Microevolutionary Processes Chapter 21

Microevolutionary Processes Microevolution is a change in allele frequencies in a population over generations Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by: Natural selection Gene flow Genetic drift

Populations Evolve Biological evolution does not change individuals It changes a population Traits in a population vary among individuals Evolution is change in frequency of traits

Gene Pools and Allele Frequencies Population localized group of individuals capable of interbreeding and producing fertile offspring Gene Pool consists of all the alleles in a population 4

Gene Mutations Infrequent but inevitable Each gene has own mutation rate The average is about one mutation in every 100,000 genes per generation Mutation rates are often lower in prokaryotes and higher in viruses Short generation times allow mutations to accumulate rapidly in prokaryotes and viruses Lethal mutations Neutral mutations Advantageous mutations

Variation in Phenotype Each kind of gene in gene pool may have two or more alleles Individuals inherit different allele combinations This leads to variation in phenotype Offspring inherit genes, not phenotypes

Variation in Phenotype

Variation in Phenotype

Variation in Phenotype

What Determines Alleles in New Individuals? Mutation Crossing over at meiosis I Independent assortment Fertilization Change in chromosome number or structure

The Hardy-Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving 11

Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population 12

Hardy-Weinberg Rule At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation: p2 + 2pq + q2 = 1 Frequency of allele A = p Frequency of allele a = q

Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a hypothetical population that is not evolving In real populations, allele and genotype frequencies do change over time 14

Conditions for Hardy-Weinberg Equilibrium The five conditions for non-evolving populations are rarely met in nature No mutations Random mating No natural selection Extremely large population size No gene flow 15

Applying the Hardy-Weinberg Principle We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that The PKU gene mutation rate is low Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions The population is large Migration has no effect, as many other populations have similar allele frequencies 16

Applying the Hardy-Weinberg Principle The occurrence of PKU is 1 per 10,000 births q2  0.0001 q  0.01 The frequency of normal alleles is p  1 – q  1 – 0.01  0.99 The frequency of carriers is 2pq  2  0.99  0.01  0.0198 or approximately 2% of the U.S. population 17

Altering Allele Frequencies in a Population Three major factors alter allele frequencies and bring about most evolutionary change Natural selection Genetic drift Gene flow 18

1.) Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions Evolution by natural selection involves both chance and “sorting” New genetic variations arise by chance Beneficial alleles are “sorted” and favored by natural selection Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness 19

Results of Natural Selection Three possible outcomes: A shift in the range of values for a given trait in some direction Stabilization of an existing range of values Disruption of an existing range of values

Directional Selection Number of individuals in the population Favors individuals at one end of the phenotypic range Allele frequencies shift in one direction Range of values for the trait at time 1 Number of individuals in the population Range of values for the trait at time 2 Number of individuals in the population Range of values for the trait at time 3

Peppered Moths Prior to industrial revolution, most common phenotype was light colored After industrial revolution, dark phenotype became more common PEPPERED MOTHS - KETTLEWELL

Adaptation Darwin noticed that every species seemed well adapted to the life it leads Adaptations physical and behavioral characteristics combine to help the organism catch food, handle harsh conditions and reproduce

Examples of Adaptations Camouflage: a structural adaptation enabling an organism to blend in with its environment Octopus Cuttlefish Mimicry: a structural adaptation that provides protection for an organism by copying the appearance of another species

black on yellow kill fellow red on black friend of jack

Pesticide Resistance Pesticides kill susceptible insects Resistant insects survive and reproduce If resistance has heritable basis, it becomes more common with each generation

Antibiotic Resistance First came into use in the 1940s Overuse has led to increase in resistant forms Most susceptible cells died out and were replaced by resistant forms

Stabilizing Selection Number of individuals in the population Intermediate forms are favored and extremes are eliminated Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3

Stabilizing Selection Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes Natural selection will tend to maintain two or more alleles at that locus For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance 30

Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% Figure 21.17 Key Frequencies of the sickle-cell allele 0–2.5% Figure 21.17 Mapping malaria and the sickle-cell allele 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% 10.0–12.5% 12.5% 31

Disruptive Selection Number of individuals in the population Forms at both ends of the range of variation are favored Intermediate forms are selected against Range of values for the trait at time 1 Number of individuals in the population Range of values for the trait at time 2 Number of individuals in the population Range of values for the trait at time 3

Sexual Selection Sexual selection natural selection for mating success can result in sexual dimorphism marked differences between the sexes in secondary sexual characteristics 33

Figure 21.15 Sexual dimorphism and sexual selection 34

2.) Genetic Drift Genetic Drift how allele frequencies fluctuate unpredictably from one generation to the next tends to reduce genetic variation through losses of alleles, especially in small populations 35

Effects of Genetic Drift Genetic drift is significant in small populations Genetic drift can cause allele frequencies to change at random Genetic drift can lead to a loss of genetic variation within populations Genetic drift can cause harmful alleles to become fixed 36

Founder Effect Founder effect occurs when a few individuals become isolated from a larger population allele frequencies in the small founder population can be different from those in the larger parent population due to chance

Bottleneck Bottleneck effect results from a drastic reduction in population size due to a sudden environmental change if the population remains small, it may be further affected by genetic drift understanding the bottleneck effect can increase understanding of how human activity affects other species

3.) Gene Flow Gene flow movement of alleles among populations alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) tends to reduce genetic variation among populations over time 39

Barriers to Gene Flow Whether or not a physical barrier deters gene flow depends upon: Organism’s mode of dispersal or locomotion Duration of time organism can move

Inbreeding Nonrandom mating between related individuals Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed Amish, cheetahs