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Download lecture 02 - selection on the gene, genome, trait and phenotype
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Evolution by natural selection The Modern synthesis restated Darwin’s 4 postulates: (1) Individuals in a population are variable for most traits, because mutation creates new alleles and sexual reproduction creates new allele combinations in every generation (2) Individuals pass their particular alleles to their offspring (3) More offspring are produced than can survive (4) Individuals that survive, or reproduce the most, have allele combinations that best adapt them to their environment Natural selection & fitness Natural selection is the sole process that produces adaptation to the environment - an adaptation is a trait that makes you better suited to your ecological niche, and increases your fitness Alleles or allele combinations, and the traits they produce, determine fitness of an individual: # of offspring that survive to reproduce - if you live forever but produce no offspring, your fitness = 0 Allele combinations resulting in higher fitness are passed to more offspring, and thus those alleles rise in frequency over time (becoming more common) - Natural selection & fitness Two general points to consider: 1) there is a trade-off between offspring size and number - the same amount of resources can be divided into more but smaller offspring, or fewer, bigger offspring maternal energy (e.g., available egg yolk) If life is easy, and offspring survival rates are high, which strategy do you predict selection will favor? Natural selection & fitness Two general points to consider: 1) there is a trade-off between offspring size and number - the same amount of resources can be divided into more but smaller offspring, or fewer, bigger offspring - measuring “fitness” is not trivial; it’s not always just # of babies made 2) an individual’s fitness is determined by lifetime reproductive output, not just the # of offspring laid in a given year Lack’s hypothesis: How many offspring? Assume the more eggs you lay, the lower the odds of surviving are for each individual offspring Multiplying # of offspring (= clutch size) by odds of each offspring’s survival gives the optimal clutch size (Lack’s hypothesis) Life-history evolution: How many offspring? Data from study on birds showed that the mean # of surviving baby birds was highest for clutches of 12 eggs However, birds laid 8-9 eggs per clutch in any given year, to save energy for future reproduction Natural selection acts at many levels In a given environment, natural selection may act strongly on: 1) one genetic locus (physical spot on chromosome) 2) multiple, adjacent loci molecular evolution 3) one trait in isolation phenotypic evolution 4) the correlation between two or more traits Selection on one locus Selection favoring one allele at a given locus is termed purifying selection - “purifying” because at this level, selection acts against all but the most favorable allele - will decrease genetic polymorphism at that locus if selection strongly favors “C” allele of the C gene... A B C D E F a b c d e f a b c d e f a b c d e f Selection on one locus Selection favoring one allele at a given locus is termed purifying selection - “purifying” because at this level, selection acts against all but the most favorable allele - will decrease genetic polymorphism at that locus Purifying selection reduces polymorphism at the C gene, by removing other alleles from the population A B C D E F A B C D E F A B C D E F A B C D E F A B C D E F A B C D E F Indirect selection on nearby loci Selection favoring one allele at a given locus is purifying selection Case 1: Bacteria (~ no recombination during reproduction) What effect will purifying selection on the C gene have on level of polymorphism at the D gene? A B C D E F a b c d e f a b c d e f a b c d e f Indirect selection on nearby loci Selection favoring one allele will also tend to drag alleles at nearby or linked loci to high frequency if selection strongly favors “big C” allele of the C gene... A B C D E F a b c d e f ...it will also tend to favor “B” and “D” alleles, if they happen to be linked to “C” on a chromosome in asexual organisms, alleles get inherited as a team due to linkage; what’s good for one allele is good for its neighbors genetic hitch-hiking: neighboring alleles benefit from purifying selection, even if they aren’t under selection at all Indirect selection on nearby loci Case 1: Bacteria (~ no recombination during reproduction) if selection strongly favors “big C” allele of the C gene... A a B b C c D E F Q d e f q What effect will purifying selection on the C gene have on polymorphism at the distant Q gene? Selective sweeps Bacteria (and mitochondria, their descendants) have circular chromosomes and limited recombination A B C D E F Are therefore prone to selective sweeps 1) purifying selection favors one allele of one gene, pushing it to fixation (100% frequency in the population) 2) at the same time, selection reduces polymorphism at all genes throughout the genome 3) whole population ends up with reduced (if any) polymorphism, until mutation or migration introduces new alleles Indirect selection on nearby loci Selection favoring one allele will also tend to drag alleles at nearby or linked loci to high frequency if selection strongly favors “big C” allele of the C gene... A B C D E F a b c d e f ...all these alleles will be lost, unless they can get onto the “winning team” i.e., any chromosome with a C allele Indirect selection on nearby loci Selection favoring one allele at a given locus is purifying selection Case 2: Eukaryotes (recombination during reproduction) What effect will purifying selection on the C gene have on level of polymorphism at (1) the D gene?... (2) the F gene? A B C D E F A B C D E F a b c d e f a b c d e f Indirect selection on nearby loci recombination allows linked loci to escape the effects of purifying selection on nearby genes crossing over “breaks up the team” Even if selection strongly favors C allele... alleles of other genes can cross over onto C chromosomes A B C D E F a b c d e f A B C d e A crossing over event can rescue the “d” allele by moving it next to the winning “C” allele, favored by purifying selection f Indirect selection on nearby loci Selection favoring one allele at a given locus is purifying selection Case 2: Eukaryotes (recombination during reproduction) A B C D E F a b c d e f A B C D E f However, a crossing over event becomes more likely as you move farther away on the chromosome “f” is more likely to get recombined onto a “C” chromosome than “d” because the F gene is farther away Natural selection acts at many levels In a given environment, natural selection may act strongly on: 1) one genetic locus (physical spot on chromosome) 2) multiple, adjacent loci molecular evolution 3) one trait in isolation phenotypic evolution 4) the correlation between two or more traits Modeling selection on quantitative traits 50 45 40 35 30 25 20 15 10 5 0 0 5 10 15 20 25 30 weight How would you model the relationship between weight and fitness, given these data? Modeling selection on quantitative traits 50 45 y = 1.6(x) - 1.9 40 35 30 25 20 A model is an equation expressing the relationship between a trait and some measure of fitness 15 10 5 0 0 5 10 15 20 25 30 weight For each unit increase in body size (weight), you get ~1.6 offspring - the slope (coefficient associated with trait value) tells you how strongly that trait affects fitness Modeling selection on quantitative traits 50 45 y = 1.6(x) - 1.9 40 35 30 25 20 A model is an equation expressing the relationship between a trait and some measure of fitness 15 10 5 0 0 5 10 15 20 25 30 weight How would you expect this trait (size) to evolve over time? Directional selection 50 45 y = mx + b 40 A linear relationship between the value of a trait and fitness is termed directional selection 35 30 25 20 15 The model is the equation of the best-fit line through the data 10 5 0 0 5 10 15 20 25 30 trait “fitness” is generally estimated as probability of adult survival, or # of offspring produced over some interval (ideally, lifetime) The mean value of a trait under directional selection should evolve in one direction over time... - mean value should increase if slope (m) is positive Directional selection Directional selection can also be inferred when the distribution of trait values in a population shifts in one direction over time # of individuals 25 20 15 Remember: directional selection 10 a) changes the mean value of a trait (+ or -) 5 0 0 5 10 15 20 # of value individuals trait 25 b) decreases the variance around the mean value of a trait (trims one tail off the distribution) Directional selection Directional selection can also be inferred when the distribution of trait values in a population shifts in one direction over time # of individuals 25 20 15 Remember: directional selection 10 a) changes the mean value of a trait (+ or -) 5 0 0 5 10 15 20 # of value individuals trait 25 b) decreases the variance around the mean value of a trait (trims one tail off the distribution) Stabilizing selection (non-linear) 25 y = -0.1x2 + 2.3x + 1.4 20 Stabilizing selection favors individuals with trait values close to the population mean 15 10 5 0 0 5 10 15 20 25 30 weight Model is non-linear, meaning not a straight line (curved) Quadratic equation includes (a) the trait value (x), and also (b) the square of the trait value (x2), which is what makes it a non-linear relationship Stabilizing selection (non-linear) Stabilizing selection maintains trait values over generations close to the same mean # of individuals 25 20 Remember: stabilizing selection 15 a) no change to mean value of trait 10 b) decreases variance 5 (trims both tails off distribution) 0 0 5 10 15 20 trait # of value individuals 25 Stabilizing selection (non-linear) Stabilizing selection maintains trait values over generations close to the same mean # of individuals 25 20 Remember: stabilizing selection 15 a) no change to mean value of trait 10 b) decreases variance 5 (trims both tails off distribution) 0 0 5 10 15 20 trait # of value individuals 25 Disruptive selection (non-linear) 30 y = 0.1x2 - 2.3x + 27 25 Disruptive selection favors individuals with trait values far from the population mean 20 15 10 5 0 0 5 10 15 20 25 30 weight Model is also a quadratic equation, but now: (a) coefficient associated with the trait value (x) is negative (-2.3) (b) coefficient of squared term (x2) is positive (0.1) Disruptive selection (non-linear) Disruptive selection splits one group into two sub-groups of different ecotypes or specialists # of individuals 25 20 Remember: disruptive selection 15 a) no change to mean value of trait 10 b) increases variance 5 (shifts distribution towards tails) 0 0 5 10 15 20 value #trait of individuals 25 Disruptive selection (non-linear) Disruptive selection splits one group into two sub-groups of different ecotypes or specialists # of individuals 25 20 Remember: disruptive selection 15 a) no change to mean value of trait 10 b) increases variance 5 (shifts distribution towards tails) 0 0 5 10 15 20 value #trait of individuals 25 Disruptive selection (non-linear) Disruptive selection is of special interest as a potential agent of ecological speciation # of individuals 25 20 15 10 5 0 0 5 10 15 20 25 # trait of individuals value A generalist species of bird will feed on seeds of many sizes If the most common plants produce either large or small seeds, this can impose disruptive selection on beak size Disruptive selection (non-linear) tiny beaks = handle tiny seeds # of individuals 25 20 big beaks = crack large seeds 15 10 5 0 0 5 10 15 20 25 # of individuals trait value Disruptive selection may thus split one generalist species into two specialist ecotypes that over time, under the right conditions, may evolve into separate species Selection-mutation balance mutation migration standing genetic variation in a population selection (usually reduces polymorphism) Multivariate selection Rather than acting on one trait in isolation, natural selection often acts on suites of traits that contribute to the overall phenotype - multivariate meaning multiple traits (variables) We model multivariate selection using equations that include the variables themselves (trait values), as well as the correlation between two variables 50 50 positive correlation 45 40 35 40 35 30 30 25 25 20 20 15 15 10 10 5 5 0 0 0 5 10 15 trait 1 negative correlation 45 20 25 30 0 5 10 15 trait 1 20 25 30 Multivariate selection Example: In the sea slug that my lab studies, finger-like projections on the back of the slug (cerata) convulse to circulate body fluid, instead of a beating heart cerata Under normal conditions, selection favors a negative correlation between (a) # of cerata, and (b) the rate at which cerata beat 50 few, fast-beating cerata 45 40 35 30 many, slow-beating cerata 25 20 15 two different ways to achieve same basic “heart rate” most eggs 10 5 0 0 5 10 15 # of cerata 20 25 30 Multivariate selection When it rains, slugs with many cerata have large surface area over which freshwater rushes into body; they explode Slugs with fast circulation (high beat rate) also take on too much water, and explode Rainfall selects for a positive correlation between # of cerata, and beat rate 50 45 few, slow-beating cerata confers high fitness 40 35 30 25 - favors few & slowbeating cerata 20 15 10 5 (no slugs with many, fastbeating cerata; all dead) 0 0 5 10 15 # of cerata 20 25 30 Multivariate selection Marshall & Munroe (2012) studied how competition affected multi-variate selection on colonies of a marine bryozoan - bryozoans grow as colonies like a honeycomb, where each unit is an individual that (a) can reproduce, and (b) will at some point senesce (stop reproducing; basically die) - colonies differ in: (a) size of individual units; (b) offspring size; (c) how fast units go into senescence senesced (dead) units Multivariate selection Under normal conditions, colonies with intermediate trait values produced the most offspring (= had highest fitness) - medium unit size - medium offspring size - medium death rate = stabilizing multivariate selection Multivariate selection Strong competition from neighboring colonies changed the shape of selection on trait correlations crowding favored a negative correlation between offspring size and senescence rate, regardless of unit size = disruptive selection Correlated selection Consider: Suppose fish like to bite off cerata, which look like worms (fish bait). This harms the bitten slugs. Studies suggest slugs with more cerata get attacked by fish more often. cerata 1) What kind of selection does fish predation impose on cerata #, as a trait in isolation? 2) How would you predict cerata # to evolve? 3) Would you expect cerata beat rate to evolve in response to fish predation? Why, or why not?