OxfordAQA A Level Biology: Diversity of Living Organisms — Revision Notes

Condensed for the final weeks. For the full explanation, use the Diversity of Living Organisms study guide.

Classification

Hierarchy: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.

Each group is contained within the one above and there is no overlap — that is what “hierarchical” means, and it is worth a mark.

Species definition: a group of organisms with similar characteristics that can interbreed to produce fertile offspring. The word fertile is essential — a mule is living proof, being the infertile offspring of two different species.

Binomial naming: Genus species — genus capitalised, species lower case, both italicised or underlined. A universal system avoids the confusion of local common names.

Phylogenetic classification groups organisms by evolutionary relationships and common ancestry, rather than by superficial similarity. Modern evidence comes from DNA base sequences, mRNA sequences and amino acid sequences in proteins — the more similar the sequences, the more recently the species shared a common ancestor. This is more reliable than appearance, because unrelated species can look alike through convergent evolution.

Biodiversity

Level	Meaning
Species richness	The number of different species present
Species diversity	Number of species and the number of individuals in each
Genetic diversity	Variety of alleles within the gene pool of a population

index of diversity   d = N(N-1) / sum[ n(n-1) ]

N = total number of organisms of ALL species
n = number of organisms of EACH species

A higher index means greater diversity – a larger number of species and/or a more even distribution of individuals across them. The specification defines the index only in these terms; the idea that greater diversity makes an ecosystem more stable is a widely discussed hypothesis, not something the specification asserts, so do not present it as an examinable fact.

Farming reduces diversity through monoculture, hedgerow removal, pesticides and herbicides. Conservation measures include maintaining hedgerows, planting field margins, crop rotation, and reducing pesticide use. There is a genuine conflict between food production and conservation, and questions usually want both sides.

Genetic diversity

Caused by mutation, meiosis (crossing over and independent assortment) and random fertilisation.

Measuring genetic diversity: compare the frequency of measurable characteristics, DNA base sequences, mRNA base sequences, or amino acid sequences. Sequence comparison is more accurate than observable characteristics, which are influenced by the environment as well as by genes.

(The genetic bottleneck, founder effect and the natural selection content below are Unit 3 material, not part of Unit 1 — included here as forward references only.)

Natural selection (Unit 3 content, not part of Unit 1)

The sequence that must appear in full:

1. Variation exists within a population, caused by random mutation.
2. There is competition for limited resources; more offspring are produced than can survive.
3. Individuals with advantageous alleles are more likely to survive and reproduce.
4. They pass those alleles to their offspring.
5. Over many generations the frequency of the advantageous allele increases in the population.

Frequency of alleles, over generations is the phrase that scores. Two errors are fatal in mark schemes:

• Saying organisms “adapt” or “try to” change. They do not — the variation exists first, by chance, and selection acts on it.
• Talking about individuals changing rather than allele frequencies shifting in a population.

Type	Effect
Directional	Favours one extreme — allele frequency shifts; seen in changing environments, e.g. antibiotic resistance (a common illustration, not itself named in the specification)
Stabilising	Favours the mean, selects against both extremes; seen in stable environments, e.g. human birth mass

Investigating diversity in the field

The specification’s own sampling content covers only random sampling, the role of chance, and the importance of an appropriate sample size; quadrats and transects are widely used field techniques for applying it, not terms the specification itself names. Random sampling using quadrats placed by random coordinates avoids bias. The sample size must be large enough for reliability, and a mean is taken across all quadrats. Where an environmental gradient exists — for example, moving away from a hedge or up a shoreline — a transect is a commonly used technique instead, because the variation being studied follows that gradient rather than being randomly distributed.

Worked example — index of diversity

A community contains 3 species with 10, 4 and 1 individuals.

N = 15, so N(N - 1) = 15 x 14 = 210

n(n - 1) terms:  10 x 9 = 90
                  4 x 3 = 12
                  1 x 0 = 0
sum = 102

d = 210 / 102 = 2.06

Species richness alone would simply report “3 species” and miss that one species heavily dominates the community — exactly why the index, which weights by relative abundance, is preferred over a raw species count.

Exam traps

• Omitting “fertile” from the species definition.
• Saying organisms adapt in response to their environment.
• Confusing species richness with species diversity.
• Using n for the total instead of N in the diversity index.
• Describing evolution as change in an individual.

Self-test

1. Define a species, and say why one word in that definition is critical.
2. Give the taxonomic hierarchy in order.
3. What does a high index of diversity indicate?
4. Describe two methods used to measure genetic diversity within a species.
5. Why is comparing DNA sequences more reliable than comparing appearance?

Answers: 1. A group of organisms with similar characteristics that can interbreed to produce fertile offspring; without “fertile”, different species such as horse and donkey would be classed as one. 2. Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. 3. Greater diversity – a larger number of species and/or a more even distribution of individuals between them. 4. Compare allele frequencies at specific gene loci between individuals (or the proportion of gene loci that are polymorphic); or compare DNA base sequences or amino acid sequences between individuals. 5. Appearance is influenced by the environment as well as by genes, and unrelated species can resemble each other through convergent evolution.