Genetic Variation & Natural Selection

Cactus

1. Genetic variation

 In a population of cacti, some have longer roots than others. In the wet season they flower.

2. Overproduction

The cacti produce large numbers of offspring.

3. The Struggle for Existence

During the dry season there is competition for water.

4. Survival of the Fittest

The cacti with the longest roots are able to obtain water, while the others die from dehydration.

5. Natural Selection 

Advantageous characteristics are passes on to the offspring.

What is a mutation?

A mutation is a change in a gene or chromosome. During meiosis, chromosomes are copied and new gametes are formed. If the chromosomes are not copied exactly a mutation occurs. This can occur naturally for no apparent reason. However, it is known that ionising radiation and some chemicals may increase the occurrence of mutations.

Sometimes a mutation causes new characteristics that help the organism to survive. For example, plants can become resistant to drought or moths are born with a darker colour that helps camouflage them. In these cases the mutated organisms not only survive but natural selection (the survival of organisms best fitted to the conditions in which they live) helps them to live more successfully.

Darker and lighter coloured moths