Week 4 Flashcards
What is natural selection?
Individuals with favorable traits (phenotypes) are more likely to survive and reproduce than those with unfavorable traits
What is adaptive evolution?
If the trait is heritable the genotypes/alleles associated with it will increase in frequency over generations
What does natural selection forget?
Components of natural selection that may affect the fitness of a sexually reproducing organism
What are the chances of natural selection?
Natural selection is not random there is no plan. There is no design in natural selection
What are the stages of organism development and the selection placed on at that stage?
Zygote to adut –> viability selection
Adult to parents –> sexual selection
Parents to gametes –> Fecundity selection and gametic selection
Gametes to zygotes –> Compatibilty selection
What is adaptation?
Characteristic that enhances the survival or reproduction of
organisms that bear it, relative to alternative states
(especially the ancestral condition)
What was the idea of Jean-Baptiste Lamarck in the 1800’s?
Inheritance of acquired characteristics ie giraffe had long neck because the stretch for food or if you bodybuild all the time then your baby would be muscular
What was the correct idea of evolution?
Wallace and Darwin were correct
Selection of quantitative genetic variation in a trait
What is adaptive radiation?
The evolution of ecological and phenotypic diversity within a
rapidly multiplying lineage
What is a classic example of adaptive radiation?
Darwins finches –> descended from one finch species and form the natural variation in the population have become 14 different species each with their specialised niches
What does natural selection act on?
Acts on phenotypes but selects for genotypes
What causes quantitative variation?
Quantitative variation underpinned by genetic component
What is directional selection?
Favours phenotype of one extreme
What is diversifying selection?
Same asdisruptive selection
Favours different extreme phenotypes
What is stabilising selection?
Selects against extreme variants.
Leads to reduced phenotypic variation
What is an example of stabilising selection?
Body size in cliff swallows
Measured after a cold year population was shown to bigger than previous year
Why cant the directional selection of cliff swallows constantly select for being larger?
Trade offs with growing bigger eg more weight when flying so more energetically costly
If year is warm then the population will shrink in size creating a stabalising effect
What is an example of diversifying selection?
Bill morphology in black-bellied seedcracker, Pyrenestes ostrinus
Diversifying selection arising from the superior fitness of different phenotypes on different resources
What is an example of natural selection?
Birth weight in humans
Very small and very large babies are most likely to die, leaving a narrower distrubution of birthweights
What are the effects of natural selection?
Natural selection will change the genotype frequencies and
(consequently) the allele frequencies in a population
What will the frequency of genotype of zygotes relative to frequency of each genotype?
They will be in Hardy-Weinburg equilibrium
What happens to the frequency of the different alleles in the genotype of the zygote population?
Viability selection occurs killing off members of a genotype
What happens to the genotype frequency after the zygote stage?
As the populaiton demographic has changed this removes the alleles from HWE. Then when the current population goes to reproduce this will create a new ratio for the alleles so the next generation zygotes will be in a new HWE
What is the overview of selection against recessive alleles?
When recessive is common = many ‘aa’ – therefore rapidly selected against
But when rare - most copies of ‘a’ are in Hz form ‘Aa’ - hidden from selection so slow rate of selection against
Hard to eliminate recessive alleles and get maximum fitness in a population
What could eliminate recessive alleles at low frequency?
Genetic drift, the recessive allele is more at risk due to its prescence in a low number of genotypes
What is the overview of selection for recessive alleles?
New recessive allele increases in frequency slowly at first because mainly in ‘Aa’
Hz form - hidden from selection
Increases rapidly once it has reached a certain frequency (because more aa’s)
Recessive alleles selected for will go to fixation once past a certain initial frequency
What can be an issue with selection for recessive alleles?
At low frequency positive recessives = very susceptible to loss through genetic drift
What does selection operate on?
Variation to produce adaptation
What is a problem with variation operating on adaptation?
Selection eliminates variation!
Genetic drift also removes variation
How is variation maintained if its a paradox with selection and drift removing selection?
Heterozygote advantage
Negative frequency dependent selection
Fluctuating selection
What is heterozygote advantage?
Overdominance - both alleles will be kept in the population
in successive generations
What is an example of heterozygote advantage?
Sickle cell trait - resitant to malaria
Beta hemoglobin gene
A = normal; S = sickle allele (codominant)
What is the trade off with sickle cell trait?
Two sickle cell traits cause sickle cell disease which can result anemia and frequent pain needing blood transfusion
What is negative frequency-dependant selection?
The fitness of a genotype is not constant but depends on the genotype frequency in the population
Negative Frequency Dependence = rarer form at advantage
(rare allele advantage)
Why would a rare allele be at an advantage?
Leads to oscillations in the phenotype frequency (and underlying alleles) in a population (maintains variation)
What is an example of rare allele advantage?
Competition for food (eating chunks of prey!) in two morphs of a cichlid species
What was the overview of the two morphs of a cichlid species?
Left genotype of predator is common to begin with (mapped over time)
Less competition/prey awareness for the right form. So these predator morphs feed/breed well and increase in frequency. (so freq of the left form alleles decreases)
Right form of predator becomes common and selection then favours the rarer left form
What is an example of fluctuating selection?
Gene for resistance to powdery mildew in Arabidopsis thaliana (RPW8) is polymorphic in natural populations
Why is the RPW8 gene polymorphic?
In the absence of pathogen infection, weight and seed yield are higher in plants without the resistance gene.
A cost of having the resistance gene!
Resistant plants do better when pathogen is abundant… but poorly when it is rare.
Temporal and spatial variation in pathogen presence – determined by external factors (e.g rainfall) - maintains the polymorphism
What are factors that can explain the occurance of a trait?
Genetic drift
Correlated evolution (pleiotropy or linkage)
An ancestral character – a result of phylogenetic history
Natural selection – adaptation
What are methods used to infer adaptation?
Complexity / apparent design
Experiments
The comparative method
What is the use of experiments inferring adaptation?
Assess the effect a change in a single, well-defined factor
has on a phenotypic trait
Test how it affects fitness
What is an example of an experiment inferring adaptation?
The adaptive value of the RPW8 resistance gene
Produce transgenic plants
Individuals differing only with respect to the presence or absence of the resistance gene.
Assess fitness
What is the comparative method?
Adaptations inferred from patterns observed across species,
correlations among traits, or correlations between traits and the environment.
What is the rationale behind the comparaitve method?
Rationale: if a certain relationship between trait and environment has evolved repeatedly, then it is likely to be adaptive
What is an example of using the comparative method?
Testes size in bat species
Hypothesis – larger group size more sperm competition (sexual selection)
Correlation showing the evolition of bigger testes evolve
What is something that you need to look out for when using the comparative method?
A simple scatter plot may provide only weak evidence that two traits evolve in tandem
Problem - a lack of independence of the data points
2 groups that share common ancestry rather than independent adaptive evolution - this would only count as 2 data points due to only 2 independent convergent evolutionary transitions
What is an example of the comparative methiod with independant contrasts?
Felstein’s method for evaluating phylogenetically independent contrasts
What is Felstein’s method?
Plot only the relative difference in traits between each species/node and their closest common ancestor (or sister spp)
Lines indicate independent evolutionary change in a trait
Then force the lines through the origin
What did Felstein’s method show about bat group and testes size?
There is a relationshipe between group size and testes size
What are life history traits?
Timing and duration of key events in an organism’s lifetime
What are demographic traits that can vary?
Number and size of offspring
Age distribution of reproduction
Life span
Alternative mating strategies
Dispersal
Mode of reproduction
What is the function of life history theory?
Life history theory addresses the conditions that favour the
evolution of variation in demographic traits.
What are the different varations in number and size of offspring with examples for juxaposed lifestyle?
Many small offspring — a continuum —- few large offspring
Blue tit, Parus caeruleus – 14 small eggs
Kiwi, Apteryx – single egg weighing 25% of its mother
What are the different varations in age distrubution of reproduction with examples for juxaposed lifestyle?
Periodical Cicada, Magicacada septendecim can what 17 years before fully maturing and reproducing
Milkweed Aphids, Aphis nerii (parthogenic reproduction)
What are the different varations in lifespan with examples for juxaposed lifestyle?
Relatively fixed lifespans - varies between populations and species
Fruit flies, Drosophila melanogaster - 26 days
Bristlecone pines, Pinus aristata - 4600 years
What letter is used to demoninate fitness?
Fitness = the per capita rate of increase of a genotype = r
How can you work out the letter ‘r’?
r = per capita rate of birth - per capita rate of death
What does the letter ‘r’ mean for evolution?
Relative values of r determine the course of evolution by natural selection
What happens if there is a evolutionary change in a demographic feature?
Evolutionary change in a demographic feature, e.g. reproductive lifespan, causes a change in the components of fitness
What happens if there is evolution in other features?
Evolution of other features (e.g. body size) occur because they affect one of the demographic traits, i.e. number / size of offspring
What life history traits would maximise fitness?
Higher survival up to and through the reproductive ages
Earlier age of first reproduction
Higher fecundity at each reproductive age
Higher fecundity early in life
Longer reproductive lifespan
What is the relationship with evoluton and higher survival up and through reproductive ages?
Natural selection does not favour post-reproductive survival unless there is transgenerational childcare
Why will fitness increase with earlier age of first reproduction?
Higher chance of surviving to reproductive age
Reduced generation time
Offspring produced at an earlier age contribute more to population growth
What are the different types of constraints?
Phylogenetic constraints
Genetic constraints
Trade offs
What are phylogenetic contraints?
Arise from their evolutionary history - e.g. silkworm moths lack mouthparts
What are genetic constraints?
Lack of genetic variation or constraints among traits due to genetic pleiotropy
What are trade-offs?
Advantage of a change in one life history trait causes disadvantage elsewhere
Type of Antagonistic pleiotropy – negative correlation between traits, because of allocation trade-offs
What is a hypothetical example of a trade off?
Denotypes (B and B’) differ in investment in reproduction Versus maintenance/growth
B increased fecundity but decreased growth or survival
B’ decreases fecundity but increases growth and survival
What can make trade-offs complicated?
As could also be differences in resource ]
Acquisition (A and A’)
Why are trade-offs important?
Trade-offs are central to LHS diversity
Cost and benefits to any strategy
Depend on the environment / ecology
What is the trade-off with reproduction?
Trade-off between reproductive effort and growth & survival
What is an example of the trade-off with reproduction?
Female Anolis sagrei with ovaries removed (OVX) grew
bigger than controls (sham) (SVL= snout to vent length)
OVX females also lived longer than the Sham females
which were still producing eggs
What is an example of an experiment with Drosophilia about age of repoduction and trade-offs?
Selection of laboratory populations of Drosophila for age at reproduction
Those selected to reproduce when old - had lower mortality rates
However, they also had lower egg production (especially when young)
What is the trade-off with greater age and reproduction?
Evolution of survival to a greater age occurred at the expense of reproduction early in life and vice versa
What is senescence?
accelerated physiological degeneration with age
Increased likelihood of death (actuarial senescence)
What are the factors impacting the evolution of senescence and life span?
Antagonistic pleiotropy - Williams (1957)
Accumulation of deleterious mutations - Medewar (1952)
What is antagonistic pleiotropy?
Because of greater contribution of earlier age classes to fitness, alleles that provide an advantage in early life have a selective advantage… even if deleterious later in life e.g. alleles for investment in early reproduction but reduced maintenance/repair
What is the overview of accumulation of deleterious mutations causing senescence?
Deleterious mutations that only affect later age classes accumulate in populations because selection against them is weak.
Why is selection weaker for older age classes?
Better to reproduce earlier
Extrinsic mortality mean less individuals survive/breed at older ages
Sometimes referred to as the selection shadow
What is a cause of senescence/lifespan?
Senescence / lifespan arises partly due to selection for earlier reproduction
What does selection favour for age of reproduction?
Should be advantageous to reproduce as early as possible Especially if fecundity increases with size
What will high extrinsic mortality led to?
High extrinsic mortality on adults will select for high reproductive effort early in life
What will low extrinsic mortality led to?
Low extrinsic mortality may select for delayed maturation and later reproduction
What was an experiment showing the relationship between extrinisic mortaility and reproduction age?
16 reptile species
The lower the annual adult extrinsic mortality (due to predation), the later reproduction begins
What are the different schedules of reproduction?
Semelparity – reproduce once and die
Iteroparity – reproduce repeatedly
What is an example of semelparity?
Antechinuses
What is an example of iteropartiy?
Sticklebacks
What is the overview of the semelparity strategy?
Gorw to maturity rapidly then invest all in one reproductive attempt
When is semelparous advantagous?
Survival increases with body size
Exponential relationship between body mass and reproduction
If reproduction is very stressful or risky
When is iteroparous advantagous?
Increases chance of success in fluctuating environments
If adult mortality (especially linked to breeding) is low
If greater fecundity achieved by saving some resources and deferring some reproduction
What is the variation in number and size of offspring?
Reproductive effort invested in:many small offspring or few large offspring
All else being equal, genotype with higher fecundity will have
higher fitness than one with lower fecundity
What determines the optimal number of offspring?
Maximises the number of surviving (recruited) offspring
Increasing or decreasing the number of offspring reduces parents fitness
What is the trade-off with number of offspring?
Trade-off between the number and resources dedicated to offspring
Finite resources to invest egg/seed/embryo/young
What selects for the best number of offspring?
Best solution depends on the environment
What does intrinsic rate of increase (rm) mean?
no density dependent (competition) effects on birth/death rates
What does Instantaneous rate of increase (r)?
actual rate – reduced by density dependence/competition
Whyt does genotypes change differently?
Different genotypes - different rates of increase at different densities
What would happen if trait ‘B’ invests more per offspring?
B has lower rate of increase at low density (lower intrinisic rate = rm,B) but a selective advantage (higher rate of increase) at higher density (nearing carrying capacity = KB
At high density, dead individuals are more frequently replaced by B rather than A individuals.
What are r- and k-selected strategies?
Relate to the selection of traits that allow success in particular environments
What are r-selected organisms?
In unstable or unpredictable environments with low density dependence, r-selection predominates - the ability to reproduce a lot is crucial.
Characteristic traits include high fecundity, small size, short generation time, and the ability to disperse offspring widely
What are k-selected organisms?
In stable environments subject to strong density dependence, K-selection predominates - ability to compete for limited resources is crucial
Tend to have long life span, and to produce fewer, well cared for offspring
Is r-and k-selected strategies still relevant?
Concept - criticised and fallen out of fashion as it is polarised and does not cover all the important elements of life-history theory
What are the disadvantages of sexual reproduction?
Halves the potential reproductive rate
Means you share your genetic reproduction
Break up co-adapted gene complexes
Cost of finding mates
What are the advantages of sexual reproduction?
Half genome transmitted from each parent with recombination
Stops accumulation of deleterious mutations Fisher-Muller hypothesis
Generates offspring variability to combat rapidly co-adapting pathogens Red Queen hypothesis
What is anisogamy?
Difference in investment in sperm and eggs
What is sexual selection?
Differs between the sexes
Females = limiting sex, males = limited sex
Selection on males to compete… And females to choose
Male reproductive success - intense competition for mates
What is the overview of the evolution of sexual selection?
Direct competition between males
Mate choice by females
Leads LHS between sexes e.g. sexual dimorphism
What are the consequences of sexual selection on males?
Males that are larger or invest more energy in competition for females are usually more successful
Competition is very costly
Can also lead to differences in LHS between individuals of same sex in a single species (different strategies)
What are alternative tactics in males?
Adopted by males that differ in size or morphological characteristics e.g. territorial males and sneaker (or satellite) males
Making the best of a bad job?
Equally effective alternative strategy?
What is sequential hermaphroditism?
Sex change
Female to male – protogyny
Male to female – protandry
Associated with changes in the relative reproductive success of the two sexes as an individual gets bigger
How does sequantial hermaphroditism occur in bluehead wrasse?
2 pathways by which terminal-phase males develop in the bluehead wrasse, Thalassoma fasciatum
Initial male phase - resembles female and sneakily mates until big enough to control an area
Initial female - will sex change when big enough to control a territory
