Life Histories

Lecture 9∙ October 2, 2018

today’s objectives

Explain why we tend to see tradeoffs in life history traits and how those tradeoffs evolve

Propose experiments to test hypotheses about the evolution of life history traits

Understand how to classify populations based on their life history traits

“Life history theory seeks to explain how natural selection and other evolutionary forces shape organisms to optimize their survival and reproduction in the face of ecological challenges posed by the environment”

What is life history evolution?

Overview of life history evolution

What is life history evolution?

Overview of life history evolution

> 1,500 years old

< 1 day old

What is life history evolution?

Overview of life history evolution

Life history traits = fitness components

Size at birth

Growth pattern

Age and size at maturity

Number, size, and sex of offspring

Age-, stage- or size-specific reproductive effort

Age-, stage- or size-specific rates of survival

Lifespan

What is the optimal combination of these factors that maximizes reproductive success?

What is life history evolution?

Overview of life history evolution

What is life history evolution?

Overview of life history evolution

Mortality risk at every stage

Why don’t all organisms just reproduce as soon as they are born, produce an infinite number of offspring, and live forever?

Environmental limitations

Intrinsic limitations  tradeoffs!

What is life history evolution?

Overview of life history evolution

Intrinsic tradeoffs:

Increase in one life-history trait is linked to a decrease in another life-history trait

The fitness gains that would have been made by the former are counteracted by the fitness loss caused by the latter

These are usually caused by genetic or phenotypic correlations between life history traits among individuals in a population

What is life history evolution?

Overview of life history evolution

Some life history traits are “phenotypically plastic”

Phenotypic plasticity = the ability of a single genotype to produce different phenotypes across environments

Reaction norm = describes the plasticity of a genotype

Overview of life history evolution

What is life history evolution?

Overview of life history evolution

The pie represents the total energy and resources that organisms can access

Principle of allocation

Intrinsic tradeoffs (negative correlations):

Number vs size of offspring

Reproduction vs survival

Current vs future reproduction

Current reproduction vs parental growth

Current reproduction vs parental condition

What is life history evolution?

Overview of life history evolution

What is life history evolution?

Intrinsic tradeoffs (negative correlations):

Reproduction vs survival

Current vs future reproduction

Current reproduction vs parental growth

Current reproduction vs parental condition

Number vs size of offspring

Overview of life history evolution

What is life history evolution?

Overview of life history evolution

Positively correlated traits

Age at reproductive maturity & adult survival

Development time & body size

offspring number vs offspring size

Organisms have limited access to energy and resources

In any given reproductive event, there is a tradeoff between the number of offspring produced and the size of those offspring

Invest in egg yolk, blood supply, provisions

Fecundity = number of eggs or seeds produced by an organism

Offspring number & offspring size

Case study: clutch size dynamics in darters

Darters of the genus Etheostoma are a large group of fish (135 species) with highly variable clutch patterns

Tom Turner and Joel Trexler 1998 investigated the tradeoffs associated with this diversity

Offspring number & offspring size

Case study: clutch size dynamics in darters

Number of eggs laid per clutch is positively correlated with female size

Number of eggs laid per clutch is negatively correlated with egg size

Offspring number & offspring size

Case study: clutch size dynamics in darters

Gene flow = a measure of shared genetic material

Gene flow between populations of each species is negatively correlated with egg size

What kind of relationship do you expect for gene flow and clutch size?

Negative correlation

Positive correlation

No relationship

Offspring number & offspring size

Each data point represents average gene flow between population of a species; The different colors represent two different methods for estimating gene flow

Case study: clutch size dynamics in darters

Gene flow = a measure of shared genetic material

Gene flow between populations of each species is negatively correlated with egg size

What kind of relationship do you expect for gene flow and clutch size?

Negative correlation

Positive correlation

No relationship

Offspring number & offspring size

Why?

What does this have to do with ecology?

Variation in life history traits are influencing the distribution and abundance of populations through dispersal, which is mediated by reproductive strategies!

Offspring number & offspring size

Plants also show a tradeoff in offspring number and offspring size

Across many different types of plants, number of seeds per plant is negatively correlated with average seed size

And seed size is correlated with rapid growth

…but plants are complicated

Offspring number & offspring size

Plants also show a tradeoff in offspring number and offspring size

Across many different types of plants, number of seeds per plant is negatively correlated with average seed size

And seed size is positively correlated with rapid growth

why?

…but plants are complicated

Offspring number & offspring size

reproductive strategies vs lifespan

Reproductive effort = the allocation of energy, time, and other resources to the production and care of offspring

Reproductive strategies & lifespan

How do organisms allocate this effort in consideration of different portions of their life cycle?

Neanderthals had long childhoods too!

Big brains take a long time to develop

Chimps mature much faster than humans

The “live fast, die young” hypothesis suggested that humans are unique in our slow growth, giving time for bigger brains

A 49,000 year old Neanderthal skeleton challenges this hypothesis!

Child of age 7.7 years had 87% average brain size

Human children age 5 have 90% brain size

What are the limitations of this study?

reproductive strategies vs lifespan

Iteroparous = reproduce several times

Adult survival is high

Adult fecundity is low

Juvenile survival is low

Ratio of adult:juvenile survival

Iteroparity is favored when high or variable

Semelparous = reproduce 1x

Typically have higher reproductive effort

Evolved independently several times

Reproductive strategies & lifespan

reproductive strategies vs lifespan

Iteroparous = reproduce several times

Adult survival is high

Adult fecundity is low

Ratio of adult:juvenile survival

Iteroparity is favored when high or variable

Semelparous = reproduce 1x

Typically have higher reproductive effort

Evolved independently several times

Reproductive strategies & lifespan

reproductive strategies vs lifespan

Most of our grain crops are semelparous

The closest relatives to rice and maize are iteroparous

This suggests the earliest stages of domestication might have selected for semelparity

Why?

Reproductive strategies & lifespan

Environmental factors influence how reproductive strategies are favored by selection

Under what conditions does the increase in fecundity associated with semelparity make up for the loss of future reproductive episodes?

Demographic model – If adult survival is very low, then individuals that put a lot of investment into reproduction early in life will have a fitness advantage over those that spread their reproductive investment over time

Bet-hedging model – If adult survival is highly variable, then individuals that spread out their investment over multiple reproductive events will have a fitness advantage over those that invest all of their reproductive resources in a single event

Non-linearity model – If most of the costs of reproduction are realized, even at low levels of reproductive effort, then individuals that invest in a single, massive reproductive event are likely to have a fitness advantage over those that invest in multiple reproductive events

All of these assume a tradeoff between survival and reproduction!

Reproductive strategies & lifespan

Demographic model of reproductive strategies

Reproductive strategies & lifespan

If iteroparous populations experience 50% adult mortality each year, then the average individual will produce 200 seeds

Environmental factors influence how reproductive strategies are favored by selection

Under what conditions does the increase in fecundity associated with semelparity make up for the loss of future reproductive episodes?

Demographic model – If adult survival is very low, then individuals that put a lot of investment into reproduction early in life will have a fitness advantage over those that spread their reproductive investment over time

Bet-hedging model – If adult survival is highly variable, then individuals that spread out their investment over multiple reproductive events will have a fitness advantage over those that invest all of their reproductive resources in a single event

Non-linearity model – If most of the costs of reproduction are realized, even at low levels of reproductive effort, then individuals that invest in a single, massive reproductive event are likely to have a fitness advantage over those that invest in multiple reproductive events

All of these assume a tradeoff between survival and reproduction!

Reproductive strategies & lifespan

Environmental factors influence how reproductive strategies are favored by selection

Under what conditions does the increase in fecundity associated with semelparity make up for the loss of future reproductive episodes?

Demographic model – If adult survival is very low, then individuals that put a lot of investment into reproduction early in life will have a fitness advantage over those that spread their reproductive investment over time

Bet-hedging model – If adult survival is highly variable, then individuals that spread out their investment over multiple reproductive events will have a fitness advantage over those that invest all of their reproductive resources in a single event

Non-linearity model – If most of the costs of reproduction are realized, even at low levels of reproductive effort, then individuals that invest in a single, massive reproductive event are likely to have a fitness advantage over those that invest in multiple reproductive events

All of these assume a tradeoff between survival and reproduction!

Reproductive strategies & lifespan

Environmental factors influence how reproductive strategies are favored by selection

Under what conditions does the increase in fecundity associated with semelparity make up for the loss of future reproductive episodes?

Demographic model – If adult survival is very low, then individuals that put a lot of investment into reproduction early in life will have a fitness advantage over those that spread their reproductive investment over time  favors semelparity

Bet-hedging model – If adult survival is highly variable, then individuals that spread out their investment over multiple reproductive events will have a fitness advantage over those that invest all of their reproductive resources in a single event  favors iteroparity

Non-linearity model – If most of the costs of reproduction are realized, even at low levels of reproductive effort, then individuals that invest in a single, massive reproductive event are likely to have a fitness advantage over those that invest in multiple reproductive events  favors semelparity

All of these assume a tradeoff between survival and reproduction!

Reproductive strategies & lifespan

Evolution of life history

David Reznick, Trinidadian guppies (Poecilia reticulata)

Predictions from theory:

Reduced adult survival, relative to juvenile, will select for increased reproductive effort

Reduced juvenile survival, relative to adult, will select for decreased reproductive effort

Reproductive strategies & lifespan

Evolution of life history

David Reznick, Trinidadian guppies (Poecilia reticulata)

Natural variation in predators among tributaries

Cichlids prey on adults

Killifish prey on juveniles

Transplanted guppies from tributaries with cichlids to tributaries with no cichlids, but lots of killifish

Measured size, fecundity and offspring size after 11 years (30-60 generations)

Reproductive strategies & lifespan

Evolution of life history

David Reznick, Trinidadian guppies (Poecilia reticulata)

Offspring size increased

Fecundity decreased

Reproductive allotment (percent of dry mass that consists of developing embryos) decreased

Reproductive strategies & lifespan

Years since introduction

Solid lines = transplants

Dashed lines = controls

Environmental factors influence how reproductive strategies are favored by selection

The cost of current reproduction is the decrease that current reproduction causes in future reproduction

What you invest now you can’t invest later

This cost typically decreases with age

Less likely to invest later as you get older

What effect is this likely to have on reproductive effort?

The decreasing cost of current reproduction with age is likely to lead to increased reproductive effort with age

The decreasing cost of current reproduction with age is likely to lead to decreased reproductive effort with age

The decreasing cost of current reproduction with age is not likely to influence reproductive effort with age

Reproductive strategies & lifespan

Environmental factors influence how reproductive strategies are favored by selection

The cost of current reproduction is the decrease that current reproduction causes in future reproduction

What you invest now you can’t invest later

This cost typically decreases with age

Less likely to invest later as you get older

What effect is this likely to have on reproductive effort?

The decreasing cost of current reproduction with age is likely to lead to increased reproductive effort with age

The decreasing cost of current reproduction with age is likely to lead to decreased reproductive effort with age

The decreasing cost of current reproduction with age is not likely to influence reproductive effort with age

Reproductive strategies & lifespan

We can classify species based on life history traits

MacArthur and Wilson

r selection (per capita rate of increase)

Characteristic high population growth rate.

Strongest in species colonizing new or disturbed habitats.

K selection (carrying capacity)

Characteristic efficient resource use.

Most prominent in species whose populations are near the carrying capacity much of the time.

Classifying life histories

Most species fall somewhere in between these two extremes!

We can classify species based on life history traits

Population attribute r selection K selection
Intrinsic rate of increase, rmax High Low
Competitive ability Not strongly favored Highly favored
Development Rapid Slow
Reproduction Early Late
Body size Small Large
Reproduction Single, semelparity Repeated, iteroparity
Offspring Many, small Few, large

Classifying life histories

today’s objectives

Explain why we tend to see tradeoffs in life history traits and how those tradeoffs evolve

Propose experiments to test hypotheses about the evolution of life history traits

Understand how to classify populations based on their life history traits