5.1.4 Natural Selection

• The number of offspring, or young, produced in each breeding event differs between species
  • Some species produce small numbers of young, e.g. elephants usually give birth to just one baby per pregnancy
  • Some species produce many offspring e.g. some species of ant can lay 3-4 million eggs in one go

• It is more usual for organisms to produce multiple offspring, to the extent that there are more offspring produced than can be supported by the surrounding environment
  • Darwin noticed this, and named the phenomenon 'overproduction of offspring'

• Overproduction of offspring means that there is always competition for resources

• In any habitat there are environmental factors that affect survival chances
  • E.g. predation, competition for food, and disease
  • Environmental factors that influence survival chances are said to act as selection pressures

• In any population, due to the variation present, some individuals will have characteristics that make them better adapted for survival
  • For example, lions that are stronger and faster are more likely to be able to catch prey and therefore more likely to survive
  • This is sometimes described as 'survival of the fittest'

• Individuals that are well adapted and survive into adulthood are more likely to find a mate and reproduce, producing many offspring
• Individuals that are less well adapted do not survive long into adulthood are likely to reproduce less often than those that survive for longer, so producing fewer offspring
  • These individuals may not reach adulthood and so do not get the chance to reproduce at all

• Many of the characteristics that affect an individual's chances of survival are determined by the alleles of genes present in their DNA
• Characteristics that are determined by alleles are heritable
  • Heritable characteristics can be physical e.g. the length of a giraffe's neck, or behavioural e.g. the innate behaviour of a woodlouse moving towards a dark hiding place

• Individuals with characteristics that increase their chances of survival are likely to produce more offspring
• This means that they are more likely to pass on the alleles that code for these advantageous characteristics to their offspring
• Note that non-heritable characteristics are not passed on to offspring
  • Non-heritable characteristics are those acquired during the lifetime of an organism e.g. gaining weight after eating lots of nuts and berries in autumn, or being injured by a predator

• Natural selection can be defined as
  • The process by which organisms that are better adapted to their environment survive, reproduce, and pass on their advantageous alleles, causing advantageous characteristics to increase in frequency within a population

• The increased survival chances of individuals with advantageous alleles mean that advantageous characteristics are more likely to be passed down through the generations
• The number of individuals in a population with a particular favourable characteristic will increase over time; the characteristic is said to increase in frequency
• Eventually this favourable characteristic will become the most common of its kind in the population; the population can be said to have adapted to its environment by the process of natural selection
• While favourable characteristics increase in frequency by natural selection, unfavourable characteristics decrease in frequency by the same process
  • Individuals with unfavourable characteristics are less likely to survive, reproduce, and pass on the alleles for their characteristics, so unfavourable characteristics are eventually lost from the population

An example of natural selection in rabbits

• Variation in fur colour exists within a rabbit population
  • One allele codes for brown fur and another for white fur

• Rabbits have natural predators such as foxes which act as a selection pressure
• The brown rabbits are more likely to survive and reproduce due to having more effective camouflage
• When the brown rabbits reproduce they pass on their alleles to their offspring
• The frequency of brown fur alleles in the population will increase
• Over many generations, the frequency of brown fur will increase and the frequency of white fur will decrease

Selection pressures acting on a rabbit population for one generation; predation by foxes causes the frequency of brown fur in rabbits to increase and the frequency of white fur in rabbits to decrease