Reproductive Success

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

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 (condition-dependent 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

• 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