AQA A-level Biology: Species, Taxonomy and Investigating Diversity (7402) – Revision Notes

These notes condense AQA A-level Biology (7402) sections 3.4.5, Species and taxonomy, and 3.4.7, Investigating diversity, from the AQA AS and A-level Biology specification for first teaching 2015. For full explanations and worked examples, read the study guide, then try the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. If you are unsure where to start, take a free diagnostic. All data on this page are invented for revision. Topic 4 is AS content, and A-level papers on topics 1 to 4 can assess it.

Key definitions

Term	Meaning
Species	Organisms able to produce fertile offspring together
Courtship behaviour	Actions before mating; a necessary precursor to successful mating and a means of species recognition
Phylogenetic classification	Grouping species by evolutionary origins and relationships, in a hierarchy with no overlap between groups
Taxon (plural taxa)	One group in the hierarchy
Binomial	Genus name plus species name, universal for each species
Random sample	A sample in which every individual or position has an equal chance of selection
Mean	The average of the values
Standard deviation	A measure of how spread out the values are around the mean

Species and courtship

• The test is fertile offspring. Offspring that are sterile do not count.
• The definition cannot be applied to asexual organisms or fossils.
• Courtship is species specific, so a partner who responds correctly is probably the same species.
• Role of courtship: it allows species recognition, so gametes are not wasted on a mating that would give no offspring or sterile ones, and it is a necessary precursor to successful mating.

The hierarchy

The eight ranks run domain, kingdom, phylum, class, order, family, genus, species. Smaller groups sit inside larger groups.

Make up your own memory sentence from the initials D, K, P, C, O, F, G, S.

Rank	Humans	Brown hare
Domain	Eukarya	Eukarya
Kingdom	Animalia	Animalia
Phylum	Chordata	Chordata
Class	Mammalia	Mammalia
Order	Primates	Lagomorpha
Family	Hominidae	Leporidae
Genus	Homo	Lepus
Species	sapiens	europaeus

Humans and the brown hare share taxa down to class and diverge at order. Therefore they are less closely related than two species in one genus.

Binomial rules

1. Genus first, with a capital letter.
2. Species second, in lower case.
3. Italics, or underline each word if handwritten.
4. Example: Lepus europaeus.

Immunology and genome sequencing

• Immunology: an antibody made against a protein from one species binds more strongly to the same protein from a close relative. Greater binding suggests a closer relationship.
• Genome sequencing: gives long base sequences that can be compared directly and counted.
• Both help to clarify evolutionary relationships and can correct a classification based on appearance alone.

Comparing genetic diversity

What is compared	What it shows	Weakness
Frequency of a characteristic	Whether populations differ	Environment can alter it; only some genes are sampled
DNA base sequence	Every difference, including those that do not change the protein	Needs long sequences for strong conclusions
mRNA base sequence	Bases that are actually translated	Shows the coding sequence only
Amino acid sequence	Differences that changed the protein	Misses changes hidden by the degenerate code

Gene technology changed the method: differences were once inferred from observable characteristics and are now read directly from the DNA. You do not need to know the technologies.

Method: percentage difference in sequences

1. Align the two sequences, base by base.
2. Count the positions that differ.
3. Divide by the number of positions compared and multiply by 100.
4. The smaller the percentage, the more closely related the species are likely to be.

Worked reminder. Two species differ at 3 of 30 aligned bases: 3 / 30 x 100 = 10 %.

Method: DNA to mRNA

Complementary base pairing from the template strand, with U in place of T. Template TAC GGA TTT gives mRNA AUG CCU AAA.

Limits of sequence evidence

• A short stretch of one gene is a small sample of the genome, so a conclusion from it is tentative.
• Two pairs can show the same number of differences at different positions, so the count alone is a simple measure.
• Equal numbers of differences do not prove equal time since divergence, but fewer differences suggest a more recent common ancestor.
• Compare the same gene, aligned in the same order, in every organism.

Random sampling and variation

1. Divide the area into a coordinate grid.
2. Use random numbers to choose coordinates.
3. Sample at each point and take enough samples.

Reason: it removes bias from the person sampling, so the sample represents the population.

Standard deviation. Small value = values clustered near the mean. Large value = widely spread. If two means have standard deviations that give overlapping ranges, the difference may be due to chance. Example: mean 25, standard deviation 2, so one standard deviation either side is 23 to 27. You only need to interpret it, not calculate it.

More worked reminders

Frequency of a characteristic. A random sample of 80 plants from site 1 has 30 with a pale flower, and 80 plants from site 2 have 56.

Site 1: 30 / 80 x 100 = 37.5 %
Site 2: 56 / 80 x 100 = 70.0 %
Difference: 70.0 - 37.5 = 32.5 percentage points

The populations may differ genetically. Say “may”, because the environment can also influence the characteristic.

Sequence difference. Two species differ at 6 of 24 aligned bases: 6 / 24 x 100 = 25.0 %. A third species that differs from the first by 4 of 24 (16.7 %) is the closer relative.

Reading mean and standard deviation. Leaf length (mm), same number of leaves per sample:

Sample	Mean	Standard deviation	One standard deviation either side
A	10.2	1.4	8.8 to 11.6
B	11.0	3.9	7.1 to 14.9
C	12.6	0.9	11.7 to 13.5

Sample B is the most variable. The ranges for A and C do not overlap, so their difference is more likely to be real. The range for B overlaps both, so its mean tells you little about how it differs from A or C.

Answering questions on this topic

• Define: give the exact wording, including “fertile”.
• Explain the role of courtship: link the behaviour to species recognition and to a successful mating.
• Suggest relationships from data: quote the numbers or percentages, state which pair is most similar, then state what that means for common ancestry.
• Compare two means: comment on both the means and the spread, and say whether the ranges overlap.
• Describe random sampling: give a way of using chance, such as random number coordinates, and say that you take enough samples.

Must-know distinctions

• Species vs genus: the species name is the second word of the binomial; the genus is the first.
• Taxon vs taxa: singular and plural.
• Mean vs standard deviation: the mean is the middle of the data; the standard deviation is the spread.
• DNA vs protein comparison: a DNA difference may not be a protein difference.
• Random vs systematic: random uses chance; a line of evenly spaced points does not.

Quick self-test

1. Define a species.
2. Give two reasons why courtship matters.
3. List the taxa from domain to species.
4. Correct this name: quercus Robur.
5. Why can a species not belong to two families?
6. Two species differ at 2 of 30 aligned bases. Give the percentage difference.
7. Species X differs from Y at 2 of 30 bases and from Z at 9 of 30. Which is closer to X?
8. Two species have identical protein sequences but different DNA. Explain how.
9. A mean is 25 and the standard deviation is 2. What range lies within one standard deviation?
10. Why choose sampling points at random?

Answers

1. Organisms able to produce fertile offspring.
2. It is a necessary precursor to successful mating, and it allows species recognition.
3. Domain, kingdom, phylum, class, order, family, genus, species.
4. Quercus robur: capital for the genus, lower case for the species, italics.
5. Taxa nest and do not overlap, so each species sits in one family.
6. 2 / 30 x 100 = 6.7 %.
7. Y: 2 of 30 is 6.7 %, whereas 9 of 30 is 30 %.
8. The genetic code is degenerate, so a base change can code for the same amino acid.
9. 23 to 27.
10. To remove bias, so every position has an equal chance of selection.

Where marks are usually lost

• Leaving out “fertile” in the definition of a species.
• Forgetting italics or capital letters in a binomial.
• Describing courtship as “attracting a mate” without linking it to species recognition.
• Saying the hierarchy has overlapping groups.
• Reading a large number of differences as a close relationship.
• Writing that protein similarity proves DNA similarity.
• Saying “random” without saying how chance is used, for example random number coordinates.
• Treating a standard deviation as an error or as the mean.
• Concluding that two means differ without commenting on spread.

Official syllabus

These notes follow sections 3.4.5 and 3.4.7 of the AQA AS and A-level Biology specification (7401 and 7402) for first teaching 2015, published by AQA.