lec10.2.pdf

Let?s put this all together. The economy of sex ? Most gonochoric (dioecious) populations have a sex ratio ~ 50/50 ? Hermaphrodites may have more males than females ? Its ?cheaper? to be a male, so ?male? is a better strategy for any individual hermaphrodite ? Females determine the reproductive output of a population ? Any individual female typically has higher mating success than any individual male Asexual reproduction avoids some of these costs ? All (or most) members of the population can reproduce ? Reproductive output of the population higher because MORE individuals reproduce What evidence is there that sex is actually good from an evolutionary standpoint? ? Ubiquity ? Syngamy seen in most eukaryotes ? Other modes of recombination prevalent in prokaryotes ? Longevity of sexual lineages compared to asexual lineages ? Most exclusively or primarily asexual organisms relatively recently derived from a sexual ancestor So why does everybody do it? Sex is advantageous because recombination is advantageous in the long term if not in the short term More variability ? Sex facilitates the generation of novel combinations of alleles more readily than mutation alone ? Sex disrupts linkage disequilibrium through crossing over and thus facilitates novel allele combinations more than mutation alone Variability hypotheses are associated with variable or fluctuating environments Fewer lethals ? Sex may help to hide or purge deleterious alleles (or even those that just have low fitness) from the general population ? Muller?s ratchet: Deleterious alleles will accumulate in an asexual population over time if mutations from ?bad? back to ?good? or ?neutral? are rare. The number of individuals with 0 low-fitness mutations will decrease from generation to generation as mutation load increases. In the first generation, there are a few individuals with no deleterious mutations Over time, all subsets of the population acquire mutations , so the ?zero? class gets eliminated, and the ?1? class becomes the strain with the smallest mutation load. Over MORE time, all subsets of the population acquire mutations , so the ?1? class gets eliminated, and the ?2? class becomes the strain with the smallest mutation load. Consequences of Muller?s Ratchet ? Over time, the proportion of the population comprised of low-fitness genotypes increases ? This will decrease the size of the population (as genotypes ?slide off? the end of the scale) ? As population size declines, genetic drift increases in importance, and the rarer, lower-mutation genotypes may get lost Mutation meltdown and sex ? The ?speed? of the ratchet increases with decreasing population size ? Combined effect of Muller?s ratchet and drift leads to ?mutation meltdown? and extinction ? Sex ?reconstitutes? low-load genotypes through recombination and independent assortment Kondrashov?s model of lethals 1 0 3 2 4 5 Number of deleterious mutations Fitness 6 7 8 9 10 1 0 3 2 4 5 Frequency 1 0 3 2 4 5 Frequency In a sexual population, the distribution of genotypes is broad because of recombination In an asexual population, the distribution of genotypes is narrower because of Muller?s ratchet and linkage equilibrium 2 2 3 4 4 3 10 3 6 5 ? Deleterious mutations are purged more effectively because relatively more individuals are beyond the threshold ? Does not require drift, so it ?works? even in large populations Kondrashov?s model of lethals Migration (gene flow) Migration violates Hardy-Weinberg - by introducing or removing alleles from the population - Migration can maintain genetic diversity in the face of natural selection or genetic drift Large ?source? population Small ?sink? population 1. Gene flow from source creates genetic variation 2. Selection (or drift) erodes genetic variation 3. Gene flow restores genetic variation Molecular sequences and random changes ? Mutations arise in gene sequences by chance ? Mutations may spread through a population by random processes ? If the mutation is selectively neutral ? If the population is small and drift is important ? More mutations in a larger population, but each has a lower chance of fixation The ?molecular clock? ? Zuckerkandl and Pauling noticed that the rate of change in amino acid sequence for some genes seems to be constant over time and across lineages ? This is HIGHLY IMPROBABLE under a selectonist paradigm ? This rate, if calibrated to the fossil record, can be used to infer the divergence time of organisms Evidence for the clock Amount of sequence difference correlates positively and constantly with fossil-based estimates of divergence time The molecular clock in ?real time? ? Substitutions accumulate over time at a constant rate ? If selection WERE acting on this sequence, you might expect very little change (negative selection) or accelerated change (positive selection) How the molecular clock is applied A E B C D 1.0 unit of time 1.0 unit of time 0.5 unit of time 0.5 unit of time The neutral theory of molecular evolution ? If the rate of mutation tracks the divergence time, then selection may not be the appropriate model of sequence change ? If selection were the driving force in sequence change, then rates would differ across and within lineages, depending on whether and how that gene was implicated in the speciation events The Neutral Theory, continued ? Substitutions that have no impact on function are neutral, and will be passed along at no cost ? For these kinds of changes, the important determinant is mutation rate, not fitness consequences ? Mutation rate of neutral mutation sets an standard for the rate of change ? Drift is the mechanism by which fixation occurs ? The fixation of these neutral mutations depends on time, not population size ? More mutation in larger population, but chance of fixation lower Mootoo Kimura, 1968 Neutral theory forms the null hypothesis of molecular evolution If we see deviations from the predictions of neutral theory, we infer that selection (or something else) has occurred Predictions of neutral theory ? Advantageous mutations are rare ? The rate of evolution for a gene will be equal to the rate of mutation ? Sites that have functional importance should change faster or slower than silent sites How do you know if selection is happening in genes? Synonymous substitutions Non-synonymous substitutions Synonymous substitutions Non-synonymous substitutions Synonymous substitutions Non-synonymous substitutions = 1 < 1 > 1 No selection Negative selection Positive selection What can?t neutral theory explain? Is evolution ?random?? Some aspects of evolution are random 1. Mutation: ultimate source of all genetic variation 2. Genetic drift: random change in gene frequencies between generation 3. Migration/Gene flow: can result in random gene flow in/out of population These are important processes that are crucial to evolution? Natural selection is not random 4. Natural selection: differential (non-random) survival and reproduction ?but only natural selection can explain adaptive evolution. These are the 4 mechanisms for microevolutionary change = change in gene frequencies across generations James Chiucchi PowerPoint Presentation