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
• CFavours both extreme phenotypes and selects against the intermediate
• DActs only on phenotypes that are not heritable

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 always have higher mutation rates
• BChance events have a proportionally larger effect on allele frequencies when fewer individuals are involved✓
• CNatural selection cannot act in small populations
• DGene flow is always greater in small populations

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 of each species in an ecosystem
• 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, which makes calculation faster
• 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
• 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 to one another
• BCan interbreed to produce fertile, viable offspring✓
• CLive in the same geographical area
• DShare more than half of 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.