Life-history evolution

For quantitative traits, the products of different genes combine to produce one trait

How do multiple traits interact together, to produce…..adaptation..fitness..life histories

The combination of all traits in a species produces the unique life history for that organism - age at maturity, # of offspring, survival rate, life span

- integrates fitness over the whole lifespan of the organism

Life-history evolution: How many offspring?

Assume the more eggs you lay, the lower the odds of surviving are for each individual offspring

Multiplying # of offspring (= clutch size) by odds of each offspring’s survival gives the optimal clutch size (Lack’s hypothesis)

Life-history evolution: How many offspring?

Data from study on birds showed that the mean # of surviving baby birds was highest for clutches of 12 eggs

However, birds laid 8-9 eggs per clutch – why?

Life-history evolution: How many offspring?

The bird’s life history had evolved to maximize total lifetime reproductive success

- birds that laid 12 eggs in their 1st clutch did great that year, but used up so much energy caring for that many babies, they burned out and didn’t reproduce in future years

- birds that laid only 8-9 eggs per clutch saved energy for more future reproduction, so had higher lifetime fitness

The adaptive evolution of life histories is often limited by trade-offs, a consequence of many traits interacting and competing for limited resources during development

- must sacrifice effort in one area to invest in another

Results from partitioning finite resources (energy) into alternative traits or life-history strategies

- growth vs. defense

- disperse or reproduce

- many small offspring vs. few large offspring

Roadblocks for adaptive evolution

Trade-offs 1: growth vs. defense

A classic trade-off in many organisms is growth vs. defense

option 1) invest energy in rapid growth and fast reproduction

- pro: you’re likely to reproduce before you get eaten - con: you will probably be eaten after you reproduce once example: fast-growing grasses

5 days 5 days later

eaten by cow

makeseeds

Trade-offs 1: growth vs. defense

A classic trade-off in many organisms is growth vs. defense

option 2) grow slowly and put energy into defenses

- pro: if well-defended, you survive & reproduce for many years - con: it’s risky to delay reproduction, especially when small example: slow-growing cactus

5 years later

initially, makespines instead of growing bigger

eventually, bigspiny adult that lives for decades

Trade-offs 2: developmental dimorphisms

Many animals can develop along 1 of 2 possible pathways, producing alternative adult morphologies

- termed developmental dimorphisms, they often reflect basic trade-offs

- 2 equal and balanced options, but each usually has an edge under some set of environmental conditions

short-winged versuslong-winged morphsin crickets

Trade-offs 2: developmental dimorphisms

Short-winged crickets: - have more energy for egg production - males make more mating

calls - cannot migrate to new

food patches, however

Long-winged crickets: - use up energy building, fueling large flight muscles - reproduce less, but... - can fly to new food patches when conditions go bad

Many animals can develop along 1 of 2 possible pathways, producing alternative adult morphologies

Trade-offs 3: reproductive investment

Another common trade-off is in reproduction-- how many kids?

option 1) invest energy in many tiny offspring, which must take care of themselves and survive until they’re fully grown adults

- pro: you produce many offspring, with little invested in each - con: only a few will survive to reproduce themselves

mother’senergy

many tiny offspring

most offspringdie during thegrowth period

small %survive

Trade-offs 3: reproductive investment

Another common trade-off in reproduction is, how many kids?

option 2) put energy into a few large offspring, which mature quickly due to maternal provisioning

- pro: many offspring will survive to reproduce - con: you can only make a few in the first place

mother’senergy

few big offspring

most offspringsurvive

big %survive

Trade-offs 3: reproductive investment

Marine invertebrates can exhibit planktotrophy or lecithotrophy

planktotrophy: make many tiny larvae which must feed in the plankton for a month before they are big enough to settle down

lecithotrophy: make a few big larvae that metamorphose right after hatching Importance for understanding trade-offs: a given species can do one or the other, but --

(1) can’t do both (except in rare cases)

(2) in-between strategies do not work (no species makes a medium # of medium-sized larvae)

Maximizing fitness in a changing world

Selection can result in a population adapting to changing environmental conditions over many generations

However, individuals (and thus, individual genotypes) often face conditions that change during their lifetime Fluctuating conditions favor the ability to change your phenotype on the run

- Note: this sounds like Lamarck, or “directed mutation” – organisms change in response to conditions they experience

however, these changes are not inherited by offspring – although the ability to change phenotype is heritable

Maximizing fitness in a changing world

Selection can result in a population adapting to changing environmental conditions over many generations

However, individuals (and thus, individual genotypes) often face conditions that change during their lifetime Fluctuating conditions favor the ability to change your phenotype on the run Two ways an individual genotype can maximize its fitness in fluctuating environments are:

(1) Bet-hedging

(2) Phenotypic plasticity

Bet-hedging

After a touchdown in football, a team has two choices:

1) kick for 1 extra point (almost 100% successful)

2) try to throw or carry ball into endzone for 2 extra points

however, teams rarely ever go for two points... why?

- variance in success for 2-point conversions is extremely high - most teams either complete 100% or 0% of attempts

thus, high chance that instead of a sure-thing 1 point, you end up with nothing

Taking the guaranteed 1 point is termed bet-hedging: give up the chance for more points, to ensure that you get something no matter what

Chicago Bears 100Minnesota Vikings 100Pittsburgh Steelers 100Denver Broncos 100Buffalo Bills 100St. Louis Rams 100Miami Dolphins 100Cincinnati Bengals 100Atlanta Falcons 50Carolina Panthers 50Arizona Cardinals 40Green Bay Packers 33Detroit Lions 33Baltimore Ravens 25Jacksonville Jaguars 25Washington Redskins 0New York Jets 0New England Patriots 0New York Giants 0Cleveland Browns 0Seattle Seahawks 0Indianapolis Colts 0Tennessee Titans 0Kansas City Chiefs 0New Orleans Saints 0

% success at attempted 2-point conversions by NFL team

Bet-hedging genotypes

Bet-hedging strategies increase the growth rate of a genotype by decreasing fitness variance between generations

- a bet-hedging genotype trades a reduction in fitness during good seasons for an advantage in bad seasons

This way, you avoid complete wipe-outs

- over many generations, this strategy will always result in the greatest growth rate for a genotype (how fast its allelesrise in frequency)

- out-competes genotypes that do great in good years, but bust during bad years

Bet-hedging dispersal strategy in Alderia

My slugs employ a bet-hedging strategy to make sure some of their offspring disperse to find new seaweed patches, while others stay close to home

one-third

two-thirds

drift for days until they locatethe seaweed that adults eat

settle immediately, no matter what’s nearby

tiny, swimming larvaeeither...

Bet-hedging germination in plantsMany plants produce a mixture of seeds: some germinate and start to grow after one winter, while others don’t germinate until they have experienced two winters

This scatters offspring in time – if there is a drought, odds are good that at least some offspring will survive by sleeping through it

However, cannot exploit a really good year by having all seeds germinate when it’s nice and rainy

2009 2010 2011

Phenotypic plasticity

An individual can regulate its phenotype in response to cues that may indicate impending changes in the environment

- Is winter coming?? Is a predator nearby??

This is phenotypic plasticity, the condition-sensitive expression of alternative phenotypes

- a phenotype may show a continuous variation in response to an environmental factor (such as temperature)

- phenotypic plasticity is itself a genetically variable trait, and can evolve in response to natural selection

Phenotypic plasticity 1: Inducible defenses

Rather than invest initially in energetically costly defenses, an organism can wait until it senses a threat

- only produce defenses (spines, toxins) when danger is near

Sort of like-- start out life as a fast-growing grass, but if you sense danger, turn into a prickly cactus

or, change colorwith the seasonsto always blend in, even when the background changes aroundyou

summer winter

snowshoe hare

Phenotypic plasticity 1: Inducible defenses

Colonies of a marine animal called a bryozoan grow spines when they smell a predatory sea slug nearby

colony smells asea slug

1-2 days later

Phenotypic plasticity 1: Inducible defenses

Example: mosquito larvae normally eat the single-celled protozoan Lambornella

- Protozoans that smell nearby mosquito larvae in time can turn into “death spheres” - burrow into the larvae, explode them from within!

Lambornella

Life-history evolution: Why do we age?

The optimal life history might be immortality, but everything shows less reproduction and higher mortality over time (age)

- so why don’t we evolve a “solution” to aging?

Two possibilities:

1) Constraint – we lack any remaining genetic variation to respond to selection against aging

2) Trade-off – we trade off getting old and dying in exchange for a reproductive advantage in our youth

Life-history evolution: Why do we age?

Hypothesis 1: Rate-of-living

- says DNA + tissue damage is caused by metabolism

- faster you metabolize, faster you age - lifespan of some animals is doubled on a starvation diet

Thus, this hypothesis predicts we age because we lack additive genetic variation to evolve better repair systems

- however, this hypothesis was not supported by:

- comparisons of metabolic rate across mammals (no connection between life span and metabolic rate)

- artificial selection expts showing lifespan can double in Drosophila in response to selection for late reproduction

Life-history evolution: Why do we age?

Hypothesis 1: Rate-of-living

May instead be due to # of cell divisions that can be completed

proposed that the progressive loss of telomeres from end of chromosomes limits # of possible cell divisions, hence lifespan

- again, not supported by experimental studies

most research indicates organisms can evolve longer lives than what they normally express

- this implies a long life is not necessarily favored by natural selection

Life-history evolution: Why do we age?

Hypothesis 2: Trade-off between reproduction & repair

Proposes that damage could be perfectly repaired, but at too great a cost to early reproduction

- selection favors investment in reproduction over repair - too costly to invest in fixing problems when young, even though it would lead to a much longer life

Mutations that increase early reproduction at a cost of shorter life are favored under natural conditions in flies and worms

Life-history evolution: Why do we age?Flies homozygous for the methuselah allele live 35% longer than wild-type flies

% s

urvi

val

However, double-mutants lay fewer eggs early in life, leading to reduction in total lifetime reproduction

thus, methuselah allele trades off early reproduction for long life and resistance to environmental stress