Reproductive success
Why do organisms vary?
• Vary in life cycles
• Vary in reproductive strategy
– Reproductive output
– Reproductive strategy (e.g. hermaphrodites)
• Vary in developmental patterns
Life history
• Everything an organism is or does
• The study of trade-offs, especially regarding growth, reproduction and death
– Remember that there is allocation of a limited pool of resources!
Number and size of offspring
• Some fishes: millions of tiny eggs each spawn
• Orchids: 10,000,000,000 seeds each fruit, size of dust specks
• Blue whale: single offspring the size of an elephant
• Coconut: ~ 12 nuts/year, each several kilos
Age at Reproduction
• Aphids: embryos in female before her birth
• Periodical cicadas are larvae for 13-17 years
• Marmosets sexually mature in ~ 1 year
• Humans sexually mature ~ 13 years
Timing of reproduction
• Semelparity: salmon, poppies, reproduce once and die
• Iteroparity: humans, alders, reproduce repeatedly throughout life
Life span
• Rotifers, mites: days
• Corals, clams: centuries
• Giant sequoia: thousands of years
• Clonal plants (huckleberry): tens of thousands of years
For semelparous species,
• Measuring fitness of a genotype is simple:
– R = LM, where L is the probability of the female’s survival to reproductive
age and M is the average number of offspring per survivor
For iteroparous species,
• Use a life table
Life tables
• Increasing survival (lx) during reproductive ages increases r
• Increasing fecundity (mx) at any age increases r
• Offspring produced early increase fitness more, because they contribute more
to population growth
Life tables
• Increasing survival (lx) during reproductive ages increases r
• Increasing fecundity (mx) at any age increases r
• Offspring produced early increase fitness more, because they contribute more
to population growth
Constraints to life history evolution
• Phylogenetic constraints constrain the evolution of life history characters
• Tradeoffs
– The advantage in a change in a character is correlated with a disadvantage
in other respects
• E.g. guppies and tungara frogs
Antagonistic pleiotropy
• What is pleiotropy?
• Genotypes have an inverse relationship between different components of
fitness
– E.g. increased fecundity may result in lower growth
• Allocational tradeoff
Senescence
• A late-life decline in fertility and probability of survival
Hippo senescence
Theories about senescence
• Mutation accumulation
• Antagonistic pleiotropy
Mutation accumulation hypothesis
• Mutations with negative effects late in life will not be selected against
– Hereditary nonpolyposis colon cancer
– Huntington’s disease
• Mutation can even cause death: the later it acts, the weaker selection against
it is
Antagonistic pleiotropy
• Mutation with positive effect early has negative consequences later; e.g. early
reproduction with early death
Trade-off between lifespan & fecundity
Iteroparity versus semelparity
• Why don’t all species reproduce early?
– Reproducing at early age may increase risk of death, decrease growth or
decrease future fecundity
– Repeated reproduction is more likely to evolve if adults have high survival
rates from one age class to the next
Offspring size and number
Offspring size and number
Lack’s clutch size hypothesis
• Selection will favor the clutch size that results in the most surviving offspring
• Assumes trade-off between survival of individual offspring and total offspring
number
Lack’s clutch size hypothesis
Other life-history phenomena
• Sex changing in fish, shrimp, and plants
• Sex ratio of offspring
• Evolution of novel traits via selection on life history
• Suboptimal life histories
Sex changing
• Sequential hermaphroditism
• If reproductive output is related to size, expect different strategies at different
sizes
• If growth is indeterminate, expect individual organisms will exhibit different
strategies during their lives
Sex change model
Blue-headed wrasse
• Males defend territories on coral reefs: male reproductive success related to
size
• Small (young) fish are drab females OR small (sneaker) males; large fish are
‘terminal’ males with territories
• If male removed, largest female switches to male
Jack-in-the-pulpit
• As in most plants, fruit/seed production limited by resources (= leaf area)
• Larger individuals are female
• After fruit production switch back to male until re-grow
Sex ratio
• Expect a ratio of 1:1
– Every organism has one mother, one father
– If one sex is rare, an allele that leads to production of rarer sex will be
favored (these individuals will have more than one mate, on average)
Sex ratio
• Assumption: parents invest equally in both sexes
• Expect deviation when:
– Offspring fitness is size-dependent, and this differs for two sexes (conditiondependent sex allocation)
– Offspring will mate only with siblings (local mate competition)
Condition-dependent sex allocation
• In polygynous species, small males seldom reproduce
• Should produce large sons; if can’t, then should produce daughters
Condition-dependent sex allocation
• Red deer, Cervus megalocerus
• Polygynous, males compete for access to groups of females
• Females in dominance hierarchy; large, well-fed, strong females more
dominant
Condition-dependent sex allocation
Local mate competition
• When individuals compete with a limited part of the population for mates
(rather than random mating)
• Extreme examples: parasitoids that mate only with siblings, self-pollinating
plants
• Expect female-biased sex allocation; production of males wasteful if they are
just competing with each other
Local mate competition
• Nasonia wasps parasitize maggots
• Wasps mate with siblings right after emergence
• Sometimes ‘superparasitism’ occurs
• Expect sex ratio of eggs laid by second female to be adjusted depending on
amount of local mate competition
Local mate competition
Local mate competition
• In self-pollinating, hermaphroditic plants, expect reduced allocation to pollen
(male gametes) relative to outcrossing plants
Local mate competition
• Blue-eyed Mary populations vary in flower size and outcrossing rate
• Expect small-flowered, selfing populations to have reduced male allocation
relative to large-flowered, outcrossing populations
Local mate competition
Inbreeding and outcrossing
• Inbreeding increases homozygosity and is often accompanied by inbreeding
depression
• Mean fitness of an inbred line may actually increase due to purging of
deleterious recessive alleles
Eichornia paniculata
•Outcrossing Brazilian population and self-fertilizing Jamaican population
•Inbred each for 5 generations then outcrossed the inbred lines
No evidence of heterosis in self-fertilizing population