Shaping Life History

What are the benefits of reproducing early vs. late and how does this affect when an organism matures?

• Reproducing late → higher fecundity via size and resource acquisition; increased quality of offspring via parental care (lower juvenile mortality)

• Reproducing early → reduces probability of dying before reproducing; reduces generation time

Costs and benefits of each results in an intermediate optimum

If adult mortality is high, what age of reproduction will selection favor?

Selection will favor earlier reproduction to increase the probability that an organism reproduces before it dies

If adult mortality is low, what age of reproduction will selection favor?

Selection will favor delayed maturation and later reproduction because this will allow for greater accumulation of resources for offspring

What is the main factor that influences optimum age of reproduction in organisms?

Mortality schedule

What is an example of natural variation in aging between a single species

Island opposums live in a car-free habitat, whereas mainland opposums have a higher likelihood of being hit by a car; island mothers have a slower physiological rate of aging than mainland mothers; mainland mothers invest more in their first clutch, whereas island mothers invest the same amount into all clutches

If there are two guppy populations and the lower stream population has higher predation than the upper stream population, what are some predictions about the life history traits of the lower stream population?

Earlier reproduction, larger first brood size, smaller size at maturity, smaller offspring

If you move guppies from the high predation lower stream and move them to the low predation upper stream, what would you predict will happen to their life history traits?

Evolve later maturity, larger size at maturation, bigger offspring, and smaller first clutch size

What is an example of anthropogenic selection on life history?

Swedish hunting laws prohibit the shooting of bears with cubs; female bears with cubs are 4x less likely to be shot; this resulted in selection to extend the period of parental care by a year, lengthening generation time and increasing net reproductive rate; this is an example of reproduction habits changing mortality rates

What is optimality?

a modeling approach based on economics that predicts the “optimal strategy” is the one that provides the highest fitness relative to alternatives; this predicts that the number of offspring one should produce is the number that yields the most surviving offspring

What is the Lack Clutch Size model?

This is a very simple, basic model for predicting how many offspring one should produce; the more offspring you produce (bigger clutch size), the lower the probability of survival is for offspring (negative relationship between # of offspring and probability of survival); to predict optimal # of offspring, multiple clutch size by probability of survival of offspring; this model doesn’t work very well because it doesn’t consider many important factors such as the fact that many organisms reproduce multiple times

What is the Smith and Fretwell model?

This is a very widely accepted model; there are multiple parts; one part is that there is a tradeoff between investment per offspring (size) and total number of offspring (negative relationship); the second part is how does size predict correlate to fitness of offspring? This is a positive diminishing returns curve, meaning that more bigger size correlates to higher probability, but only up to a certain point; more is always better, but at some point not a whole lot better; this creates a different “optimum” fitness for mom and offspring, leading to parent-offspring conflict

What is parent-offspring conflict?

Based on the Smith and Fretwell model, female fitness is maximized at an intermediate investment level that is LESS than the optimum investment level for offspring; kids always want more than the parent can give, so there must be a balance

How does environment affect offspring fitness and optimum investment by mother?

The offspring fitness function varies with environment, so in a poor environment, more investment is required by the mother to reach the same level of fitness for the offspring; this predicts a higher optimum investment by mother in poor environments; ex. seed beetles mothers produce larger eggs on poor hosts

If a single seed beetle mom was moved from a good host to a bad host, what would you expect to happen to her egg size, if egg size is plastic?

Egg size would increase because optimum investment becomes higher in poor environments

If a single seed beetle mom was moved from a good host to a bad host, what would you expect to happen to her egg size, if egg size is NOT plastic?

Egg size would remain the same since it is not plastic and cannot change within a single generation

Do males and females have the same interest in investment per offspring?

No, males generally benefit from more investment in current offspring because future offspring may not be his (low certainty of paternity); mothers, on the other hand, benefit from less investment in current offspring than the father because they want to save some energy for future reproduction because future offspring are certain to be hers

What is a good explanation for why gene imprinting occurs in mammals?

Conflict between sexes in terms of interest in investment; males and females silence different genes expressed in fetuses; gene copies from the mother are more likely to silence genes that stimulate growth (requires more investment), whereas gene copies from the father are more likely to express genes that stimulate growth

What is gene imprinting?

When gene copies from only parent are expressed; ex. methylation “silences” genes