Natural Selection & Evolution (WJEC GCSE Biology: Combined Science)

Natural Selection

• In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce
  • This results in natural selection
• Individuals in a species show a range of variation caused by differences in genes, caused by mutations
• When organisms reproduce, they produce more offspring than the environment can support
• This leads to competition for food and other resources which results in a ‘struggle for survival’
• Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
• Therefore the alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
• This means that in the next generation, there will be a greater number of individuals with the better-adapted variations in characteristics
  • This theory of natural selection was put forward by Charles Darwin and became known as ‘survival of the fittest’
• If the process of natural selection occurs too slowly within a population, it may lead to extinction
  • This is because the organisms will not be well enough adapted to their environment

Natural selection in snails diagram

Natural selection illustrated by snail shell colour

• Within the population of snails there is variation in shell colour
• Normal varieties of shell colours in this snail species is black or grey (as evidenced by the first picture)
• Chance mutations lead to a small number of snails / one snail having a white shell
• This ‘small number’ is shown in the second diagram where there are less white shelled snails than black or grey shelled snails
• The white shelled snail(s) survive longer
  • This is the ‘survival of the fittest’, a term used to explain why some organisms succeed in the competitive struggle for survival against other members of their population
• The reason the white shelled snail(s) survive longer is because they are better camouflaged
• This means that they are less likely to be seen by predators and eaten
• As they survive longer they get more opportunities to reproduce
• And so the allele for white shells is passed onto offspring more frequently than the alleles for black or grey shells
• Over generations, this is repeated until the majority of snails in the population have white shells

 Natural selection in moths diagram

Another good example of natural selection is the evolution of the peppered moths

Using models to illustrate natural selection

• We can use models to study natural scenarios e.g. natural selection
• However, there are often limitations with models which reduce the value of the illustrations drawn

A bird model example

• Start with a population of "birds" with different beaks
  • Use tweezers of two different lengths to represent the two different variations of a trait, long beaks and short beaks
• Scatter some small objects (like beads or seeds) in a container. These represent the food source
  • Add a lid to make it slightly more difficult to get the food
• Set a timer for a short period, representing a generation time
• Instruct participants to act as "birds" and try to collect food with the tweezers
• Simulate natural selection by removing the birds with the least food
  • For example, if the lid makes it easier for birds with long beaks to access food, birds with short beaks will have less food and be removed from the population
• After each generation time (timer runs out), count the number of surviving birds of each beak type
• After the birds have been removed from each generation, reproduction can be modelled by doubling the number of short and long tweezers in the population (keeping the ratios the same)
• Repeat the process for several generations, adjusting the parameters (such as the difficulty of accessing food or the percentage of birds removed) to simulate different selective pressures
• Observe how the proportions of the different birds change over time due to natural selection

Limitations of the model

• Simplification: The model is very simplistic e.g. it only takes into account the effect of beak length on survival. In reality, organisms possess a range of traits which affect their survival and reproductive success
• Fixed parameters: Abiotic and biotic factors in real-world environments are constantly fluctuating, whereas this model is fairly static
• Lack of genetic variation: In the model, birds do not exhibit genetic variation or inheritance of traits, which are crucial aspects of natural selection in real organisms
• Inability to explore complex interactions: The model does not consider interdependence between organisms, including biotic factors such as competition, predation, and symbiosis
• Time constraints: Evolution occurs over many generations in real populations whereas this model operates within a short period, therefore giving an unrealistic impression of the speed of evolution

The Work of Darwin and Wallace

Charles Robert Darwin

• Charles Darwin spent five years on a voyage around the world on a ship called HMS Beagle
• During the voyage, he studied the plants and animals at all the different locations around the world that the ship visited
• He noticed that there was variation in members of the same species
  • He also noted that those individuals with characteristics most suited to their environment were more likely to survive, reproduce and, therefore, pass on their characteristics to their offspring
• To explain his observations, Darwin developed his theory of evolution by natural selection

Alfred Russel Wallace

• Alfred Russel Wallace was a scientist who, after conducting his own travels around the world and gathering much evidence, independently developed his own theory of evolution based on the process of natural selection
• He published scientific papers on this theory with Darwin in 1858 (Darwin published his book, On the Origin of Species, the following year)
• Wallace is best known for:
  • His work studying the warning colouration of species (particularly butterflies) and how this must be an example of a beneficial characteristic that had evolved by natural selection, as the warning colouration helps to deter predators
  • Developing the theory of speciation

Evolution

•  If the environment does not change, selection does not change
• This will favour individuals with the same characteristics as their parents
• If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele
• So the individuals that survive and reproduce will have a different set of alleles that they pass on to their offspring
• Over time, this will bring about a change in the characteristics of the species - it will produce evolution
• Evolution is defined as the change in adaptive features of a population over time as a result of natural selection
• Natural selection results in a process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common
• This means populations of organisms become better suited to their environment
  • A good example of this is the development of antibiotic resistance by bacteria, warfarin resistance in rats and pesticide resistance in insects