AP Biology: Evolution and Ecology

Evolution and ecology sit at the core of AP Biology because they explain both the history of life and the rules that govern how living systems work today. Evolution accounts for the diversity of organisms across time, while ecology explains how those organisms interact with one another and with their environment. Together, they connect molecular biology to whole ecosystems, and they give students a framework for interpreting biological data rather than memorizing isolated facts.

Evidence for Evolution

Evolution is supported by multiple independent lines of evidence, which is one reason it is considered a foundational theory in biology.

Fossils and transitional forms

The fossil record documents changes in organisms over geological time. While it is incomplete, it includes many transitional forms that show intermediate features between major groups. Fossils also allow scientists to place evolutionary change in a timeline and connect it to environmental shifts, such as climate change or mass extinctions.

Comparative anatomy and development

Homologous structures are anatomical features inherited from a common ancestor, even if their functions differ today. The classic example is the shared bone pattern in vertebrate forelimbs. Vestigial structures, such as pelvic bones in some whales, make sense as evolutionary remnants. Comparative embryology also shows that related organisms often share early developmental patterns, reflecting shared ancestry.

Molecular evidence

DNA and protein sequences provide direct, quantifiable evidence of relatedness. Closely related species tend to have more similar sequences. This is especially powerful because it allows comparisons across all life forms, including microbes, and it can be used to estimate divergence times when paired with mutation rates and fossil calibration points.

Mechanisms of Evolution

Evolution is defined as a change in allele frequencies in a population across generations. Several mechanisms can drive those changes, and AP Biology focuses on how each one operates and how they can be detected.

Natural selection

Natural selection occurs when individuals with heritable traits that increase fitness leave more offspring. Fitness in biology refers to reproductive success in a given environment, not strength or health in a general sense.

Natural selection depends on three conditions:

1. Variation exists within the population.
2. Some of that variation is heritable.
3. Individuals with certain traits have higher reproductive success.

Selection can take different forms:

• Directional selection favors one extreme phenotype, often when environments shift.
• Stabilizing selection favors intermediate phenotypes, reducing variation.
• Disruptive selection favors both extremes, which can increase variation and contribute to speciation.

Genetic drift

Genetic drift is random change in allele frequencies, strongest in small populations. Two common scenarios illustrate drift:

• The bottleneck effect occurs when a population is sharply reduced by an event, leaving a non-representative gene pool.
• The founder effect occurs when a small group starts a new population, carrying only a subset of the original genetic variation.

Unlike natural selection, drift is not adaptive. Alleles can become common or disappear purely by chance.

Gene flow

Gene flow is the movement of alleles between populations through migration and interbreeding. It tends to reduce genetic differences between populations. Gene flow can introduce beneficial alleles, but it can also prevent local adaptation by continuously mixing gene pools.

Mutation and recombination

Mutation creates new alleles, providing raw material for evolution. Most mutations are neutral or harmful, but some are beneficial in specific environments. Recombination during meiosis reshuffles alleles and increases genetic variation, which can enhance the potential response to selection.

Population Genetics and the Hardy-Weinberg Framework

Population genetics links microevolution to measurable genetic patterns. The Hardy-Weinberg model provides a null hypothesis: if certain conditions are met, allele frequencies remain constant across generations.

If a gene has two alleles with frequencies $p$ and $q$, then:

• $p+q=1$
• Genotype frequencies are $p^2$ (homozygous dominant), $2pq$ (heterozygous), and $q^2$ (homozygous recessive)

The Hardy-Weinberg conditions include large population size, random mating, no mutation, no migration, and no natural selection. Real populations rarely meet all conditions, so deviations help biologists infer which evolutionary mechanisms are acting.

Speciation and the Origin of Biodiversity

Speciation is the process by which new species arise. It typically requires reproductive isolation, meaning that gene flow between populations is reduced or eliminated.

Allopatric and sympatric speciation

• Allopatric speciation occurs when a population is separated by a geographic barrier, such as a mountain range or river. Over time, mutation, selection, and drift can lead to divergence.
• Sympatric speciation occurs without geographic separation. It can happen through mechanisms like polyploidy in plants or strong disruptive selection combined with assortative mating.

Reproductive isolating mechanisms

Isolating mechanisms can be prezygotic or postzygotic:

• Prezygotic barriers prevent fertilization, such as differences in mating behavior, timing, habitat, or mechanical incompatibility.
• Postzygotic barriers occur after fertilization, such as reduced hybrid viability or sterility (a classic example is the mule).

Phylogenetics: Reconstructing Evolutionary Relationships

Phylogenetics uses data to build evolutionary trees (phylogenies) that represent hypotheses about relationships.

Reading phylogenetic trees

A key skill is understanding that relatedness is determined by the most recent common ancestor, not by how “similar” organisms appear. Nodes represent common ancestors, and clades include an ancestor and all its descendants. Shared derived traits, called synapomorphies, support clade formation.

Molecular clocks and data sources

DNA sequences, protein sequences, and sometimes morphological traits are used to build trees. Molecular clocks can estimate divergence times by assuming mutations accumulate at roughly constant rates in certain regions of the genome, but these estimates require calibration and careful interpretation.

Population Ecology

Population ecology examines how population size changes and what limits growth.

Exponential and logistic growth

When resources are abundant, populations may grow exponentially. If $N$ is population size and $r$ is intrinsic growth rate, exponential growth can be modeled as:

$$\frac{dN}{dt}=rN$$

As resources become limiting, growth slows, producing logistic growth:

$$\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right)$$

Here, $K$ is carrying capacity, the maximum population size the environment can sustainably support.

Density-dependent and density-independent factors

• Density-dependent factors intensify as population density rises, such as competition, disease, and predation.
• Density-independent factors affect populations regardless of density, such as droughts, freezes, or storms.

Community Ecology

Communities include multiple interacting species, and those interactions shape evolution and ecosystem function.

Species interactions

• Competition can reduce fitness when species share limiting resources. Competitive exclusion predicts that two species cannot occupy the same niche indefinitely.
• Predation and herbivory can drive evolutionary “arms races,” influencing defenses and hunting strategies.
• Symbiosis includes mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, one harmed).

Succession and disturbance

Ecological succession describes how communities change after disturbance. Primary succession begins without soil, while secondary succession begins where soil remains. Disturbance regimes, such as periodic fires, can maintain biodiversity by preventing dominance by a single species.

Ecosystems, Energy Flow, and Biodiversity

Ecosystems integrate living communities with abiotic factors such as nutrients, water, and climate.

Energy flow and trophic structure

Energy enters most ecosystems through photosynthesis and moves through trophic levels. Because energy transfer is inefficient, higher trophic levels support less biomass. Food webs capture the complexity of feeding relationships more accurately than single food chains.

Nutrient cycling

Matter cycles through ecosystems via biogeochemical cycles like the carbon and nitrogen cycles. Unlike energy, which flows through and is lost as heat, nutrients are recycled, though human activity can disrupt these cycles through fertilizer use, fossil fuel combustion, and land-use change.

Biodiversity and resilience

Biodiversity includes genetic diversity within species, species diversity within communities, and ecosystem diversity across landscapes. Higher biodiversity often increases resilience, the ability of a system to withstand and recover from disturbances. Conservation biology applies these principles to protect habitats, maintain population viability, and preserve ecological interactions that sustain ecosystems.

Bringing It Together for AP Biology

Evolution explains how populations change and diversify, while ecology explains how those populations persist within environmental constraints. In AP Biology, the most important move is to connect mechanisms to evidence: natural selection to observed trait shifts, genetic drift to small-population patterns, phylogenies to molecular data, and population dynamics to resource limits. When those connections are clear, evolution and ecology become less about terminology and more about reasoning through real biological systems.