Of course! Here is a comprehensive study guide for AP Biology on the topic of Natural Selection, structured to align with the AP curriculum’s key concepts, learning objectives, and question styles.
AP Biology Study Guide: Natural Selection (Unit 7: Natural Selection)
I. The Big Picture: What is Natural Selection?
Natural selection is the primary mechanism for evolution, the process of change in the heritable characteristics of biological populations over successive generations. It is the process by which organisms with traits better suited to their environment tend to survive and reproduce more successfully, passing those advantageous traits to their offspring.
Key Idea: Natural selection acts on individuals, but populations evolve.
II. The Four Core Principles of Natural Selection (Darwin’s Postulates)
For natural selection to occur, four conditions must be met. A great way to remember them is with the acronym V.I.S.T.
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V - Variation:
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What it is: Individuals within a population exhibit variation in their traits (phenotypes).
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Why it’s important: If all individuals were identical, there would be no “better” traits to select for.
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Where it comes from:
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Mutations: Random changes in DNA. This is the ultimate source of all new alleles.
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Sexual Reproduction: Crossing over, independent assortment, and random fertilization shuffle existing alleles into new combinations.
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I - Inheritance:
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What it is: Traits are passed down from parents to offspring through genes.
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Why it’s important: For a trait to become more common in a population, it must be heritable. Acquired characteristics (like a bodybuilder’s muscles) are not passed on.
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S - Selection (Differential Survival and Reproduction):
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What it is: More offspring are produced than can survive, leading to competition for limited resources. Some individuals have traits (adaptations) that give them an advantage in this struggle.
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Why it’s important: This is the “selection” part. The environment acts as a selective pressure, determining which traits are favorable.
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Fitness: In biology, fitness is not about strength. It is a measure of an organism’s reproductive success—how many viable, fertile offspring it produces. Higher fitness = more offspring.
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T - Time:
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What it is: Over many generations, the heritable traits that increase fitness will become more common in the population.
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Why it’s important: This process leads to a change in the population’s allele frequencies over time, which is the definition of microevolution.
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III. Key Vocabulary
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Adaptation: A heritable trait that increases an organism’s fitness in its specific environment.
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Fitness: The relative ability of an organism to survive and produce fertile offspring.
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Selective Pressure: Any factor in the environment that influences survival or reproduction (e.g., predators, lack of food, climate change, disease).
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Population: A group of individuals of the same species living in the same area that can interbreed.
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Allele Frequency: The proportion of a specific allele (e.g., ‘A’ or ‘a’) within a population’s gene pool. Evolution is a change in these frequencies over time.
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Gene Pool: All the alleles for all the genes in a population.
IV. Types of Natural Selection
Natural selection can alter the frequency distribution of heritable traits in three main ways. It’s essential to be able to recognize, describe, and graph these.
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Directional Selection:
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What it does: Favors one extreme phenotype.
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The Graph: The population’s bell curve shifts in one direction.
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Example: Antibiotic resistance. The presence of antibiotics is a strong selective pressure that kills non-resistant bacteria. The few bacteria with resistance (one extreme) survive and reproduce, shifting the entire population’s phenotype towards being resistant. Another classic example is the peppered moth during the industrial revolution.
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Stabilizing Selection:
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What it does: Favors the intermediate phenotype and selects against the extremes.
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The Graph: The bell curve narrows and becomes taller in the middle.
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Example: Human birth weight. Babies that are very small or very large have lower survival rates than babies of an average, intermediate weight.
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Disruptive (or Diversifying) Selection:
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What it does: Favors both extreme phenotypes and selects against the intermediate phenotype.
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The Graph: The bell curve splits into two peaks.
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Example: A population of finches lives in an environment with only very large, hard seeds and very small, soft seeds. Finches with large beaks are good at cracking large seeds, and finches with small beaks are good at handling small seeds. Finches with intermediate beaks are not efficient at either, so they are selected against. This can lead to the formation of new species (speciation).
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V. Other Mechanisms of Evolution (That Are NOT Natural Selection)
Natural selection is not the only way allele frequencies can change.
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Genetic Drift: Changes in allele frequencies due to random chance. It has a much stronger effect in small populations.
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Bottleneck Effect: A drastic reduction in population size (e.g., from a natural disaster) results in a new population whose gene pool may be very different from the original.
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Founder Effect: A small group of individuals breaks off from a larger population to start a new, isolated one. The new population’s gene pool is limited to the alleles of the founders.
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Gene Flow: The movement of alleles between populations due to migration. It tends to reduce the genetic differences between populations, making them more similar.
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Sexual Selection: A type of natural selection where individuals with certain inherited characteristics are more likely than other individuals to obtain mates. This can lead to sexual dimorphism (differences between males and females).
VI. Evidence for Evolution
The AP exam expects you to use evidence to support the theory of evolution.
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Direct Observation: We can see evolution happening in real-time (e.g., the evolution of drug-resistant viruses and bacteria).
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Fossil Record: Shows changes in organisms over time and the existence of extinct species. Transitional fossilsshow links between different groups (e.g., Archaeopteryx).
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Biogeography: The geographic distribution of species. Island species often closely resemble species on the nearest mainland, suggesting they share a common ancestor.
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Comparative Anatomy:
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Homologous Structures: Similar structures in different species due to a shared common ancestor, but with different functions (e.g., the forelimbs of a human, cat, whale, and bat).
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Analogous Structures: Structures with similar functions but that evolved independently, not from a common ancestor (e.g., the wings of a bird and an insect). This is a result of convergent evolution.
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Vestigial Structures: Remnants of features that served a function in an organism’s ancestors but have little to no use today (e.g., the pelvic bones in whales).
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Molecular Biology: The strongest evidence.
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Universal Genetic Code: All life uses DNA and RNA, and the genetic code is nearly universal, suggesting a very early common ancestor.
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DNA/Protein Comparisons: The more similar the DNA or amino acid sequences are between two species, the more closely related they are.
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VII. Practice Questions
Multiple Choice Question (MCQ):
A population of squirrels is preyed on by hawks. Very large squirrels are too heavy to climb high branches to escape, and very small squirrels are not fast enough to run away. Mid-sized squirrels have the best chance of survival. Which pattern of natural selection would be expected in this squirrel population?
(A) Disruptive selection
(B) Directional selection
(C) Stabilizing selection
(D) Sexual selection
Answer: (C) Stabilizing selection. The intermediate phenotype (mid-sized) is favored, while both extremes are selected against.
Free Response Question (FRQ) Style Prompt:
A species of grass lives in a field. The field is contaminated with heavy metals from a nearby factory. Some individual grass plants have a rare, random mutation that allows them to tolerate these metals.
(a) DESCRIBE the role of variation in the response of the grass population to the environmental change.
(b) EXPLAIN how natural selection would act on this population over several generations. Be sure to include the concepts of selective pressure and differential reproduction.
(c) PREDICT how the allele frequencies in the grass population will change over time.
Sample Answer Breakdown:
(a) Description (1 point): The initial population of grass has genetic variation. Most plants do not have the mutation for metal tolerance, but a few individuals do. This pre-existing variation is crucial because it provides the raw material for selection to act upon.
(b) Explanation (2 points): The heavy metals in the soil act as a strong selective pressure. The non-tolerant grass plants will struggle to grow and reproduce, or they will die. The few plants with the tolerance mutation will survive and reproduce more successfully (differential reproduction). Because the trait is heritable, they will pass the tolerance allele to their offspring.
(c) Prediction (1 point): Over time, the frequency of the allele for metal tolerance will increase in the population. The frequency of the allele for non-tolerance will decrease. Eventually, most or all of the grass plants in the field may be metal-tolerant.
VIII. Key Takeaways & Common Misconceptions
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Individuals don’t evolve; populations do.
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Natural selection is not random. The mutations that cause variation are random, but the selection of those traits by the environment is not.
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“Survival of the Fittest” means survival of the best-reproducing, not the strongest.
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Organisms do not evolve “on purpose” or because they “need” to. Selection can only act on the variation that is already present.