Unit 7: Natural Selection

Evolution as a Change in Populations (Not Individuals)

Evolution is best defined as a change in the genetic makeup of a population over time, specifically a change in allele frequencies in the population’s gene pool. This framing matters because everyday language (for example, “bacteria evolved to resist antibiotics”) can sound like individuals choose to adapt. In biology, individuals are born with a genotype and generally keep it for life; populations evolve across generations when some heritable traits become more common.

A population is a group of interbreeding organisms of the same species living in the same area. A population’s gene pool is the total collection of alleles present across all individuals. When the relative frequencies of alleles in that pool change from one generation to the next, evolution has occurred.

Why individuals don’t “evolve”

An individual can’t evolve because evolution requires changes in allele frequencies across generations. What individuals can do is acclimate (a short-term physiological adjustment, like producing more red blood cells at high altitude) or develop (changing phenotype as they grow). These changes typically do not alter the DNA passed to offspring. Natural selection acts on individuals’ phenotypes (what they express), but evolution is measured as genetic change in the population.

Genetic variability (the foundation of evolution)

The differences among individuals in a population are called genetic variability (genetic variation). Natural selection can only occur if some individuals, by chance, already have heritable variants that increase evolutionary fitness in that environment. The more genetic variation a population has, the more likely it is that some variant will be “the lifesaver” when conditions change.

Where heritable variation comes from

Natural selection cannot create useful traits on demand; it sorts existing heritable variation. Variation must exist first.

Key sources of genetic variation include:

• Mutation: random changes in DNA sequence that create new alleles. Most mutations are neutral or harmful in a given environment; occasionally one is beneficial. Mutations are treated as random with respect to need (the environment does not “cause helpful mutations”).
• Recombination in sexual reproduction: produces new combinations of existing alleles.
  • Crossing over in meiosis (prophase I)
  • Independent assortment of homologous chromosomes
  • Random fertilization

A related idea is that as long as a mutation does not prevent an organism from surviving to reproduce, it may be passed on to the next generation.

Phenotype, genotype, and the environment

A genotype is an organism’s allele combination; a phenotype is the observable trait. Many traits depend on both genes and environment: genes provide potential, and the environment influences expression. AP Biology emphasizes that selection acts on phenotypes, but only heritable (genetically influenced) phenotypic differences can cause evolutionary change.

Example: heritable vs. non-heritable differences

Imagine a population of plants where taller plants make more seeds because they get more sunlight.

If height differences are mostly genetic, allele frequencies associated with tallness can increase. If height differences are mostly due to soil quality (environment), taller plants might still make more seeds, but the population won’t necessarily evolve because offspring won’t inherit the “good soil.”

Exam Focus

• Typical question patterns:
  • Distinguish “evolution in a population” from “changes in an individual.”
  • Identify which traits can respond to natural selection (must be heritable).
  • Explain how mutations and recombination generate variation.
• Common mistakes:
  • Saying organisms “mutate because they need to.” Instead, mutations arise randomly; selection changes frequencies.
  • Treating acclimation as evolution.
  • Forgetting that selection acts on phenotype but evolution is measured genetically.

Natural Selection: Darwin’s Logic, Mechanism, and Patterns

Natural selection is a mechanism of evolution in which individuals with certain heritable traits survive and reproduce more successfully in a particular environment. Over many generations, this causes alleles associated with those traits to increase in frequency. Natural selection is defined and measured at the population level (allele frequencies), but it occurs through differences among individuals in survival and reproduction.

Much of modern evolutionary theory traces back to Charles Darwin’s explanation for how adaptive traits become common without intentional design.

Darwin’s key observations

Darwin’s reasoning can be summarized as a set of observations and inferences:

• Each species produces more offspring than can survive.
• These offspring compete with one another for limited resources.
• Organisms in every population vary.
• Individuals with favorable heritable traits tend to survive and leave more offspring, so those traits are more likely to be passed to subsequent generations.

The four conditions for natural selection

A reliable way to analyze selection scenarios is to check four conditions:

1. Variation: individuals differ in a trait.
2. Heritability: some of that variation is genetic and can be passed to offspring.
3. Overproduction/competition: more offspring are produced than can survive; resources are limited.
4. Differential reproductive success: individuals with certain traits leave more viable, fertile offspring.

If any condition is missing, natural selection (as an evolutionary mechanism) cannot operate.

Fitness (evolutionary fitness)

Evolutionary fitness is measured by reproductive success, meaning how many viable, fertile offspring an organism contributes to the next generation (often relative to others in the same population). Fitness is environment-dependent and can change when biotic and abiotic factors change. Different genetic variations can be selected for in different generations.

Importantly, “fitness” is not just strength or longevity. A trait can reduce survival yet still be favored if it increases reproduction (for example, showy mating traits).

Adaptations are population-level outcomes

An adaptation is a heritable trait that increases fitness in a specific environment. You infer an adaptation by showing it has a genetic basis, increases survival or reproduction under those conditions, and becomes more common over generations. Not every trait is an adaptation; some are neutral, historical leftovers, or side effects of other selected traits.

Lamarck and acquired traits (historical contrast)

In Darwin’s day, Jean-Baptiste de Lamarck proposed that acquired traits (changes gained during an organism’s lifetime) were inherited and passed to offspring. Modern genetics does not support this as a general mechanism of evolution: most acquired changes do not alter the heritable DNA passed on, which is why AP Biology focuses on heritable variation and differential reproductive success.

Types of selection on a trait distribution

AP Biology frequently tests recognition and interpretation of selection patterns on trait-distribution graphs.

Directional selection

Directional selection favors one extreme phenotype, shifting the population mean.

• Classic example: peppered moth coloration during industrial pollution.
• Another example idea: increased beak depth during drought if harder seeds dominate.

Stabilizing selection

Stabilizing selection favors intermediate phenotypes and reduces variation.

• Example idea: human birth weight (historically, very small or very large babies had lower survival).

Disruptive selection

Disruptive selection favors both extremes over intermediates, potentially increasing variation and sometimes contributing to speciation.

• Example idea: birds with either small or large beaks succeed if food is mostly small seeds and large seeds, but medium seeds are rare.

Disruptive selection does not automatically produce speciation; speciation also requires reduced gene flow and the evolution of reproductive isolation.

Sexual selection (a special case)

Sexual selection favors traits that improve mating success.

• Intrasexual selection: competition within one sex (often male-male competition).
• Intersexual selection: mate choice (often female choice).

A classic description is females choosing to mate with males that have a large, beautiful tail. Such traits can seem maladaptive for survival, but persist if they substantially increase reproductive success.

Worked selection examples

Example 1: color variation under predation

Suppose insects vary from light to dark, and birds more easily spot light insects on dark tree bark. A strong explanation explicitly links environmental pressure to differential reproductive success and then to allele-frequency change:

• Variation: color differs.
• Heritability: color has a genetic component.
• Competition/predation: birds remove more visible insects.
• Differential reproduction: darker insects survive more and reproduce more.
  Over time, alleles for darker coloration increase.

Example 2: the peppered moths (data-driven narrative)

In a population of peppered moths, exactly half were dark and carried alleles for dark coloring, and half were light and carried alleles for light coloring (a 1:1 phenotype ratio). This ratio persisted until air pollution (primarily from burning coal) changed the environment. As tree surfaces darkened, light moths became easier to spot and were removed at higher rates by predators, while dark moths survived and reproduced. Over time, the gene pool reached about 90 percent dark alleles and 10 percent light alleles. This is an example of directional selection.

Exam Focus

• Typical question patterns:
  • Given data (survival rates, offspring counts), explain how natural selection changes allele frequencies.
  • Interpret trait distribution graphs and identify selection type (directional, stabilizing, disruptive).
  • Distinguish survival advantage from reproductive advantage (fitness) and connect fitness to specific biotic/abiotic pressures.
  • Explain why Lamarck’s acquired-traits idea does not produce population-level allele-frequency change.
• Common mistakes:
  • Describing selection as organisms “trying” to adapt.
  • Ignoring heritability (selection on a non-heritable trait won’t cause evolution).
  • Confusing “most common trait” with “most fit trait” (fitness depends on environment).
  • Overstating disruptive selection as “always” causing speciation.

Artificial Selection and Human-Driven Evolutionary Change

Artificial selection occurs when humans intentionally choose which individuals reproduce to increase the frequency of desired traits. It is evolution because allele frequencies change across generations, but the selecting agent is human preference rather than the natural environment.

How artificial selection works

The mechanism parallels natural selection: there is heritable variation, some individuals contribute disproportionately to the next generation (because humans breed them), and alleles associated with desired traits become more common. Artificial selection often reduces genetic diversity, especially when breeders use a narrow subset of individuals repeatedly.

Examples and what they demonstrate

Dog breeds show how substantial differences in body shape and behavior can arise by selecting variants already present (plus new mutations over time) without necessarily creating new species. Crop domestication (for example, selecting for larger fruits/seeds, reduced bitterness, or non-shattering seed heads) demonstrates how quickly selection on heritable traits can reshape plant populations.

A key trade-off is that rapid phenotypic change can come with costs: inbreeding, genetic disease, and vulnerability to new pathogens.

Human impacts that aren’t artificial selection (but still cause evolution)

Not all human-caused evolution is artificial selection. Antibiotic resistance evolves by natural selection even though humans created the antibiotic environment; humans are not choosing which bacteria reproduce, the antibiotic is the selective pressure. Pesticide resistance evolves similarly.

Example: selective breeding vs. resistance

• Selective breeding: a farmer chooses cows with the highest milk yield to breed.
• Natural selection in a human-altered environment: a population of insects is sprayed with pesticide; resistant insects survive and reproduce.

Both change allele frequencies, but only the first is artificial selection.

Exam Focus

• Typical question patterns:
  • Compare natural selection and artificial selection (same logic, different selecting agent).
  • Predict consequences of selective breeding on genetic variation.
  • Identify whether a scenario is artificial selection or natural selection caused by humans.
• Common mistakes:
  • Labeling antibiotic resistance as artificial selection.
  • Forgetting that selection requires heritable variation.
  • Assuming artificially selected traits always increase fitness in nature.

Population Genetics: Measuring Evolution with Allele and Genotype Frequencies

Population genetics connects Mendelian genetics to evolution by tracking how allele frequencies change over time. Mendel’s laws (segregation and independent assortment) scale up to populations, allowing predictions about genotype frequencies when specific conditions are met.

Allele frequencies and genotype frequencies

For a gene with two alleles, A and a:

• Allele frequency is the fraction of all alleles in the gene pool that are A versus a.
• Genotype frequency is the fraction of individuals that are AA, Aa, or aa.

Because diploid individuals carry two alleles, allele frequencies are often calculated by counting alleles:

• Each AA individual contributes two A alleles.
• Each Aa contributes one A and one a.
• Each aa contributes two a alleles.

Hardy-Weinberg equilibrium (HWE) as a baseline model

The Hardy-Weinberg model describes an ideal population in which allele frequencies do not change from generation to generation; genotype frequencies are constant over time even with “shuffling of genes” through reproduction. In AP Biology, HWE functions as a null hypothesis: if observed genotype frequencies match HWE expectations (given the assumptions), there is no evidence of evolution at that gene under those conditions. If observed frequencies differ, one or more assumptions are violated.

The Hardy-Weinberg equations

For a two-allele system with allele frequencies p (A) and q (a):

p + q = 1

Expected genotype frequencies under HWE:

p^2 + 2pq + q^2 = 1

Where:

• p^2 is the expected frequency of AA (homozygous dominant)
• 2pq is the expected frequency of Aa (heterozygous)
• q^2 is the expected frequency of aa (homozygous recessive)

Assumptions of Hardy-Weinberg (and consequences if violated)

HWE requires:

• Very large population size (minimizes random sampling effects).
• Random mating with respect to the gene.
• No natural selection among genotypes.
• No migration (gene flow).
• No mutation (or mutation negligible over the timeframe).

It’s important to understand what happens if these conditions are not met:

1. If the population is small, it is more susceptible to random environmental impacts and sampling effects; allele frequencies can change by chance (genetic drift).
2. If mutations occur, new alleles are introduced and genetic equilibrium is disturbed.
3. If immigration or emigration occurs, individuals entering or leaving bring or remove alleles.
4. If mating is non-random, individuals choose partners based on certain traits; genotype frequencies shift (often more homozygosity), even if allele frequencies may not immediately change.
5. If natural selection occurs, organisms better adapted to the environment survive and reproduce more, so their alleles become more common.

Worked example: using HWE to estimate carriers

Many AP questions give a recessive phenotype frequency and ask for carrier frequency. Suppose a recessive trait is caused by genotype aa, and 9% of the population shows the trait.

1) Recessive phenotype frequency corresponds to q^2:

q^2 = 0.09

2) Solve for q:

q = 0.3

3) Find p using p + q = 1:

p = 1 - 0.3 = 0.7

4) Carrier frequency is heterozygotes 2pq:

2pq = 2(0.7)(0.3) = 0.42

So about 42% are carriers (Aa). A common misconception is treating 9% as q instead of q^2; always identify whether a value represents an allele frequency, genotype frequency, or phenotype frequency.

Detecting evolution: comparing observed and expected frequencies

A common task is to compute expected genotype frequencies from p and q, compare them to observed genotype frequencies, and then interpret which HWE assumption(s) may have been violated (selection, drift, gene flow, nonrandom mating, mutation). If observed frequencies differ substantially from expectations, the population may not be in HWE for that gene.

Exam Focus

• Typical question patterns:
  • Given genotype counts, calculate allele frequencies and test HWE predictions.
  • Given recessive phenotype frequency, calculate p, q, and carrier frequency.
  • Interpret what a deviation from HWE suggests biologically and connect it to a violated assumption.
• Common mistakes:
  • Confusing allele frequencies with genotype frequencies.
  • Using HWE equations without checking what the given value represents (allele vs genotype vs phenotype).
  • Claiming “the population is evolving” without explaining which assumption is violated and how that changes allele frequencies.

Mechanisms of Evolution Besides Natural Selection

Allele frequencies can change through several mechanisms. AP Biology often asks you to compare mechanisms or identify which best explains a pattern.

Genetic drift (evolution by chance)

Genetic drift is a random change in allele frequencies due to chance events, especially strong in small populations. Drift can reduce genetic variation and can cause alleles (even harmful or neutral ones) to become fixed or lost. Drift is not “random mutation”; it is random sampling of which alleles get passed on.

Two important cases of genetic drift are:

Bottleneck effect

A bottleneck occurs when population size is drastically reduced (disaster, overhunting, habitat loss). The survivors may not represent the original gene pool, so allele frequencies can shift sharply.

Founder effect

A founder effect occurs when a small group colonizes a new area. The new population’s allele frequencies reflect the founders’ alleles, not necessarily the source population.

In both cases, reduced variation can increase inbreeding and reduce a population’s ability to adapt to future environmental changes.

Gene flow (migration)

Gene flow is the transfer of alleles between populations via migration and interbreeding. It can increase genetic variation within a population and reduce differences between populations (counteracting divergence). However, gene flow is not automatically beneficial: migrants can introduce alleles poorly suited to the local environment, reducing local adaptation.

Mutation (source of new alleles)

Mutation introduces new alleles into a gene pool. Mutation rates per gene are usually low, so mutation alone often changes allele frequencies slowly in the short term, but it is essential over long timescales because it replenishes variation.

Nonrandom mating

Nonrandom mating (inbreeding, assortative mating) changes genotype frequencies by increasing homozygosity, even if allele frequencies do not necessarily change. This is a key reason random mating is a Hardy-Weinberg assumption.

Distinguishing drift vs. selection in data patterns

A useful pattern-recognition rule:

• If an allele increases consistently in multiple populations under the same environmental pressure, that pattern suggests natural selection.
• If allele frequencies fluctuate unpredictably, especially in small populations or after a bottleneck, that suggests genetic drift.

Exam Focus

• Typical question patterns:
  • Identify whether drift, gene flow, mutation, nonrandom mating, or selection best explains an allele-frequency change.
  • Explain consequences of bottlenecks/founder events on variation.
  • Predict how gene flow affects differences between populations.
• Common mistakes:
  • Treating drift as “selection that happens randomly.” Drift is non-adaptive; selection is differential fitness.
  • Assuming gene flow always increases adaptation.
  • Forgetting that nonrandom mating mainly changes genotype frequencies.

Evidence for Evolution and Common Ancestry

Evolution is supported by multiple independent lines of evidence that converge on the conclusion of common ancestry: life shares ancestors and has diversified through descent with modification.

Fossil evidence, paleontology, and transitional features

The fossil record documents that species have changed over time and that extinct organisms existed. Although fossilization is rare and biased (hard parts fossilize more; some environments preserve better), the record reveals consistent sequences and transitional features.

Paleontology has also provided methods for dating fossils, including:

• estimating the age of the rocks where a fossil is found,
• measuring the rate of decay of isotopes (including carbon-14),
• using geographical data.

Transitional features link groups by showing intermediate characteristics. “Transitional” does not mean “half-evolved” or inferior; it indicates traits that help reconstruct evolutionary change across lineages.

Biogeography

Biogeography is the study of the distribution of flora (plants) and fauna (animals). Patterns such as related species appearing in widely separated regions can be explained by common ancestry combined with migration, continental drift, and isolation.

Embryology

Embryology is the study of development. A classic comparative observation is that vertebrate embryos (fish, amphibians, reptiles, birds, mammals including humans) show fishlike features called gill slits (pharyngeal pouches). Similar developmental patterns are consistent with shared ancestry.

Comparative anatomy: homologous vs. analogous structures

Morphological homologies focus on anatomical structures shared across species.

• Homologous structures are similar due to shared ancestry, even if they serve different functions (for example, forelimb bone patterns in mammals). Homology supports common ancestry.
• Analogous structures are similar due to similar selective pressures, not shared ancestry (convergent evolution). A classic example is wings of birds and insects.

Vestigial structures

Vestigial structures are reduced remnants of features functional in ancestors. They support evolution because they make sense as historical leftovers rather than optimal design.

Molecular biology (DNA and protein comparisons)

Molecular evidence is often the most compelling because all organisms use DNA/RNA and a largely universal genetic code. Mutations accumulate over time, and closely related species generally share more sequence similarity. AP problems may present sequences or counts of differences and ask which species are most closely related.

Common ancestry and phylogenetic trees (cladograms)

Common ancestry means that some original life-form is an ancestor of all life, with lineages branching over time. Phylogenetic trees (often called cladograms in many classroom contexts) are used to study relationships among organisms and are built using fossil and/or molecular data.

Some diagrams are drawn with even spacing between species (often called cladograms), while others use uneven branch lengths or distances (often called phylogenetic trees) to represent differing amounts of change or time. Regardless of drawing style, they begin with a common ancestor and branch outward; each fork (node) represents a common ancestor.

Key interpretation rules:

• A node represents a common ancestor.
• Two taxa that share a more recent common ancestor are more closely related.
• Rotating branches around a node does not change relationships; only the branching pattern matters.
• Trees are not “ladders of progress,” and taxa at the tips are not “more evolved” than others.

Convergent evolution and why similarity can be misleading

Convergent evolution occurs when similar environments select for similar traits in unrelated lineages, producing analogous structures. This explains why similar function does not necessarily imply close relatedness.

Continuing evolution

Evolution is constantly occurring. We can observe small changes in DNA, track changes in allele frequencies in real time, and also see consistent patterns of change in the fossil record.

Exam Focus

• Typical question patterns:
  • Distinguish homologous vs. analogous traits in scenarios.
  • Use molecular sequence comparisons to infer relatedness.
  • Interpret phylogenetic trees/cladograms to determine most recent common ancestors.
  • Identify which lines of evidence (fossils, biogeography, embryology, anatomy, molecular data) best support a claim.
• Common mistakes:
  • Equating “similar function” with homology.
  • Reading phylogenies as a ranking of advancement.
  • Assuming the fossil record must be complete to be valid evidence.

Speciation: How New Species Form

Speciation is the process by which populations evolve into distinct species. Using the biological species concept, species are groups of populations that can interbreed in nature and produce viable, fertile offspring, and are reproductively isolated from other such groups.

Speciation connects microevolution (allele-frequency change) to macroevolution (patterns above the species level) because accumulated genetic differences and reproductive barriers split lineages, increasing biodiversity.

Reproductive isolation (the core of speciation)

Reproductive isolation prevents gene flow between populations. Once gene flow is reduced or stopped, populations can diverge genetically through selection, drift, and mutation.

Reproductive barriers are commonly categorized as:

Prezygotic barriers (before fertilization)

Prezygotic barriers prevent fertilization.

• Habitat isolation
• Temporal isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation

Postzygotic barriers (after fertilization)

Postzygotic barriers relate to problems after fertilization, often involving hybrid survival or reproduction.

• Reduced hybrid viability
• Reduced hybrid fertility (a classic example is mules)
• Hybrid breakdown

Allopatric vs. sympatric speciation

There are two major geographic pathways:

Allopatric speciation

Allopatric speciation occurs when populations become separated by a geographic barrier (mountains, rivers, distance) so the two populations cannot interbreed. Gene flow drops, and divergence can occur via selection and drift until reproductive isolation evolves.

Sympatric speciation

Sympatric speciation occurs without a geographic barrier. Gene flow must be reduced through mechanisms such as strong selection, habitat differentiation, sexual selection, or chromosomal changes. In plants, polyploidy (extra chromosome sets) can create near-instant reproductive isolation because polyploid individuals may not produce fertile offspring with the original diploid population.

Divergent evolution and tempo (including punctuated equilibrium)

When populations experience different variation and different environmental pressures, they can change in different ways and become unable to mate; this pattern is often described as divergent evolution.

A model describing evolutionary tempo is punctuated equilibrium: divergent evolution that occurs relatively quickly after a period of stasis (little net change).

Reinforcement and hybrid zones

If diverging populations come into secondary contact:

• If hybrids have low fitness, selection may favor stronger prezygotic isolation (reinforcement).
• If hybrids are fit, populations might merge.
• A stable hybrid zone may persist where hybrids continue to occur.

Example: analyzing a speciation scenario

If a river splits a population of rodents into two isolated groups, gene flow drops sharply. Mutations accumulate independently, drift can shift allele frequencies differently on each side, and different environments may favor different traits (selection). Eventually, even if the river disappears, the populations may no longer interbreed successfully because reproductive barriers evolved. The key causal chain is: isolation reduces gene flow, reduced gene flow allows divergence, divergence leads to reproductive isolation.

Exam Focus

• Typical question patterns:
  • Identify prezygotic vs. postzygotic barriers in examples.
  • Predict whether speciation is more likely allopatric or sympatric given a scenario.
  • Explain how reduced gene flow leads to speciation and how reinforcement can strengthen barriers.
  • Interpret punctuated equilibrium vs. gradual change in terms of stasis and rapid divergence.
• Common mistakes:
  • Treating speciation as individuals “deciding” not to mate (barriers evolve via genetic changes affecting traits like timing, behavior, or compatibility).
  • Treating speciation in animals as a single sudden mutation (usually gradual accumulation; polyploidy is the major “instant” case and mostly in plants).
  • Confusing “hybrid” with “new species” (hybrids can be sterile or unstable).
  • Forgetting that gene flow counters divergence.

Continuing Evolution in Real Time: Resistance, Pathogens, Climate Change, and Conservation

Modern examples show that evolution is ongoing and observable. These scenarios are common on AP exams because they require you to apply the logic of natural selection to data and to connect evolution to medicine, agriculture, and conservation.

Antibiotic resistance

In a bacterial population, some cells may already carry alleles that provide resistance (often arising from prior mutation). When antibiotics are applied, susceptible bacteria die or reproduce less, resistant bacteria survive and reproduce, and resistance alleles increase in frequency. The antibiotic does not “teach” bacteria to resist; it changes which bacteria leave descendants.

Misuse (overuse or incomplete courses) accelerates resistance by repeatedly applying selective pressure while leaving survivors to repopulate.

Pesticide and herbicide resistance

The same selection logic applies to agricultural pests and weeds. Resistance can evolve quickly when population sizes are huge, generation times are short, and selection pressure is intense. Strategies such as rotating chemicals or maintaining refuges (areas without the pesticide) can slow resistance by keeping susceptible alleles in the gene pool, reducing the speed at which resistance fixes.

Evolution in response to climate change

As climates shift, selection pressures change. Populations may respond by evolving new trait distributions (if sufficient genetic variation exists), shifting geographic ranges (a non-evolutionary response that changes where populations persist), or declining if they cannot adapt fast enough. Rapid environmental change can also reduce population size, increasing drift and reducing variation, which makes adaptation harder.

Conservation genetics

Genetic diversity functions like a toolkit: more diversity increases the chance that some individuals carry alleles helpful under new stresses (disease outbreaks, temperature changes). Small, isolated populations face increased inbreeding (more homozygosity and expression of deleterious recessives), stronger drift (loss of alleles by chance), and reduced adaptive potential. Conservation plans often aim to maintain habitat connectivity to support gene flow, while also considering risks such as outbreeding depression in some contexts.

Example: interpreting selection from a data table

If data show genotype RR survives pesticide exposure at much higher rates than Rr and rr, and after exposure the frequency of allele R increases, your explanation should include: differential survival and reproduction by genotype (selection), heritability of resistance, and the resulting allele-frequency change.

Exam Focus

• Typical question patterns:
  • Explain antibiotic/pesticide resistance using natural selection steps.
  • Use before-and-after data to infer which genotype has the highest fitness.
  • Apply evolutionary reasoning to conservation scenarios (small populations, gene flow, inbreeding, drift).
• Common mistakes:
  • Claiming individuals “become resistant” during their lifetime.
  • Ignoring that resistance alleles must exist (via mutation/standing variation) before selection can increase them.
  • Confusing population decline (ecology) with allele-frequency change (evolution), though they can interact.

Origins of Life on Earth (Background Ideas Often Tied to Evolution)

Several hypotheses and experiments address how the first life could have arisen from nonliving chemistry on early Earth.

Oparin-Haldane hypothesis (early atmosphere chemistry)

Alexander Oparin and J. B. S. Haldane proposed that the primitive atmosphere contained mostly inorganic molecules and was rich in methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O), with almost no free oxygen (O2). This reducing atmosphere was proposed to facilitate the formation of organic molecules.

Miller-Urey experiment

Stanley Miller and Harold Urey simulated conditions thought to resemble primitive Earth by placing the proposed atmospheric gases into a flask and using electrical charges to mimic lightning. Organic compounds similar to amino acids appeared, supporting the idea that simple organic building blocks can form under plausible early-Earth conditions.

RNA-world hypothesis

A common hypothesis is that the earliest life-forms were simple molecules of RNA. The RNA-world hypothesis suggests RNA could have served both as genetic material and as a catalyst (ribozymes), potentially preceding DNA and protein-based life.

Exam Focus

• Typical question patterns:
  • Describe what the Miller-Urey experiment tested and what its results imply.
  • Identify the gases proposed in the Oparin-Haldane model and the significance of low O2.
  • Explain the logic of the RNA-world hypothesis (why RNA is plausible as an early biomolecule).
• Common mistakes:
  • Treating Miller-Urey as “proving” exactly how life began (it supports plausibility of abiotic synthesis under certain conditions).
  • Assuming early Earth had abundant O2 (the model emphasizes little free oxygen).