Life History

Evolution of Life Histories Involves Trade-offso Life history– lifetime pattern of growth, development and reproductiono Trade offs: when a beneficial change in one trait is associated with a detrimental

change in anothero Each individual has a limited amount of resources that it can allocate to specific

tasks. Its allocation to one task reduces the resources available for the others. Thus, allocation to reproduction reduces the amount of resources available

for growth o Trade-offs are imposed by constraints of physiology, energetics, and the

prevailing physical and biotic environment—the organism’s habitat As such, the evolution of an organism’s life history reflects the interaction

between intrinsic and extrinsic factors Extrinsic ecological factors such as the physical environment and the

presence of predators or competitors directly influence age specific rates of mortality and survivorship

Intrinsic factors relating to phylogeny (the evolutionary history of the species), patterns of development, genetics, and physiology impose constraints resulting in trade-offs among traits

Reproduction involves both benefits and costs to individual fitnesso Behavioral, physiological, and energetic activities involved in reproduction

extract some sort of cost to future reproductive success in the form of reduced survival, fecundity, and/or growth

o 1. activities associated with the acquisition of a mateo 2. defense of a breeding territoryo 3. feeding and protection of young, as well as the direct physiological costs of

reproductiono reproduction can also directly reduce an individual’s ability to produce future

offspring. The current reproductive expenditure might leave the individual with insufficient energy resources to produce the same number of offspring during future periods of reproduction

o increased allocation of resources to reproduction relative to growth diminished future fecundity.

Allocation to reproduction has been shown to reduce allocation to growtho Individual reproducing earlier in age will produce fewer offspring per

reproductive period than an individual that postpones reproduction in favor of additional growth

act of reproduction at a given age therefore has potential implications to both age-specific patterns of mortality (survivorship) and fecundity (birth rate) moving forward.

For this reason, the age at which reproduction begins—the age at maturity—is a key aspect of the organism’s life history.

Age at maturity is influenced by patterns of age-specific mortality

o If natural selection functions to maximize the relative fitness of the individual, then the age and size at maturity are optimized when the difference between the costs and benefits of maturation at different ages/sizes is maximize

o Both extrinsic and intrinsic factors influence evolution of age at maturity Natural selection will favor the age at maturity that results in the greatest

number of offspring produced over the lifetime of an individualo Primary fitness advantage of delaying maturity is the larger initial body size

obtained by individuals when they first reproduce primary cost of delaying reproduction (late maturity) is the increased risk

of death prior to reproduction, or death before the advantage of increased fecundity as a result of delayed maturity are fully realized

o Selection should be expected to favor genotypes that mature earlier (prior to those ages), thus increasing their probability of contributing genes to future generations

Increased age at maturity was accompanied by a larger average size at age of maturity for females, and the production of fewer, but larger offspring

Reproductive effort is governed by trade offs o Fecundity is the number of offspring produced per unit of time (bx), but the

energetic costs of reproduction include a wide variety of physiological and behavioral activities in addition to the energy and nutrient demands of the reproductive event, including gonad development, movement to spawning area, competition for mates, nesting, and parental care.

Together, the total energetic costs of reproduction per unit time are referred to as an individual’s reproductive effort

o Amount of energy organisms invests in reproduction varieso Probability of future survival (and therefore future reproduction) is low, so early

maturity and high reproductive effort will maximize individual fit- ness. Conversely, an increased juvenile mortality results in delayed maturity and reduced reproductive effort

o Female inhabiting the high mortality environment have a significantly higher allocation to reproduction than those inhabiting low mor- tality environments.

o Variation in allocation to reproduction were found to be related to patterns of mortality caused by extrinsic factors (predation or extreme temperatures).

o Allocation to reproduction at any time during the life of an individual involves trade-offs between current benefits from the production of offspring and costs in terms of potential reduction in future reproduction. Natural selection should function to optimize the trade-off between present and future reproduction.

o Optimized life history is one that resolves conflicts between the competing demands for survival and reproduction to the best advantage of the individual in terms of fitness

o Increased allocation to reproduction (energy expenditure to the feeding and caring of offspring) resulted in a reduction in the probability of future survival of parents, and therefore future reproduction

As a result of increase in energy expenditure, survival rate of patents decreased with increasing brood size

Nestling survival rate declined with increasing brood size

o 1. as reproductive effort increased, the number of offspring increased, but the probability of offspring survival decreased

current reproductive success is the product of the two: number of offspring produced multiplied by the probability of their survival. As a result of the inverse relationship between the number of offspring and their probability of survival

o 2. As reproductive effort increased, parental survival decreased