Biol 4 Speciation

Allelle frequency is affected by Differential Reproductive Success

• Sometimes the frequency of an allele within a population changes. This can happen when the allele codes for a characteristic that affects the chances of an organism surviving.
• Not all individuals are as likely to reproduce as each other. There's differential reproductive success in a population - individuals that have an allele that increases their chances of survival are more likely to survive, reproduce and pass on their genes (including the beneficial allele), than individuals with different alleles.
• This means that a greater proportion of the next generation inherit the beneficial allele.
• They in turn are more likely to survive, reproduce and pass on their genes.
• So the frequency of the beneficial allele increases form generation to generation.
• This process is called Natural Selection.

Different Types of Natural Selection lead to different Frequency Patterns

Stabilising Selection - where individuals with alleles towards the middle of the range are more likely to survive and reproduce. it occurs when the environment isn't chenging and reduces the range of possible phenotypes. For example, in a mammal population the is a range of fur lengths. In a stable climate, having fur at the extremes of this range reduces the chances of surviving as its harder to maintain the right body temperature. Animals with alleles for average fur length are the most likely to survive, reproduce and pass on their alleles. so these alleles increase in frequency. the proportion of the population with average fur length increases and the range of fur length decreases.

Directional Selection - where individuals with alleles for characteristics for the extreme type are more likely to survive and reproduce. this could be in respense to an environmental change. For example, cheetahs are the fastest animals on land. It's likely that this characteristic was developed through directyional selection, as individuals that have the alleles for speed are more likely to catch prey than slower individuals. So they're more likely to survive, reproduce and pass on their allels. Over time, the frequency of alleles for high speed increases and the population becomes faster.

Geographical Isolation and Natural Selection lead to Speciation

• Speciation is the development of a new species. It occurs when populations of the same species are reproductively isolated.
• this can happen when a physical barrier, e.g. a flood or an earthquake, divides the population of a species, causing some individuals to become seperated form the main population. this is known as Geographical Isolation.
• Populations that are seperated will experience slightly different conditions. For example, there might be a different climate on each side of the physical barrier.
• The populations will experience different selective pressures and so different changes in allele frequencies.
• The changes in allele frequencies will lead to differences accumulating in the gene pools of the seperated populations, causing changes in the phenotype frequencies.
• Eventually the individuals from the different populatiuons will have changed so much that they wont be able to breed with one another to produce fertile offspring - this means that they have become reproductively isolated.
• The two groups have now become seperate species.