Download • Sexual selection thought to be equally important as natural

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
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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
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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
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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
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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.
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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
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