The process of natural selection

In a nutshell

Evolution is driven by a process called natural selection, which is how a species changes slowly over time so they become better adapted to their environment. This helps them compete against other organisms in the population more successfully or adapt to a change in their environment more easily.

Natural selection

key word	definition
Natural selection	The process behind evolution, in which the characteristics of a species changes over time in response to competition with other organisms or to changes in the environment.
Evolution	The change in inherited characteristics of an organism that occur over a long period of time and leads to the formation of new species.
Species	A group of organisms that can interbreed to produce fertile offspring.
Population	A group of organisms of one species living in the same area at the same time.
Variation	Differences between individuals of the same species.
Competition	The negative interaction between organisms that try to use the same resources that are in short supply.
Gene	A short section of DNA that codes for a specific characteristic of a living organism.

The process of natural selection

Natural selection is thought to be the process behind evolution. Natural selection occurs within a population of organisms in the same habitat.

1.	Individuals in a population show a wide range of variation in characteristics due to differences in genes.
2.	Individuals in a population have to compete for resources such as food or shelter. They may also have to adapt to a change in their environment.
3.	Individuals with characteristics that are best suited to the environment or that help the individual compete more successfully are more likely to survive and reproduce. Their genes are passed onto the next generation.
4.	Individuals with characteristics that are not well suited to the environment or do not help the individual compete are poorly adapted. They are less likely to survive and therefore less likely to reproduce. Their genes are less likely to be passed down to the next generation.
5.	The genes responsible for useful characteristics are inherited by the offspring.
6.	Over many generations, these genes become more common in the population so the characteristic becomes more common in the population.

Note: Only genetic variation in characteristics can be inherited by offspring and future generations through reproduction. Environmental variation cannot be inherited, so it does not play a role in natural selection.

Example

Over time, giraffes have evolved to have longer necks. This is due to natural selection.

Science; Genetics and evolution; KS3 Year 7; The process of natural selection

1.	Giraffes show variation in neck length due to genetic differences.
2.	Giraffes eat leaves from trees. As the population of giraffes grew, competition for food - the leaves on trees - increased, and leaves on lower branches were scarce.
3.	Giraffes with a slightly longer neck could reach and eat leaves a little bit higher on the trees - they could compete better for food. These giraffes were more likely to survive and reproduce.
4.	Giraffes with shorter necks could not reach these leaves, so they starved and ultimately died. They therefore were less likely to have reproduced before dying.
5.	The genes responsible for a longer neck were inherited by the offspring. These offspring could reach higher up leaves and so continued to survive and reproduce.
6.	Over many generations, the genes responsible for a longer neck became more common in the population so all giraffes eventually had long necks.