Chapter 23 The Evolution of Populations

Natural selection acts on individuals But remember individuals do not evolve Yet populations do evolve (over time) Microevolution Change in allele frequencies in a population over generations Important Things To Remember About Evolution

1.Natural selection Individuals with certain inherited traits survive and reproduce better Only natural selection causes adaptive evolution 2.Genetic drift Chance events that alter allele frequency 3.Gene flow Transfer of alleles between populations 3 Mechanisms for Microevolution

Evolution requires variation in heritable traits Individuals have differences in their genes (DNA sequences) Genetic Variation

Genetic variation measured by: Gene variability: Measured by average % of loci in population that are heterozygous (average heterozygosity) Nucleotide variability: Measured by comparing DNA sequences directly Genetic Variation

Geographic variation Difference between genetic material of separate populations Some geographic separation is complete (e.g. separate islands) while others are more gradual Cline Graded change in a character along a geographic axis Species will show gradual phenotypic and/or genetic differences over the geographic area due to gradual changes in environment Variation Between Populations

Example of Cline Based On Temperature Change with Climate Maine Cold (6°C) Latitude (ºN) Georgia Warm (21ºC) Ldh-B b allele frequency 30

New alleles can arise by mutation or gene duplication Mutations are changes in DNA nucleotide sequence Only mutations in germ line cells passed to offspring Many mutations are silent due to redundancy or changes in non-coding regions Some mutations are harmful, some may be beneficial Sources of Genetic Variation

We can use the Hardy-Weinberg Equation which allows us to compare allele frequency between what would be expected if evolution was not occurring. How can we tell if a population is evolving?

Population: Localized group of individuals capable of interbreeding and producing fertile offspring Gene pool: all the alleles for all loci in a population A locus is fixed if all individuals in population homozygous for same allele e.g. All individuals either AA or aa If there are 2 or more alleles however, individuals in the population can be homozygous or heterozygous e.g. AA, aa or Aa Gene Pool and Allele Frequencies

Calculating allele frequency in populations Diploid organisms: Total # of alleles at a locus is the total # of individuals x 2 Total # of dominant alleles at a locus = 2 alleles for each homozygous dominant individual + 1 allele for each heterozygous Total # of recessive alleles at a locus = 2 alleles for each homozygous dominant individual + 1 allele for each heterozygous Allele Frequencies

In lobsters, there is a gene C with 2 alleles: C R and C L C R codes for right handed claws C L codes for left handed claws C R C R lobsters will have larger right claws C L C L lobsters will have larger left claws C R C L will have both claws the same size Example for Allele Frequencies

In lobsters, there is a gene C with 2 alleles: C R and C L C R codes for right handed claws C L codes for left handed claws C R C R lobsters will have larger right claws C L C L lobsters will have larger left claws C R C L will have both claws the same size In a population of 10,000 lobsters, there are: 7,500 right handed 2,000 of equal size 500 left handed Example for Allele Frequencies

If there are 10,000 lobsters, there are 20,000 copies of the C gene (remember diploid?) Calculate the % of each allele based on the phenotype frequency C R = 7,500 (C R C R ) x 2 = 15, ,000 (C R C L ) = 17,000 C L = 500 (C L C L ) x 2 = 1, ,000 (C R C L ) = 3,000 17, ,000 = 20,000 (matches # of C gene seen above) C R frequency is 0.85 or 85% (17,000/20,000) C L frequency is 0.15 or 15% (3,000/20,000) = 1 (sum of alleles is always 1) Example for Allele Frequencies

Hardy-Weinberg principle describes a population that is NOT evolving States that frequencies of alleles and genotypes in a population remain constant from generation to generation If gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population Getting back to Hardy-Weinberg

1.No mutations 2.Random mating 3.No natural selection 4.Extremely large population size 5.No gene flow 5 Conditions for Non-evolving Populations

Consider a population of 500 wildflowers (1,000 alleles) with the following allele frequencies: C R (red flowers) = 0.8 = p C W (white flowers) = 0.2 = q p and q by convention represent the allele frequencies Using Hardy-Weinberg

Selection of Alleles at Random from a Gene Pool Alleles in the population Gametes produced Each egg: Each sperm: 80% chance 20% chance 80% chance 20% chance Frequencies of alleles p = frequency of q = frequency of C W allele = 0.2 C R allele = 0.8

Frequency of genotypes can be calculated C R C R = p 2 = (0.8) 2 = 0.64 (64%) C R C W = 2pq = 2 (0.8)(0.2) = 0.32 (32%) C W C W = q 2 = (0.2) 2 = 0.04 (4%) Frequencies of genotypes confirmed by Punnett Square Hardy-Weinberg Equilibrium

80% C R (p = 0.8) (80%) (20%) Sperm 20% C W (q = 0.2) CRCR CWCW (80%) (20%) CRCR CWCW Eggs 64% (p 2 ) C R 16% (pq) C R C W 16% (qp) C R C W 4% (q 2 ) C W 64% C R C R, 32% C R C W, and 4% C W C W Gametes of this generation: 64% C R (from C R C R plants) 4% C W (from C W C W plants) 16% C R (from C R C W plants) + + Genotypes in the next generation: 16% C W (from C R C W plants) = = 80% C R = 0.8 = p 20% C W = 0.2 = q 64% C R C R, 32% C R C W, and 4% C W C W plants

Remember Hardy-Weinberg theorem describes a hypothetical population that is NOT evolving 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow But in real populations, allele frequencies DO change! 5 Conditions for Non-evolving Populations

If p and q represent the frequencies of the only two possible alleles in a population at a particular locus, then: p 2 + 2pq + q 2 = 1 p 2 and q 2 are frequencies of homozygotes 2pq is frequency of heterozygotes Hardy-Weinberg Equilibrium

In a population of pigs, there are 4 black pigs and 12 pink pigs. The pink allele is dominant and the black allele is recessive. What is the percentage of the pigs that are heterozygotes? Step 1: What is the frequency of the black pigs? 4/16 pigs or 25% (0.25) are black = q 2 Step 2: What is the frequency of the black allele? Square root of 0.25 = 0.5 = q Step 3: What is the frequency of the pink allele? = 0.5 = p Practice Hardy Weinberg Problem

In a population of pigs, there are 4 black pigs and 12 pink pigs. The pink allele is dominant and the black allele is recessive. What is the percentage of the pigs that are heterozygotes? Step 4: What is the frequency of heterozygotes? 2pq = 2 x 0.5 x 0.5 = 0.5 or 50% Overall: 25% are homozygous recessive (black), 25% are homozygous dominant (pink) and 50% are heterozygous (pink) Practice Hardy Weinberg Problem

Remember 3 major factors can alter allele frequencies: Natural selection Genetic drift Gene flow How Factors Can Alter Allele Frequencies in Populations

Smaller a sample, the greater the chance of deviation from a predicted result (violates condition #4, i.e. Large populations) Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next Genetic drift tends to reduce genetic variation through loss of alleles Genetic Drift

5 plants leave off- spring Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW Generation 2 p = 0.5 q = plants leave off- spring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 3 p = 1.0 q = 0.0

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 Causes of Genetic Drift

Bottleneck effect Sudden reduction in population size due to a change in the environment The resulting gene pool may no longer be reflective of the original population’s gene pool If the population remains small, it may be further affected by genetic drift Causes of Genetic Drift

Bottleneck Effect Original population Bottlenecking event Surviving population

Human actions can cause serious bottlenecks for other species Ex. Northern elephant seals have significantly reduced genetic variation most likely due to excessive hunting by humans By end of 19 th c., there were only about 20 individuals Population size back up to over 30,000 but still much less genetic variation compared to lesser hunted Southern elephant seal Humans Influence on Bottlenecks

Genetic drift is significant in small populations Genetic drift causes 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 Summary of Genetic Drift

Gene flow is movement of alleles between populations Alleles can be transferred by individuals moving or gametes (for example, pollen) Gene flow reduces genetic variation over time Ex. Organisms of many social species will disperse when reaching reproductive age, leaving their original family group and finding new territories Gene Flow

Sometimes gene flow can increase the fitness of population Spread of alleles that may carry an advantage to a new population of the species Ex. Resistance to insecticides Some populations of mosquitoes have evolved alleles that protect them from insecticides As these mosquitoes move to new areas, this beneficial allele moves with them Gene Flow

Phrases “struggle for existence” and “survival of the fittest” misleading Imply direct competition among individuals Reproductive success is generally more subtle and depends on many factors Relative fitness is contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative Fitness

Sexual selection Natural selection for mating success May result in sexual dimorphism Marked differences between the sexes in secondary sexual characteristics Sexual Selection

Intrasexual selection Competition among individuals of one sex (often males) for mates of the opposite sex Types of Sexual Selection

Intersexual selection (mate choice) Occurs when individuals of one sex (usually females) are choosy in selecting their mates Types of Sexual Selection

Male showiness can increase a male’s chances of attracting a female But can also decrease his chances of survival Types of Sexual Selection

Perfection unattainable Selection can act only on existing variations Evolution is limited by historical constraints Adaptations are often compromises Chance, natural selection, and the environment interact Why Aren’t Organisms Perfect?