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Evolution of Sex • Vast majority of organisms reproduce sexually…why? • Must be a benefit to counter costs • Enhances genetic diversity • Clones tend to be of average fitness • Increasing genetic variability produces a wider array of phenotypes – more likely to produce “superfit” individual. Sexual Selection • Sexual selection thought to be equally important as natural selection. Individuals compete for mating opportunities as any other resource. • Intrasexual – direct male-male competition • Intersexual - female choice for males • Sexual dimorphism Fitness and offspring number • Fitness was defined as reproductive output – why do some species produce few young? • Fitness not same as fecundity • Tradeoff – many small vs. few large •Opportunist (r-selected) •Unpredictable environment •Variable population size •Low competitive ability •Small body •Rapid maturation •Short life span •Many, small young (high fecundity) •Parental care rare •Type III survivorship •Competitor (K-selected) •Predictable environment •Stable population size (near K) •High competitive ability •Large body •Slow maturation •Short life span •Few, large young (low fecundity) •Parental care common •Type I survivorship 1 Variability in Clutch Size • Evolution is often about tradeoffs – A mutation is beneficial in one aspect will be costly in another – Eg. thicker fur is an advantage in colder climates, but a disadvantage in warmer ones • Clutch size = number of offspring produced by one individual at one time • Tradeoffs involved in clutch size – Produce few young • • • • Can invest more in each offspring Larger offspring Parental care Lower mortality in young – Produce many young • Large number of offspring • Relatively little energy invested per offspring Variability in clutch size • Latitudinal gradient in mean clutch size • Clutch size, fecundity are determined by genetic and environmental factors – Species and population differences indicate genetic component – Individual annual or seasonal variability indicate environmental component Other reproductive strategies • Semelparous – One lifetime reproductive bout – All eggs in one basket – Allocate all resources at once to reproduction • Iteroparous – Multiple reproductive bouts – Spawn in multiple years, allocate more/less in good or bad years = bet hedging strategy 2 Energy investment in reproduction • Semelparous organisms - Up to 50% of body mass dedicated to reproductive structures • Other expenses – Secondary sexual characteristics – Territory defense and maintenance – Mating calls – Nesting – Parental care – Migration Territory Size – trade offs • Large territory – More resources – More mates – More area to defend – More competitors • Small territory – Easier to defend – Fewer competitors – Fewer mates Reproductive migrations • Many species, especially semelparous ones, migrate long distance to spawn • Homing behavior in salmon, birds, others – Return to natal breeding grounds • - possibility of inbreeding • + known “good” breeding ground Monogamy, polygyny and polyandry • More sexually dimorphic species are also more polygamous and males have larger harems 3 Sneaker male strategy • Male costs (alpha male) – Bright colors – Nest – Territory defense – Larger body size – Polygamous • Alternate male strategy – Sneaker male – Female mimic How do you measure population size? • Census • Other species more difficult. – How to sample? – What defines the population? Life History and Demographics • Nt+1 = Nt + B + I - D - E – N = population size at time (t) – B = births – I = immigration – D = deaths – E = emigration • Population size and dynamics – What constitutes your population? – What do you count? Population Size • What do you measure? • Unitary vs modular organisms – Genet – Ramet – Module – Individual 4 Life history and demographics Baby Boomers • Population size is more than # of individuals • Must incorporate age categories (or life stages) Human Population Demographic Trends Natality • Natality - equivalent to birth rate, also includes production of new individuals by hatching, germination, fission etc. • Two basic aspects of natality: – Fertility -- number of individuals born, hatched, etc.; the actual level of output – Fecundity -- potential level of natality, physical capacity to produce mature ova for a female. 5 Lifespan • physiological longevity = average potential longevity of individuals in a population under optimum conditions – Death due to senescence • ecological longevity = average longevity of individuals in a population under natural conditions – Death from disease, predation, other stress Population dynamics • Population size through time should be predictable • Nt+1 = Nt + B + I - D - E • Time 1 – N = 100 – 20 births – 25 deaths – 10 immigrants – 15 emmigrants • Time 2 – 100 + 20 +10 – 25 – 15 = 90 6