Natural Selection (OCR AS Biology)

Genetic variation

• Organisms of the same species have very similar genomes, but two individuals (even twins) will have differences between their DNA base sequences
• These differences in DNA base sequences between individual organisms within a species population are called genetic variation
• Genetic variation is transferred from one generation to the next and results in genetic diversity within a species

Effects of genetic variation

• There needs to be some level of genetic diversity within a population for natural selection to occur

• Differences in the alleles possessed by individuals within a population result in differences in phenotypes
  • Some phenotypes may be advantageous, disadvantageous or neutral, compared to other phenotypes

• Selection pressures increase the chance of individuals with a specific (more advantageous) phenotype surviving and reproducing over others
• The individuals with the favoured phenotypes are described as having a higher fitness
  • The fitness of an organism is defined as its ability to survive and pass on its alleles to offspring
  • Organisms with higher fitness possess adaptations that make them better suited to their environment

• A population with a large gene pool or high genetic diversity has a strong ability to adapt to change
• If a population has a small gene pool or very low genetic diversity then they are much less able to adapt to changes in the environment and so can become vulnerable to extinction
  • Cheetahs are an example of a species with a small gene pool
  • They experienced a very large decline in numbers approximately 10,000 years ago
  • This left small, fragmented populations of individuals remaining
  • There was no mixing between populations and large amounts of inbreeding occurred
  • This is problematic for conservation as low genetic variation means the species are less likely to be able to respond (survive) in the event of any environmental changes

Environmental factors

• Environmental factors affect the chance of survival of an organism - they act as a selection pressure
• Every individual within a species population has the potential to reproduce and have offspring which contribute to population growth
• If all the offspring of every individual survived to adulthood and reproduced, the population would experience exponential growth
  • This type of growth only happens when there are no environmental factors or population checks acting on the population (for example, when there are plentiful resources and no disease)
  • One well known but rare example of exponential growth in a population is the introduction of 24 European rabbits into Australia in the 1800s. The rabbits had an abundance of resources, little or no competition and no natural predators. This meant the population increased rapidly and they became a major pest
  • In reality, there are several environmental factors that prevent every individual in a population making it to adulthood and reproducing

Exponential growth in a population of rabbits that have no environmental checks

Natural selection

• The main processes resulting in natural selection are as follows:
  • Random mutation can produce new alleles of a gene
  • Many mutations are harmful or neutral but, under certain environmental conditions, the new alleles may benefit their possessor, leading to an increased chance of survival and increased reproductive success
  • The advantageous allele is passed onto the next generation
  • As a result, over several generations, the new allele will increase in frequency in the population

Example of natural selection in rabbits

• Variation in fur colour exists within rabbit populations
• At a single gene locus, normal brown fur is produced by a dominant allele whereas white fur is produced by a recessive allele in a homozygous individual
• Rabbits have natural predators like foxes which act as a selection pressure
• Rabbits with a white coat do not camouflage as well as rabbits with brown fur, meaning predators are more likely to see white rabbits when hunting
• As a result, rabbits with white fur are less likely to survive than rabbits with brown fur
• Therefore, the rabbits with brown fur have a selection advantage, so they are more likely to survive to reproductive age and be able to pass on their alleles to their offspring
• Over many generations, the frequency of alleles for brown fur will increase and the frequency of alleles for white fur will decrease

Selective pressures acting on a rabbit population for one generation. Predation by foxes causes the frequency of brown fur alleles in rabbits to increase and the frequency of white fur alleles in rabbits to decrease.