AQA A Level Biology

Revision Notes

7.3.3 Selective Advantage

Selective Advantage

  • Every individual within a species population has the potential to reproduce and have offspring
  • In reality, however, there are many environmental factors that prevent every individual in a population from surviving to adulthood and reproducing

Examples of environmental factors limiting survival

  • For African lions living in the wild, there are several environmental factors that ensure not all individuals survive and reproduce:
  • Competition for food
    • There is a limited supply of prey: other lions and carnivores will also be hunting the same prey. If a lion is not able to hunt and feed it will die from starvation
  • Competition for a reproductive mate
    • Female lions will often outnumber male lions in a population. This means the males compete with each other to mate with the females. When a male is in a contest with another male, one (or both) could be injured or killed. Whoever loses the contest won’t be able to mate with the females in a pride and so won’t pass on his genes to any offspring
  • Supply of water
    • African habitats can be very arid during the dry season. The water sources that the lions drink from can be miles apart. If a lake or source of water dries up then they can die due to dehydration
  • Temperature
    • The extreme heat experienced in the lion’s African habitat can cause them to overheat and die. It can also prevent them from hunting for long periods during the day, meaning they are less likely to get the food they need to survive

Selective advantage

  • Environmental factors affect an individual’s chance of survival
    • They are said to act as selection pressures
  • In any species population, some individuals will have phenotypes that make them better adapted to survive (and more likely to reproduce)
    • This is due to the genetic variation present within populations
  • For example, lions with phenotypes that are fitter and faster are more likely to be able to catch prey, survive and reproduce
    • Their phenotype is said to provide them with a selective advantage
  • Male lions with stronger and more aggressive phenotypes are more likely to outcompete other males for access to females, ensuring they can reproduce
    •  Again, their phenotype provides them with a selective advantage
  • Individuals with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation
  • Over time, this differential reproductive success has an effect on the allele frequencies within a gene pool
    • Alleles that are favourable (provide a selective advantage) will increase in frequency in a gene pool
    • Alleles that are not favourable (don’t provide a selective advantage) will decrease in frequency in a gene pool

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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