Chapter 23: The Evolution of a Population

Population Group of individuals in a geographic location, capable of interbreeding and producing fertile offspring Natural selection acts on individuals, but only populations evolve

Galapagos Islands Population of medium ground finches on Daphne Major Island During a drought, large-beaked birds were more likely to crack large seeds and survive Evolution by natural selection

Types of Evolution Microevolution= change in the gene pool of a population over many generations 4 Methods of Microevolution Mutations Natural Selection Genetic Drift= chance events cause genetic changes from one population to the next Gene Flow= individuals or gametes move to a different population

Genetic Drift Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next Bottleneck Effect= event kills a large number of individuals and only a small subset of population is left Founder Effect= Small number of individuals colonize new location

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat= severe reduction in the population of greater prairie chickens in Illinois Low levels of genetic variation, only 50% of their eggs hatched Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Greater prairie chicken Range of greater prairie chicken

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck Results showed a loss of alleles at several loci

Number of alleles per locus Percentage of eggs hatched Population size Figure 23.11b Number of alleles per locus Percentage of eggs hatched Population size Location Illinois 1930–1960s 1993 1,000–25,000 <50 5.2 3.7 93 <50 Kansas, 1998 (no bottleneck) 750,000 5.8 99 Figure 23.11 Genetic drift and loss of genetic variation. Nebraska, 1998 (no bottleneck) 75,000– 200,000 5.8 96

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Researchers introduced greater prairie chickens from population in other states Introduced new alleles into population Increased the egg hatch rate to 90%

Genetic Drift: Summary 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

Gene Flow Gene flow = movement of alleles among populations Alleles can be transferred through: Movement of fertile individuals Gametes Gene flow tends to reduce variation among populations over time Barriers to dispersal can limit gene flow between populations

Geographic Variation Most species exhibit geographic variation Differences between gene pools of separate populations Mice in Madeira Island in Atlantic Ocean Several isolated populations of non-native mice Mountain range prevents gene flow Fusion of chromosomes

Geographic Variation Variation among populations is due to drift, not natural selection

Geographic Variation Cline= graded change in a trait along a geographic axis Mummichog Fish and Cold-Adapted Allele Effect of natural selection

Natural Selection Evolution by natural selection involves both change 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

Evolution Evolution by natural selection is possible because of genetic variation in population Gene pool= all the alleles for all loci in a population Genetic Variation = differences in DNA sequences Gene variability Average heterozygosity= average percent of loci that are heterozygous in a population Fixed loci= all individuals in a population have same allele Nucleotide variability Measured by comparing the DNA sequences of pairs of individuals

Evolution Phenotype= product of inherited genotype + environmental influences Discrete characters= classified on an either-or basis Flower color: red or white Quantitative characters= vary along a continuum within a population Skin color in humans Natural selection can only act on variation with a genetic component

Evolution Genetic variation primarily comes from 2 sources: Mutation and Gene Duplication Original source of new alleles or genes May be neutral before it becomes an advantage “raw material” of evolution Sexual Reproduction= unique combination of genes following crossing over, independent assortment of chromosomes, and random fertilization

Evolution Genetic drift and gene flow do not consistently lead to adaptive evolution Can increase or decrease the match between an organism and its environment Natural selection increases the frequencies of alleles that enhance survival and reproduction Adaptive evolution occurs as the match between an organism and its environment increases Because the environment can change, adaptive evolution is a continuous process

Evolution of a Population Types of Natural Selection Directional Selection Highest reproduction in one extreme phenotype Stabilizing Selection Highest reproduction of intermediate phenotypes Disruptive Selection Highest reproduction of two extreme phenotypes Generations 1 2 3

Frequency of individuals Figure 23.13 Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Figure 23.13 Modes of selection. (a) Directional selection (b) Disruptive selection (c) Stabilizing selection

Sexual Selection Form of natural selection Individuals with certain traits are more likely to mate Sexual Dimorphism= differences in appearance of males and females Vertebrates= males usually “showier” of sexes or engage in competition for females Characteristics may be disadvantage Male birds with bright feathers more obvious to predators

Sexual Selection Intrasexual selection= competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection (mate choice)= individuals of one sex (usually females) are choosy in selecting their mates

Sexual Selection How do female preferences evolve? Good genes hypothesis= if a trait is related to male health, both the male trait and female preference for that trait should increase in frequency

Example of Sexual Selection Females select males based on traits indicating defenses against parasites and pathogens Bird, Mammal, and Fish Species Female stickleback fish and Major Histocompatibility Complex (MHC) Higher reproductive success increases frequency of defense trait in next generation

Genetic Variation in Populations Neutral variation= genetic variation that does not confer a selective advantage or disadvantage Various mechanisms help to preserve genetic variation in a population Diploidy= maintains genetic variation in the form of hidden recessive alleles Heterozygotes can carry recessive alleles that are hidden from the effects of selection

Genetic Variation in Populations Balancing selection= natural selection maintains stable frequencies of two or more phenotypic forms in a population Balancing selection includes Heterozygote advantage Frequency-dependent selection

Heterozygote Advantage Malaria caused by protist, Plasmodium 1-2 million people die/yr from disease Modifies red blood cells to obtain nutrients, escape destruction by spleen

Heterozygote Advantage Sickle-Cell Allele: recessive allele Produces abnormal hemoglobin proteins Normal Blood Cell Sickle-Cell

Heterozygote Advantage Homozygous Dominant No sickle cell allele Susceptible to malaria Homozygous Recessive Sickle-cell disease Resistant to malaria Heterozygotes= co-dominant alleles, both types of blood cells Heterozygotes have decreased symptoms of malaria and decreased symptoms of sickle-cell disease Higher survival

Frequency-Dependent Selection Fitness of a phenotype declines if it becomes too common in the population Selection can favor whichever phenotype is less common in a population Example: Predators can form a “search image” of their prey Most common phenotype Rare phenotypes may avoid detection by predators, increasing survival and reproduction

Limits of Natural Selection Selection can only act on existing variation in a population. New alleles do not appear when needed Evolution is limited by historical constraints. Ancestral structures are adapted to new situations Adaptations are usually compromises. One characteristic may be an adaptation in one situation, a disadvantage in another Natural selection interacts with chance/random events and the environment. Chance events can alter allele frequencies in population Environment can change

Evolution of Populations Measured by calculating changes in gene pool over time Frequency of an allele in a population Diploid organisms: total number of alleles at a locus is the total number of individuals times 2 Homozygous Dominant= 2 dominant alleles Heterozygous= 1 dominant, 1 recessive allele Homozygous Recessive= 2 recessive alleles

Frequency of Alleles in Population For a characteristic with 2 alleles, we can use p and q to represent their frequencies Frequency of Alleles: p= frequency of “A” allele (dominant) Total number of “A” alleles/total number of alleles q= frequency of “a” allele (recessive) Total number of “a” alleles/total number of alleles Frequency of Alleles= p+ q =1

Frequency of Alleles in Population Population of wildflowers that is incompletely dominant for color 320 red flowers (CRCR) 160 pink flowers (CRCW) 20 white flowers (CWCW) Calculate the number of copies of each allele: CR  (number of homozygotes for CR X 2) + number of heterozygotes (320  2)  160  800 CW  (number of homozygotes for CW x 2) + number of heterozygotes (20  2)  160  200

Frequency of Alleles in Population To calculate the frequency of each allele: p  freq CR  number of CR alleles/total number of alleles p = 800 / (800  200)  0.8 q  freq CW  number of CW alleles/total number of alleles q= 200 / (800  200)  0.2 The sum of alleles is always 1 0.8  0.2  1

Hardy-Weinberg Principle The Hardy-Weinberg principle describes a population that is not evolving frequency of alleles will not change from generation to generation A “null hypothesis” to check for evidence of evolution If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

Hardy-Weinberg Principle Figure 23.7 Hardy-Weinberg Principle Alleles in the population Frequencies of alleles Gametes produced p = frequency of Each egg: Each sperm: CR allele  = 0.8 q = frequency of 80% chance 20% chance 80% chance 20% chance CW allele  = 0.2 Figure 23.7 Selecting alleles at random from a gene pool. 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

Hardy-Weinberg Principle When allele frequencies remain constant from generation to generation, the population is in Hardy-Weinberg equilibrium Assumptions: No natural selection No mutation No gene flow Random mating Large population

Hardy-Weinberg Equilibrium Hardy-Weinberg Equilibrium Equations Frequency of Alleles: p + q = 1 Frequency of Genotypes: p2 + 2pq + q2 = 1

Hardy-Weinberg Equilibrium The variables “p” and “q” come from Punnett Square for populations p= frequency of “A” allele (dominant) q= frequency of “a” allele (recessive)

Hardy-Weinberg Equilibrium Hardy-Weinberg Equilibrium Equations Frequency of Alleles: p + q = 1 p= 0.7 q=0.3 Frequency of Genotypes: p2 + 2pq + q2 = 1 p2 = frequency of “AA” genotype Ex: 0.72 = 0.49 2pq = frequency of “Aa” genotype Ex: 2(0.7)(0.3)= 0.42 q2 = frequency of “aa” genotype Ex: 0.32 = 0.09 Frequency of Genotypes= 0.49 + 0.42 + 0.09 = 1

Population Variables: Hardy-Weinberg Equilibrium Once scientists know what p and q are in the population, they can track the population through time and see if population is at equilibrium or changing If p and q change through time, one of the Hardy-Weinberg Equilibrium assumptions are not being met Evolution is occurring Populations can be at equilibrium at some loci, but not at other loci

Real-World Example: Hardy-Weinberg Equilibrium and Fisheries Management Kelp Grouper (Epinephelus bruneus) Commercial fish species in Korea Recent declines in landings 2007: IUCN Red List- Vulnerable Study by An et al. 2012 Genotyped 12 gene loci from 30 fish 3 of 12 loci showed deviations from Hardy-Weinberg Equilibrium