Speciation and genetic drift

What is a species?

• A species is a group of individuals that are capable of breeding with one another to produce fertile offspring

  • Individuals within a species share the same genes but typically have different alleles

What is speciation?

• Speciation is the process by which new species are formed

How does speciation occur?

• Speciation occurs when populations of the same species become reproductively isolated which can result in the accumulation of difference in their gene pools.

  • This can lead to changes in allele frequency which leads to evolution.

  • New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring. In this way, new species arise from existing species

What are the two ways speciation can take place?

• Speciation can occur due to a physical barrier, e.g. a flood or an earthquake, divides a population of a species, causing some individuals to become separated from the main population

  • This is geographical isolation and leads to allopatric speciation

• Alternatively speciation can also occur when a population becomes reproductively isolated without any physical separation 

  • This leads to sympatric speciation

Explain allopatric speciation

• Some members of a population are geographically separated from the rest by a physical barrier like a mountain or river

• This means the populations will experience different selection pressures and so different changes in allele frequencies could occur

• The gene pools of the two populations are slightly different, by chance

• Mutations occur in each population, and natural selection favours different phenotypes 

• Different alleles will be more advantageous in the different populations

• Eventually individuals from the different populations will have changed so much that they won’t be able to breed with another to produce fertile offspring so are reproductively isolated

• And so they have become two separate species

Explain sympatric speciation

• Speciation takes place within the same geographical location

• Random mutations could occur within a population, preventing members of that population breeding with other members of the species

  • An example of this is: most eukaryotic organisms are diploid - they have two sets of homologous (matched) chromosomes. This is known as polyploidy

  • Individuals with different numbers of chromosomes can’t reproduce sexually to give fertile offspring - so if a polypoid organism emerges in a diploid population, the polypoid organism will be reproductively isolated from the diploid organism

• Reproductive isolation prevents gene flow, and leads to genetic divergence

What is genetic drift and why does it affect smaller populations more?

• In small populations, there is a random element called genetic drift that affects allele frequencies

  • Genetic drift does not have a great influence on large populations because any chance variations in allele frequency tend to even out across the whole population

  • This is when chance, rather than environmental factors, dictates which individuals survive, breed and pass on their alleles

Give an example of genetic drift?

• One example is a genetic bottleneck, which is when most of a population dies, either through disease, climatic changes, predation or hunting. The organisms that survive this near extinction will, by chance, have a much smaller variety of alleles than the original population, and the allele frequencies will be different. 

  • Even if the population increases again, the variety of alleles will be much smaller than in the original population, and the frequencies of these alleles may be very different

Give another example of genetic drift

• Another example is the founder effect. This is when a small proportion of a population colonise a new area. These few organisms again have a much smaller variety of alleles than the original larger population.

  • Even when there is a much larger population in the future, they will still have a very different range of alleles, and frequency of those alleles, than the original large population.