Difficulty: Introductory to Intermediate | Prerequisites: Basic understanding of DNA, alleles, and heredity.
Evolution is the central unifying concept in biology. It explains how populations change over time, how new species arise, and why organisms are adapted to their environments. This section covers the four mechanisms that drive evolution (natural selection, genetic drift, gene flow, mutation), how populations split into new species (speciation), and how human activities threaten the process. The exam draws heavily on the leopard frog scenario to test these concepts.
Conservation biology sits at the applied end of this material. If you understand how evolution works, you can understand why habitat fragmentation and climate change are so damaging: they disrupt the very processes that allow species to adapt.
Evolution happens through four mechanisms: natural selection (differential survival and reproduction based on traits), genetic drift (random changes in allele frequency, especially in small populations), gene flow (movement of alleles between populations), and mutation (new genetic variation). New species form when populations become reproductively isolated, either by geography (allopatric) or within the same area (sympatric). Habitat fragmentation and climate change threaten biodiversity by blocking gene flow and reducing populations' ability to adapt.
Natural selection
The process by which individuals with heritable traits that improve survival and reproduction in a given environment leave more offspring, causing those traits to become more common over generations. Think of it as "survival of the fittest" where "fittest" means best-suited to the current environment, not strongest.
Genetic drift
Random changes in allele frequencies in a population, most significant in small populations where chance events have outsized effects. Think of it as evolution by luck rather than by advantage.
Founder effect
A type of genetic drift that occurs when a small group of individuals colonises a new habitat, carrying only a subset of the original population's genetic variation. The new population's allele frequencies may differ substantially from the source population purely by chance.
Gene flow
The transfer of alleles between populations through migration of individuals or gametes. It tends to make populations more genetically similar to each other.
Mutation
A change in the DNA sequence that creates new alleles. Mutations are the ultimate source of all genetic variation but are random with respect to fitness.
Allopatric speciation
Speciation that occurs when populations are geographically separated by a physical barrier (a river, a mountain range, a highway). Over time, the isolated populations diverge genetically and develop reproductive barriers. Think of it as speciation by physical separation.
Sympatric speciation
Speciation that occurs within the same geographic area, without a physical barrier. It can arise through polyploidy (especially in plants) or ecological specialisation. Less common than allopatric speciation in animals.
Reproductive barriers
Mechanisms that prevent two populations from interbreeding, even if they come back into contact. Can be pre-zygotic (before fertilisation: habitat isolation, temporal isolation, behavioural isolation) or post-zygotic (after fertilisation: hybrid inviability, hybrid sterility).
Adaptive radiation
The rapid diversification of a single ancestral lineage into many species, each adapted to a different ecological niche. Requires ecological opportunity: available niches (empty or underused), reduced competition, and heritable variation to exploit those niches.
Habitat fragmentation
The breaking up of continuous habitat into smaller, isolated patches by human activities (roads, agriculture, development). It reduces population connectivity, limits gene flow, increases genetic drift, and makes populations more vulnerable to local extinction.
Biodiversity
The variety of life at all levels: genetic diversity within species, species diversity within communities, and ecosystem diversity across landscapes.
Natural selection is directional: it favours specific traits that increase fitness in a given environment. It requires heritable variation in a trait, differential survival or reproduction linked to that trait, and the trait being passed to offspring. Example from the exam: barnacle shell shape conferring higher reproductive success in the lower intertidal zone.
Genetic drift is random: allele frequencies change by chance, not because certain alleles are better. Its effects are strongest in small populations. Example from the exam: a founding population of only 50 frogs colonising a new pond. With so few individuals, the allele frequencies in the founders may not represent the source population, and random events (which frogs happen to survive and reproduce) can shift allele frequencies substantially.
The founder effect is a specific case of genetic drift: the 50 frogs that founded the pond population carried a random, incomplete sample of the original gene pool.
A new highway isolates a group of leopard frogs from the main pond population.
The two populations can no longer exchange individuals or genes (gene flow is cut off).
Over time, each population independently accumulates genetic changes through natural selection (adapting to different local conditions) and genetic drift.
Eventually, reproductive barriers develop. If the populations meet again, they can no longer interbreed. They are now separate species.
This is allopatric speciation: geographic isolation followed by genetic divergence.
Adaptive radiation occurs when a lineage rapidly diversifies into many species, each filling a different ecological niche.
Conditions required: ecological opportunity (empty niches, reduced competition), sufficient genetic variation, and enough time.
Using the frogs as an example: if frogs colonised a new environment with many unoccupied niches (different food sources, microhabitats), different populations could specialise and eventually diverge into distinct species.
Classic real-world examples: Darwin's finches, Hawaiian honeycreepers, cichlid fishes.
The exam presents a dominant allele for bright red feathers in hummingbirds.
A heterozygous male (Rr) crosses with a heterozygous female (Rr).
Punnett square: RR, Rr, Rr, rr.
Genotype ratio: 1 RR : 2 Rr : 1 rr.
Phenotype ratio: 3 red (RR + Rr + Rr) : 1 not-red (rr) = 3:1.
The 9:3:3:1 ratio applies to dihybrid crosses (two traits), not monohybrid crosses like this one.
The highway scenario is specifically an example of habitat fragmentation: continuous habitat broken into isolated patches.
Consequences: reduced gene flow between populations, smaller effective population sizes (increasing genetic drift), loss of genetic diversity, increased vulnerability to local extinction.
Climate change compounds the problem: shrinking ponds reduce carrying capacity (K) and per capita growth rate (r). Fragmented populations cannot migrate to track shifting conditions.
Conservation strategies from the exam:
Wildlife corridors (underpasses, overpasses) to reconnect fragmented populations and restore gene flow.
Habitat restoration and protection to maintain or expand carrying capacity.
Assisted migration or translocation programmes for populations that cannot move on their own.
R | r | |
|---|---|---|
R | RR | Rr |
r | Rr | rr |
Genotype ratio: 1 RR : 2 Rr : 1 rr
Phenotype ratio (with complete dominance): 3 dominant : 1 recessive = 3:1
Habitat fragmentation is one of the leading causes of biodiversity loss worldwide. Roads, farmland, and urban development isolate populations of everything from amphibians to large mammals. Wildlife corridors (such as highway underpasses for wildlife) are a direct application of gene flow theory: reconnecting populations maintains genetic diversity and evolutionary potential. Conservation genetics uses principles of drift and effective population size to determine minimum viable population sizes for endangered species.
Students often confuse natural selection with genetic drift. Natural selection is non-random and directional (traits that help survival spread). Genetic drift is random and has no direction (allele frequencies change by chance).
Students sometimes think a 3:1 phenotype ratio applies to all crosses. It only applies to a monohybrid cross between two heterozygotes with complete dominance. A dihybrid cross gives 9:3:3:1.
A common error is thinking speciation requires a physical barrier. Sympatric speciation can occur without geographic isolation, though it is less common in animals.
Students may confuse habitat fragmentation with habitat loss. They overlap but are distinct: fragmentation is about breaking habitat into pieces (reducing connectivity), while loss is about the total amount of habitat disappearing. Both are harmful; fragmentation specifically disrupts gene flow.
⚠️ The distinction between natural selection and genetic drift is tested in both multiple choice and long answer. Be able to define each and give an example from the exam scenarios.
⚠️ The frog + highway scenario = allopatric speciation. The highway = habitat fragmentation. Both terms are tested.
⚠️ The Rr × Rr cross gives a 3:1 phenotype ratio, not 9:3:3:1 (which is dihybrid) or 1:2:1 (which is genotype ratio, not phenotype).
⚠️ Long-answer questions may ask you to explain adaptive radiation, including the ecological conditions required. Know the formula: ecological opportunity + genetic variation + time = adaptive radiation.
⚠️ Conservation questions ask for specific strategies. "Build wildlife corridors" and "restore or protect habitat" are strong answers. Be concrete.
True or False: Genetic drift has the strongest effect in large populations. (False, it is strongest in small populations.)
Fill in the blank: Speciation caused by geographic isolation is called ______ speciation. (allopatric)
True or False: A cross between two heterozygotes (Rr × Rr) produces a 9:3:3:1 phenotype ratio. (False, it produces a 3:1 ratio; 9:3:3:1 is for dihybrid crosses.)
Fill in the blank: A highway that splits a frog population into two isolated groups is an example of habitat ______. (fragmentation)
True or False: Mutation is the only mechanism that creates new alleles. (True.)
Q: If a group of leopard frogs were geographically isolated from the main pond by a new highway and eventually developed reproductive barriers, what type of speciation is this?
A: Allopatric speciation. The highway acts as a geographic barrier, preventing gene flow and allowing the two populations to diverge independently.
Q: The highway reducing connectivity between populations is primarily a threat to biodiversity known as what?
A: Habitat fragmentation.
Q: Distinguish between natural selection and genetic drift. How might founding a frog population with only 50 individuals lead to genetic drift?
A: Natural selection is non-random: individuals with traits that improve fitness in their environment survive and reproduce at higher rates, shifting allele frequencies directionally. Genetic drift is random: allele frequencies change by chance alone, with no connection to fitness. With only 50 founders, the frogs carry a small, random sample of the source population's genetic variation (founder effect). In this small group, chance events (which individuals happen to mate, which die young) can shift allele frequencies substantially, even fixing alleles that offer no advantage.
Q: A mutation produces a dominant allele for bright red feathers. A heterozygous male mates with a heterozygous female. What is the expected phenotype ratio?
A: 3:1 (three red : one non-red).
Q: If colonisation of the lower intertidal zone by C. stellatus leads to higher reproductive success for individuals with flatter shells, and this trait is heritable, which evolutionary mechanism is occurring?
A: Natural selection. The trait (flatter shells) confers a fitness advantage (higher reproductive success), it varies among individuals, and it is passed to offspring.
Q: Explain adaptive radiation using the frogs as an example. What ecological conditions are necessary?
A: Adaptive radiation is the rapid diversification of a single lineage into many species, each occupying a different niche. If leopard frogs colonised a new environment with many unoccupied niches (different food sources, microhabitats, breeding sites), natural selection would favour different specialisations in different subpopulations, eventually leading to distinct species. The conditions required are ecological opportunity (available niches with reduced competition), heritable genetic variation, and sufficient time for divergence.
Q: Predict the effect of pond shrinkage on carrying capacity and per capita growth rate. How does habitat fragmentation reduce a species' ability to adapt to climate change?
A: Pond shrinkage reduces the total resources available (food, breeding sites), which lowers carrying capacity (K). As K drops, the population is forced to a smaller size, which reduces per capita growth rate as density-dependent pressures intensify at a lower threshold. Habitat fragmentation isolates populations, preventing gene flow. Small, isolated populations have less genetic diversity and are more vulnerable to drift. With fewer alleles to draw on, they have a reduced capacity to produce adaptive variation in response to changing conditions. They cannot move to track shifting habitats because the fragments are disconnected.
Evolutionary mechanisms connect directly to interspecific competition (natural selection drives character displacement and niche differentiation). Population ecology provides the population sizes and growth rates that determine how strongly drift operates versus selection. Cell biology underpins genetics: DNA replication, mutation, and protein expression are the molecular basis of heritable variation. Conservation biology is the applied synthesis of all these concepts.
natural selection, genetic drift, founder effect, bottleneck effect, gene flow, migration, mutation, allele frequency, allopatric speciation, sympatric speciation, geographic isolation, reproductive isolation, reproductive barriers, pre-zygotic, post-zygotic, adaptive radiation, Mendelian genetics, monohybrid cross, Punnett square, dominant, recessive, phenotype ratio, 3:1 ratio, habitat fragmentation, biodiversity, conservation biology, wildlife corridors, carrying capacity and climate change, effective population size, genetic diversity