Selection Pressures (DP IB Biology: HL): Revision Note

Selection Pressures: Abiotic Factors

• In Biology, ‘abiotic’ means non-living

• An abiotic factor is a non-living factor within an ecosystem

  • Some abiotic factors are density-independent

    • E.g. temperature or rainfall

•  Abiotic factors can act as selection pressures

  • They affect the survival of individuals in a population, causing the population size to fluctuate

Abiotic Factors Table

Selection Pressures: Sexual Selection

• Sexual selection is another type of selection pressure that can affect the evolution of animal species

• It can be defined as:

  • A form of selection that occurs due to the preference of one sex for certain characteristics in individuals of the other sex

• In order for a selection pressure to have an effect, there needs to be variation within the population

  • Within the male cohort of a population, there will be variation in physical and behavioural traits which are visible to the female cohort (and vice versa)

• These differences are sometimes viewed as indicators of overall fitness by the females within the population

• As a result, they can affect an individual's success in attracting a mate and drive the evolution of an animal population

What are the effects of sexual selection?

• Sexual selection within a population can cause:

  • Reproductive isolation

  • Sexual dimorphism

What is reproductive isolation?

• Reproductive isolation occurs when changes in the alleles and phenotypes of some individuals in a species prevent them from successfully breeding with other individuals that don't have these changed alleles or phenotypes

• For example, certain alleles may result in changes in male courtship behaviour meaning they are no longer attractive to females

  • The females no longer choose to mate with these males, creating reproductive isolation

What is sexual dimorphism?

• Sexual dimorphism is the distinct difference in size or appearance between the sexes of an animal species

•  Sexual dimorphism is commonly seen in Birds of Paradise

  • The male birds are brightly coloured and can perform intricate courtship displays

  • The female's plumage consists of greys and browns

How does sexual dimorphism occur?

• A physical and/or behavioural trait within the male phenotype is used as an indicator of fitness by the females in the population

  • E.g. a male peacock with vibrant, healthy tail feathers is likely to have a lower disease burden compared to a male with dull, sparse tail feathers

• Females are more likely to be attracted to and mate with males that display the desired traits

• Over time, the genes associated with these traits are inherited by subsequent generations and they become more prominent within the species

• As a result, sexual selection can impact mating success, driving the evolution of an animal population

What is the difference between natural selection and sexual selection?

• Natural selection occurs due to competition for resources whereas sexual selection occurs due to competition for mates

  • On islands where resources are plentiful and predators rare, females are often the primary selection pressure which determines how males evolve

• Sexual selection results in animals with enhanced mating success whereas natural selection tends to result in populations of individuals that are well-adapted to their environment

• Sexual selection does not result in individuals that are well-adapted to their environment

  • E.g. Peacocks possess iridescent tail feathers with a specific eye-spot pattern which are used heavily during courtship displays to females

  • Over time, sexual selection in peacocks has resulted in males with longer tail feathers and more elaborate patterns

  • These traits actually make the bird more prone to predation, reducing their chances of survival

    • A long tail reduces agility, ability to fly and makes the bird easier to spot 

• Sexual selection can be a more prominent evolutionary force than natural selection as variation in mating success can:

  • Amplify selection

  • Maintain new genetic variation among individuals

•  Both of which can result in rapid evolutionary change

Peacock and Peahen

https://commons.wikimedia.org/wiki/File:Peacock_with_peahen.jpg

A male peacock proudly displays his elaborate tail feathers in an attempt to attract a female mate

• The concept of sexual selection is viewed as Darwin's second-greatest insight

• Darwin was aware that the existence of traits that were not favoured by natural selection needed a reasonable explanation