ADAPTATION

INTRODUCTION

• Adaptation

  • A trait that increases the fitness of its possessor

  • Change in allele frequency that increases population mean fitness (a process)

  • Result from natural selection

  • Have genetic basis (not acclimation)

• Non-adaptive variation

  • Causes

    • Direct effects of environment

    • Genetic drift -- random changes in allele frequency due to sampling error

    • Multiple adaptive phenotypes -- e.g. camouflage in grouse chicks

    • Laws of physics or chemistry -- e.g. flower colour in hydrangea

  • Constraints on adaptation

    • Trade-offs

      • Time and resources (energy, water, nutrients) are finite

    • Pleiotropy

      • One gene affects >1 trait

      • Selection on one traits causes change in a second

    • Developmental constraints

      • E.g. pollinated flowers remain attached until abscission layer forms

• Demonstrating adaptation

  • is hard

    • What is the function of the trait in a focal organism? ----- e.g. hooks on a pigeon beaks reduce parasite loads

    • Do similar traits fulfill the same function in multiple species?

    • Did the trait originate for its current function?

    • Is the trait maintained by natural selection or merely inherited from ancestors?

  • Expecting a trait to be adaptive just because it is there often leads to flawed evolutionary thinking

    • Our default explanation should be that a trait is not adaptive… this is the null that needs to be rejected

  • Goal: show that trait developed or is maintained through natural selection

    • Type of study: experimental, observational, theoretical

    • Level of study: population, comparative (among species)

    • Good experiments:

      • Test the effect of changes in a single trait on fitness or components of fitness

      • Control for direct effects of manipulations

      • Test several hypotheses that may explain traits function

    • Good observational studies:

      • Use natural variation to test alternative hypotheses

      • Measure potential confounding variables

      • Require detailed information about natural history

    • Theoretical studies

      • Consider benefits and costs of contrasting phenotypes

      • Assign numerical values to benefits and costs or define relationship between trait value and benefit

      • Use mathematical approaches to predict the best phenotype under a given set of conditions

Comparative method

• Each species is a data point

  • Usually observational

  • Greater generality than single species studies

  • Greater potential for confounding factors

• Test for association between traits and environment or correlations among traits

  • Potential problem: closely related species may resemble one another due to shared inheritance of traits rather than because they experience similar selection pressures

Left: - seed size evolved once Right: - seed size evolved 6x 6 "sister species comparisons" explains that trait evolved each generation/natural selection is present - changes in seed size are consistently associated with changes in habitat - stronger support stronger evidence of adaptive trait

• Phenotypic resemblance due to shared ancestry is accounted for by examining evolutionary change since a common ancestor

• Sister species comparisons

  • Require many species, frequent independent changes in hypothesized casual variable

  • Only involve comparisons of living species

  If ABC and DEF are all related share the same trait we should replace the data with data from the common ancestor

  • Phylogenetically independent contrasts (PICs)

    • Compare changes in casual variable with changes in dependent variable

    • Often involved estimation of ancestral phenotypes

    • Look for patterns of divergence as sister species evolve independently away from a common ancestor

    Independent contrasts - discrete or continuous traits - imply perfectly known phylogeny - imply accurate reconstruction of ancestral phenotypes - assume similarity among relatives is not maintained by natural selection we don’t say there is no evidence for natural selection --- we say that there is less evidence for natural selection

• Phylogenetics vs Comparative Method

  Phylogenetics  Goal: describe ancestral relationships Can be confounded by independent/parallel evolution Strategy: use many neutral characters

  Comparative Method Goal: describe adaptation Can be confounded by shared ancestry Strategy: consider independent/parallel evolution (PICs) E.g. importance of wings vs feathers Requires phylogeny based on characters that are independent of the study focus

LIFE-HISTORY EVOLUTION

• Life histories

  • Age-specific schedule of survival and reproduction

  • Life history traits

    • "components of lifetime survival and reproduction" -- combine to determine lifetime reproductive success

    • Much variation in: lifespan, reproductive cycle, size & number of offspring

      • Big species reproduce less often because they take more time and more energy to grow up and the resources are limited

  • Principle of allocation

    • "Darwinian demon"

      • A hypothetical organism that can maximize all elements of its fitness simultaneously

      • Reproduce at bird, infinite life span, infinite reproductive episodes, large numbers of viable offspring

      • Why don’t we see these?

        • Finite time & resources cause trade-offs

    • Reproductive success decreases when survivability increases because you invest in yourself more than to reproducing

• Rate of living

  • Senescence: late life decline in reproduction and probability of survival

    • There is a physiological limit to cell and tissue repair

  • "Evolutionary" Hypotheses

    1. Mutation accumulation

    2. Trade-offs between reproduction and repair

• Mutation accumulation

  • Late-acting deleterious mutations accumulate because selections becomes weaker with age

  • Theory supporting mutation accumulation

    • Mutation causing death could be only mildly deleterious if it affects late life

    • Deleterious mutations that act late in life will accumulate because selection against them is weak (e.g. genes causing cancer/Alzheimer's)

  • Natural selection weakens age

    • One an organism has reproduced, there is less selective pressure to maintain itself

      • Mutations that have harmful effects late in life (after reproduction) can accumulate in the population, since they don’t strongly affect an individual’s reproductive success

      • Over time, late-acting mutations lead to physiological decline and aging

      • Investing heavily in early reproduction can reduce investment in repair and maintenance, leading to increased cumulative lifetime damage (senescence)

      • Species with fast life histories (short-lived, early-reproducing) experience stronger effects of mutation accumulation because selection after reproduction is very weak

      • Slow life history species (long-lived, delayed reproduction) experience stronger selection later in life, so they often evolve better maintenance and repair mechanisms, and thus age more slowly

• Trade-offs

  • Strong selection for early reproduction

    • Mutation for early breeding can have negative effects late in life and still be advantageous

  • Trade-off between reproduction early in life and survival

    • Increased reproduction reduces subsequent growth and survival in many animals and plants

  • Contributions to mortality and aging

    • Intrinsic factors

      • Accumulation of late acting deleterious mutations

      • Trade-offs between reproduction and repair

    • Extrinsic factors

      • Predation, disease, resource depletion, resource accumulation

  • Logic of trade-offs applied to many aspects of life history

    • Reproductive effort, mate attraction vs gamete production, offspring size & number, reproduction vs defense

• Evolution of aging

  • Strength of natural selection decreases over life-span --- accumulation of late-acting deleterious mutations

    • E.g. Menopause

      • Non-adaptive hypotheses: increased longevity with modern health care, social supports

      • Adaptive hypotheses

        • Prolonged childhood - time for children to reach maturity

        • Grandmother hypothesis - time/energy to care for grandchildren

        • Sexual selection - assumes female reproduction limited by access to mates

• Life histories can give lots of information because they reflect how a species balances growth, survival and reproduction in its environment

• Life history traits tell us a lots about a species' ecology, evolution, and vulnerability

• Pace of life and ecology

  Fast life histories Early maturity, short lifespan, many offspring Thrive in unstable or unpredictable environments Can rebound quickly after disturbance Tend to have large populations and small body sizes Population size and density: maintain large populations and high densities

  Slow life histories Late maturity, long lifespan, few offspring Adapted to stable environments Recover slowly from population declines Often large-bodied with low population densities Population size and density: have smaller populations and lower densities, which make them more vulnerable to chance events (demographic or environmental stochasticity)

• Range size and movement

  Fast species Often have smaller home ranges and limited dispersal, but high population turnover allows them to colonize new areas quickly

  Slow species usually have larger home ranges and greater movement, sometimes covering vast areas to find food or mates

• Extinction risk and conservation

  Fast species Typically more resilient and recover faster after disturbance

  Slow species Generally more at risk of extinction because: They reproduce slowly and recover poorly after declines They require more space and resource They often face strong human pressures (e.g. hunting, habitat loss, etc.)

• Genetic and demographic consequences

  Fast-reproducing species Maintain larger effective population sizes and can, in general, adapt more readily to change This links life history to evolutionary potential and conservation genetics

  Slow-reproducing species With small populations tend to have lower genetic diversity and a higher risk of inbreeding

• Life history strategies summarize the evolutionary "choices" species make between living fast and dying young, or living slow and investing in longevity and survival

EVOLUTION OF SEX

• Sex

  • Polygenic traits - continuous variation also reflects the environment

  • Domesticated species

    • Anyone with patience to breed organisms can see populations change over time

    • Size advantage model: changing sex at the right size maximizes lifetime reproductive success

      • E.g. reproductive success differs with size for males and females in fish -- smaller = reproduce successfully as females (producing eggs), larger = reproduce successfully as males (defending harems and fertilizing many eggs)

  • Biological sexes are defined by different gametic. Strategies for reproduction

  • Sexes are regions of phenotypic space which implement different gametic reproductive strategies

  • Sexes are life-history stages rather than applying to organisms throughout their lifespans

  • There is a difference between sex as a reproductive system and sex as an individual trait

    • At the species level: most sexually reproducing species are anisogamous (they have two distinct gamete types: large nutrient-rich eggs and small motile sperm

    • At the individual level: nature does not always produce a clean binary division of individuals into "male" or "female"

  • Concept of sex

    • Helps maintain how organisms reproduce, evolve, and allocate resources -- essential for understanding life histories, selection, and genetic diversity

• Paradox of sex

  • Sexual -> new genotypes (mating types, male and female)

  • Asexual -> clones (vegetative propagation, apomixis, parthenogenesis)

  • Sex poses as a paradox

    • Sex is complicated, costly, and dangerous --- searching for a mate takes time, energy, and increases exposure to STDs --- once potential mate is found there is usually additional exertion required to 'woo' ………. All this and there is no guarantee that a mate will prove fertile

    • Asexual species can be highly successful (e.g. dandelion)

  • Breaking the paradox

    • Sexual reproduction introduces genetic recombination, which can:

      • New combinations of genetic variation --- more raw material for selection

      • Combine beneficial mutations from different individuals

      • Purge deleterious mutations more effectively (Muller's ratchet)

      • Help populations adapt faster to changing environments

• Long-term advantages of sex

  • Sex predominant

  • Most asexual lineages recently evolved

  • Complete asexuality -> extinction

    • Asexual strategies tend to go into extinction than sexual

• Short-term advantages

  • Many theories, some evidence

  • Theoretical explanations are not mutually exclusive

  • All assume recombination and selection:

    • Decreased frequency of non optimal genotypes

    • Increased frequency of superior genotypes

  1. Temporal fluctuations in selection pressure

     • Avoidance of parasites/disease

     • Sex continually generates new phenotypes

     • "Red Queen Hypothesis" - continuous adaptation is needed for a species to maintain its relative fitness among the other species and systems it is co-evolving with

       • Sexual reproducers were more fit when there were parasites

       • Asexual reproducers were more fit when there were no parasites

       • Sex adds variability in the offspring

  2. Heterogeneous environments (spatial variation in selection pressure)

     • Lottery model

       • Offspring disperse into various patches (sexual: many diff tickets; asexual: many copies of 1 ticket)

     • Asexual taxa often occur in biotically simple environments

     • Sibling competition

       • Genotypes differ in resource use

       • Diverse offspring compete less than identical offsprings

  3. Eliminating mutations

     1. Muller's ratchet

        • Can never lose mutations in selfing population and individuals with the fewest mutations will be lost by drift

        • Forward mutations are much more frequent than reverse mutations

        • Asexual offspring inherit all existing mutations and experience additional mutations

        • Mildly deleterious mutations accumulate in small clonal populations over time

        • Multiple mutations eventually have a drastic effect on fitness

     2. Mutation threshold

        • Fitness is severely reduced in individuals with more mutations than a threshold number

          • Mutations are most rapidly eliminated when they occur with other mutations

        • Before selection, sexual reproduction recombination generates a wider range of mutations per individual than does asexual reproduction

SEXUAL SELECTION

• Evolution of males and females

  • Isogamy - gametes with identical morphology; mutations affecting size

  • Anisogamy - functional divergence

    • Large gametes --- have higher survival capacity

    • Small gametes --- produced in higher numbers

    • W = offspring number x offspring survival

• Sexual selection

  • "selection on mating success"

  • May oppose viability selection, but is still a component of natural selection

  • Females & males invest unequally in each offspring

    • "Batemans principle"

      • Male fitness is usually limited by number of mates; therefore males will usually compete for mates

      • Female fitness is usually limited by resources; therefore females will usually be choosy