Life History Evolution

Transcription

1 Life history evolution References Stearns (1992) The Evolution of Life Histories Roff (2002) Review Partridge & Harvey (1988) Science 241:

2 Overview Life history traits Life history : how individuals live their lives Life history traits The Darwinian Demon Constraints and trade-offs Reproductive schedules The evolution of repeated breeding Life history traits: components of fitness traits which, when all is held equal, show a correlation with fitness evolve to maximise fitness Key features of populations but are part of individual s phenotype Examples include: growth rate adult size age at maturity age specific fecundity number and size of offspring mortality rate and ageing 2

3 Life history traits Reproductive schedules: Consider two genotypes, that reproduce clonally Each produces only two offspring in a lifetime before dying Timing of reproduction differs between them: a has one offspring in one year, and another in the next b has none in first year and two in second Clone b a Age Life history traits Reproductive schedules: Consider two genotypes, that reproduce clonally Each produces only two offspring in a lifetime before dying Timing of reproduction differs between them: a has one offspring in one year, and another in the next b has none in first year and two in second 160 Early reproduction is good Number of descendants Year 3

4 Darwinian demon Darwinian demon Limits to Darwinian demon Constraints e.g. no organisms can acquire food fast enough to produce a colossal number of offspring immediately upon birth. Trade offs between life history traits e.g. structures or behaviours that increase fecundity may prove detrimental to survival. Begins reproducing at birth Produces many, large offspring Never dies Leads to two questions central to life history evolution What are the constraints: what life histories are feasible? What circumstances favour the evolution of a greater reproductive output at the expense of longevity and vice versa? 4

5 Constraints Trade-offs Phylogenetic: aquatic mammals do not evolve into fish Survival Partridge & Harvey (1988) Science 241: Physiological, biomechanical, developmental: insects are size-limited by the exoskeleton s ability to supply oxygen Reproduction hippos need bigger legs than mice a foetal ear does not become an adult eye Live fast die young? Grow old gracefully? 5

6 Cole s Paradox (1954): for an annual species, the absolute gain in intrinsic population growth which could be achieved by changing to the perennial reproductive habit would be exactly equivalent to adding one individual to the average litter size semelparous iteroparous Cole (1954) Quarterly Review of Biology 29: i.e. giving birth to x+1 offspring and then dying is the same as producing x offspring every year, and living infinitely Cole (1954) Quarterly Review of Biology 29: x+1 x 6

7 Annual (semelparous) X Perennial (iteroparous) x+1 x Cole (1954) Quarterly Review of Biology 29:

8 Hidden assumption offspring have the same survival as adults Cole s paradox resolved Annual (semelparous) X Perennial (iteroparous) Charnov & Shaffer (1973) Am. Nat. 107: If p is adult (parent s) survival and y is offspring survival, annual must add p/y offspring to litter size x to be equivalent to perennial s x i.e. litter size must be x + p/y (only x + 1 when their survival is equal). Cole (1954) Quarterly Review of Biology 29: x+1 100% adult survival (p) 100% juvenile survival (y) x p/y = 1 Emphasises importance of relative survival. Predictions: When p/y ratio is large, expect repeat breeding (iteroparity) When p/y ratio is small, expect semelparity i.e. when parents survival is low relative to that of offspring, expect high reproductive investment. 8

9 Annual (semelparous) X Perennial (iteroparous) X Annual (semelparous) X Perennial (iteroparous) Cole (1954) Quarterly Review of Biology 29: x+1 0% adult survival (p) 100% juvenile survival (y) x p/y is small Cole (1954) Quarterly Review of Biology 29: X X XX 100% adult survival X x+1 x 20% juvenile survival X X p/y is large Semelparity favoured Iteroparity favoured 9

10 Annual meadowgrass, Poa annua Habitat types: Opportunistic: disturbed; mortality of adults and juveniles presumed to be high i.e. p/y small Pasture: undistrubed; presumed adult mortality low but juvenile mortality high i.e. p/y large Annual meadowgrass, Poa annua Habitat types: Opportunistic: disturbed; mortality of adults and juveniles presumed to be high i.e. p/y small Pasture: undistrubed; presumed adult mortality low but juvenile mortality high i.e. p/y large Law et al. (1977) Evolution 31: Compared to those from pasture, plants from opportunistic habitat tended to: Grow less i.e. were smaller Start reproducing faster Produce more offspring in the first breeding season but fewer in the second Die quicker i.e. lived fast, died young Law et al. (1977) Evolution 31: Compared to those from pasture, plants from opportunistic habitat tended to: Grow less i.e. were smaller Start reproducing faster Produce more offspring in the first breeding season but fewer in the second Die quicker i.e. lived fast, died young Agrees with theoretical predictions: When p/y ratio is small, expect semelparity i.e. high reproductive investment. When p/y ratio is large, expect repeat breeding (iteroparity) 10

11 Trade-offs Agreement with theoretical prediction: Semelparous species should have higher fecundity than closely related iteroparous species Reproduction Semelparity Survival Iteroparity Prediction: Semelparous species should have higher fecundity than closely related iteroparous species Young (1990) Evol. Ecol. 4: Species fecundity ratio Semel-/Iteroparous Oryza perennis 2.9 Oryza perennis 5.3 Ipomopsis aggregata Gentiana spp Helianthus spp Hypochoeris spp Lupinus spp Sesbiana spp Temperate herbs Old field herbs

12 Overview Life history traits The Darwinian Demon Constraints and trade-offs Reproductive schedules The evolution of repeated breeding 12