Introduction
1. One obstacle to understanding evolution is the common misconception that organisms evolve.
a. Natural selection does act on individuals by impacting their chances of survival and their reproductive success.
b. However, the evolutionary impact of natural selection is only apparent in tracking how a population of organisms changes over time.
c. It is the population, not the individual, that evolves.
2. A population of marine snails has genetic variation in shell coloration.
a. The light shells may be more visible against background and those snails may be eaten more frequently by predators.
b. Individuals with light shells will become less frequent or disappear from the population.
A. Population Genetics
1. A population�s gene pool is defined by its allele frequencies.
a. A population is a localized group of individuals that belong to the same species.
1. One definition of a species is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in nature.
b. Populations of a species may be isolated from each other, such that they exchange genetic material rarely, or they may live near each other and have a low density in an intermediate region.
c. Members of a population are far more likely to breed with members of the same population than with members of other populations. (Fig. 23.2)
d. The total aggregate of genes in a population at any one time is called the population�s gene pool.
1. It consists of all alleles in all individuals of a population.
2. Often, there are two or more alleles for a gene, each contributing a relative frequency in the gene pool.
B. Causes of Microevolution
1. Microevolution is a generation-to-generation change in a population�s allele frequencies.
a. Microevolution occurs even if the frequencies of alleles are changing for only a single gene in a population while the others are at equilibrium.
2. Four causes of microevolution
a. Four factors can alter the allele frequencies in a population:
1. Genetic drift
2. Natural selection
3. Gene flow
4. Mutation
b. Genetic drift
1. Genetic drift is changes in gene frequencies from one generation to another because of chance events in small populations.
a. The gene pool of a small population may not be accurately represented in the next generation.
b. For example, in a small wildflower population with a stable size of only ten plants, genetic drift can completely eliminate some alleles. (Fig. 23.4)
2. Genetic drift in small populations often occurs as a result of two situations: the bottleneck effect or the founder effect.
a. The bottleneck effect occurs when the numbers of individuals in a larger population are drastically reduced by a disaster (flood, etc).
1. By chance, some alleles may be overrepresented and others underrepresented among the survivors.
2. Some alleles may be eliminated altogether.
3. Genetic drift will continue to impact the gene pool until the population is large enough to minimize the impact. (Fig. 23.5)
4. Bottlenecking is an important concept in conservation biology of endangered species.
a. Populations that have suffered bottleneck incidents have lost at least some alleles from the gene pool.
b. This reduces individual variation and adaptability.
c. For example, the genetic variation in the three small surviving wild populations of cheetahs is very low when compared to other mammals.
1. Their genetic variation is similar to highly inbred lab mice.
b. The founder effect occurs when a new population is started by only a few individuals that do not represent the gene pool of the larger source population.
1. At an extreme, a population could be started on an island by a single pregnant female or a single seed with only a tiny fraction of the genetic variation of the source population.
2. Genetic drift would continue from generation to generation until the population grew larger.
3. Founder effects have been demonstrated in human populations that started from a small group of colonists.
c. Natural selection
1. In a population with variable individuals, natural selection will lead some individuals to leave more offspring than others.
2. Selection results in some alleles being passed along to the next generation in numbers disproportionate to their frequencies in the present generation.
3. Natural selection accumulates and maintains favorable genotypes in a population.
d. Gene flow
1. Gene flow is genetic exchange due to migration of fertile individuals or gametes between populations.
2. For example, if a wildflower population consisted entirely of white flowers (rr), its pollen (r alleles only) could be carried into a neighboring population of RR, Rr and rr plants.
a. This would increase the frequency of r alleles in the target population in the next generation.
3. Gene flow tends to reduce differences between populations.
4. If extensive enough, gene flow can amalgamate neighboring populations into a single population with a common genetic structure.
e. Mutation
1. A mutation is a change in an organism�s DNA.
2. A new mutation that is transmitted in gametes can immediately change the gene pool of a population by substituting the mutated allele for the older allele.
3. For any single gene, mutation alone does not have much quantitative effect on a large population in a single generation.
4. An individual mutant allele may have greater impacts later through increases in its relative frequencies as a result of natural selection or genetic drift.
5. While mutations in an individual gene are a rare event, the cumulative impact of mutations in many genes can be significant.
a. Each individual has thousands of genes, any one of which could experience a mutation.
b. Populations are composed of thousands or millions of individuals that may have experienced mutations.
6. Over the long term, mutation is very important to evolution because it is the original source of genetic variation that serves as the raw material for natural selection.
C. Genetic Variation, the Substrate for Natural Selection
1. Genetic variation can be heritable or non-heritable.
a. The variation among individuals in a population is a combination of heritable and non-heritable traits.
1. Phenotype, the observable characteristics of an organism, is the cumulative product of an inherited genotype and a multitude of environmental influences.
2. For example, these butterflies are genetically identical at the genes for coloration, but they emerge at different seasons and have seasonal variations in hormones.
b. Only the genetic component of variation can have evolutionary consequences as a result of natural selection.
1. This is because only heritable traits pass from generation to generation.
2. Mutation and sexual recombination generate genetic variation.
a. New alleles originate only by mutation.
1. Mutations are changes in the nucleotide sequence of DNA.
2. Mutations of individual genes are rare and random.
3. Mutations in somatic cells are lost when the individual dies.
4. Only mutations in cells that produce gametes can be passed along to offspring.
b. Most point mutations, those affecting a single base of DNA, are probably harmless.
1. Most eukaryotic DNA does not code for proteins and mutations in these areas are likely to have little impact on phenotype.
2. Even mutations in genes that code for proteins may lead to little effect because of redundancy in the genetic code. Example of redundancy: AAA and AAG both code for phenylalanine so a point mutation that substitutes a G for the last A in AAA would have no effect on the resulting protein because it would still code for phenylalanine.
3. However, some single point mutations can have a significant impact on phenotype.
a. Example: sickle-cell disease is caused by a single point mutation.
c. Mutations that alter the structure of a protein enough to impact its function are more likely to be harmful than beneficial.
1. A random change is unlikely to improve a genome that is the product of thousands of generations of selection.
2. Rarely, a mutant allele may enable an organism to fit its environment better and increase reproductive success.
3. This is especially likely if the environment is changing.
a. Example: mutations that enable HIV to resist antiviral drugs are selected against under normal conditions because they slow reproductive rate of the virus, but are favorable under drug treatment.
d. Chromosomal mutations, including rearrangements of chromosomes, affect many genes and are likely to disrupt proper development of an organism.
1. However, occasionally, these dislocations link genes together such that the phenotype is improved when inherited as a package.
e. Duplications of chromosome segments, whole chromosomes, or sets of chromosomes are nearly always harmful.
1. However, when they are not harmful, the duplicates provide an expanded genome.
2. These extra genes can now mutate to take on new functions.
f. In organisms with sexual reproduction, most of the genetic differences among individuals are due to unique recombinations of the existing alleles from the population gene pool.
1. Crossing over, random segregation of homologous chromosomes and random union of gametes creates a unique assortment of alleles in each individual.
2. Sexual reproduction recombines old alleles into fresh assortments every generation.
D. Natural Selection as the Mechanism of Adaptive Evolution
1. The effect of selection on a varying characteristic can be directional, diversifying, or stabilizing
a. Directional selection is most common during periods of environmental change or when members of a population migrate to a new habitat with different environmental conditions.
1. Directional selection shifts the frequency curve for a phenotypic character in one direction by favoring what had been rare individuals. (Fig. 23.12)
2. Peter and Rosemary Grant documented directional evolution in beak size for the medium ground finch in the Galapagos Islands. (Fig. 23.13)
a. During wet years when seeds are abundant, all individuals consume relatively few large seeds.
b. However, during dry years when seeds are scarce, the small seeds are quickly depleted and birds with larger, stronger beaks that can crack large seeds are at an advantage, and their genes increase in the population.
b. Diversifying selection occurs when environmental conditions favor individuals at both extremes of the phenotypic range over intermediate phenotypes.
1. For example, two distinct bill types are present in black-bellied seedcrackers in which larger-billed birds are more efficient when feeding on hard seeds and smaller-billed birds are more efficient when feeding on soft seeds. (Fig. 23.14)
c. Stabilizing selection favors intermediate variants and acts against extreme phenotypes.
1. Stabilizing selection reduces variation and maintains the predominant phenotypes.
2. Human birth weight is subject to stabilizing selection.
a. Babies much larger or smaller than 5-9 lb have higher infant mortality.
2. Sexual selection may lead to pronounced secondary differences between the sexes
a. Males and females of a species differ not only in their reproductive organs, but often also in secondary sexual characteristics that are not directly associated with reproduction.
1. These differences, termed sexual dimorphism, may include size differences, coloration differences, enlarged or exaggerated features, or other adornments.
2. Males are usually the larger and showier sex, at least among vertebrates.
b. Sexual dimorphism is a product of sexual selection.
1. Intrasexual selection is direct competition among individuals of one sex (usually males) for mates of the opposite sex.
a. Competition may take the form of direct physical battles between individuals.
b. The stronger individuals gain status.
c. More commonly, ritualized displays discourage lesser competitors and determine dominance.
2. Intersexual selection or mate choice occurs when members of one sex (usually females) are choosy in selecting among individuals of the other sex.
a. Males with the most masculine features are the most attractive to females.
b. Interestingly, these features may not be adaptive in other ways and expose these individuals to extra risks.
c. Even if these extravagant features have some costs, individuals that possess them will have enhanced reproductive success if they help an individual gain a mate.
1. Every time a female chooses a mate based on appearance or behavior, she perpetuates the alleles that caused her to make that choice. This is sexual selection.
2. She also allows a male with that particular phenotype to perpetuate his alleles.
d. The underlying basis of female choice is probably not aesthetic.
1. Current research is investigating the hypothesis that females use these sexual advertisements to measure the general health of a male.
2. Individuals with infections or other problems are likely to have a relatively dull, disheveled plumage.
3. These individuals are unlikely to win many females.
4. For the female that chooses a healthy mate, the benefit is a greater probability of having healthy offspring.