Ch. 23 The Evolution of Populations

Common Misconception and what actually happens

Individual organisms are thought to evolve, but it is populations that evolve

Microevolution

a change in allele frequencies in a population over generations

What causes allele frequency to change (3)

1. Natural Selection

2. Genetic Drift

3. Gene Flow

Genetic Variation

caused by differences in genes or other DNA segments

• prerequisite for evolution by natural selection

• measured as gene variable + nucleotide variability

• diffs occur in noncoding regions (90%) + coding regions (10%)

Phenotype

the product of inherited genotype and environmental influences

• differences determined by single gene (“either-or” basis) + influence of two or more genes

Gene Variability

average heterozygosity measures the average percent of loci that are heterozygous in a population

• can have evolutionary consequences + sexual reproduction by recombing existing alleles

Nucleotide Variability

§measured by comparing the DNA sequences of 2 or more individuals

Phenotypic Variation

results from environmental influences

What causes new alleles and genes?

Alleles: mutation (change in nucleotide sequence of DNA) + gene duplication

Genes: gene duplication

Point Mutation

a change in a single nucleotide in a DNA sequence

• alter phenotypes or neutral variation (redundcy in genetic code)

Chromosomal mutations

delete, disrupt, or rearrange many loci are typically harmful

• duplication of DNA increases genome size = less harmful

Rapid Reproduction

Mutation is low in animals and plants (100,000/gen) + prokaryotes

Sexual Reproduction

most genetic variation results from recombination of alleles

1. Crossing over: most variation

2. Indep assortment: random

3. Fertilization

Population

localized group of individuals capable of interbreeding and producing fertile offspring

Gene Pool

consists of all the alleles for all loci in a population

• locus = fixed when homozygous (2 or more alleles = homo or hetero)

Hardy-Weinberg Equation

the genetic makeup we expect for a population that is not evolving at a particular locus

• describes constant freq of alleles + tests if evolution is occuring

• p² + 2pq + q² = 1

Conditions for Hardy-Weinberg Equilibirum + Consequence if not met (5)

1. No mutations (gene pool modified)

2. Random mating (inbreeding)

3. No natural selection (allele frew change when indiv with diff genotypes show diff survival)

4. Extremely large pop size (genetic drift)

5. No gene flow (alter allele freq)

PKU

caused by a defect in a gene that helps create the enzyme needed to break down the amino acid phenylalanine in the body

• low mutation rate, random mate selection, large pop, no migration affects

Natural Selection

Selection results in alleles being passed to the next generation in proportions that differ from those in the present generation

• causes adaptive evolution

Adaptive Evolution

a process in which traits that enhance survival or reproduction increase in frequency over time

Genetic Drift

describes how allele frequencies fluctuate unpredictably from one gen to the next

reduces genetic variation + smaller a sample = greater chane of random deviation

1. founder effect

2. pop bottleneck

Founder Effect

occurs when a few individuals become isolated from a larger population or leave a population and move to a new area

• alleles in smaller founder can be different than large parent pop

Bottleneck Effects

occurs when there is a drastic reduction in population size due to a sudden change in the environment

• resulting gene pool no longer reflective

Gene Flow

Consists of alleles moving between populations from fertile indivs or gametes

• reduces pop variation over time + inc fitness of pop

• important in evolution change

Ex: resistance of bugs to insecticides

What does evolution by natural selection involve?

-only NS is consistent: reproductive advantage + adaptive evolution

Chance and Sorting

• new genetic variations by chance

• benefical alleles are sorted and favored

Relative Fitness

the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

• Higher = favored by NS + inc in # faster

• Lower = removed by NS + dec in # faster

Three modes of selection

1. Directional: favor indiv at 1 extreme end of phenotypic range

2. Disruptive: favor indiv at both extremes of phenotypic range

• eliminates stabalizing

3. Stabilizing: favors intermediate variant + acts against exremes

Sexual Selection

Process in which individuals with certain inherited characteristics are more likely to acquire mates than other individuals of the same sex

• results in sexual dimorphism (marked diffs btwn sexes)

Intra-sexual selection

direct competition among individuals of one sex (often males) for mates of the opposite sex

Inter-sexual selection “mate choice”

occurs when individuals of one sex (usually females) are choosy in selecting their mates

• showiness attracts females

Diploidy

maintains genetic variation in the form of recessive alleles hidden from selection in heterozygotes

Balancing Selection

occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

1. Freq-dep selection: fitness of pheno depends on commonality (no advantage)

2. Heterozygote advantage

Heterozygote Advantage

occurs when heterozygotes have a higher fitness than either homozygous (AA or aa)

• results from stabilizing directional selection

Why Natural Selection Cannot Fashion Perfect Organisms (4)

1. Selection can act only on existing variations

2. Evolution is limited by historical constraints

3. Adaptations are often compromises

4. Chance, natural selection, and the environment interact