16.1 Genes, Populations, and Evolution
1. A population is all of the members of a single species occupying a certain area at the same time.
2. Microevolution relates to evolutionary change within populations.
A. Microevolution in the Peppered Moth
1. Population genetics studies the variation in alleles in a gene pool.
2. Moths (diploid organisms) carry two copies of each chromosome.
a. A single gene for body color has two alleles, D (dark color and dominant), and d (light color and recessive).
b. There are three possible genotypes for the color gene:
i. DD (homozygous dominant)
ii. Dd (heterozygous)
iii. dd (homozygous recessive)
The gene pool is the total of all the alleles in a population; it is described in terms of gene frequencies.
The allele frequency is the proportion of each allele in a population’s gene pool.
The Hardy-Weinberg principle 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.
No mutation: no allelic changes occur, or changes in one direction are balanced by changes in the other direction.
No gene flow: migration of alleles into or out of the population does not occur.
Random mating: individuals pair by chance and not according to their genotypes or phenotypes.
No genetic drift: the population is large so changes in allele frequencies due to chance are insignificant.
No selection: no selective force favors one genotype over another.
In real life, 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.
Any change of allele frequencies in a gene pool of a population signifies that evolution has occurred.
The Hardy-Weinberg law tells us what factors cause evolution—those that violate the conditions listed.
A Hardy-Weinberg equilibrium provides a baseline by which to judge whether evolution has occurred.
Hardy-Weinberg equilibrium is a constancy of gene pool frequencies that remains across generations.
B. Microevolution and Equilibrium
Mutations are permanent genetic changes.
Without mutations, there could be no inheritable phenotypic variations among members of a population.
Mutations are the primary source of genetic differences among prokaryotes that produce asexually.
In sexual reproducing organisms, both mutations and sexual recombination are important in generating phenotypic differences.
2. Gene flow (gene migration) is the movement of alleles among populations by migration of breeding individuals.
a. Gene flow can increase variation within a population by introducing novel alleles produced by mutation in another population.
b. Continued gene flow decreases diversity among populations, causing gene pools to become similar.
c. Gene flow among populations can prevent speciation from occurring.
3. Genetic drift refers to changes in allele frequencies of a gene pool due to chance rather than selection by the environment.
a. Genetic drift occurs in both large and small populations; small populations are more likely to show the effects of drift.
b. Genetic drift occurs when founders start a new population, or after a genetic bottleneck with interbreeding.
c. The bottleneck effect prevents most genotypes from participating in production of the next generation.
1) The bottleneck effect is caused by a severe reduction in population size due to a natural disaster, predation, or habitat reduction.
2) The bottleneck effect causes a severe reduction in the total genetic diversity of the original gene pool.
3) The cheetah bottleneck causes relative infertility because alleles were lost due to intense inbreeding when populations were reduced in earlier times.
c. The founder effect is an example of genetic drift where rare alleles or combinations occur in higher frequency in a population isolated from the general population.
1) This is due to founding individuals containing a fraction of total genetic diversity of the original population.
2) Which particular alleles are carried by the founders is dictated by chance alone.
d. When population sizes are so small that there are few, if any, available unrelated mates, inbreeding can occur.
1) Inbreeding does not affect the frequency of alleles and does not cause a population to evolve.
2) Recessive disorders can arise more frequently when inbreeding occurs.
e. Random mating involves individuals pairing by chance, not according to genotype or phenotype.
1) Nonrandom mating occurs when certain genotypes or phenotypes mate with one another.
2) Assortative mating is a type of nonrandom mating that occurs when individuals tend to mate with those having the same phenotype with respect to a certain characteristic.
3) Assortative mating divides a population into two phenotypic classes with reduced gene exchange.
4) Homozygotes for gene loci that control a trait increase, and heterozygotes for these loci decrease.
16.2 Natural Selection
A. Types of Natural Selection
1. Stabilizing selection occurs when extreme phenotypes are eliminated and the intermediate phenotype is favored.
a. The average number of eggs laid by Swiss starlings is four or five.
b. If the female lays more or less than this number, fewer survive.
c. Genes determining the physiology of yolk production and behavior are involved in clutch size.
2. Directional selection occurs when an extreme phenotype is favored; the distribution curve shifts that direction.
A shift to more colorful, later maturing male guppies with exposure to no predators compared to drab colored, early maturing male guppies.
3. Disruptive selection occurs when extreme phenotypes are favored and can lead to more than one distinct form.
a. British snails (Cepaea nemoralis) vary because a wide range causes natural selection to vary.
b. In forest areas, thrushes feed on snails with light bands.
c. In low-vegetation areas, thrushes feed on snails with dark shells that lack light bands.
B. Sexual Selection
1. Sexual selection refers to adaptive changes in males and females that lead to an increased ability to secure a mate.
In males, this may result in an increased ability to compete with other males for a mate.
Females may select a mate with the best fitness (ability to produce surviving offspring), thereby increasing her own fitness.
2. Female Choice
a. Two hypothesis regarding a female’s choice of a mate are:
1) The good genes hypothesis contends that females choose mates based on traits for improving the survival of offspring.
2) The runaway hypothesis states that females chose mates on the basis of traits that attract them to females; the trait can then become exaggerated until it is a handicap.
The Raggiana Bird of Paradise exhibits sexual dimorphism (males and females differ in size and other traits)
1) The ornateness of the male is a factor in selection by a female.
2) More feathery Raggiana are parasite-free, and their selection by females would increase the chance for survival.
3. Male Competition
a. A cost-benefit analysis can be applied to determine if the benefit of access to mating is worth the cost of competition among males.
b. Baboons have a dominance hierarchy.
1) A dominance hierarchy is a ranking within a group where the higher ranking individuals acquire more resources.
2) Dominance is determined by confrontation where one animal gives way to the other.
3) Baboons are dimorphic: males are larger and have large canine teeth; they decide when the troop moves, and they defend it.
4) Females mate with dominant males when ovulation is near; the dominant males then protect all young.
5) The drawbacks to being large and in danger are outweighed by the chance of fathering young.
6) The subordinate males have less chance to mate but they do have avenues to have some offspring.
c. Red deer stake out a territory, an area that is defended against competitors.
Territoriality involves the type of behavior needed to defend a particular territory.
A stag competes for females that form a harem that mate only with him.
A stag remains at peak fighting ability for only a short time; one stag can only father about two dozen offspring.
C. Inbreeding in Populations (Nature of Science reading)
1. Inbreeding, or mating with close relatives, often occurs when a
population size is small, perhaps as a result of a bottleneck of founder’s effect.
2. A consequence of inbreeding is the increase of homozygous genotypes in a population.
3. An example of inbreeding is the occurrence of a rare form of non-sex-linked colorblindness, achromatopsia, in Pingelap in the Pacific Ocean.
a. Achromatopsia in Pingelap resulted from a population crash in 1775 when a typhoon killed 90% of the population, and 20 people survived.
b. Achromatopsia began to appear frequently four generations after the typhoon.
c. Inbreeding can be long term and one in 12 Pingelapese suffer from achromatopsia.
16.3 Maintenance of Diversity
1. Populations always show some genotypic variation; populations that lack variation may not be able to adapt to new conditions.
A. Natural Selection
1. Imperfections are common because compromises have had to be made. For example, the success of humans is attributable to their dexterous hands, but the spine is subject to injury because the vertebrate spine was not originally designed to stand erect.
2. If a feature has successfully evolved, then the benefit outweighs the cost.
3. The environment also plays a role in maintaining variations. For example, disruptive selection promotes polymorphisms in a population.
4. The environment also includes specific selecting agents that help maintain variations.
B. Heterozygote Advantage
1. Heterozygote advantage occurs when the heterozygote is favored over the two homozygotes.
2. Sickle-Cell Disease
In sickle-cell disease, heterozygotes are more fit in malaria areas because the sickle-cell trait does not express unless the oxygen content of the environment is low; but the malaria agent causes red blood cells to die when it infects them (loss of potassium).
Some homozygous dominants are maintained in the population but they die at an early age from sickle-cell disease.
Some homozygotes are maintained in the population for normal red blood cells, but they are vulnerable to malaria.
C. Cystic Fibrosis
In cystic fibrosis the recessive allele causes the person to have a defective plasma membrane protein.
The agent that causes typhoid fever can use the normal version of this protein, but not the defective one, to enter cells.
Heterozygote superiority caused the recessive allele to be maintained in the population.