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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

  1.  plays a role in natural selection
  2. differences can lead to changes in the population
  3. 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


1. selective pressure

2. 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


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

  1.  want their sons to be attractive for future females
  2.  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

  1.  good genes are expected in addition to attractiveness (crucial for survival)
  2. explains why male ornaments are attractive to females 


Does Batemans principle always apply?

How could it be refined ?


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 ?


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 ?

1. Territory holding vs mobility (fighting)

2. 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

1. Any population that deviates from the 50:50 sex ratio will be shifted back to it by natural selection

2. 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 ?

1. Heterozygote advantage

2. Negative frequency-dependent selection

3. Force of mutation

4. Inconsistent selection

5. 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