ecology reflection
Alyssa Johnson
F18Lecture9LifeHistories.pptxLife 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?
http:// www.sciencemag.org/news/2017/09/neandertals-humans-may-have-had-long-childhoods
Rosas et al. 2017 Science (22 Sep 2017)
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
