Natural selection

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

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:

• Biotic factors involve other living organisms

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

Abiotic factors

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

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

Variation:

• 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

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

Selection pressures

• 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)