AQA A-Level Biology: Biodiversity and Investigating Variation
A clear revision guide to biodiversity and variation for AQA A-Level Biology: the index of diversity, how farming affects biodiversity, measuring genetic diversity, and types of variation.
Biodiversity measures the variety of life, and this guide covers how it is defined and measured, how farming affects it, and how variation within and between species is investigated. It ties together the ideas of genetic diversity from across Topic 4.
What biodiversity means
Biodiversity is the variety of living organisms, and it can be considered at any scale, from a single pond up to the whole planet. It has three components: genetic diversity (the variety of alleles within a species), species diversity, and ecosystem diversity.
A few related terms are worth getting straight:
- A habitat is the place where an organism lives.
- A population is a group of organisms of the same species in a particular place at a particular time that can interbreed.
- A community is all the populations of different species living in an area.
- Species richness is simply the number of different species in a community.
- Species diversity takes account of both the number of species and the number of individuals of each species.
The index of diversity
Species richness alone can be misleading, because it ignores how many individuals belong to each species. A community dominated by one species is far less diverse than one where individuals are spread evenly, even if both contain the same number of species. The index of diversity captures this by taking both into account.
It is calculated as:
- d = N(N - 1) divided by the sum of n(n - 1)
where N is the total number of organisms of all species, and n is the number of organisms of each individual species. A high index means many species are present and are evenly represented, while a low index means the habitat is dominated by one or a few species. Because it includes the number of individuals in each species, the index of diversity is a more useful measure than species richness on its own.
Farming and biodiversity
Farming tends to reduce biodiversity, because it simplifies an ecosystem to maximise the yield of one crop or animal. Clearing woodland and hedgerows removes habitats, niches and food sources for other species. Monoculture, growing a single crop over a large area, reduces the variety of plants directly. Herbicides remove competing weeds, further cutting plant variety, and pesticides kill insect pests, which also reduces the food available to their predators. The overall effect is fewer plant species, fewer habitats and fewer food sources, so fewer animal species too.
There is a genuine tension here. Conservation measures increase biodiversity, but on a farm they can reduce the land available for crops and lower yields, cutting a farmer's income. To offset this, farmers are often given financial incentives or grants to adopt conservation practices, because those practices make food slightly more expensive to produce. Practical measures include restoring hedgerows and field margins, reducing pesticide use, growing several crops together (intercropping), and using crop rotation with nitrogen-fixing plants instead of relying on fertilisers.
Measuring genetic diversity
Genetic diversity within or between species can be measured in several ways. The oldest is to compare observable characteristics, but this is unreliable, because many features are controlled by several genes and are also influenced by the environment, so differences may not reflect the genes at all.
Modern methods compare the molecules directly: the DNA base sequence, the mRNA base sequence, or the amino acid sequence of a particular protein. The logic is the same as for classification: because mutations accumulate over time, more differences in these sequences mean organisms are more distantly related. Gene technologies made this direct comparison possible, replacing the older reliance on observable features.
Types of variation
Variation is the differences in characteristics between individuals, whether within a species (intraspecific) or between species (interspecific). It is caused by genes (different alleles), by the environment, or, most often, by a combination of both. An organism's observable characteristics, its phenotype, are the result of that combination.
Variation comes in two forms:
| Feature | Continuous variation | Discontinuous variation |
|---|---|---|
| Categories | A range with no distinct categories | Distinct, separate categories |
| Data type | Quantitative | Qualitative |
| Genetic control | Many genes | One or a few genes |
| Environmental influence | Strongly influenced | Little or no influence |
| Example | Height | Blood group |
When comparing variation between two groups, the standard deviation is useful: it measures the spread of values around the mean, so a larger standard deviation means more variation. If the standard deviations of two sets of data overlap a great deal, any difference between their means may simply be due to chance, and a statistical test is used to judge whether a difference is genuinely significant.
Allele frequency and the gene pool
Finally, two terms describe variation at the level of a whole population. A gene pool is the complete set of alleles in an interbreeding population, and the allele frequency is the number of times a particular allele occurs within that gene pool. A change in allele frequency over generations is, as the natural selection guide explains, exactly what evolution is.