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Selection Pressures (HL IB Biology)

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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 Abiotic factors that affect a community table, IGCSE & GCSE Biology revision notes

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 Peahenpeacock-with-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

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.