Mader Evolution of Poplulations Chapter 23

Mader Evolutionary Study Microevolution –describes the small-scale changes within gene pools of populations over generations and how new species originate Macroevolution –Patterns of changes in form of groups of related species over broad periods of geologic time determine phylogeny, the evolutionary relationships among species and groups of species

Mader Population Genetics Populations are individuals belonging to the same species. Species are individuals in a population who have the potential to interbreed and produce fertile offspring. A gene pool is the total aggregate of genes in a population at any one time –All of the alleles in all individuals in the population

Mader Allele Frequency Each individual has 2 alleles or more per gene. (homozygous or heterozygous) Each allele has a relative frequency in the gene pool. –Calculation of allele frequency: Pink (A) is dominant over white (a). 500 plants (480 are pink & 20 are white) –How many “a” alleles are there in the population? –How many “A” alleles are there in the population?

Mader If the following were true… 320 are Homozygous Dominant 160 are Heterozygous – what is the allele frequency for A? –What is the allele frequency for a?

Mader Genetic Structure Allele frequncy for A = 0.8 (80%) Allele frequency for a = 0.2 (20%) Knowing allele frequencies can help you determine genotype frequencies. –What is the genotype frequency of AA? –Aa? –aa?

Mader Genetic Structure AA = 320 out of 500 = 0.64 (64%) Aa = 160 out of 500 = 0.32 (32%) Aa = 20 out of 500 = 0.04 (4%)

Mader Causes of Changes in Allelic Frequencies Natural selection Mutations Gene flow Genetic drift –Founder effect –Bottleneck effect Nonrandom mating –Inbreeding –Sexual selection

Mader Genetic Equilibrium The allele frequencies in a population remain constant from generation to generation –All traits are selectively neutral (no natural selection) –Mutations do no occur –The population must be isolated from other populations (no gene flow) –The population is large (no genetic drift) –Mating is random

Mader Hardy-Weinberg The Hardy-Weinberg principle: –Allele frequencies in a population will remain constant assuming: No Mutations No Gene Flow Random Mating No Genetic Drift No Selection

Mader Industrial Melanism and Microevolution

Mader Hardy-Weinberg Required conditions are rarely (if ever) met –Changes in gene pool frequencies are likely –When gene pool frequencies change, microevolution has occurred Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place

Mader Hardy-Weinberg Allele frequencies for each allele (p = A, q =a) Frequency of homozygotes (p 2 = AA, q 2 = aa) Frequency of heterozygotes (pq + qp = 2pq = Aa) p + q = 1 (all alleles sum to 100%) p 2 + 2pq + q 2 = 1 (all individuals sum to 100%)

Mader Calculating Gene Pool Frequencies Using the Hardy-Weinberg Equation

Mader Hardy-Weinberg Equation Allows us to calculate the frequencies of alleles in a gene pool Ex: 1 of babies have PKU –The disease is caused by a recessive allele –q 2 = frequency of the homozygous recessive genotype for PKU –Given q 2 = –What is the frequency of the PKU allele in the population? –What is the frequency of the dominant normal allele?

Mader q =0.01 p = = 0.99

Mader Causes of Microevolution Genetic Mutations –The raw material for evolutionary change –Provides new combinations of alleles –Some might be more adaptive than others

Mader Causes of Microevolution Gene Flow –Movement of alleles between populations when: Gametes or seeds (in plants) are carried into another population Breeding individuals migrate into or out of population –Immigration or Emmigration –Continual gene flow reduces genetic divergence between populations

Mader Gene Flow

Mader Causes of Microevolution Nonrandom Mating –When individuals do not choose mates randomly Assortative mating: –Individuals select mates with their phenotype –Individuals reject mates with differing phenotype Sexual selection: –Males compete for the right to reproduce –Females choose with males possessing a particular phenotype –Both of these cause an increase in homozygotes

Mader Causes of Microevolution Genetic Drift –Occurs by disproportionate random sampling from population –Can cause the gene pools of two isolated populations to become dissimilar –Some alleles are lost and others become fixed (unopposed) –Likely to occur: After a bottleneck When severe inbreeding occurs, or When founders start a new population –Stronger effect in small populations

Mader Genetic Drift

Mader Genetic Drift Bottleneck Effect –A random event prevents a majority of individuals from entering the next generation –Distaster occurs changing the environment drastically –Some alleles end up overrepresented or underrepresented –Ex: elephant seals hunted to almost extinction –Next generation consists of alleles that just happened to make it.

Mader Genetic Drift Founder Effect –When a new population is started from just a few individuals (isolation) –The alleles carried by population founders are dictated by chance –Formerly rare alleles will either: Occur at a higher frequency in the new population, or Be absent in new population

Mader Founder Effect

Mader Natural Selection Adaptation of a population to the biotic and abiotic environment –Requires: Variation - The members of a population differ from one another Inheritance - Many differences are heritable genetic differences Differential Adaptiveness - Some differences affect survivability Differential Reproduction – Some differences affect likelihood of successful reproduction

Mader Natural Selection Results in: –A change in allele frequencies the gene pool –Improved fitness of the population Major cause of microevolution

Mader Types of Selection Most traits are polygenic - variations in the trait result in a bell-shaped curve Three types of selection occur: –(1) Directional Selection The curve shifts in one direction Ex - when bacteria become resistant to antibiotics

Mader Directional Selection

Mader Types of Selection Three types of selection occur (cont): –(2) Stabilizing Selection The peak of the curve increases and tails decrease Ex - when human babies with low or high birth weight are less likely to survive –(3) Disruptive The curve has two peaks Ex – When Cepaea snails vary because a wide geographic range causes selection to vary

Mader Stabilizing Selection

Mader Disruptive Selection

Mader Maintenance of Variations Genetic variability –Populations with limited variation may not be able to adapt to new conditions –Maintenance of variability is advantageous to population Only exposed alleles are subject to natural selection

Mader Maintenance of Variations Recessive alleles: –Heterozygotes shelter recessive alleles from selection –Allows even lethal alleles to remain in population at low frequencies virtually forever –Lethal recessive alleles may confer advantage to heterozygotes Sickle cell anemia is detrimental in homozygote However, heterozygotes more likely to survive malaria Sickle cell allele occurs at higher than expected frequency in malaria prone areas

Mader Sickle-cell Disease