Factors Affecting Evolution (OCR A Level Biology)

• Environmental factors that affect the chance of survival of an organism are known as selection pressures
  • For example, there could be high competition for food between lions if there is not plentiful prey available; this environmental factor ‘selects for' faster, more powerful lions that are better hunters

• These selection pressures can have different effects on the allele frequencies of a population through natural selection
• There are two main types of selection:
  • Stabilising selection
  • Directional selection

Stabilising selection

• Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations
• This means things stay as they are unless there is a change in the environment
• A classic example of stabilising selection can be seen in human birth weights
  • Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights

Stabilising selection on human birth weight

Directional selection

• Directional selection is natural selection that produces a gradual change in allele frequencies over several generations
• This usually happens when there is a change in environment (and therefore a change in selection pressures) or a new allele has appeared in the population that is advantageous
• Directional selection occurs via the following process:
  • There is always phenotypic variation within a population
  • There is a selection pressure that favours a particular phenotype
  • The phenotype is produced by particular alleles
  • Individuals with the favoured phenotype are fitter and so more likely to reproduce and pass on the advantageous alleles to their offspring
  • Those who do not possess the advantageous allele or phenotype are less likely to survive and pass on their alleles to their offspring
  • So over time and several generations the frequency of the advantageous allele increases and the frequency of other alleles decreases

• For example, a recent finding has shown that climate change is having an effect on fish size in certain habitats:
  • The increase in temperature is selecting for smaller body size and against larger body size
  • Warmer seas cause fish metabolism to speed up and so increases their need for oxygen; oxygen levels are lower in warmer seas
  • Larger fish have greater metabolic needs than smaller fish, and so they feel the effect of increased temperatures more strongly
  • Organisms are sensitive to changes in temperature primarily because of the effect that temperature can have on enzyme activity
  • Fish with a smaller body size are therefore fitter and better adapted to living in seas experiencing increased temperatures
  • Fish body size is determined by both genetic and environmental factors
  • Fish of a smaller size are more likely to reproduce and pass on their alleles to offspring
  • Over generations, this leads to an increase in the frequency of alleles that produce a small body size and a decrease in the frequency of alleles that produce a larger body size

Directional selection acting on fish body size

Other factors that can affect the evolution of a species

• There are other factors or processes that can affect allele frequencies in a population and therefore the evolution of a species
• These include:
  • Genetic drift
  • Genetic bottleneck
  • Founder effect

Genetic drift

• This is when chance (instead of environmental selection pressures) affects which individuals in a population survive, breed and pass on their alleles
• For example, 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, genetic drift is occurring
• In large populations, genetic drift is less likely to have an effect because any chance variations in allele frequencies usually even out across the whole population
  • Natural selection has a much more influential role in the evolution of large populations

Example of genetic drift in plants

• In a small population of plants growing near a playground with a rubber floor:
  • Half of the plants have alleles for blue flowers
  • Half of the plants have alleles for pink flowers

• By chance, most of the seeds from the pink-flowered plants land on the rubber floor of the playground (where they are not able to germinate and grow), whereas all the seeds from the blue-flowered plants land on fresh fertile soil (where they are able to germinate and grow)
• Over several generations, the alleles for the pink flowers may disappear from this population due to chance (because the seeds from the original population carrying pink alleles for flower colour were not able to germinate on rubber)

An example of how genetic drift can occur

Genetic bottleneck

• The bottleneck effect is similar to the Founder effect
• It occurs when a previously large population suffers a dramatic fall in numbers
• A major environmental event can massively reduce the number of individuals in a population which in turn reduces the genetic diversity in the population as alleles are lost
• The surviving individuals end up breeding and reproducing with close relatives

Example of the bottleneck effect

• A clear example of a genetic bottleneck can be seen in cheetahs today
• Roughly 10,000 years ago there was a large and genetically diverse cheetah population
• Most of the population was suddenly killed off when the climate changed drastically at the end of the Ice Age
• As a result, the surviving cheetahs were isolated in small populations and lots of inbreeding occurred
• This meant that the cheetah population today has a serious lack of genetic variation
• This is problematic for conservation as genetic variation within a species increases the likelihood that the species is able to respond (survive) in the event of any environmental changes
  • Remember the environment exerts a selection pressure on organisms

The bottleneck effect in cheetahs after the Ice Age

Founder effect

• The Founder effect occurs when only a small number of individuals from a large parent population start a new population
• As the new population is made up of only a few individuals from the original population only some of the total alleles from the parent population will be present
• In other words, not all of the gene pool is present in the smaller population
  • A gene pool is the complete range of DNA sequences (alleles) that exist in all the individuals of a population or species

• Which alleles end up in the new founding population is completely up to chance
• As a result, the changes in allele frequencies may occur in a different direction for the new small population vs the larger parent population

The founder effect in lizards

• Anole lizards inhabit most Caribbean Islands and they can travel from one island to another via floating debris or vegetation
• The individual lizards that arrive on an island, as well as the alleles they carry, is completely up to chance
• They may only carry a small selection of alleles, with many more alleles present in the lizard population on the original island
• The lizards on the original island could display a range of scale colours from white to yellow and the two individual lizards that arrived on the island have white scales
  • This means that the whole population that grows on that island might only have individuals with white scales
  • In comparison, the original island population has a mixture of white and yellow scaled individuals. This difference between the two populations is completely due to chance

The founder effect on lizards and their scale colour

Processes Affecting Allele Frequencies Summary Table