Scottish Advanced Higher Biology Syllabus

Why can steroid (lipid-soluble) hormones diffuse directly into the cell, and where are their receptors located?

Because they are hydrophobic, steroid hormones pass directly through the phospholipid bilayer. Their receptors are intracellular (in the cytoplasm or nucleus), and the hormone–receptor complex typically acts as a transcription factor regulating gene expression.

What is the difference between hydrophilic and hydrophobic signalling molecules in terms of how they reach their receptors?

Hydrophilic (water-soluble) signals cannot cross the membrane and bind receptors on the cell surface. Hydrophobic (lipid-soluble) signals diffuse through the membrane to bind intracellular receptors.

Define a quadrat and state when it is the appropriate sampling tool.

A quadrat is a frame of known area placed on the ground to sample the abundance of sessile (non-mobile) or slow-moving organisms, such as plants, by counting or estimating percentage cover within it.

Describe the mark–release–recapture (Lincoln index) method and give the formula for estimating population size.

A sample of animals is captured, marked and released; later a second sample is taken and the number of marked individuals recaptured is recorded. Population size is estimated by $$N = \frac{M \times C}{R}$$ where $M$ = number first marked, $C$ = total in second sample, $R$ = marked individuals recaptured.

What is a transect, and when is a belt transect used in preference to a line transect?

A transect is a line across a habitat along which organisms are recorded to study distribution along an environmental gradient. A belt transect (a strip of defined width, e.g. using quadrats) is used when abundance as well as presence/distribution is required.

List three ethical or practical considerations when capturing and sampling animals in the field.

Minimise harm/stress to organisms (use humane traps, handle quickly), avoid affecting the population or habitat (e.g. recapture distortion, marking that increases predation), and obtain any necessary permissions/licences for protected species.

Why may using a point frame or counting along randomly placed quadrats reduce sampling bias?

Random placement (e.g. using coordinates from random numbers) ensures every part of the area has an equal chance of being sampled, preventing the observer from unconsciously selecting unrepresentative or convenient areas.

State the difference between a taxonomic and a phylogenetic approach to identifying organisms, including the use of model organisms.

A taxonomic approach classifies organisms by observable characteristics (often using keys), while a phylogenetic approach groups organisms by evolutionary relatedness using molecular/sequence data. Model organisms are well-studied species used as representatives to infer information about related organisms.

Define natural selection.

Natural selection is the non-random increase in frequency of advantageous (DNA) variation and the decrease in frequency of deleterious variation, driven by differential survival and reproduction of organisms best suited to their environment.

Distinguish between stabilising, directional and disruptive selection.

Stabilising selection favours intermediate phenotypes and reduces variation. Directional selection favours one extreme phenotype, shifting the mean. Disruptive selection favours both extremes against the intermediate, potentially splitting a population.

What is the difference between allopatric and sympatric speciation?

Allopatric speciation occurs when populations are separated by a geographical barrier (geographical isolation). Sympatric speciation occurs without geographical separation, through behavioural, ecological or polyploidy/reproductive isolation within the same area.

Define genetic drift and explain why it has a greater effect on small populations.

Genetic drift is the random change in allele frequencies between generations due to chance. Small populations are more affected because random sampling of alleles causes proportionally larger fluctuations, more readily fixing or losing alleles (e.g. founder effect, bottleneck).

Distinguish between sexual selection by male–male rivalry and female choice.

Male–male rivalry: males compete directly (often via fighting or display) for access to females, favouring traits such as size and weaponry. Female choice: females select mates based on honest signals of fitness (e.g. ornaments), favouring those traits in males.

Explain the evolutionary cost and benefit of sexual reproduction compared with asexual reproduction.

Cost: sexual reproduction is slower, requires a mate, and only half a parent's genes pass to each offspring. Benefit: it generates genetic variation (through independent assortment, crossing over and random fertilisation), helping populations adapt to changing environments and resist parasites (Red Queen hypothesis).

What is the difference between a narrow and a wide hybrid zone, and what does each indicate?

A narrow hybrid zone indicates strong selection against hybrids (low hybrid fitness), keeping the zone thin. A wide hybrid zone indicates weaker selection against hybrids, allowing more gene flow between the parental populations.

Define inbreeding and outbreeding, and state the typical consequence of each.

Inbreeding is mating between closely related individuals, increasing homozygosity and the risk of inbreeding depression (expression of deleterious recessive alleles). Outbreeding is mating between unrelated individuals, increasing heterozygosity and often improving fitness (hybrid vigour/heterosis).

What is the difference between a r-selected and a K-selected reproductive strategy?

r-selected species produce many small offspring with little parental care in unstable environments (rapid population growth). K-selected species produce few large offspring with much parental care in stable environments near carrying capacity.

Define altruism in the context of social behaviour and explain it using kin selection.

Altruism is behaviour that harms the donor's own fitness while benefiting the recipient. Kin selection explains it because helping relatives, who share genes, increases the donor's inclusive fitness, allowing 'altruistic' alleles to be favoured if the benefit to relatives outweighs the cost.

State Hamilton's rule for the spread of an altruistic allele by kin selection.

An altruistic allele spreads when $$rB > C$$ where $r$ is the coefficient of relatedness between donor and recipient, $B$ is the benefit to the recipient, and $C$ is the cost to the donor.

What is the difference between a dominance hierarchy and reciprocal altruism?

A dominance hierarchy is a ranked social order reducing conflict and energy spent fighting, where higher-ranked individuals gain priority access to resources. Reciprocal altruism is the exchange of altruistic acts between (often unrelated) individuals on the expectation of a returned favour later.

Describe the advantages of living in a social group such as cooperative hunting and predator avoidance.

Social groups can hunt cooperatively to capture larger prey, share information, provide collective defence and dilution effect against predators, and rear young communally; costs include increased competition, disease spread and conspicuousness to predators.

In eusocial insects, why do sterile workers help raise the queen's offspring rather than reproduce themselves?

Because of their haplodiploid genetics, female workers share a higher relatedness ($r = 0.75$) with full sisters than they would with their own offspring ($r = 0.5$), so by kin selection they propagate more of their genes by helping the queen raise sisters.

State the steps of the scientific method as a logical process.

Observation, formulation of a testable hypothesis, prediction, designed experiment, collection and analysis of data, drawing conclusions, and either supporting or refuting the hypothesis (leading to revision and further testing).

Distinguish between the independent, dependent and controlled (confounding) variables in an experiment.

The independent variable is deliberately changed by the experimenter; the dependent variable is measured as the outcome; controlled variables are kept constant so they do not confound the results.

What is the difference between a negative control and a positive control in an experiment?

A negative control is a treatment expected to give no effect, confirming the response is due to the independent variable. A positive control is a treatment known to give a definite effect, confirming the experimental system can detect a response.

Explain the difference between a placebo and a double-blind procedure in trials.

A placebo is a dummy treatment with no active agent, used to control for psychological effects. A double-blind procedure ensures neither the subjects nor the experimenters know who receives the treatment or placebo, preventing bias in administration and assessment.

Define accuracy, precision and reliability in the context of measurement.

Accuracy is how close a measurement is to the true value. Precision is how close repeated measurements are to one another (small spread). Reliability is the consistency of results when an experiment is repeated, increased by repetition and adequate replicates.

What is the difference between a systematic error and a random error, and how is each minimised?

A systematic error is a consistent bias (e.g. a mis-calibrated instrument) shifting all readings the same way; it is reduced by calibrating equipment and improving method. A random error causes unpredictable scatter; it is reduced by taking repeated measurements and averaging.

State the difference between the mean, median and mode as measures of central tendency.

The mean is the arithmetic average ($\bar{x} = \frac{\sum x}{n}$). The median is the middle value when data are ordered. The mode is the most frequently occurring value.

What does the standard deviation measure, and what is shown by error bars on a graph?

Standard deviation measures the spread (dispersion) of data about the mean. Error bars (e.g. $\pm$ one standard deviation or standard error) show the variability/uncertainty of each mean; non-overlapping error bars suggest a real difference between groups.

When carrying out a literature review for a dissertation, what is the difference between a primary and a secondary source?

A primary source reports original research/data first-hand (e.g. a peer-reviewed research paper). A secondary source summarises, reviews or interprets primary work (e.g. a review article, textbook or website), and is generally considered less authoritative for original evidence.