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Unformatted text preview: Lectures 15 Sexual Selection Today's theme
If natural selection acts on random mutations that occur at low frequencies, (where drift may act to eliminate them), then how do organisms change through time? Is it always a long slow process? If natural selection acts through differential mortality of individuals carrying poor genotypes for a particular environment, shouldn't evolution always be slow? shouldn' Yet, paleoecologists tell us that some evolutionary events have happened very quickly. How? Today's theme
Density (Frequency) Independent Selection: changes in Selection: allele frequencies driven by differential mortality set against a backdrop of non-selective mortality. Example: nonlethal recessive alleles. Also called `hard selection.' --selection.' generally considered a slow process. Density (Frequency) Dependent Selection: The density Selection: or frequency of a character determines it's relative it' success. This includes differential reproductive contribution to the next generation. Often called "soft generation. selection' and allows alleles to spread in populations selection' much faster than hard selection. A HARD hypothesis: genes that control cell nuclei are under constant attack from viral infection. Thus, the portion of the genome that encodes for this function must be highly adaptable, maybe even with high mutation rates, or high frequencies of positive mutations. These mutations can drive differentiation quickly. A `soft' hypothesis soft'
Frequency dependent selection based on traits that confer mating advantages (disproportionate contribution to the gene pool of the next generation) can drive selection on quantitative characters over an brief period of (evolutionary) time. Causes of Evolution, Natural Selection Sexual Selection
Darwin proposed the theory of sexual selection in The Descent of Man, and Selection in Relation to Sex, 1871: Sex, 1871: Why do the males of some species have such elaborate decorations? The myriad ways that natural selection may act on breeding opportunity Why are the females of some species so choosy in picking mating opportunities? Lec 21-22, Selection 2 & Sexual Selection 1 From a far more recent book Causes of Evolution, Natural Selection Sexual Selection Darwin proposed that elaborate traits and behaviors may help drive reproductive success, and may therefore be favored by selection Sexual Selection on a trait is defined as differences in reproductive success generated by differential success in competition for mates, related to the expression of some trait or suite of traits. Causes of Evolution, Natural Selection Sexual Selection, Unbalanced Investment
Anisogamy = unequal gamete size between the sexes There is frequently a difference in Parental Investment between the sexes: Beyond gamete production: more female investment bias
Differential Gamete Investment (Anisogamy) (Anisogamy) Differential cost of maturing fertilized eggs Differential investment in rearing young Small, mobile sperm are cheap to produce in large numbers (humans = 1000 sperm/sec) Sperm don't contribute to don' offspring survival Shorter reproductive cycle Large eggs with yolk stores expensive to produce (humans = 1 egg/month) Larger egg increases offspring survival Longer reproductive cycle Causes of Evolution, Natural Selection Sexual Selection, Sex Roles
Bateman's curve Bateman' (Bateman 1948)
6 5 Fitness 4 3 2 1 0 0 1 2 3 4 5 Number of different mates FEMALES MALES Causes of Evolution, Natural Selection Sexual Selection, Sex roles: Males
Robert Trivers (1972) proposed that differences in Parental Investment shape sex roles: Males have lower parental investment in each offspring and have much to gain from multiple mating. Hence, males should mate as often as possible. If there is generally a 1:1 sex ratio, ratio, then this means competing for females by fighting or signaling Higher fitness means more genes in the next generation, so natural selection typically favors: Low investors: seek many mates High investors: seek few, high quality mates Equal investors:???? investors:???? Neither a moral statement, nor an excuse! The Naturalistic Fallacy** ** Moore 1903, Principia Ethica Lec 21-22, Selection 2 & Sexual Selection 2 Causes of Evolution, Natural Selection Sexual Selection, Sex Roles, females
Robert Trivers (1972) proposed that differences in Parental Investment shape sex roles: Causes of Evolution, Natural Selection Sexual Selection, Female Parental Investment
Examples of how parental investment shapes sex roles: Females that have high reproductive investment and low reproductive potential should mate more rarely and be selective, favoring males that have resources that benefit selective, them or their offspring Manakins are polygynous. polygynous. Males do not provide parental care. Females invest heavily in eggs and care: PI = higher for females Bulwer's Pheasant Bulwer' Thus sexual selection is very strong, with elaborate male signals and very choosy females Manakins Causes of Evolution, Natural Selection Sexual Selection, Male Parental Investment
Examples of how parental investment shapes sex roles:
Pipefish (and seahorses) have a male brood pouch that provides nutrients to the embryos PI = higher for males Female pipefish are larger, have ornaments, and compete aggressively for males; males are choosy and prefer larger more ornamented females Male pouch Ornamented female Causes of Evolution, Natural Selection Sexual Selection Two classic types
Darwin proposed 2 mechanisms for sexual selection: a. INTRASEXUAL selection: Direct competition among males for access to females (e.g. weapons & signals of fighting ability) b. INTERSEXUAL selection: Indirect competition among males to attract females; involves female choice You discover a new species where males and females look very different--females are camouflaged and males are different-- bright and colorful Which of the following factors likely did NOT contribute to this situation? A. Eggs are larger than sperm, so females invest more in each offspring B. Males use their bright colors to convince females to mate C. Females have a higher Potential Reproductive Rate than males D. Males use their bright colors to intimidate other males E. Dull colors help females to avoid predators Lec 21-22, Selection 2 & Sexual Selection 3 Causes of Evolution, Sexual Selection, Natural Selection Intrasexual selection Intrasexual Why do giraffes have long necks? Observation 1: when food is scarce, 1: giraffes often bend down to feed on scrub. They don't always reach higher & don' higher in trees. Observation 2: As giraffes grow, only their 2: necks get disproportionately long, not their legs & torso Observation 3: males competing for 3: females engage in extensive fights with their necks. Yup it's called NECKING. it' Hypothesis: the evolution of long necks in Hypothesis: giraffes was (also) driven by male-male malecompetition for access to females. Intrasexual selection: Direct competition among males for access to females (e.g. weapons & signals of fighting ability) giraffes necking
Elephant Seals Observation: males with longer necks generally outneck their opponent A plethora of less direct mechanisms for intrasexual selection
Spermatophore removal Blue damselfly
Blue milkweed beetle Mate guarding Causes of Evolution, Sexual Selection, Natural Selection Intersexual selection Intersexual If females get to choose among males, then how do male sexual selection traits evolve? What are the benefits of choosiness? The three primary models of sexual selection by female choice: 1) Direct benefit 2) Good genes 3) Runaway sexual selection Each one implies that the choosy sex needs a specific type of information about potential mates, so different types of traits (signals) evolve under each model Models are not exclusive! Causes of Evolution, Natural Selection Sexual Selection, Direct Benefits
Females choose mates based on elaborate traits (signals) that honestly indicate material benefits provided by the male. Direct benefits may include: access to the male's territory male' male parental investment protection help in raising offspring avoidance of parasites (directly transmitted parasites, like STDs, lice) Causes of Evolution, Natural Selection Sexual Selection, Case Study
Direct Benefits example: ~90% of bird species are socially
monogamous; the main benefits of mate choice are probably direct.
5 SONG RATE 5 SONG RATE 4 4 3 15 May 25 1 June 10 3 10 20 30 PAIRING DATE % TIME FORAGING Males with higher song rates pair earlier = preferred by females Song rate is higher for males on better quality territories = song indicates good territories These are non-genetic benefits but may imply a genetic quality non- Female willow warblers prefer males that sing at a high rate to obtain good territories: Lec 21-22, Selection 2 & Sexual Selection 4 Causes of Evolution, Natural Selection Sexual Selection, Good genes?
Females in many birds (and other taxa), receive no direct benefits from males, so good genes or runaway sexual selection could each be benefits of mate choice Causes of Evolution, Natural Selection Sexual Selection, Good genes vs runaway selection
Two Options: Genes for sexy males spread since the bearers are preferred by choosy females (sexy males mate more; spreading the sexy make genes----- runaway sexual selection. genes----Genes for quality spread because they provide the bearer a fitness advantage: choosy females have healthier kids, and so more surviving offspring. Good genes or runaway selection? We see the strongest sexual selection in species where males provide no parental care (few direct benefits) PROBLEM: These are NOT mutually exclusive, both may be operating Causes of Evolution, Natural Selection Sexual Selection, Intersexual selection Intersexual
Good Genes example: Tail length in barn swallows is costly but indicates good immunity (heritable quality handicap)
(Moller 1989, 1990, 1995) The classic runaway selection argument
Costs: A) Energy to build the peacocks mating display. B) Predator vulnerability. Thus, it is a maladaptive waste of time and energy. Benefit: Females choose on the basis of the signal, reinforcing the signal A. Males with elongated tails mate faster B. Tail growth reflects mite infestation C. Mite immunity is heritable So, here may be a poor male laboring under the cost of female whimsy whimsy Causes of Evolution, Natural Selection Sexual Selection, Runaway selection
So do females have arbitrary aesthetic preferences? There is some evidence for female preference for arbitrary male traits that could allow runaway to get started
60 Causes of Evolution, Natural Selection Sexual Selection, runaway selection
The main evidence offered for runaway is the sheer bizarreness and extravagance of male traits!
Many people offer the apparently arbitrary plumage elaboration in Birds of Paradise as evidence of Runaway No data to support this... It just seems reasonable Zebra finches with novel crests
Percent of social time females spent with each stimulus type 50 40 30 20 10 0 Artificial male ornaments (N. Burley) white crest none red green crest crest Lec 21-22, Selection 2 & Sexual Selection 5 Causes of Evolution, Natural Selection Sexual Selection, Predicting behavior
Robert Trivers (1972) proposed that differences in Parental Investment shape sex roles: Parental Investment is a continuum, continuum, The more similar the PI between the sexes, the more similar the sex roles. Predicting sexual selection
Do you predict higher sexual dimorphism in: r-selected species or K-selected species? KWhat do predict to be the relationship between male weaponry (horns, antlers, etc.) and male parental care? Can flowers be considered a form of sexual selection now acting through pollinators as the surrogate choosy partner? Can males that advertise direct benefits provide dishonest signals? What is the likely cost of defending a large harem for the male elephant seal? Causes of Evolution, Natural Selection Sexual Selection, Intersexual selection Intersexual Clarification of definitions
EPISTASIS: When the phenotype is affected by genes at more than one locus and they interact in a non-additive way non For example, the combination of genes A and B is not just A + B, rather B gene modifies expression of A gene -- the B genes shuts off the A gene, or causes it to express more or less The phrase I gave to help you remember epistasis ("multiple genes, one effect") vs. pleiotropy ("one gene, multiple effects") is effect" effects" just to help you remember...it's not the complete definition remember...it' POLYGENIC: When more than one (typically many) genes control one trait; leads to quantitative variation in traits Epistasis is a special case of polygenic inheritance where the genes control the traits in a non-additive way non- 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 If you're interested, take EVE107! you' Principles of Animal Communication Which of the following is NOT true about inbreeding A. Inbreeding leads to an over-representation of homozygotes in overthe population B. Inbreeding leads to an over-representation of heterozygotes overin the population C. Inbreeding is otherwise known as positive assortative mating D. The same effects of inbreeding occur when genotypes prefer others of the same kind regardless of whether they're close they' relatives E. Inbreeding leads to inbreeding depression Lec 21-22, Selection 2 & Sexual Selection 6 ...
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