Lecture 15_Discussion 5, Selection_NOTES

Lecture 15_Discussion 5, Selection_NOTES - Lecture 15:...

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Unformatted text preview: Lecture 15: Natural Selection Lecture Galapagos Finch II. Causes of Evolution So far... So Analysis of effects of... A. mutation B. gene flow between populations C. finite population size/genetic drift D. nonrandom mating E. Now it’s time to analyze the types of selection, including... 1. stabilizing selection 2. directional selection 3. disruptive selection 4. frequency-dependent selection 5. heterozygote advantage 6. sexual selection 7. kin selection (we already covered this) II. Causes of Evolution, E. Natural Selection Human height Human is a polygenic and quantitative character character 0.16 Phenotype Frequency 0.12 Mean = 5.6 0.08 0.04 0 5.08 5.17 5.00 5.25 5.33 A Histogram showing Histogram a normal distribution of height (bell curve) height (sometimes has “%” or sometimes “number of individuals” on y axis) on 5.92 5.42 5.50 5.58 5.67 5.75 5.83 6.00 6.08 6.33 Height (feet) Height 6.17 6.25 6.42 II. Causes of Evolution, E. Natural Selection 0.16 Phenotype Frequency 0.12 0.08 0.04 0 5.75 5.92 6.08 6.25 5.33 5.42 5.50 5.58 5.67 5.83 6.00 6.17 6.33 5.08 5.17 5.00 5.25 6.42 What if we get rid of What everyone over 6’ and everyone under 5.3’? everyone Height (feet) II. Causes of Evolution, E. Natural Selection 0.16 Phenotype Frequency 0.12 What if we get rid of What everyone over 5.75’? everyone 0.08 0.04 0 5.75 5.92 6.08 6.25 5.33 5.42 5.50 5.58 5.67 5.83 6.00 6.17 6.33 5.08 5.17 5.00 5.25 6.42 Height (feet) II. Causes of Evolution, E. Natural Selection 0.16 Phenotype Frequency 0.12 0.08 0.04 0 5.75 5.92 6.08 6.25 5.33 5.42 5.50 5.58 5.67 5.83 6.00 6.17 6.33 5.08 5.17 5.00 5.25 6.42 What if we get rid of the What middle—everyone over 5.42’ and under 5.83’? 5.42’ Height (feet) II. Causes of Evolution, E. Natural Selection A. _stabilizing selection___ favors average phenotype A. favors B. __________________ favors individuals that are EITHER above OR below the average phenotype (not both at the same time) below C. __________________________________ favors individuals that are BOTH above AND below the average BOTH II. Causes of Evolution, E. Natural Selection 1.Stabilizing Selection Stabilizing selection: Stabilizing 1. favors individuals with the mean value of a trait__ 2. _____________________________________ for that trait FITNESS FUNCTION Relative Fitness frequency RESPONSE FUNCTION before selection Phenotypic trait (z) Phenotypic trait (z) Stabilizing Selection does not significantly change the mean; it reduces Stabilizing the phenotypic variance the II. Causes of Evolution, E. Natural Selection II. 1.Stabilizing Selection Stabilizing selection on human birth weight Stabilizing RESPONSE RESPONSE FUNCTION FUNCTION FITNESS FITNESS FUNCTION FUNCTION Infants pre-mature Infants Infants can’t get out! get “Many obstetricians and some midwives recommend inducing labor if you are near or at full-term, and they Many think the baby is larger than average -- macrosomia, literally, “big body.” Typically, they use an estimation that the baby weighs or will soon weigh 4,000 grams (8 lbs. 13 oz.) as the threshold.” that from parenting.ivillage.com parenting.ivillage.com II. Causes of Evolution, E. Natural Selection 2. Directional Selection Directional selection: Directional 1. shifts the mean value of a trait up or down._not both, one or other. 2. usually reduces phenotypic variance for that trait (shift in mean is most important response.) (shift FITNESS FUNCTION What if this had What been a positive slope? slope? RESPONSE FUNCTION before selection Directional Selection favors more extreme individuals (e.g. largest or Directional extreme smallest) and leads to a change in mean smallest) II. Causes of Evolution, E. Natural Selection 2. Directional Selection Directional selection in the peppered moth, Biston betularia Directional Biston H.B.D. Kettlewell “typica” form “carbonaria” form II. Causes of Evolution, E. Natural Selection 2. Directional Selection Directional selection in peppered moths: as soot declines so does the Directional ___________________________________________________ ___________________________________________________ 100 % Melanic Moths 80 60 40 20 0 1959 1963 1967 1971 1975 1979 1983 1987 1991 1995 amount of soot Directional Directional selection acting over many generations can lead to major evolutionary change change Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways II. Causes of Evolution, E. Natural Selection 3. Disruptive Selection Disruptive/diversifying selection favors individuals at the extremes of a phenotypic distribution; is a type of ______________________ (maintains ______________________ genetic variation) genetic FITNESS FUNCTION Fitness Frequency RESPONSE FUNCTION BEFORE Phenotypic trait (z) Phenotypic trait (z) Disruptive/diversifying selection (and frequency-dependent selection): 1. have little effect on the mean value of a trait have 2. usually increase phenotypic variance for that trait 2. 3. _____________________________ in a population II. Causes of Evolution, E. Natural Selection 3. Disruptive Selection Disruptive selectionin black-bellied seed crackers: individuals at Disruptive either extreme are more successful than average individuals either Probability of survival Efficient at Efficient cracking soft, small seeds, but can’t open large ones ones Can open Can large, hard seeds, but bad at small ones ones Female lower bill width (mm) II. Causes of Evolution, E. Natural Selection II. 3. Disruptive Selection FITNESS FUNCTION RESPONSE FUNCTION Probability of survival Female lower bill width (mm) II. Causes of Evolution, E. Natural Selection 4. Frequency-Dependent Selection Frequency-dependent selection: the fitness of a phenotype is dependent on Frequency-dependent its frequency relative to other phenotypes in a given population; is another type of balancing selection. _rare individuals have an advantage.__ _rare Example: scale-eating cichlids (Perissodus microlepis) Example: (Perissodus Right-mouthed Right-mouthed P. microlepis P. attack left flank of prey of Left-mouthed P. Left-mouthed microlepis attack microlepis right flank of prey right Hori (1993) Science 260: 216 Frequency left­mouthed fish Rare type has an advantage Sample Year II. Causes of Evolution, E. Natural Selection 5. Heterozygote Advantage Heterozygote Advantage (a.k.a. Heterosis): heterozygotes do better than Heterozygote either homozygote; another form of __balancing selection__ __balancing Example: Warfarin resistance in Norway rats • Vitamin K complexes with coagulants in Vitamin the blood to stop bleeding the % RESISTANT RATS • Warfarin is an anticoagulant/blood thinner Warfarin that prevents vitamin K from complexing with coagulants in the blood with • Non-resistant rats that eat warfarin bleed Non-resistant to death to • In 1959, warfarin-resistant rats appeared In in rat populations in the United Kingdom in • Warfarin resistance conferred by a single Warfarin mutation at the W-locus: mutation Ws ⇒ Wr WsWs homozygotes susceptible WsWr heterozygotes resistant WrWr homozygotes resistant 100 + warfarin 80 60 40 20 0 - warfarin 1950 1959 1966 1980 II. Causes of Evolution, E. Natural Selection 5. Heterozygote Advantage Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways Why do WrWs heterozygotes have higher fitness than either homozygote? 1. Rats need vitamin K in their food, and vitamin K is almost always Rats in short dietary supply. in 2. The allele that confers warfarin resistace (Wr) also makes it more also difficult to absorb dietary vitamin K. difficult 1. __so individuals carrying the Wr allele needs lots more vitamin __so ka in diets than individuals with the Ws_ ka Genotype Resistance to warfarin Resistance Vitamin K requirement WsWs W s Wr WrWr susceptible resistant resistant 1x (normal amount) 2-3x 20x II. Causes of Evolution, E. Natural Selection 5. Heterozygote Advantage Heterozygote advantage & warfarin resistance in Norway rats: the data: Heterozygote Observed f(Ws) = 0.339 Observed f(Wr) = 0.661 WsWs H-W Expected H-W Frequencies Frequencies Observed Frequencies 0.115 0.054 Ws W r 0.448 0.568 Wr W r 0.437 0.378 TOTAL 1.000 1.000 _there are 12% more heterozygotes than expected at HW equil._ Heterozygote advantage: • leads to _over-representation of heterozygotes__in the population _over-representation • Maintains both alleles, even with selection against homozygotes Maintains II. Causes of Evolution, E. Natural Selection 6. Sexual Selection Darwin proposed the theory of sexual selection in The Descent of Man, and Selection in Relation to Sex, 1871: and He proposed that elaborate male traits may help in reproduction, and He may therefore be favored by selection may Sexual selection on a trait is defined as _is defined as differences in reproductive success, caused by competition over mates, and related to the expression of traits. , you become a bigger competitor for mates mates Sexual selection is a type of natural selection, where the selective Sexual advantage of the trait is in improving the ability to compete over mates; both males and females can have sexually-selected traits mates; II. Causes of Evolution, E. Natural Selection 6. Sexual Selection Darwin proposed 2 mechanisms for sexual selection: Darwin a. _intrasexual__selection: Direct competition among males for a. _intrasexual__selection: access to females (e.g. weapons & signals of fighting ability) (e.g. b. intersexual__selection: Indirect competition among males to selection: attract females; involves female choice (females are picky, males instead of fighting are trying to be more attractive to females to get a mate) to Intrasexual selection was not very controversial (deer with antlers), Intrasexual but intersexual selection was very controversial (one of reasons is hard to think that animals, let alone females, had the ability to ability choose(during darwins time)); we now have very good evidence that females can be choosy, so this is no longer controversial either that II. Causes of Evolution, E. Natural Selection II. 6. Sexual Selection, b. Intersexual selection 6. Intersexual Males can increase their fitness by being sexy if females are choosy, but why are females choosy in the first place? but The primary models of sexual selection by female choice: The 1. 1. _direct benefits (what female gets directly)___ _direct 2. _genetic benefits__(she is passing on good genes to her _genetic offspring)__ offspring)__ a. Good genes b. Runaway sexual selection Models are not exclusive! All can happen at same time,. Models II. Causes of Evolution, E. Natural Selection II. 6. Sexual Selection, b. Intersexual selection 6. Intersexual Direct Benefits = Females choose mates based on elaborate traits (signals) that honestly indicate material benefits provided by the male that _directly help the female___ (non-genetic) _directly Direct benefits include access to the male’s territory, male Direct parental investment (nourishment, increased fecundity, protection, help in raising offspring) and avoidance of parasites (directly transmitted parasites, like STDs, lice) (directly Female willow warblers prefer males that sing at a Female high rate to obtain good territories high • Males on good territories sing longer each day Males • Females who mate with these males have more kids, and surviving young. II. Causes of Evolution, E. Natural Selection 6. Sexual Selection, b. Intersexual selection 6. Intersexual Good genes: Females prefer elaborate traits that are correlated with Good _some heritable aspects of male quality and provide good genes_ aspects to the female’s offspring to The gene(s) for female preference spread because they gives the The bearer a fitness advantage (choosy females have healthier kids, and so more surviving offspring = higher fitness) and Tail length in barn Tail swallows indicates good immunity good (Moller 1989, 1990, 1995) Tail growth Tail reflects mite infestation infestation Mite Mite immunity is heritable heritable II. Causes of Evolution, E. Natural Selection 6. Sexual Selection, b. Intersexual selection 6. Intersexual Runaway selection (a.k.a. Sexy Son model): elaborate traits that provide _no quality information or direct benefit__ to the female evolve _no because of the mating advantages achieved by sons (Fisher 1930) because A summary of the mechanism: If there are some females in the population who prefer males with a If trait (e.g. long tails), then these males have a mating advantage over males with short tails, who can only mate with non-choosy females females Females with the preference have _sexier sons__, so the preference Females _sexier so and trait both spread (genetic linkage) and An analogy: It’s easy to think of human fashion following the same basic An pattern. Why do men need to wear ties to get a job? Because bosses will only hire men who wear ties... Why? That’s just the way it is... it So maybe males are stuck with the baggage of female aesthetic whims II. Causes of Evolution, E. Natural Selection 6. Sexual Selection, b. Intersexual selection 6. Intersexual So do females have arbitrary aesthetic preferences? Yes There is some evidence for female preference for _apparently arbitrary male traits which could allow runaway to get started male Zebra finches with novel crests Percent of Percent social time females spent with each stimulus type type (N. Burley) white white crest crest none red green red green crest crest crest crest 60 50 40 30 20 10 0 Artificial male Artificial ornaments ornaments II. Causes of Evolution, E. Natural Selection 6. Sexual Selection, b. Intersexual selection 6. Intersexual The main evidence offered for runaway is the sheer bizarreness and The main extravagance of male traits! extravagance Many people offer the apparently arbitrary plumage elaboration in Birds of Paradise and manakins as evidence of Runaway Runaway II. Causes of Evolution, E. Natural Selection 6. Sexual Selection, c. Sex Roles Bateman’s curve (Bateman 1948) (Bateman 6 5 Fitness 4 3 2 1 0 0 1 2 3 4 5 Number of different mates Males and females typically differ in Males typically __potential reproductive rate___ __potential This is due to females typically having This typically larger gametes , bigger eggs,(anisogamy) and higher parental investment and So selection typically favors: So typically • Males that seek multiple partners_ Males (each partner increases male fitness) (each • Females that mate once or twice (there’s no fitness advantage of more) (there’s Not a political or moral statement, and not not an excuse! Assuming that something is “right” just because it is “natural” is called the Naturalistic Fallacy the II. Causes of Evolution, E. Natural Selection II. 6. Sexual Selection, c. Sex Roles Parental investment (and therefore potential reproductive rate) is a continuum, but females typically invest more than males continuum When parental investment is more similar between the sexes, When both sexes may be showy and choosy (e.g. _monogamy_) _monogamy_ When males invest more than females, __sex roles are reversed. When Pipefish (and seahorses) have a male Pipefish brood pouch that provides nutrients to the embryos Female pipefish are larger, have ornaments, and compete aggressively for males; males are choosy and prefer larger more ornamented females larger Male pouch Ornamented Ornamented female female II. Causes of Evolution, E. Natural Selection 6. Sexual Selection, c. Sex Roles 135 years after Darwin proposed sexual selection, it is still a very active and controversial area of research in animal behavior, evolutionary biology, psychology and anthropology psychology If you’re interested, take EVE107 next Fall! 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