A3.1 Diversity of organisms

Life on Earth is staggeringly varied, and A3.1 is about how biologists make sense of that variety: how we define a species, how we name organisms so scientists everywhere mean the same thing, and how we measure diversity. The recurring tension is that nature does not come in neat boxes — species blur at the edges, individuals within a species differ, and our naming systems are human attempts to impose order on a continuum. Understanding both the usefulness and the limits of these classifications is exactly what the syllabus rewards.

Defining a species

The most widely used definition is the biological species concept: a species is a group of organisms that can interbreed and produce fertile offspring. This neatly explains why a horse and a donkey, though they can mate, are different species — their offspring, the mule, is sterile.

However, this definition has real limits, which the syllabus expects you to discuss. It cannot be applied to organisms that reproduce asexually, to extinct species known only from fossils, or to populations that never meet in the wild. Hybrids further complicate matters: some species do occasionally produce fertile hybrids, blurring the boundary. These difficulties show that “species” is a useful but imperfect human category applied to a continuous natural world.

Naming and classifying: binomial nomenclature

To communicate unambiguously, biologists use binomial nomenclature, a two-part Latin name introduced by Linnaeus. The first part is the genus (capitalised) and the second is the species (lower case); the whole name is italicised, for example Homo sapiens. After first use it may be abbreviated, as in H. sapiens.

This system is governed by international rules so that every organism has one universally recognised scientific name, avoiding the confusion of local common names (a single common name may refer to different species in different places). Be precise about the conventions — capitalisation and italics are frequently tested.

Variation within and between species

Individuals of the same species are not identical; they show variation. The syllabus distinguishes two patterns. Continuous variation shows a smooth range of values, such as human height or body mass, and is typically influenced by many genes together with the environment. Discrete (discontinuous) variation falls into distinct categories with no intermediates, such as ABO blood groups, and is usually controlled by one or a few genes.

The syllabus also notes that members of a species can differ in their chromosome numbers and arrangements, and that the number of chromosomes is a characteristic feature of a species rather than a measure of complexity. Differences in chromosomes between populations can contribute to reproductive isolation, linking variation to the formation of new species.

Measuring and assessing biodiversity

To compare habitats or track environmental change, ecologists need to quantify diversity, not just list species. Diversity has two components: species richness (the number of different species present) and evenness (how equally individuals are distributed among those species). A community dominated by one species is less diverse than one where individuals are spread evenly, even if both contain the same number of species.

The syllabus introduces a diversity index, such as Simpson’s reciprocal index, which combines richness and evenness into a single value: a higher value indicates greater diversity. You are expected to appreciate why such an index is more informative than a simple species count and to interpret values when comparing communities, for example to assess the impact of human activity or conservation efforts.