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Evolution Sexual Selection III Monogamy, Polygyny, Extra-pair Paternity, and Post-copulatory Sexual Selection Why be monogamous? Why be polygynous? Monogamy If females remain receptive after mating, males that can prevent a partner from accepting sperm from another male should have higher fitness by mating monogamously than by trying to be polygynous Males remain with a single female because parental care and protection of offspring are key Monogamy Monogamy Timing maximizes RS Polygamy and Promiscuity Polygyny – One male, multiple females Polyandry – One female, multiple males Promiscuity – mixed mating system generally involving extra-pair paternity Why Seek EPCS? • For males, easy opportunity for increasing RS, but could face female abandonment • For females, slightly more complicated, as there are several downsides: o Mate abandonment o Decreased parental investment o Harassment by other males/females o Sexually-transmitted diseases o No increase in offspring number Evidence for Genetic Compatibility Evidence for Genetic Compatibility Extra-pair offspring sired by female bluethroats are also more heterozygous and have higher immune responses than within-pair offspring. What about humans? *cited 360 times…. Or….the infamous Sweaty T-shirt Experiment Sperm Competition Competition among males to fertilize females doesn’t stop at ejaculation 1st male advantage – often seen in external fertilizing species, as well as species with sperm storage capabilities Last male advantage – Common in species that can remove competing sperm Sperm competition Selection can act on sperm morphology, sperm number, as well as females’ ability to control fertilization success Drosophila bifurca have 58mm long sperm! Wood mice sperm have an apical hook used to attach to other sperm and create mobile trains, which have higher fertilization success than individual sperm A splendid fairy wren male may have 8 billion sperm at any given time Across taxonomic groups, in species that have a high potential for sperm competition, there are relationships between sperm competition and ejaculate quality/sperm production In birds, there is a direct relationship between levels of extra-pair paternity and testis mass Sperm Competition Selection on sperm can also occur within a species Older, territorial males nesting in the interior of a colony produce ejaculates with more sperm that swim faster than territorial males on the periphery, giving them a fertilization advantage Sneakers may release their sperm at exactly the same time, but sneakers will fertilize more eggs Sperm competition occurs across the animal kingdom Removing competing sperm Male black-winged damselflies use a spiky, modified penis to scrub out and remove gametes from the female’s sperm storage organ before transferring their own sperm. Male dunnocks peck at the cloaca of their partners if they find another male near her. This behaviour results in her ejecting a droplet of ejaculate from the other male. Copulatory plugs Observed in mammals, spiders, reptiles, and insects, copulatory plugs are inserted just after copulation in order to limit subsequent copulations by another male. The golden orb spider, Nephila fenestra males take this to a whole new level Females still hold the cards Females may store the sperm of their social partner, but instead use recently received sperm from an extra-pair partner to fertilize the egg. Tree swallows are socially monogamous migratory passerines Both males and females provide parental care to offspring Have one of the highest rates of extra-pair paternity in any bird species (83% of nests, 47% of nestlings in an Ontario Population; Stapelton et al. 2007) Thus, selection will act strongly on males to assure fertilization success. Copulation attempts drop quickly after the first egg is laid Frequent copulations decrease the risk of cuckoldry in presence of sperm competition Life History Evolution Life Histories Life history traits: a trait directly associated with reproduction and survival, including size at birth, growth rate, age and size at maturity, number of offspring, frequency of reproduction, and life span. Life history traits Darwinian demon - the ideal organism matures at birth and leaves an infinite number of immortal offspring Conservation of mass states that this is impossible Life history evolution balances the life history traits and trade-offs are selected Life history: The big questions How should it live its life? When and how big should it be when reproducing? How much energy should it allocate to reproduction? Should it produce many or few, large or small offspring? How many offspring should be male or female? Factors involved in life history evolution 1) The demography of the population 2) Risk of reproductive failure 3) Heritability and plasticity of the traits 4) Tradeoffs among traits 5) Phylogenetic history of the species 1) Demography Connects age and size-specific variation in survival and fecundity to variation in fitness “Small organisms are usually small not because smallness improves fecundity or lowers mortality. They are small because it takes time to grow large, and with heavy mortality the investment in growth would never be paid back as increased fecundity” – Kozlowski 1992 Age Early maturation reduces juvenile mortality and generation time, but produces fewer offspring with higher mortality We can observe huge variation in life history characteristics within lineages Murellets - Fledging date - Clutch size Marbled Murrelet Ancient Murrelet 2) Risk of reproductive failure If a bird puts all its eggs into one nesting event and loses them to a predator = zero reproduction Best to reduce risk by spreading risk across a set of independent events Seed types of Heterotheca latifolia I’m a Darwinian Dolt! 3) Heritability/Plasticity Life history traits are controlled by many genes (i.e., quantitative traits) Heritabilities for life history traits: ~0.05-0.4 Trinidadian guppies cichlid Killifish Guppy 4) Tradeoffs Trade-off: a change in one trait that increases fitness is linked to a change in another trait that decreases fitness - # offspring & survival rates of offspring - attractiveness & predation - reproductive investment & survival - current & future reproduction Detecting tradeoffs If both traits improve fitness when increased and both reduce fitness when decreased, then a positive genetic correlation between traits would not imply a tradeoff If one trait improves fitness when increased and the other improves fitness when decreased, then a positive genetic correlation between traits would imply a tradeoff 5) Phylogeny Phylogenetic effects are the contribution to traits shared by all individuals because they belong to a species or larger taxonomic group. Explaining the evolution of life-history traits Comparative method can help estimate how much of the pattern is attributable to history and how much is attributable to microevolutionary processes Clutch size evolution & reproductive investment 1. How does reproductive investment vary with age? 2. How large should each offspring be? 3. How many offspring should be produced in each reproductive attempt? 4. How much parental care (if any) should be invested to bring the offspring to independence? Kiwi lays an egg that is 5 times larger than chicken egg Can weigh ¼ the weight of the female Largest clutch sizes Tiny eggs relative to body size The eggs of the fly, Zenilla pullata, are only 0.027 by 0.02 mm Orchid seeds weigh less than a microgram and have a chance of 1 in 1 billion to survive In contrast, dung beetles raise 4-5 young that have survival rates similar to whales Caeceilians can produce clutches of more than 100 eggs weighing up to 65% of the mother’s post-birth weight. Females also provide parental care to offspring. Common pipestrel bats produce twins that weigh 50% of the mother’s post birth weight. Ageing Lifespan and rate of aging Lifespans evolve The soma is mortal, and the germ line is immortal Death at old age does not impact fitness Life spans evolve as trade-offs with life histories: o Extrinsic mortality o Intrinsic mortality Longer lifespans evolve if extrinsic mortality rates decrease in Fixed effects olderoorganisms, increasing the value of older organisms because of increased contribution to RS