Measuring biodiversity

Biodiversity can be considered at three levels: the diversity of habitats, the number of species, and the genetic diversity within a species. An endemic species is one found naturally in one geographical area and nowhere else — endemics are especially vulnerable to extinction.

Species richness is just a count of species. An index of diversity is better because it also takes abundance into account: a wood with 100 oaks and 1 birch is far less diverse than one with 50 of each, even though both have a richness of 2.

Your turn — heterozygosity index

Your turn — index of diversity

Niche, adaptation and natural selection

A niche is the role and position a species has in its environment: what it eats, when it is active, what eats it, what it tolerates. If two species occupy the same niche, one will out-compete the other (competitive exclusion) — so coexisting species always differ somewhere.

Adaptations come in three flavours:

• Anatomical — e.g. the marram grass rolled leaf, sunken stomata and thick cuticle that reduce transpiration.
• Physiological — e.g. the kangaroo rat producing extremely concentrated urine.
• Behavioural — e.g. nocturnality to avoid daytime heat.

Natural selection, in order: (1) there is genetic variation caused by mutation; (2) a selection pressure acts; (3) individuals with the advantageous allele are more likely to survive and reproduce; (4) they pass the allele on, so the allele frequency in the population increases. Evolution is that change in allele frequency.

Never write that an organism "adapts to" or "wants to" survive. Individuals do not adapt; populations evolve, because some individuals already happened to carry a useful allele.

The Hardy–Weinberg equation

Hardy–Weinberg lets you calculate allele and genotype frequencies and, crucially, lets you test whether a population is evolving.

You almost always start from q², because the homozygous recessive is the only genotype you can identify from the phenotype alone.

The assumptions matter: a large population, random mating, no migration in or out, no mutation, and no selection. If the observed frequencies differ from the predicted ones, one of those conditions has been broken — the population is evolving. Reproductive isolation lets allele frequencies in two populations diverge, which is the first step towards speciation.

Your turn — Hardy–Weinberg

Testing the assumptions

Classification and the three domains

Taxonomic hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Each group is nested inside the one above, and each species has a binomial name (genus + species), e.g. Homo sapiens.

Woese’s comparison of ribosomal RNA sequences showed that the "bacteria" were really two profoundly different groups, so a level above kingdom was added — the three domains: Bacteria, Archaea and Eukarya.

How science works: the change was not accepted because Woese said so. It was accepted after the data were published, peer reviewed, presented at conferences, criticised, and independently reproduced using other molecular evidence (DNA sequences, protein sequences, immunology). That is the process SNAB wants you to describe.

Plant cell ultrastructure, starch and cellulose

On top of the standard eukaryotic organelles, a plant cell has: a cellulose cell wall with a middle lamella (calcium pectate) gluing adjacent walls together, pits and plasmodesmata (cytoplasmic channels between cells), chloroplasts (grana of thylakoids in a stroma), amyloplasts (starch stores) and a large vacuole with a tonoplast.

• Starch — a polymer of α-glucose. Amylose has 1,4-glycosidic bonds only and coils into a compact helix; amylopectin is branched (1,6 bonds), so it can be hydrolysed quickly. Insoluble, so it does not affect water potential — a perfect store.
• Cellulose — a polymer of β-glucose. Alternate residues are flipped 180°, giving straight, unbranched chains. Many hydrogen bonds cross-link them into microfibrils, which are laid down in different directions to give enormous tensile strength.

Core practical 6/7/8 territory: the tensile strength of plant fibres; the effect of mineral deficiency (nitrate for amino acids and nucleotides, calcium for the middle lamella, magnesium for chlorophyll) on plant growth; and the antimicrobial properties of plant extracts.

Starch or cellulose?

Tap a statement, then tap where it belongs.

Match the measure to its meaning

Tap a definition on the left, then the correct term.

From folklore to clinical trials

Historically, drugs were tested by trial and error — William Withering used a digitalis soup from foxglove, adjusting the dose until it worked without killing the patient. Modern testing is far more rigorous:

• Pre-clinical — the drug is tested on cells, tissues and then animals to check for basic toxicity and to establish a dose.
• Phase I — a small number of healthy volunteers: is it safe?
• Phase II — a larger group of patients: does it work, and at what dose?
• Phase III — a very large, randomised, double-blind, placebo-controlled trial comparing it with the current best treatment.

Double-blind means neither the patient nor the doctor knows who has the drug — that removes both the placebo effect and unconscious bias in reporting outcomes.

Your turn — bacterial growth

Growing microbes and conserving species

Bacterial growth (4.14) requires nutrients (a carbon source, nitrogen, minerals), a suitable temperature and pH, and — for aerobes — oxygen. A culture shows a lag phase (enzymes being made), an exponential (log) phase, a stationary phase (nutrients running out, waste accumulating) and a death phase. Aseptic technique keeps the culture pure. Core practical 9 uses this to test the antimicrobial activity of plant extracts, measuring the zone of inhibition.

Conservation (4.16):

• Zoos — captive breeding, studbooks to maximise genetic diversity, reintroduction, research and education. Criticisms: the small gene pool, behavioural problems in captivity, poor reintroduction success, and cost.
• Seed banks — cheap, store enormous genetic diversity in a tiny space, and seeds stay viable for decades when dried and frozen. Limitations: some species (e.g. many tropical trees) have recalcitrant seeds that cannot be dried; viability must be tested by periodic germination; a seed bank preserves the species but not the habitat.

Judging a conservation strategy

Why is cellulose so strong?

The big ideas to know

You have covered the whole of SNAB Topic 4. Press Finish to see your score.