IB Biology: Genetics and Evolution HL

Understanding genetics and evolution at the Higher Level is not just about memorizing terms; it's about grasping the fundamental processes that generate Earth's breathtaking biodiversity. This knowledge allows you to explain why antibiotic resistance emerges, how new crop species are developed, and what our DNA reveals about our shared ancestry with all life on the planet. It synthesizes molecular biology with population dynamics to form the cornerstone of modern biological science.

The Gene Pool: A Population's Genetic Blueprint

A gene pool is the complete set of all alleles present in an interbreeding population at a given time. Think of a population as an island and the gene pool as the total collection of all genetic "tools" available on that island. Within this pool, allele frequency is the critical metric—it represents how common a specific allele is within the population, expressed as a proportion or percentage. For example, if you are studying flower color in a population of 100 pea plants, and the allele for purple flowers (P) appears 120 times (remember, each diploid plant has two alleles), its frequency is 120 / (100 plants * 2 alleles) = 0.6 or 60%. Evolution, at its most basic genetic level, is defined as a change in these allele frequencies in a population's gene pool over successive generations. This shift indicates that the genetic makeup of the population is not static but is dynamically responding to various pressures.

The Hardy-Weinberg Principle: A Mathematical Model of Genetic Equilibrium

The Hardy-Weinberg principle provides a mathematical null model to detect when evolution is occurring. It states that allele and genotype frequencies in a population will remain constant from generation to generation if specific conditions are met. These conditions are: a very large population size, random mating, no mutations, no migration (gene flow), and no natural selection. This principle allows us to calculate expected genetic variation. The equations are: $$p+q=1$$ $$p^2+2pq+q^2=1$$ where p is the frequency of the dominant allele, q is the frequency of the recessive allele, $p^2$ is the frequency of homozygous dominant genotypes, $2pq$ is the frequency of heterozygous genotypes, and $q^2$ is the frequency of homozygous recessive genotypes.

Worked Example: In a population, 16% of individuals show a recessive trait ($q^2=0.16$). Therefore, $q=\sqrt{0.16}=0.4$. Since $p+q=1$, $p=1-0.4=0.6$. You can now calculate the expected frequency of heterozygotes: $2pq=2\ast 0.6\ast 0.4=0.48$. This means 48% of the population is expected to be heterozygous carriers. If real data deviates significantly from these expectations, you can infer that one or more of the Hardy-Weinberg conditions are being violated—meaning evolution is in action.

Mechanisms of Evolution: Disrupting Hardy-Weinberg Equilibrium

Evolution occurs through mechanisms that disrupt genetic equilibrium. Natural selection is the non-random increase in frequency of alleles that enhance survival and reproduction. Directional selection favors one extreme phenotype, shifting the population's mean trait value (e.g., larger beak size in drought conditions). Stabilising selection favors intermediate phenotypes and reduces variation, acting against extremes (e.g., human birth weight). Gene flow is the movement of alleles between populations via migration, which can introduce new alleles or change frequencies. Genetic drift is the change in allele frequencies due to chance events, especially powerful in small populations (like founder effects or bottlenecks). Mutation is the original source of all new genetic variation, though it acts slowly at the population level. Non-random mating, such as assortative mating, changes genotype frequencies but not allele frequencies directly.

Speciation: The Origin of Species

Speciation is the process by which new, distinct species evolve. It requires reproductive isolation—barriers that prevent members of different species from interbreeding. Allopatric speciation occurs when a physical geographic barrier (like a mountain range or river) splits a population, leading to genetic divergence in isolation. The classic example is Darwin's finches on different Galápagos Islands. Sympatric speciation occurs without geographic separation, often through ecological or behavioral isolation within the same habitat. A major mechanism in plants is polyploidy, where an organism has more than two complete sets of chromosomes. This can happen instantly (e.g., through meiotic failure) and creates a new plant that is reproductively isolated from its diploid parents, as chromosome pairing during meiosis is impossible.

Molecular Evidence for Evolutionary Relationships

Modern evidence for evolution extends far beyond fossils to the molecular level. Comparing DNA and protein sequences allows us to quantify relatedness. The principle is simple: the more similar the sequences, the more recently two species shared a common ancestor. For example, the protein cytochrome c, essential for cellular respiration, is found across diverse eukaryotes. The amino acid sequence differences in cytochrome c between species correlate strongly with established evolutionary distances; humans and chimpanzees have identical sequences, while humans and yeast show many differences. DNA sequence comparisons, such as analyzing mutations in mitochondrial DNA or highly conserved genes, provide powerful, objective data for constructing accurate phylogenetic trees. This molecular clock helps estimate the timing of evolutionary divergences.

Common Pitfalls

1. Misapplying Hardy-Weinberg: A common error is using the frequency of the recessive phenotype as $q$. Remember, it is $q^2$. Always start by identifying $q^2$ from the given data (the proportion of homozygous recessive individuals), then calculate $q=\sqrt{q^2}$. Another trap is assuming a population is in equilibrium; the principle is a tool to test for evolution, not an assumption that populations always obey it.
2. Confusing Speciation Mechanisms: Students often conflate sympatric and allopatric speciation. The key is geography: if a physical barrier is involved, it is allopatric. If new species arise in the same geographic area (e.g., through polyploidy, host-plant switching in insects, or sexual selection), it is sympatric. Memorize clear, specific examples for each.
3. Overlooking the Source of Variation: When explaining evolution, it's crucial to identify the correct mechanism. Remember that mutation creates the raw genetic variation upon which natural selection, genetic drift, and gene flow act. Do not state that natural selection creates new alleles; it only changes the frequency of existing ones that arose from mutation.
4. Misinterpreting Molecular Evidence: Do not conclude that two species with similar traits are always closely related—this could be convergent evolution. True evolutionary relatedness is demonstrated by homology (shared ancestry), which is best confirmed by molecular similarities in DNA or protein sequences, not just analogous (similar function) structures.

Summary

• Evolution is a genetic change in populations, defined as a shift in allele frequencies within a gene pool over time.
• The Hardy-Weinberg principle ($p^2+2pq+q^2=1$) provides a mathematical model to test for evolutionary change by comparing observed and expected genotype frequencies.
• The five main mechanisms of evolution—natural selection (including directional and stabilising), genetic drift, gene flow, mutation, and non-random mating—disrupt Hardy-Weinberg equilibrium.
• Speciation requires reproductive isolation and can occur with geographic separation (allopatric) or without it (sympatric), with polyploidy being a major sympatric mechanism in plants.
• Molecular evidence, from comparisons of DNA and protein sequences like cytochrome c, provides quantitative, objective data for constructing phylogenetic trees and confirming evolutionary relationships.