OxfordAQA A-Level Biology: The Diversity of Living Organisms (9610)

This guide covers Unit 1 The Diversity of Living Organisms, the first of five units in OxfordAQA International AS & A-Level Biology (9610), for teaching from September 2016. Units 1-2 form the International AS (50% of the full A-level content, 40% of final marks); Units 3-5 are International A2-only, with Unit 5 the synoptic paper.

Where this fits in 9610

Unit 1 opens the AS-level content alongside Unit 2 (Biological systems and disease), establishing core biological concepts – cell structure, biological molecules, and the classification of living things – that the A2-only Units 3-5 (Populations and genes; Control; and the synoptic paper, which draws on all four content units) build on.

Syllabus coverage

OXFORDAQA INTERNATIONAL AS & A-LEVEL BIOLOGY (9610) — UNIT 1 THE DIVERSITY OF LIVING ORGANISMS

Unit 1 as a whole covers eleven sections, from biological molecules and cell structure through to biodiversity (3.1.1-3.1.11). This guide covers the classification and ecology strand of Unit 1: species and taxonomy (3.1.10), biodiversity within a community and genetic diversity as measured by base, mRNA and amino acid sequences (3.1.11), and the field-sampling practical skills that go with them. Natural selection, directional and stabilising selection and the genetic bottleneck are not Unit 1 content – the specification places them in Unit 3 (Investigating cells, and Populations and genetics), which is International A2-only and is covered in a companion guide for that unit. Biological molecules, cell structure, enzymes, transport into and out of cells, gas exchange, DNA and genes, protein synthesis and meiosis (3.1.1-3.1.5 and 3.1.7-3.1.9) are covered in the companion guides for those sections.

How to approach it

Classification and biodiversity content rewards precise use of taxonomic vocabulary (species, genus, kingdom) and comfort calculating simple biodiversity indices, both of which are easy to under-revise. Field-sampling questions often ask you to justify why a technique avoids bias, so practise explaining the reasoning behind random quadrat placement and transects, not just describing the methods. (Natural selection questions, marked strictly on causal order – variation first, by chance, then selection acting on it – belong to Unit 3 and are covered in that unit’s guide.)

Official syllabus

OxfordAQA International AS & A-Level Biology (9610) specification PDF, Version 5.1 — oxfordaqa.com.

Classification and phylogeny

Organisms are classified into a hierarchy — domain, kingdom, phylum, class, order, family, genus, species — with no overlap between groups at the same rank. The binomial system names each species by genus and species, italicised, with the genus capitalised.

A species is a group of organisms able to breed to produce fertile offspring. The qualifier “fertile” is what excludes the mule.

Modern classification is phylogenetic: it reflects evolutionary relationships and common ancestry rather than superficial resemblance. Evidence comes from comparing DNA base sequences, mRNA sequences and amino acid sequences in proteins, and immunological comparison of proteins. The more similar the sequences, the more recent the common ancestor.

Biodiversity and how it is measured

Biodiversity can be considered at three levels: within a habitat (species diversity), within a species (genetic diversity), and across habitats.

Species richness counts the number of species present. It ignores abundance, so an index of diversity is more informative:

d = N(N - 1) / sum of n(n - 1)

N = total number of organisms of all species
n = total number of organisms of each species

A higher value indicates greater diversity. A community dominated by one species has low diversity even if species richness is high.

Genetic diversity within a species can be measured as the proportion of gene loci that are polymorphic, or by comparing base or amino acid sequences. It arises from mutation, from meiosis (crossing over and independent assortment), and from random fertilisation. (The genetic bottleneck and founder effect, and the natural-selection content below, belong to Unit 3 and are included here only as forward references – they are not assessed as part of Unit 1.)

Adaptation and natural selection (Unit 3 content, not part of Unit 1)

Adaptations may be anatomical, physiological or behavioural. Natural selection follows a fixed logic that examiners expect in sequence: variation exists within a population and arises from random mutation; more offspring are produced than can survive; individuals with advantageous alleles are more likely to survive and reproduce; those alleles are passed on; over many generations their frequency in the population increases.

The phrase “the organism adapted in order to survive” reverses the causation and is heavily penalised — variation arises first, by chance, and selection acts on it afterwards.

Selection can act in different ways depending on the environment. Directional selection favours one extreme of a trait, shifting the population’s allele frequencies in that direction — a commonly used illustration (not itself named in the specification, which frames selection in terms of predation, disease and competition producing differential survival and reproduction) is antibiotic resistance in bacteria under repeated antibiotic exposure, since only resistant individuals survive to reproduce as the environment changes. Stabilising selection instead favours the mean and selects against both extremes, keeping allele frequencies steady in a stable, unchanging environment — human birth mass is a commonly used illustration, since both very low and very high birth masses carry increased risk.

Agricultural practices and deforestation reduce biodiversity by removing habitats and by favouring monoculture; conservation techniques attempt to balance this against the need to feed a growing population.

Investigating diversity in the field

This specification’s own sampling content is limited to random sampling, the role of chance, and the importance of an appropriate sample size; quadrats and transects are widely used field techniques for applying that content but are not themselves named in the specification. Random sampling using quadrats placed by random coordinates avoids bias. Sample size must be large enough for reliability, and a mean is taken. Where an environmental gradient exists, a transect is a commonly used technique instead, because the variation being studied is not random.

Worked example

A community contains 3 species with 10, 4 and 1 individuals. Calculate the index of diversity.

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 report “3 species” and miss that one species dominates.

Common mistakes

Defining a species without the word fertile. Writing binomial names without italics or without capitalising the genus. Saying organisms “adapt to their environment” rather than describing selection acting on existing variation. Treating species richness as though it were diversity. Forgetting that random sampling requires random coordinates, not simply “throwing the quadrat”.

Quick revision checklist

• List the taxonomic hierarchy in order and apply the binomial system correctly.
• Define species, and explain what makes classification phylogenetic.
• Describe the molecular evidence used to establish relationships.
• Distinguish species richness from an index of diversity and calculate d.
• Design a valid random sampling investigation and know when a transect is used instead.