Problem 3 Flashcards
What is Malthus key principle ?
According to him populations could potentially grow exponentially but in practice can’t do so
–> they are therefore limited by incomplete survival and/or reproduction
Reproductive success
Refers to the number of viable descendants produced
- plays a role in natural selection
- differences can lead to changes in the population
- if competitive advantage is bigger, adaptation will be faster
Fitness (of an allele)
Refers to the population frequency of an allele in one generation relative to its frequency in the previous one
–> describes the absolute or relative reproductive success of classes of individuals
What is the general principle for changes in polygenic traits as a result of natural selection ?
The response to selection is the product of
- selective pressure
- Heritability
–> evolutionary change gets faster with increasing selective advantage + advanced heritability of the selected trait
Sexual dimorphism
Refers to phenotypic differences between males and females
According to Darwin, why does sexual dimorphism exist ?
According to him, the differences arise through sexual selection
Sexual selection
Refers to natural selection on the ability to gain mates
–> if males with a particular trait, can gain more mates than their rivals, then these traits will increase
ex.: bigger than average size
Batemans principle
States that male reproductive success increases with each additional partner mated to a greater extent than is true for females
–> females are choosy, whilst males happily engage in sexual interactions as cost is low
Why is there usually more intrasexual competition between males ?
Because, the benefit of fighting will often gain access to more mates (by ousting rivals out)
–> so the benefit is greater than the cost
–> this follows the Bateman principle
Sexy son hypothesis
(Fisher)
States that if there is any initial slight preference for males with longer tails, then the preference for the long tail and the length of the tail itself co-evolve to both become ever greater over time
- want their sons to be attractive for future females
- explains why male ornaments are attractive to females
Good genes hypothesis
Females choose males with the largest ornaments because males are proving they have the quality to do well in the current environment
- good genes are expected in addition to attractiveness (crucial for survival)
- explains why male ornaments are attractive to females
Does Batemans principle always apply?
How could it be refined ?
No,
A refined version would state that whichever sex invests less per episode of reproduction will have larger variance in reproductive success
–> they will therefore compete more for mates
Extra pair matings
Refer to matings that take place with a male other than the social partner
What is the reason for extra pair matings ?
The best males may already be paired up and so females may have to settle for a male that a male that is available and ready to choose her
Does Batemans principle apply to humans ?
Yes,
when it comes to short term relations
–> if it comes to long term relationships on the other hand, both sexes are equally choosy but in subtle different ways as both have different priorities
Which hypotheses explain why male ornaments are attractive to females ?
Good genes hypothesis + Sexy son hypothesis
Which mating strategies are available to both sexes ?
- Territory holding vs mobility (fighting)
- Extra pair mating
What does the theory of sex ratio state ?
The 50:50 sex ratio is the equilibrium that the population moves to, over evolutionary time, and then stays at
- Any population that deviates from the 50:50 sex ratio will be shifted back to it by natural selection
- 50:50 sex ratio is due to the fact that you have one mother and a father
Purifying Selection
Is natural selections that acts against mutation
–> whenever mutations arise at that locus, the mutation will have lower fitness and are weeded out
At which loci does no purifying selection occur and for what reasons ?
At loci that have no phenotypic effect
–> because they are non coding, so new alleles can spread at free will
Stabilizing selection
Is a mode of selection where the highest fitness is found in the current populations average of the trait
–> individuals higher or lower on the trait have reduced fitness
=> eliminates genetic variation
Directional selection
Is a mode of selection in which the highest fitness is found in individuals either above or below the average population value of characteristic
–> shifts the overall makeup of the population
Why is directional selection so significant ?
Because as long as the characteristic is heritable, it causes predictable changes in the population, so that the PA moves towards the optimum for the characteristic
How does directional selection work as an agent of change ?
It can produce phenotypic changes that have never been seen in ancestral generations
What is the difference between selection and mutation ?
Whilst mutation increases genetic variation within populations, selection tends to decrease it
–> they have opposing effects
How does directional selection decrease variation ?
By driving the alleles associated with highest fitness towards fixation
–> this way it reduces + eliminates competitor alleles as it goes
What does genetic variation dependent on ?
Mutation - Selection balance
What are the 5 mechanisms of maintenance that lead to persistence of variation/preserve variation at some loci ?
- Heterozygote advantage
- Negative frequency-dependent selection
- Force of mutation
- Inconsistent selection
- Sexually antagonistic selection
Heterozygote advantage
Refers to the situation where individuals with one copy of a particular allele have higher fitness than individuals with either no copies or two copies
–> Aa > AA/0
ex.: sickle cell disease
Negative frequency dependent selection
Refers to a situation where a phenotype is associated with relative high fitness when it is rare, but relatively low fitness when it is common
–> stabilizes an intermediate frequency
ex.: advantage of left-handers in interactive sports
Force of mutation
Genetic variation will persist if the force of the mutation is strengthened or the force of selection is weakened
Inconsistent selection
The selective optimum moves around from time to time/ place to place
–> genetic variation persists because selection will decrease the frequency of particular alleles in some years/ places and increase them in others
Sexually antagonistic selection
Refers to a selective regime where the optimal phenotype for a male is not the same as the optimum for a female
–> to the extent that male + female phenotype are genetically correlated
=> inconsistent selection that varies from *body to body* rather than time/ place
Why is the Sickle cell disease an example of Heterozygote advantage ?
One sickle cell allele will protect against malaria (Heterozygous)
–> BUT: when homozygous one is either not protected and thus will either get the sickle cell disease (aa) or malaria (AA)
Neutral variation
Refers to differences in the DNA sequence that don’t give an organism a selective advantage/ disadvantage
Mutation-Selection balance
Mutation introduces genetic variation into population while selection reduces it
–> the level of genetic diversity depends on the relative strength of the two
Relative fitness
Refers to the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals
–> determines how the proportions of 2 alleles competing with each other change
ex.: more efficient food collecting = more power = more eggs
According to Malthus, success in competition is related to .. ?
The phenotypic level
–> selection depends on the phenotype so the frequency of an advantageous dominant allele increases faster than that of an advantageous recessive allele
Adaptionist stance hypothesis
If some feature/ behaviour is commonly found in an organism it was probably an efficient design solution to a problem that the organism had faced
Ultimate explanation
Explains how a particular design increased ancestral fitness
–> can be agnostic about the proximate mechanisms producing the effect
Proximate explanation
Explains the genetic mechanism that led to the formation of a new characteristic
–> natural selection will favor any proximate mechanism available + reliably produce the optimal phenotype
–> cannot be deduced from ultimate explanations
Intrasexual selection
(Form of sexual selection)
Refers to selection on the ability to compete with rivals of the same sex
Intersexual selection
(Form of sexual selection
Refers to selection in the ability to attract members of the opposite sex
Sexual selection in humans
- Variance in male productive success doesn’t tend to be much higher than that in females rather the local economic + social situation modifies the extent of the difference
- Females are choosier + men more interested in gaining additional mating partners, no reversal
- When selecting a long-term partner both sexes are choosy as both have to invest a lot in upbringing (different priorities though)
- Male short-term mating vs female extra-pair mating
Why can’t organisms become perfect ?
(Constraints on optimality)
-
Time lags between environmental change and the organism responding
* adaptation is always behind - Selective regime/Trade offs
* selective or inconsistent selection causes constraints but phenotypic plasticity can help -
Genetic correlations
* changes to most genes will have many different phenotypic consequences (Pleiotropy) - Local peaks in the adaptive landscape
Phenotypic Gambit
Refers to the strategy of forming adaptionist hypotheses directly about the phenotype without needing to know what the underlying genetic or developmental mechanisms are
Genetic correlation
Refers to a situation in which selecting for one trait changes the population average of the other as well
Hitch hiking traits
Refer to changes that are not fitness enhancing themselves but genetically correlated with others that are
–> they can thus evolve alongside those
Trade-offs
If two ore more traits are conflicting but correlating, they won’t express fully in frequency but rather compromise like a trade off
Which 2 important effects does genetic correlation have on the design of organisms ?
- Hitch hiking traits
- Phenotypes come to reflect trade offs
Reverse Engineering
(Method to test adaptive hypotheses)
Examining carefully how something works, then trying to come up with ideas about what function would have led it to be designed in this way
Optimality modelling
(Method to test adaptive hypotheses)
The investigator takes one of the candidate hypotheses identified by reverse engineering and tries to find expectations about the design
Experimental manipulation + experiments of nature (Method to test adaptive hypotheses)
Involves using comparisons between individuals with more + less of the characteristic
–> this way one investigates the fitness of natural or experimentally produced mutants
Comparative evidence
(Method to test adaptive hypotheses)
Involves testing across different species that have experienced the selection pressure to differing extents
Why is generating adaptive hypotheses useful ?
How can they be tested ?
- They are a useful device for finding out more about the characteristics under study
2.
a) Reverse engineering + Optimality modelling
b) Experimental manipulation + experiments of nature
c) Comparative evidence
