Key Concepts of Evolutionary Mechanisms

Why This Matters

Evolution isn't just one process—it's a toolkit of mechanisms that shape genetic variation and drive change across populations over time. On the AP Biology exam, you're being tested on your ability to distinguish how each mechanism operates, what conditions favor one over another, and why certain outcomes (like reduced diversity or rapid adaptation) occur. The exam loves to present scenarios where you must identify which mechanism is at work, so understanding the underlying principles—not just definitions—is critical.

These concepts connect directly to Unit 7's big ideas about heredity and evolution, but they also tie back to earlier units on genetics and even cell biology. You'll see these mechanisms appear in multiple-choice questions asking you to predict outcomes, and in FRQs that require you to explain why a population changed. Don't just memorize the terms—know what each mechanism does to allele frequencies, whether it's random or selective, and how mechanisms can work together or against each other.

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Mechanisms That Increase Genetic Variation

For evolution to occur, populations need raw material—genetic differences that selection and other forces can act upon. These mechanisms introduce new alleles or combinations into populations.

Mutation

• The ultimate source of all genetic variation—without mutation, evolution would eventually stall because there would be no new alleles to select
• Can be beneficial, neutral, or harmful depending on the environment; most mutations are neutral, but beneficial ones provide the substrate for adaptation
• Occurs randomly with respect to fitness—mutations don't arise because an organism needs them, a key distinction from Lamarckian thinking

Gene Flow

• Transfer of alleles between populations through migration of individuals or movement of gametes (like pollen)
• Increases diversity within populations by introducing alleles that may not have arisen locally through mutation
• Homogenizes allele frequencies between populations, which can prevent local adaptation or counteract divergence caused by selection or drift

Horizontal Gene Transfer

• Non-reproductive transfer of genetic material, most common in prokaryotes through transformation, transduction, or conjugation
• Enables rapid acquisition of new traits—bacteria can gain antibiotic resistance genes from other species in a single generation
• Complicates phylogenetic analysis because genes don't follow vertical inheritance patterns, creating reticulate (web-like) evolutionary histories

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Random Mechanisms That Change Allele Frequencies

Not all evolutionary change is adaptive. These mechanisms alter allele frequencies through chance events, independent of whether alleles are beneficial or harmful. They're especially powerful in small populations.

Genetic Drift

• Random fluctuations in allele frequencies that occur in all populations but have the greatest impact when population size is small
• Can eliminate beneficial alleles or fix harmful ones—drift doesn't "care" about fitness, only probability
• Reduces genetic variation over time as alleles randomly reach fixation (100%) or loss (0%), limiting future adaptive potential

Genetic Bottleneck

• Drastic population reduction caused by catastrophic events (disease, natural disaster, habitat destruction) that randomly eliminates most genetic variation
• Surviving alleles may not represent original diversity—the post-bottleneck population's gene pool is determined by chance, not fitness
• Classic example: Northern elephant seals were reduced to ~20 individuals by hunting; today's population shows extremely low genetic diversity despite recovery in numbers

Founder Effect

• Small group colonizes new habitat, carrying only a subset of the original population's alleles
• Allele frequencies in new population differ from source—rare alleles in the parent population may become common, and common alleles may be absent
• Explains high rates of certain genetic disorders in isolated human populations, such as Ellis-van Creveld syndrome among the Amish

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Selective Mechanisms That Favor Certain Alleles

These mechanisms increase the frequency of alleles that confer advantages—whether for survival, reproduction, or simply transmission to the next generation. Unlike drift, selection is non-random.

Natural Selection

• Differential survival and reproduction based on heritable phenotypic variation—individuals with advantageous traits leave more offspring
• Requires three conditions: variation must exist, variation must be heritable, and variation must affect fitness
• Acts on phenotypes, not genotypes directly—selection "sees" the trait, but the underlying alleles change in frequency as a result

Sexual Selection

• A subset of natural selection where traits increase mating success rather than survival—sometimes at a survival cost
• Produces elaborate secondary sexual characteristics like peacock tails, elk antlers, and complex courtship behaviors
• Creates sexual dimorphism—males and females of the same species evolve different traits because they face different selective pressures for reproduction

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Mechanisms That Distort Inheritance Patterns

These mechanisms cause alleles to spread through populations in ways that don't follow simple Mendelian expectations. They reveal that inheritance itself can be subject to evolutionary forces.

Genetic Hitchhiking

• Neutral or harmful alleles increase in frequency because they're physically linked (on the same chromosome) to a beneficial allele under positive selection
• Reduces variation near selected loci—a "selective sweep" eliminates nearby variation as the beneficial allele spreads
• Demonstrates that the genome evolves as linked blocks, not as independent genes—recombination rate determines how much of the chromosome hitchhikes along

Meiotic Drive

• Biased transmission during meiosis where certain alleles end up in more than 50% of functional gametes
• Violates Mendel's law of segregation—the "cheating" allele spreads even if it reduces organism fitness
• Can lead to evolutionary conflict between driving elements and the rest of the genome, which may evolve suppressors

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Quick Reference Table

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Self-Check Questions

1. A population of mice on an island experiences a hurricane that kills 90% of individuals. The surviving mice happen to all have brown fur, though the original population was 50% brown and 50% white. Which mechanism explains the change in allele frequency, and why isn't this natural selection?

2. Compare and contrast how gene flow and genetic drift affect genetic variation within and between populations. Under what conditions might they have opposing effects?

3. A researcher notices that a neutral SNP has increased dramatically in frequency in a population at the same time as a nearby gene for pesticide resistance spread. Which mechanism best explains this, and what would you predict about genetic variation in the chromosomal region surrounding the resistance gene?

4. Which two mechanisms could cause a harmful allele to increase in frequency in a population? Explain the conditions under which each would operate.

5. An FRQ describes two isolated populations of the same species: one large and one small. Both experience the same selection pressure favoring a new beneficial mutation. Predict which population will show faster adaptive change and explain how the interaction between selection and drift affects your answer.