Measuring biodiversity: species richness and heterozygosity index

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Measuring biodiversity: species richness and heterozygosity indexSpec 4.17

In short

Species richness is the number of different species in a habitat, measured by sampling. Genetic diversity within a species is measured with the heterozygosity index, which is the number of heterozygotes divided by the number of individuals in the population. A higher value means more genetic diversity and a population better able to adapt.

Species richness

Species richness is the number of different species in a habitat. It is found by sampling (for example with quadrats, transects or pitfall traps), identifying each species and counting how many species there are. It is simple, but it takes no account of how many individuals there are in each species. A habitat with 10 species of which 99% of individuals are one species has the same richness as one where the 10 species are equally common.

Genetic diversity: heterozygosity index

Genetic diversity is the variety of alleles within a species. One measure is the heterozygosity index, which uses the proportion of individuals in a population that are heterozygous for a gene.

heterozygosity index = number of heterozygotes ÷ number of individuals in the population

The value is between 0 and 1. A value near 1 means many individuals are heterozygous and the population has high genetic diversity. A small isolated population, in which alleles are lost and relatives interbreed, has a low index.

Calculating a heterozygosity index

A sample of 80 beetles was genotyped for one gene. 30 beetles were AA, 36 were Aa and 14 were aa. Calculate the heterozygosity index.

Heterozygotes (Aa) = 36
Individuals in the population sample = 80
36 ÷ 80

Answer: 0.45

Maths skill:

Heterozygotes are individuals with two different alleles (Aa). Do not include homozygous individuals (AA or aa) in the top line.

Stacked bar chart of genotype frequencies for a large population with heterozygosity index 0.60 and a small isolated population with index 0.15. — Illustrative data: the small isolated population has a much lower heterozygosity index, so less genetic diversity.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the three-domain system of classification?

The three-domain system places all organisms into Archaea, Bacteria or Eukarya. It is based on molecular evidence, such as differences in ribosomal RNA, membrane lipids and DNA-associated proteins, which showed that archaea differ from bacteria as well as from eukaryotes. It replaced the earlier grouping of all prokaryotes together.

What is the difference between biodiversity and endemism?

Biodiversity is the variety of living organisms in an area, including species, genetic and habitat diversity. Endemism is when a species is found in one geographical region and nowhere else, such as lemurs in Madagascar. Endemic species are vulnerable because of their restricted range.

How is genetic diversity measured?

Genetic diversity within a species can be measured with the heterozygosity index: the number of heterozygous individuals divided by the total number of individuals in the population. The value ranges between 0 and 1, and a higher value means greater genetic diversity.

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