide variety of honeycreepers descended from one goldfinchlike ancestor. (Fig. 19.13)
4. Hawaii is also the home of the silversword plants that radiated from ancestral tarweeds.