New Variation in Populations

How does new variation arise in a population?

Lichen: an example of symbiosis, another way to produce new variants.

A population of organisms — a group of individuals of the same species — represents a pool of hereditary material that is collectively called the genome of the species. In this genome are genes, which code for all of the traits that make an organism what it is. A look at individuals in any population reveals an amazing amount of variation for any given trait — height in human beings or coat color in dogs, for example. Behind this variation are differences in the code in the genes. At any given time, a population can be characterized by having traits that vary in certain ways. But how does new variation arise?

A critical part of cell reproduction involves the replication of a cell’s entire set of chromosomes. Although replication is remarkably reliable, errors do occur and result in changes in the sequence of bases in a gene. Parts of a gene may be deleted, substituted, or doubled, for example. An error like this is referred to as a mutation. Mutation is one of the key sources of new variation in genes. Mutations may or may not be harmful.

Whether or not the mutation is expressed in the next generation of individuals in a population depends on where the mutation occurs. If it occurs in an organism’s body cells, the mutation will only appear in subsequent generations of the affected body cells. If the mutation occurs as sex cells are being produced, the mutation may be passed on to the next generation through the affected individual’s sperm or eggs. This is thought to be one of the most important ways that new variation arises in a population.

prev: DNA, chromosomes, and genes

next: scientist at work