60 Sources of Variation

Key Idea: Variation may come from changes to the genetic material (mutation), through sexual reproduction, and as a result of the effects of the environment.

Mutations are changes to the DNA Changes to the DNA modify existing genes. Mutations can create new alleles.

Mutation: Substitute T instead of C

Sexual reproduction involves meiosis and mate selection Sexual reproduction rearranges and reshuffles the genetic material into new combinations. Fertilisation unites dissimilar gametes to produce more variation.

The genetic instructions for creating the individual (the genotype) may be modified by the environment during and after development to produce the phenotype (the physical expression of the modified genetic information). Genes interact to influence the phenotypic result.

The genetic make up of the individual is its genotype. It determines the genetic potential of the individual.

1. Define the following terms:

(a) Mutation:

(b) Genotype:

(c) Phenotype:

2. What factors determine the phenotype?

3. How could two individuals with the same genotype have a different phenotype?

Environmental factors influence expression of the genotype. The external environment includes physical factors (e.g. temperature) or biotic factors (e.g. competition). The internal environment (e.g. hormones) and chemical tags and markers on the DNA may also affect genotype expression.

` Variation refers to the diversity of phenotypes or genotypes within a population or species. Variation arises through mutation (the source of new alleles) and sexual reproduction (which shuffles alleles into new combinations). ` Variation provides the raw material for adaptation and so enables populations respond to changes in their environment.

Very gradual environmental change, e.g. mountain building, allows sufficient time for populations (even those that reproduce asexually) to acquire the variability to adapt.

However rapid environmental change, e.g. the emergence of a new strain of disease, demands a more rapid response. ` Variation is important in providing adaptable defences against disease. Species with adaptations to survive a disease flourish.

Those without die out. Sexual reproduction is an adaptation to increase the chances that any one of the offspring will have the allele combinations that enable them to survive a disease. ` Variation in species that reproduce asexually is generated by mutation and sometimes (as in bacteria) by gene transfers.

Aphids can reproduce both sexually and asexually. Females hatch in spring and give birth to clones. Many generations are produced asexually. Just before autumn, the aphids reproduce sexually. The males and females mate and the females produce eggs which hatch in spring. This increases variability in the next generation.

These diagrams show how three beneficial mutations could be combined through sexual or asexual reproduction.

Variation from recombining alleles

Mutation A

Mutation B

Mutation C Clones

Mutation F

Mutation D

Mutation E

During meiosis, alleles are recombined in new combinations. Some combinations of alleles may be better suited to a particular environment than others. This variability is produced without the need for mutation. Beneficial mutations in separate lineages can be quickly combined through sexual reproduction. Some asexually reproducing organisms (e.g. bacteria) are able to exchange genes occasionally. Bacteria exchange genes with other bacteria during a process called conjugation (shown by the thicker blue line). This allows mutations arising in one lineage to be passed to another.

4. (a) What is variation and how does it arise in sexually reproducing organisms?

(b) Why is variation important in a changing environment?

5. How could a favourable mutation spread through different bacterial lineages?