Natural Selection (AQA A Level Biology)

Natural Selection

• 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

Environmental factors

• Environmental factors limit population size as they result in differential survival and reproduction (some individuals survive and reproduce, some do not)

• Environmental factors can be biotic or abiotic

• Biotic factors involve other living organisms

  • This includes things like predation, competition for resources and disease

• Abiotic factors involve the non-living parts of an environment

  • Examples of abiotic factors include light availability, water supply and soil pH

• A combination of these biotic and abiotic factors means that not all individuals within a population will survive

  • For example, if a food source is limited, some animals within a population will not get enough to eat and will starve to death

• For most populations in the wild, the number of offspring produced is much higher than the number of individuals that make it adulthood

Natural selection

• Variation exists within a species population

• This means that some individuals within the population possess different phenotypes (due to genetic variation in the alleles they possess; remember members of the same species will have the same genes)

• Environmental factors affect the chance of survival of an organism; they, therefore, act as a selection pressure

• Selection pressures increase the chance of individuals with a specific 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

• When selection pressures act over several generations of a species they can cause a change in the allele frequency and the phenotype frequency in a population through natural selection

  • Natural selection is the process by which individuals with a fitter phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations

• The allele frequency and the phenotype frequency in a population can also change due to isolation

  • If a population becomes isolated from the other populations of a species (e.g. a few lizards of the same species are transported on drift-wood to a new, uninhabited island and start a new population there) then this new population may experience new (i.e. different) selection pressures from the main population. Over many generations, this will cause the allele and phenotype frequencies in the new population to change (compared to the populations on the mainland)

An 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

• 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.

Apparatus & Techniques: Modelling Natural Selection & Genetic Drift

• Computer programs can be used to model the effects of natural selection

  • They usually start off with a simple simulated population, with no particular selection pressures acting upon it

• Natural selection can then be investigated by changing various factors and observing the effects on the simulated population

• Examples of factors that can be changed include:

  • The presence or absence of different biotic and abiotic selection pressures (such as predators, disease and food availability)

  • Making new, specifically selected mutations appear in the population

  • Changing the likelihood that a new mutation will appear in the population

  • Switching which alleles are dominant and which are recessive

  • Changing the environment the species is in

  • Changing one or more of the adaptations of the species in the simulated population

• The effects of these changing factors can be modelled by the program and the probabilities of different outcomes for the simulated population can be calculated

• One of the benefits of these computer programs is that time can be sped up

  • This means that natural selection, which for many species would normally occur over very long time periods and many many generations, can be modelled and the effects observed in much shorter and more experimentally appropriate timescales

Genetic drift

• When a population is significantly small, chance can affect which alleles get passed onto the next generation

• Over time some alleles can be lost or favoured purely by chance

• When there is a gradual change in allele frequencies in a small population due to chance and not natural selection then genetic drift is occurring

• Computer programs similar to those described above can be used to model genetic drift