Evolution & Biodiversity

Stabilising selection differs from directional selection in that stabilising selection:

• AFavours intermediate phenotypes and selects against both extremes, reducing variation✓
• BFavours one extreme phenotype, shifting the mean of the population in that direction over time
• CFavours both extreme phenotypes and selects against the intermediate, splitting the population
• DActs on phenotypes that are not heritable and so are not passed on to the next generation

Answer: Stabilising selection favours individuals with phenotypes close to the mean and selects against both extremes, reducing variation and keeping the mean constant. Directional selection, by contrast, favours one extreme and shifts the mean.

Allopatric speciation is best described as the formation of new species when:

• APopulations are separated by a geographical barrier, preventing gene flow✓
• BPopulations remain in the same area but become reproductively isolated by behavioural differences
• CA single individual undergoes a chromosome mutation and self-fertilises
• DTwo existing species hybridise to form a fertile offspring

Answer: Allopatric speciation occurs when a physical/geographical barrier separates populations, stopping gene flow. The isolated populations experience different selection pressures and mutations, diverging until they can no longer interbreed. Sympatric speciation, in contrast, happens without geographical separation.

Genetic drift has a greater effect on allele frequencies in small populations than in large ones because:

• ASmall populations accumulate mutations at a higher rate per generation (a higher mutation load), so new alleles appear faster than drift can remove them
• BChance events have a proportionally larger effect on allele frequencies when fewer individuals are involved✓
• CNatural selection stops operating once a population falls below a few hundred individuals, leaving allele frequencies fixed until numbers recover
• DSmall populations experience a higher rate of immigration, so gene flow rather than chance is what shifts their allele frequencies

Answer: Genetic drift is change in allele frequency due to random chance. In a small population, a chance event (such as which few individuals reproduce) shifts allele frequencies much more, proportionally, than the same event would in a large population.

In the modern taxonomic hierarchy used in A-level classification, which sequence lists the groups from the most inclusive (broadest) to the least inclusive?

• ASpecies, genus, family, order, class, phylum, kingdom, domain
• BKingdom, domain, class, phylum, family, order, species, genus
• CDomain, kingdom, phylum, class, order, family, genus, species✓
• DDomain, phylum, kingdom, class, family, order, genus, species

Answer: From broadest to narrowest the hierarchy is: domain, kingdom, phylum, class, order, family, genus, species. Each level contains the groups below it, with the species being the smallest unit.

A phylogenetic tree based on comparison of DNA and protein sequences primarily aims to show:

• AThe relative abundance (population size) of each species
• BThe geographical distribution of each species
• CThe carrying capacity of each species' habitat
• DThe evolutionary relationships and common ancestry between organisms✓

Answer: Phylogenetic classification groups organisms by evolutionary history. Trees built from molecular data (DNA/RNA/protein sequences) show how closely related organisms are and reveal patterns of common ancestry; more similar sequences imply more recent common ancestors.

An index of diversity (such as Simpson's index) takes into account both the number of species present and the number of individuals in each species. Why is such an index often more useful than simply counting the number of species?

• AIt ignores rare species and weights the dominant ones more heavily, which makes the calculation faster and the result easier to compare between sites
• BIt reflects species richness and the relative abundance (evenness) of species, giving a fuller picture of a community✓
• CIt measures the total biomass of the community directly, in grams per square metre
• DIt always gives a higher value for communities dominated by a single species

Answer: A diversity index combines species richness (how many species) with evenness (how individuals are distributed among species). A community dominated by one species has lower diversity than one with individuals spread evenly, even if both have the same number of species; a simple species count would miss this.

According to the biological species concept, two organisms belong to the same species if they:

• ALook physically very similar and share the same feeding niche within a habitat
• BCan interbreed to produce fertile, viable offspring✓
• CLive in the same geographical area
• DShare over half their DNA base sequence

Answer: The biological species concept defines a species as a group of organisms that can interbreed to produce fertile, viable offspring. A key limitation is that it cannot easily be applied to organisms that reproduce asexually or are only known from fossils.

What is natural selection?

• ABetter-adapted individuals develop new features during their lifetime
• BAll individuals in a population change in the same direction
• CBetter-adapted individuals survive and reproduce more successfully✓
• DIndividuals choose which characteristics to pass to offspring

Answer: Variation already exists in a population; the environment determines which variants leave more offspring, so advantageous alleles become more frequent. Characteristics acquired during life are not inherited, which is the Lamarckian idea offered as a distractor.

What is directional selection?

• AOne extreme phenotype is favoured, shifting the mean✓
• BBoth extreme phenotypes are favoured, splitting the population
• CThe intermediate phenotype is favoured, narrowing the range
• DAll phenotypes are equally favoured, keeping the mean stable

Answer: Directional selection follows a change in conditions and moves the population mean towards one extreme — antibiotic resistance being the standard example. Stabilising selection favours the middle, and disruptive selection favours both extremes.

What is speciation?

• AThe extinction of a species from a particular habitat
• BThe migration of a species into a new geographic area
• CThe formation of a new species from an existing one✓
• DThe increase in numbers of one species in an area

Answer: Speciation occurs when populations become reproductively isolated and diverge until they can no longer interbreed to produce fertile offspring. Geographic separation drives allopatric speciation; behavioural or temporal isolation drives sympatric.

How does allopatric speciation differ from sympatric speciation?

• ASympatric requires geographical separation; allopatric does not
• BAllopatric occurs only in plants; sympatric only in animals
• CAllopatric occurs rapidly; sympatric always occurs slowly
• DAllopatric requires geographical separation; sympatric does not✓

Answer: In allopatric speciation a physical barrier splits the population and the isolated groups diverge under different selection pressures. Sympatric speciation happens within one area through mechanisms such as polyploidy, which is common in plants.

What does an index of diversity take into account that species richness does not?

• AThe total number of different species present
• BThe relative abundance of each species present✓
• CThe geographical area over which sampling occurred
• DThe genetic variation found within each species

Answer: Species richness is a simple count, so a community with one dominant species and several rare ones scores the same as an even one. A diversity index weights by abundance and so reflects community structure much better.

How does genetic diversity within a species help it survive environmental change?

• ASome individuals may already possess advantageous alleles✓
• BAll individuals adapt together to the new conditions
• CIndividuals develop new alleles in response to the change
• DThe species mutates faster when conditions become harsher

Answer: Selection can only act on variation that already exists, so a genetically diverse population is more likely to contain individuals that cope. Low diversity — as in many domesticated or bottlenecked species — is a serious vulnerability.

Why does classification based on DNA give a more reliable phylogeny than one based on appearance?

• AUnrelated species can evolve similar features convergently✓
• BRelated species always look extremely similar to each other
• CDNA sequences do not change at all over evolutionary time
• DAppearance cannot be measured accurately in any organism

Answer: Convergent evolution produces analogous structures — a dolphin's fin and a shark's — in unrelated lineages, which misleads morphological classification. DNA and protein sequences reflect shared ancestry far more directly.

What is the correct order of taxonomic ranks from largest to smallest?

• AKingdom, domain, phylum, class, order, family, genus, species
• BDomain, kingdom, phylum, class, order, family, genus, species✓
• CDomain, kingdom, class, phylum, order, family, genus, species
• DDomain, phylum, kingdom, class, order, family, species, genus

Answer: Domain is the broadest rank, added after rRNA comparison split prokaryotes into Bacteria and Archaea. Each rank nests inside the one above, and the binomial name comes from the last two.

What is a hierarchy in the context of classification?

• AGroups are contained within larger groups which may overlap
• BGroups are ranked by their total number of species
• CGroups are ranked by how recently they evolved
• DGroups are contained within larger groups with no overlap✓

Answer: Each taxon sits wholly inside the next one up and no organism belongs to two groups at the same rank, which is what makes the system unambiguous. That structure is also why a change at a high rank reorganises everything beneath it.