Evolution and Biodiversity Revision Notes | A Level AQA Biology

1. Why So Many Species? The Central Question

There are an estimated 8.7 million species on Earth. From deep-sea anglerfish to alpine edelweiss, life has colonised almost every conceivable environment. The question at the heart of this lesson is deceptively simple: how did one ancestral lineage become millions of distinct species?

The answer lies in a chain of connected processes — genetic variation, natural selection, isolation, and time. Each link in that chain is something you need to be able to explain precisely in an exam. By the end of this lesson, you will be able to trace that chain from a single population all the way to a new species.

2. The Raw Material: Genetic Variation

Evolution cannot act on a population that is genetically identical. Variation is the essential starting point. Within any population, individuals differ in their alleles — the alternative forms of genes — because of mutation, meiosis (independent assortment and crossing over), and random fertilisation.

Mutations are the ultimate source of new alleles. Most are neutral or harmful, but occasionally a mutation produces a phenotype that is better suited to current environmental conditions. That individual is more likely to survive, reproduce, and pass the advantageous allele on — this is the engine of natural selection.

Picture a population of beetles living on bark. A mutation produces a darker pigmentation allele. On dark bark, darker beetles are better camouflaged from predators, so they survive and reproduce at a higher rate. Over many generations, the dark allele becomes more common — allele frequency has shifted. This is directional selection in action.

Genetic variation is therefore the raw material on which selection acts. Without it, a population cannot adapt, and without adaptation, speciation cannot begin. This connects directly to the spec requirement: evolutionary change over a long period of time depends on the continuous generation and selection of variation.

3. Natural Selection: The Mechanism of Change

Natural selection has four conditions, all of which must hold simultaneously. First, there must be variation in a heritable trait within the population. Second, that variation must affect survival or reproductive success — some phenotypes must be better suited to the environment than others. Third, individuals with the advantageous phenotype must survive and reproduce more. Fourth, offspring must inherit the advantageous alleles from their parents.

When these conditions are met, the population changes over time. Alleles that confer an advantage increase in frequency; alleles that reduce fitness decrease in frequency or disappear entirely. Over many generations, the population's phenotype shifts — this is evolutionary change.

• Directional selection: one extreme phenotype is favoured, shifting the mean of the population (e.g. antibiotic resistance in bacteria).
• Stabilising selection: intermediate phenotypes are favoured, reducing variation around the mean (e.g. birth weight in humans).
• Disruptive selection: both extremes are favoured over the intermediate, increasing variation and potentially driving populations apart.

Disruptive selection is particularly relevant to speciation. When two extreme phenotypes are both favoured — perhaps because they exploit different food sources — the population can split into two distinct groups with different adaptations. If those groups also become reproductively isolated, speciation can follow.

4. From Population to Species: Isolating Mechanisms

Natural selection alone does not produce a new species. For two populations to diverge into separate species, they must stop exchanging genes — they must become reproductively isolated. Once gene flow between populations ceases, each population accumulates different mutations and responds to different selection pressures independently. Over time, the genetic differences become so great that the two groups can no longer interbreed successfully, even if they come back into contact. At that point, they are separate species.

Isolating mechanisms fall into two broad categories: those that prevent mating from occurring at all (pre-zygotic barriers) and those that prevent a hybrid zygote from developing into a fertile adult (post-zygotic barriers). Both types reduce or eliminate gene flow between populations.

Notice that pre-zygotic barriers are generally more efficient at preventing gene flow, because no resources are wasted on producing non-viable or sterile offspring. Post-zygotic barriers still reduce gene flow, but at a higher biological cost to both populations.

5. Allopatric and Sympatric Speciation

There are two principal routes by which speciation occurs, and examiners expect you to distinguish between them clearly.

Allopatric speciation begins when a population is physically divided by a geographical barrier — a new river, a rising mountain range, a widening sea channel. The two sub-populations are now geographically isolated, so gene flow between them stops. Each sub-population faces its own local environment with its own selection pressures. Mutations accumulate independently, allele frequencies diverge, and eventually the two groups become so genetically different that they can no longer interbreed — even if the barrier is later removed. The Galapagos finches are a classic example: ancestral finches colonised different islands, each with different food sources, and diverged into 13 distinct species with different beak morphologies.

Sympatric speciation occurs within the same geographical area, without physical separation. This is less common in animals but is well documented in plants through polyploidy — the duplication of entire chromosome sets. If a plant undergoes polyploidy, its chromosome number may double (autopolyploidy) or combine two different species' genomes (allopolyploidy). The resulting polyploid individual cannot interbreed with the original diploid population because the chromosome numbers are incompatible at meiosis, so it is immediately reproductively isolated. New polyploid plant species can arise in a single generation.

Worked example: Tracing the Steps of Allopatric Speciation

A river changes course and divides a population of mice into two groups on opposite banks. Describe and explain the sequence of events that could lead to speciation.

1. The river acts as a geographical barrier, preventing migration and interbreeding between the two sub-populations. — Gene flow between the two groups ceases. This is the critical first step — without stopping gene flow, the populations cannot diverge independently.
2. Random mutations occur independently in each sub-population. Because the populations are isolated, a beneficial mutation arising in population A cannot spread to population B, and vice versa. — Over many generations, each population accumulates a different set of alleles. The genetic composition of the two groups begins to diverge.
3. The two riverbanks may have different vegetation, predators, or climate. Natural selection favours different phenotypes in each environment, accelerating the divergence of allele frequencies. — Directional selection in different directions in each population drives the two groups further apart phenotypically and genotypically.
4. Over thousands of generations, the two populations develop different allele frequencies, different adaptations, and potentially different chromosome structures or numbers. — The populations are now genetically distinct. If they were to come into contact, mating attempts might produce infertile or inviable hybrids — post-zygotic barriers are forming.
5. If the river later dries up and the two groups come into contact, they no longer interbreed successfully. Pre-zygotic barriers (e.g. different courtship behaviours) or post-zygotic barriers (hybrid sterility) prevent gene flow. — By the biological species concept, two groups that cannot interbreed to produce fertile offspring are separate species. Speciation is complete.

Answer: Geographical isolation stops gene flow → independent mutation and selection in each sub-population → allele frequencies diverge → reproductive isolation develops → two distinct species.

6. Time, Divergence, and the Accumulation of Biodiversity

The diversity of life we observe today is the cumulative result of billions of years of evolutionary change. Each speciation event produces two lineages where there was one. Those lineages can each speciate again, and again, branching like a tree. The fossil record shows that life began as simple prokaryotes approximately 3.5 billion years ago, and the diversity of forms has increased — with some mass extinctions interrupting the trend — ever since.

Adaptive radiation is the process by which a single ancestral lineage rapidly diversifies into many species, each adapted to a different ecological niche. The Galapagos finches illustrate this on a small scale; the diversification of mammals after the extinction of non-avian dinosaurs 66 million years ago illustrates it on a grand scale. When ecological opportunities open up — new habitats, reduced competition, novel food sources — natural selection can drive rapid divergence into those niches.

The key insight is that biodiversity is not a static inventory — it is the accumulated product of an ongoing process. Every species alive today is the endpoint of an unbroken chain of evolutionary change stretching back to the origin of life. The diversity we see reflects the number of times populations have become isolated, diverged, and been shaped by selection into distinct forms.

It is worth pausing to consider the timescales involved. A single generation of selection produces a tiny shift in allele frequency. But across thousands of generations — and evolution operates over millions of years — those tiny shifts compound into profound differences. The difference between a bat's wing and a human hand reflects roughly 85 million years of independent evolution from a common mammalian ancestor. Time is the multiplier that converts small selective differences into the spectacular diversity of life.

7. Exam Technique: Writing High-Mark Answers

Questions on evolution and speciation frequently use the command words 'explain', 'describe', 'compare', and 'evaluate'. Each demands a different response structure, and confusing them is one of the most common ways students lose marks.

When a question asks you to 'explain' speciation, the examiner wants a causal chain — not a list of facts. Use connective language: 'because', 'therefore', 'as a result', 'this means that'. A strong explanation links each step to the next: isolation stops gene flow, therefore mutations accumulate independently, therefore allele frequencies diverge, therefore the populations become reproductively isolated.

When asked to 'compare' allopatric and sympatric speciation, you must address both similarities and differences explicitly. A common error is to describe each type separately without ever making a direct comparison. Use phrases such as 'in contrast to allopatric speciation, sympatric speciation...' or 'both types require reproductive isolation, but they differ in that...'.