Pearson Edexcel International A Level Biology: Classification, Biodiversity and Conservation (YBI11)

This guide covers statements 4.14 to 4.21 of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology specification (YBI11), Issue 2. These statements sit in Topic 4, which is part of the International AS (IAS) content, so they belong to the first year of the course and also count towards the full International A Level. The wording below follows the specification; where a statement is split into (i), (ii) and (iii) parts, every part is covered.

Related pages: revision notes, practice questions, the Edexcel A Level Biology hub, the printable checklist and the free diagnostics.

Statements and skills

Statement	Skill
4.14 (i), (ii)	Explain classification, the species concept, and how new molecular evidence leads to new groupings, including three domains
4.15	Know that the variety of life is extensive but threatened by human activity
4.16	Define biodiversity and endemism
4.17	Measure biodiversity with species richness and the heterozygosity index
4.18	Calculate and use the index of diversity D
4.19	Explain niche; discuss behavioural, anatomical and physiological adaptations
4.20 (i)-(iii)	Use the Hardy-Weinberg equation; link allele frequency change to mutation, selection and reproductive isolation
4.21	Evaluate zoos and seed banks in conservation

4.14 Classification

Classification organises the variety of life into groups based on relationships between organisms. Those relationships are inferred from similarities and differences in phenotype (observable features such as structure) and in genotype (DNA base sequences and the proteins they code for). Classification is built around the species concept.

A common working definition of a species is a group whose members breed together and produce fertile offspring. It is useful but imperfect. It cannot be tested on fossils, it does not apply to organisms that reproduce only asexually (many bacteria), and some closely related species can occasionally hybridise. This is why molecular evidence is now used alongside appearance.

New data and the three-domain system (4.14 ii)

Scientists propose a new grouping, publish the evidence, and other scientists examine it. They check the methods, repeat the work and test whether the conclusion follows from the data. This critical evaluation by the scientific community, usually through peer review, is why a new grouping is accepted only when independent results support it.

Comparing ribosomal RNA sequences showed that the prokaryotes were not a single group. The result was the three-domain system: Archaea, Bacteria and Eukarya. Both Archaea and Bacteria are prokaryotic, yet their sequences, cell wall chemistry and membrane lipids differ, and in some molecular features Archaea resemble eukaryotes more than bacteria.

Worked example: reading sequence data. The same 200-base section of a gene is compared in four species. The number of base differences between each pair is shown.

	Species 1	Species 2	Species 3
Species 2	6
Species 3	22	20
Species 4	31	29	25

Convert to a percentage difference: 6/200 × 100 = 3.0%, 22/200 = 11%, 31/200 = 15.5%, 20/200 = 10%, 29/200 = 14.5%, 25/200 = 12.5%.

Species 1 and 2 differ least, so they are the most closely related pair. Species 3 differs from them by 10-11%, so it branched earlier. Species 4 differs most (14.5-15.5%), so it is the most distantly related. Fewer differences suggest a more recent common ancestor, assuming mutations accumulate at a similar rate in each lineage.

4.15 The variety of life and human activity

Over geological time the variety of life has become extensive: new species have arisen faster than others were lost, and there are now millions of species. This variety is now being threatened by human activity. The main pressures are habitat destruction and fragmentation, overexploitation (hunting and harvesting), pollution, introduced invasive species and climate change. You need to know the statement and be able to use these as examples in an answer, not to recall extinction figures.

4.16 Biodiversity and endemism

Biodiversity is the variety of life in a given area. It has three levels:

• species diversity: how many species there are and how evenly individuals are spread among them
• genetic diversity: the variety of alleles within a species
• habitat (ecosystem) diversity: the variety of habitats in an area

Endemism describes a species found naturally only in one particular geographical area and nowhere else. Examples include lemurs in Madagascar and the kakapo in New Zealand. Endemic species are often vulnerable because their whole population is in one small area, so one disturbance can remove the species entirely.

4.17 Measuring biodiversity

Species richness is the number of different species in a habitat. It is simple to find but ignores how many individuals each species has. A field with 50 species where one species makes up 95% of the individuals is less diverse in practice than its richness suggests. Sample using a random method, such as random coordinates for quadrats, and sample enough area to represent the habitat.

Genetic diversity within a species can be measured with the heterozygosity index:

heterozygosity index = number of heterozygotes ÷ number of individuals in the population

Worked example. A wild population of 80 animals is genotyped at one gene. 34 are heterozygous. A captive population of 45 has 9 heterozygotes.

• Wild: 34 ÷ 80 = 0.425
• Captive: 9 ÷ 45 = 0.20

The captive population has less genetic diversity at this gene. Possible reasons are a small number of founders, inbreeding and genetic drift. Low heterozygosity makes a population less able to adapt to new disease or climate change. One gene gives only a limited picture, so a conclusion for the whole genome needs many genes.

4.18 Index of diversity

The index of diversity compares habitats and takes account of both richness and evenness:

D = N(N − 1) ÷ Σn(n − 1)

Here N is the grand total of individuals counted across every species, and n is the count for a single species. A higher D means greater diversity. If every individual belongs to one species, D = 1, its lowest possible value.

Worked example. Two 30-individual samples from woodland A (mixed native trees) and woodland B (a plantation).

Woodland A: n	n(n − 1)	Woodland B: n	n(n − 1)
12	132	20	380
8	56	7	42
5	20	3	6
3	6
2	2
Σ = 216		Σ = 428

For both, N = 30, so N(N − 1) = 30 × 29 = 870.

• D for A = 870 ÷ 216 = 4.03
• D for B = 870 ÷ 428 = 2.03

Woodland A is more diverse: it has five species against three, and its individuals are spread more evenly. Notice that D is lower when a few species dominate.

Evaluating the index. D depends on sample size and on a representative, random sample. It counts individuals but not their size or biomass. It describes the sample taken, so a small sample may not describe the whole habitat. Comparisons are fair only if the same sampling effort is used in each habitat.

4.19 Niche and adaptation

A niche is the way a species fits into its ecosystem: its habitat, its food, its predators, its relationships with other species, its active period and the conditions it tolerates. Two species with an identical niche compete strongly and one is likely to be excluded, so closely related species in the same habitat usually differ in some part of their niche.

Adaptations that fit an organism to its niche are of three kinds:

Type	Meaning	Example
Behavioural	How the organism acts	Desert rodents stay in burrows by day and feed at night, avoiding the heat
Anatomical	Structural features	Marram grass leaves roll up and have sunken stomata, trapping humid air and reducing water loss
Physiological	Internal processes	Kangaroo rats produce very concentrated urine, saving water

Aim to name the adaptation, say which type it is, and explain how it helps survival or reproduction. A bare example without the “how” earns little.

4.20 Hardy-Weinberg, selection and speciation

(i) The Hardy-Weinberg equation

For a gene with two alleles, with frequency p for the dominant allele and q for the recessive allele:

• p + q = 1
• p² + 2pq + q² = 1

p² is the frequency of homozygous dominant, 2pq of heterozygous and q² of homozygous recessive. The equation predicts genotype frequencies in a population that is not changing. It assumes no selection, no mutation and no migration, together with random mating in a large population. If observed frequencies differ from the predicted ones, or the allele frequency changes between generations, then one or more of these conditions is not being met and evolution is occurring.

Worked example 1. In a plant population 9% have white flowers (recessive, aa).

• q² = 0.09, so q = √0.09 = 0.30
• p = 1 − 0.30 = 0.70
• heterozygous (2pq) = 2 × 0.70 × 0.30 = 0.42
• homozygous dominant (p²) = 0.49
• In 2000 plants, 0.42 × 2000 = 840 are heterozygous carriers

Worked example 2: is the allele frequency changing? A population of 500 is genotyped in generation 1 and again in generation 8.

Generation	AA	Aa	aa
1	245	210	45
8	160	240	100

Generation 1: A alleles = 2 × 245 + 210 = 700 of 1000, so p = 0.70 and q = 0.30. Generation 8: A alleles = 2 × 160 + 240 = 560 of 1000, so p = 0.56 and q = 0.44.

The frequency of a has risen from 0.30 to 0.44, so the allele frequency is changing. If generation 1 conditions had continued, aa would be 0.09 × 500 = 45; the observed 100 is far higher.

(ii) Causes of change

Mutation creates new alleles at a very low rate, so by itself it changes allele frequencies slowly. Natural selection changes them when individuals with certain alleles survive and reproduce more than others, so those alleles become more common. Selection acts on phenotypes, and the allele frequency changes in the next generation.

(iii) Reproductive isolation

If two populations of a species are kept apart (by a barrier such as water, by different breeding times or by different courtship behaviour), they no longer exchange alleles. Each population accumulates different mutations and experiences different selection pressures. Over many generations the genetic information diverges until members of the two populations can no longer breed together and have fertile offspring. They are now separate species.

4.21 Zoos and seed banks

To evaluate is to weigh strengths against weaknesses and reach a judgement.

Method	Strengths	Limitations
Captive breeding	Protects species from the threats they face in the wild; breeding can be planned using records to keep heterozygosity high	Small populations lose genetic diversity; animals may lose natural behaviours; expensive
Reintroduction	Restores wild populations when the cause of decline has been removed	Needs suitable habitat and protection; released animals may fail to survive
Scientific research	Gives knowledge of breeding, diet and disease that helps wild populations	Results from captive animals may not apply fully to the wild
Education	Raises public awareness and funds	Effect on behaviour is hard to measure
Seed banks	Store many species and much genetic diversity in a small space cheaply	Some seeds cannot survive drying and freezing; stored seeds must be tested and renewed; the habitat is not conserved

Good evaluations end with a judgement, such as that these methods support in situ conservation rather than replace it.

Common errors

• Writing “a species is a group that can breed” and omitting “fertile offspring”.
• Treating species richness as if it included relative numbers.
• Calculating D with N − 1 inside the sum, or using N² instead of N(N − 1).
• Calling a high D “low diversity”. For this formula, higher D means more diverse.
• Using the heterozygosity index on the number of alleles instead of the number of individuals.
• Taking q as the recessive phenotype frequency, instead of q².
• Saying Hardy-Weinberg proves evolution is occurring: it is a baseline against which change is detected.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, February 2021, Pearson Education Limited: Topic 4, statements 4.14 to 4.21.