Chapter 6 The ways of change: drift and selection Population genetics Study of the distribution of alleles in populations and causes of allele frequency changes Key Concepts Diploid individuals carry two alleles at every locus Homozygous: alleles are the same Heterozygous: alleles are different Evolution: change in allele frequencies in a population (from one generation to the next) Hardy-Weinberg equilibrium Extended Mendel to populations Obs: Population allele frequencies do not change if: Population is infinitely large Prevents effects of drift Genotypes do not differ in fitness No selection There is no mutation Mating is random No special choice at a particular locus There is no migration Predictions from Hardy-Weinberg Equilibrium Why was the Ester 1 allele so much rarer inland? Hardy and Weinberg proposed theorem in 1908 that p 2 + 2pq + q 2 = 1 Allele frequencies predict genotype frequencies Rests on the 5 preceeding assumptions If no outside forces, allele frequencies in population will not change from one generation to the next. - NO EVOLUTION 1
Key Concepts Hardy-Weinberg theorem proves that allele frequencies do not change in the absence of drift, selection, mutation, and migration H-W is also a mathematical model Mechanisms of evolution are forces that change allele frequencies drift, selection, mutation, and migration Key Concept Hardy-Weinberg serves as the fundamental null model in population genetics Genetic Drift Peter Buri started 107 cultures with 8 males and 8 females all heterozygous for bw 75 red eye and for bw white eye bw / bw 75 heterozygous orange parents All H-W assumptions met except large population size 19 generations each generation started with: Randomly selected: 8 males and 8 females Many populations had alleles that went to Extinction Fixation Other populations ranged between two extremes What happened? Genetic drift results from random sampling error Genetic drift causes evolution in finite populations Sampling error is higher with smaller sample 2
Drift reduces genetic variation in a population Key Concepts Alleles are lost at a faster rate in small populations Alternative allele is fixed Genetic drift is the random, nonrepresentative sampling of alleles from a population during breeding Alleles are lost more rapidly in small populations Evolutionary biologists have debated the importance of natural selection and genetic drift Bottlenecks reduce genetic variation R.A. Fisher Sewel Wright Motoo Kimura A bottleneck causes genetic drift Rare alleles are likely to be lost during a bottleneck Founder effect Founder effects cause genetic drift 3
High incidence of migraine headaches attributed to founder effects Key Concept Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations The concept of fitness Fitness: the reproductive success of an individual with a particular phenotype Components of fitness: Survival to reproductive age Mating success Fecundity Relative fitness: fitness of a genotype standardized by comparison to other genotypes Contribution of alleles to fitness Average excess fitness: difference between average fitness of individuals with allele vs. those without Δp = p x (a A1 /ϖ) Natural selection more powerful in large populations Pleiotropy may constrain evolution Pleiotropy: mutation in a single gene affects many phenotypic traits Can be antagonistic Net effect on fitness determines outcome of selection Drift weaker in large populations Small advantages in fitness can lead to large changes over the long term 4
Pesticide resistance and pleiotropy - Why was the Ester 1 allele so much rarer inland? Pesticide resistance and pleiotropy Key Concept Hardy-Weinberg serves as the fundamental null model in population genetics Condition that we try to falsify Test populations to see if they are in H-W equil. These are the null model frequencies Ex: Hemoglobin (Box 6.3) Cavalli-Sforza - Nigeria Hg A and S (based on difference in β-globin) Results: More SA and AA and fewer SS than H-W would predict Why are there so many hemoglobin A alleles in the population? Heterozygote Advantage S = sickle cell disease hemoglobin SS = sickle cell disease Low fitness A = normal hemoglobin AS AA = Susceptible to malaria Low fitness Protected from malaria Protected from sickle cell disease Survive and reproduce higher fitness Testing the Null Model Founder effect Amish of Lancaster, PA Ellis-van Creveld Syndrome: mutation causes dwarfism and polydactylism General population at levels less than 0.1%, Lancaster Amish the allele s level is approximately 7%. Cavalli-Sforza, 2007 Absence of upper incisors and conical lower incisors 5
Ellis-van Creveld Inherited disorder of bone growth: Very short stature (dwarfism). Short forearms and lower legs Narrow chest with short ribs. Extra fingers and toes (polydactyly), Malformed fingernails and toenails Dental abnormalities. More than half of affected individuals are born with a heart defect, which can cause serious or life-threatening health problems. Founder effect Nonrandom Mating sibling matings - Inbreeding reduces variation Fig Wasps Naked Mole rats Human Eyebrow Mites (Demodex folliculorum) Key Concept Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations How Genetic Variation is Lost Effects of Population size Genetic Drift: fixation or loss of alleles Population bottlenecks causes habitat destruction or fragmentation introduced competitors or predators disease affects Mendelian (discontinuous) characters more severely than quantitative (continuous) characters Genetic Drift Genetic Drift ex. Cheetahs Acinonyx jubatus Large scale climate change about 10,000 years ago Most populations of cheetahs went extinct in North America, Europe, Asia, and much of Africa Current animals are the result of inbreeding among the surviving few animals (perhaps a single preganant female?) Little genetic variability especially among immune system genes Habitat encroachment and poaching have further reduce cheetah numbers, consequently snuffing out even more genetic variation and leaving cheetahs even more vulnerable to extinction. 6
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Slides excerpted from: 10,000ya Cheating Cheetahs Cheating Cheetahs 2007 study, female cheetahs seem to be at least as promiscuous as their male counterparts. Females frequently mate with several different males while they are fertile and are then likely to bear a single litter of cubs fathered by multiple males making many of the cubs within a single litter only half-siblings. This discovery has important implications for the conservation of these endangered animals. Though it conflicts with the idea that cheaters never prosper, evolutionary theory suggests that, in this case, cheating may be beneficial Three hypotheses for evolution of cheating 1. Even if several of her cubs were killed by a new disease, succumbed to a novel environmental stress, or just didn't have what it took to make a living in the Serengeti, a female with a variable litter could still hope that one of her cubs would have "the right stuff" to survive. Biologists refer to this as "bet-hedging" not putting all your eggs (or in this case, cubs) in one basket. Cheating Cheetahs 2. Perhaps, multiple mating is really a strategy to avoid expending extra energy fending off would-be suitors. In other words, maybe females mate multiple times not because it ensures genetic variation in offspring, but because it's so much easier than fighting off males right and left. Web info from Berkeley evolution education site http://evolution.berkeley.edu/evolibrary/news/07070 1cheetah 8
Cheating Cheetahs Cheating Cheetahs 3. Perhaps multiple mating evolved as a way to deter infanticide. In some big cats (and in many other species), males try to kill cubs that are not their own. However, if a mother mates with many different males, it is more difficult for a male to tell whether or not a cub is his own and the male would likely be deterred from killing the cub. This third hypothesis suggests that multiple mating was favored by natural selection because it discouraged infanticide against a female's cubs, not because it increased the litter's genetic variation. This third hypothesis fits with the observation that wild cheetah males seem to rarely (if ever) commit infanticide, though it is common in lions and other big cats. Gottelli, D., Wang, J., Bashir, S., and Durant, S. M. (2007). Genetic analysis reveals promiscuity among female cheetahs. Proceedings of the Royal Society B 274(1621):1993-2001. http://dx.doi.org/10.1098/rspb.2007.0502 Menotti-Raymond, M., and O'Brien, S. J. (1993). Dating the genetic bottleneck of the African cheetah. Proceedings of the National Academy of Sciences 90(8):3172-3176. http://www.pnas.org/content/90/8/3172.abstract Natural Selection Individuals vary in the expression of their phenotypes This variation causes some individuals to perform better than others Natural Selection happens when there is differential fitness Modern definition: Natural Selection The differential survival or reproduction, on the average, of different phenotypes in a population Will lead to changes in frequencies of those phenotypes within a generation, that is,different age classes will have different phenotype frequencies. Within a generation - Darwin thought of many generations Evolution happens between generations Note: NS does not have to lead to evolution! The concept of fitness Fitness: the reproductive success of an individual with a particular phenotype Ability to get genes into future generations Components of fitness: Survival to reproductive age Mating success Fecundity Relative fitness: fitness of a genotype standardized by comparison to other genotypes Measuring Fitness Difficult, rarely possible to Record lifetime reproductive success Record how many of those offspring survive to reproduce themselves Other problems Can t follow organisms for long time Complex relationship b/w genotype and phenotype Fitness is product of entire phenotype Proxies for fitness Probability of surviving to reproductive age Measure number of offspring in a season 9
Fitness Survival Fecundity Mating Zygote adult gametes zygotes t t t t+1 t and t+1 = generations Fecundity = ability to produce gametes Contribution to the next generation = fitness If different phenotypes are due to different genotypes, and have different fitnesses, then natural selection will act and the phenotype and genotype frequencies will change. Fitness Absolute fitness of a genotype = average reproductive rate of individuals with that genotype Absolute Fitness = W Subscripts = genotypes A 1 A 1 W 11 A 1 A 2 W 12 A 2 A 2 W 22 Fitness Fitness Absolute fitnesses determine whether a population will increase or decrease in size If average absolute fitness of all individuals in the population is >1 then population increases in size If average absolute fitness is <1 then population decreases in size Population geneticists use value W W = all fitness components: survival, mating success and fecundity Describes relative contribution of individuals with one genotype compared with the average contribution of all individuals in the population Relative Fitness Average excess fitness: difference between average fitness of individuals with allele vs. those without Δp = p x (a A1 /ϖ) Contribution of alleles to fitness Average excess fitness can be used to predict how the frequency of the allele will change from one generation to the next 10