Natural Selection (AQA A Level Biology)

The Theory of Natural Selection

• Genetic variation exists within populations due to the presence of different alleles

• There is differential reproductive success between the organisms with different alleles of the same gene

• Under certain environmental conditions, individuals with certain alleles will have an increased chance of survival and reproduction

• New alleles can arise in populations through random mutation

• Natural selection can cause the frequency of alleles in a population to change over time

Principles of Natural Selection

• Random mutation can produce new alleles of a gene

• Many mutations are harmful or neutral but, under certain environmental conditions, the new alleles may benefit their possessor, leading to an increased chance of survival and increased reproductive success

• The advantageous allele is passed onto the next generation

• As a result, over several generations, the new allele will increase in frequency in the population

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

• Therefore, 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

Image showing 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.

• Natural selection causes a change in allele frequencies over time

  • Selection pressures (caused by the environment an organism is in) increase the likelihood that certain individuals with specific alleles survive to reproductive age, enabling them to pass on their alleles to their offspring

• There are other factors or processes that can affect allele frequencies in a population:

  • The founder effect

  • Genetic drift

  • The bottleneck effect

The 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

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

Example of genetic drift in plants

• In a small population of five plants growing near a playground with a rubber floor; three of the plants have blue-and-white flowers and two of the plants have pink-and-white flowers

• By chance, most of the seeds from the pink-and-white flowered plants end up on the rubber floor of the playground, whereas all the seeds from the blue-and-white flowered plants land on fresh fertile soil where they are able to germinate and grow

• Over several generations, the allele for the pink-and-white flowers may disappear from this population due to chance (because the seeds carrying pink-and-white alleles for flower colour cannot germinate on rubber)

Bottleneck effect

• 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

Processes that cause allele changes table