Comparing genetic diversity

Genetic diversity is the variation in genetic material among organisms. It can be investigated within a species or between species by comparing observable characteristics or biological sequences.

Comparing characteristics

The frequencies of measurable or observable characteristics can be compared between groups of organisms. A difference in how often a characteristic occurs may suggest a genetic difference between the groups.

Characteristics provide indirect evidence of genetic differences because the environment can also affect them. A difference in a characteristic therefore does not, by itself, show that the organisms have different DNA sequences.

Comparing sequences

Genetic diversity can also be investigated by comparing corresponding DNA base sequences, mRNA base sequences, or amino acid sequences in the proteins they encode. Differences between corresponding positions show variation at the level being compared.

A direct comparison of DNA base sequences identifies differences in the genetic material itself. Gene technology has changed investigations of genetic diversity: instead of relying on characteristics to infer DNA differences, scientists can now compare DNA sequences directly.

Comparisons at different levels may reveal different amounts of variation. A difference in a DNA base sequence can be reflected in the corresponding mRNA sequence, but it does not always change the amino acid sequence of the protein. This is because different base triplets can specify the same amino acid. Two organisms may therefore have different DNA sequences but the same amino acid sequence for a protein.

Sequence similarity and relationships

DNA and protein sequence data can suggest how organisms within a species, or organisms from different species, are related. In general, organisms with more similar corresponding sequences are inferred to be more closely related and to share a more recent common ancestor than organisms with more sequence differences.

This is an inference from the evidence: sequence similarities and differences help scientists propose relationships, rather than directly showing the ancestors themselves.

Random sampling of biological variation

Individuals of the same species can differ in measurable characteristics. To investigate this variation, researchers measure a sample: a selection of individuals from the larger population they want to study. For example, they might measure the leaf length of a sample of plants of the same species.

In a random sample, eligible individuals have an equal chance of being selected. Researchers do not deliberately favour individuals with particular characteristics, such as especially long or short leaves.

Random sampling reduces selection bias: a tendency for the way individuals are chosen to favour some characteristics over others. This helps the sample represent the population. However, a random sample may still differ from the population by chance; for example, it might happen to contain more plants with long leaves.

The measurements collected from the selected individuals provide the data used to summarise variation in the characteristic.

Mean and standard deviation in biological variation

Individuals of the same species can differ in measurable characteristics, such as leaf length. Two useful ways to describe a set of measurements are its mean, which summarises the central value, and its standard deviation, which describes how spread out the measurements are around the mean.

Mean: the central value

To calculate the mean, add all the measurements and divide the total by the number of measurements:

For example, if three leaves have lengths of $$9$$, $$10$$ and $$11\ \text{cm}$$, their mean length is $$30 \div 3 = 10\ \text{cm}$$.

Standard deviation: spread around the mean

The standard deviation describes how widely the measurements are spread around their mean. A small standard deviation means that values are more tightly clustered around the mean; a large standard deviation means that they are more spread out.

Consider a second group of leaves with lengths of $$5$$, $$10$$ and $$15\ \text{cm}$$. Its mean is also $$10\ \text{cm}$$, but its measurements are more spread out than those in the first group. It therefore has a larger standard deviation.

When comparing variation within a species, comparing means shows whether groups differ in their central values. Comparing standard deviations shows whether they differ in spread. Both are needed for a fuller account: groups can have the same mean even when individuals in one group differ much more from one another.