Natural Selection & Evolution

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Survival

Individuals must survive and reproduce to pass on their genes to their offspring. Whether an individual is capable of surviving or not is influenced by the following factors:

Predation

Predation can decrease the probability of survival.
If an individual is exposed to high predation, they are more likely to be eaten by a predator.
If an individual is better at avoiding predators, they are more likely to survive and reproduce.
The genes that allow an individual to avoid predation are more likely to be passed onto offspring.

Disease

Disease can decrease the probability of survival.
If an individual is better at combatting disease or avoiding infection, they are more likely to survive and reproduce.
The genes that allow an individual to avoid disease are more likely to be passed onto offspring.

Competition

Competition can decrease the probability of survival.
- Competition can exist between species (interspecific) or within species (intraspecific).
If an individual is better at outcompeting other individuals, they are more likely to survive and reproduce.
The genes that allow an individual to outcompete are more likely to be passed onto offspring.

Natural Selection and Evolution

Natural selection is the process where the frequency of alleles in a population changes over time. Natural selection is a process that gives rise to evolution.

Selective advantage

Genetic variation exists between individuals in a population.
Some individuals will be more likely to survive (e.g. by being better at fighting disease) than others.
Individuals that are better at surviving than others have a selective advantage.

Producing offspring

Individuals with a selective advantage are more likely to survive to reproduce than others.
This means that the genes of an individual with a selective advantage are more likely to be passed onto offspring than the genes of an individual without a selective advantage.

Increasing allele frequencies

Individuals with a selective advantage are more likely to pass on their beneficial alleles than other individuals.
The next generation is more likely to have alleles that provide a selective advantage than alleles that do not.
This generation is also more likely to survive to reproduce and pass on their genes.
This causes the alleles that provide a selective advantage to increase in frequency in the population.

Natural selection

The process where the frequency of beneficial alleles increases over time is called natural selection.
Natural selection controls the frequency of alleles in a population.
If a harmful allele develops in an individual, this individual is less likely to survive and the harmful allele will decrease in frequency. This is also natural selection.

Evolution

The process of natural selection gives rise to evolution.
Evolution is defined as a change in allele frequencies over time.
- E.g. The evolution of humans is the change in allele frequencies that has taken place over millions of years. This change has been driven by natural selection.