Source: Introduction to Biology I Midterm Examination, University of Washington
Difficulty: Introductory | Prerequisites: None, though familiarity with basic scientific method vocabulary helps.
This material covers the foundational concepts of evolutionary biology: what science is and why evolution qualifies as a scientific theory, how Darwin's four postulates explain the mechanism of natural selection, and what counts as evidence that evolution has occurred. If you are coming into this cold, start here. Every other topic in the course (phylogenetics, speciation, molecular biology) builds on these ideas. You should already be comfortable with the difference between a hypothesis and a theory in everyday language versus scientific usage.
Evolution is the change in heritable traits within a population over generations. Natural selection is one mechanism driving it: individuals with traits better suited to their environment survive and reproduce at higher rates, and those traits are passed on. A scientific theory (like evolution) is not a guess; it is a well-tested explanation backed by a large body of converging evidence.
Scientific theory
A well-tested explanation supported by a large body of evidence from multiple independent lines of inquiry. In simple terms, this means a theory is science's highest level of confidence in an explanation, not a guess or a hunch.
Natural selection
The process by which individuals with heritable traits better suited to their environment tend to survive and reproduce at higher rates. Think of it as: the environment "filters" which traits get passed on, not by design, but by which organisms happen to leave more offspring.
Darwin's four postulates
The four conditions required for natural selection to operate: (1) variation exists among individuals, (2) that variation is heritable, (3) not all individuals survive and reproduce equally, and (4) survival and reproduction are non-random with respect to traits. In simple terms, there must be differences, those differences must be passed on, some individuals must do better than others, and the reason they do better must be linked to their traits.
Fitness
An organism's ability to survive and reproduce in a given environment. Think of it as: how many offspring you leave that themselves go on to reproduce, not how strong or fast you are.
Adaptation
A heritable trait that increases an organism's fitness in a particular environment. In simple terms, a trait shaped by natural selection because it helps the organism survive or reproduce in its specific setting.
Descent with modification
The principle that species share common ancestors and change over time. Think of it as: all life is related through a branching family tree, and each branch accumulates its own changes.
Heritable variation
Differences among individuals in a population that can be passed from parents to offspring through genetic material. In simple terms, the raw material natural selection works on.
A scientific theory is not a guess. It is an explanation that has been tested repeatedly and supported by converging evidence from independent sources.
Theories can be updated or refined as new evidence emerges. They are not fixed or unchangeable.
Evolution is a scientific theory because it is testable, falsifiable, and supported by evidence from fossils, genetics, comparative anatomy, and direct observation of natural selection in action.
The everyday use of "theory" (meaning "guess" or "speculation") is different from the scientific use. This distinction is heavily tested.
Variation: Individuals in a population differ from one another in their traits.
Heritability: At least some of that variation is passed from parents to offspring.
Differential survival and reproduction: Not all individuals survive and reproduce equally; some leave more offspring than others.
Non-random survival: The differences in survival and reproduction are linked to trait differences (not purely random). Individuals with certain traits are more likely to survive and reproduce.
All four must hold for natural selection to operate. Remove any one and the mechanism breaks down.
Natural selection acts on individuals (their phenotypes), but evolution occurs at the level of populations (changes in allele frequencies over generations).
Individuals do not change their traits during their lifetime to adapt. This is a critical distinction: the population shifts because individuals with favourable traits leave more offspring, not because any individual transforms.
The classic example: Darwin's finches on the Galapagos Islands. During drought, seeds became harder and larger. Finches with deeper, stronger beaks could crack these seeds and survived at higher rates. Because beak depth is heritable, the next generation had a higher average beak depth. The population changed; no individual finch grew a bigger beak in response to drought.
Fitness is measured by reproductive output, not by strength or speed in everyday terms.
An adaptation is a heritable trait that increases fitness in a given environment. To demonstrate that a trait is an adaptation, you need evidence that (a) it is heritable and (b) it increases fitness in that environment.
A trait that appears only due to environmental changes during an organism's lifetime (a phenotypically plastic response that is not heritable) is not an adaptation in the evolutionary sense.
"Individuals adapt during their lifetime." They do not. Natural selection does not involve individuals changing their traits to suit the environment. The population shifts because individuals with better-suited traits leave more offspring.
"Evolution is just a theory, meaning it is unproven." In science, "theory" denotes the highest level of tested, evidence-backed explanation. It does not mean "guess."
"Natural selection is random." Mutations are random, but natural selection is non-random. Survival and reproduction are systematically linked to trait differences.
"Evolution has a goal or direction." Evolution does not proceed towards a predetermined goal. It is not teleological. Populations change in response to current environmental pressures, with no foresight.
⚠️ The distinction between the everyday and scientific meanings of "theory" appears in multiple question formats (multiple choice, true/false, short answer). Know the scientific definition cold.
⚠️ Darwin's four postulates are tested directly: you may be asked to list them, explain why each matters, or identify which postulate a scenario violates.
⚠️ "Natural selection acts on individuals, evolution occurs at the level of populations" is a fill-in-the-blank and short-answer favourite.
⚠️ The Darwin's finches example (drought, beak depth, heritability) is used across multiple question types. Be ready to walk through the mechanism step by step.
⚠️ Know why heritable variation matters: without it, differential survival cannot produce evolutionary change.
True or false: Scientific theories are unchanging and never updated with new evidence. (False. Theories are refined as new evidence emerges.)
True or false: In natural selection, individuals change their traits during their lifetime to adapt. (False. Populations change across generations because individuals with favourable traits leave more offspring.)
Fill in the blank: Natural selection acts on _______, but evolution occurs at the level of _______. (individuals; populations)
Fill in the blank: The combination of traits that increase an organism's survival and reproductive success in a particular environment are called _______. (adaptations)
True or false: Differential reproductive success is a key component of natural selection. (True.)
Q: Which of the following best describes the scientific definition of a theory? (a) A guess based on beliefs, (b) A well-tested explanation supported by a large body of evidence, (c) An unproven hypothesis that cannot be tested, (d) A fact that cannot be changed.
A: (b). A scientific theory is a well-tested explanation, not a guess or an immutable fact.
Q: Which principle is NOT part of Darwin's four postulates for natural selection? (a) Variation exists, (b) Traits are inherited, (c) Individuals can change their traits during their lifetime to adapt, (d) Survival and reproduction differ among individuals due to trait differences.
A: (c). Individuals do not change their own traits to adapt. Natural selection works across generations, not within an individual's lifetime.
Q: Darwin's finches developed deeper beaks during drought conditions. Explain the mechanism using Darwin's postulates.
A: Variation in beak depth existed in the population (postulate 1). Beak depth is heritable (postulate 2). During drought, large hard seeds were the main food source, so not all finches survived equally (postulate 3). Finches with deeper beaks could crack the seeds and survived at higher rates (postulate 4). The next generation inherited deeper beaks on average.
Q: Explain the nature of science and how scientific theories differ from non-scientific beliefs, using evolution as an example.
A: Science relies on testable, falsifiable explanations supported by empirical evidence. A scientific theory is not a guess; it is the product of repeated testing and converging lines of evidence. Evolution qualifies because it is supported by fossils, genetics, comparative anatomy, direct observation (e.g. antibiotic resistance, finch beak changes), and phylogenetic analysis. Non-scientific beliefs may be untestable or unfalsifiable and do not require empirical support.
Q: Why are both variation and heritability crucial for evolution by natural selection?
A: Without variation, all individuals would be identical and natural selection would have nothing to act on. Without heritability, even if some individuals survived better, their advantageous traits would not be passed to offspring, so the population could not change across generations. Both are necessary for the mechanism to produce evolutionary change.
This material connects directly to Evidence for Evolution (artificial selection, homology, vestigial structures), which provides the empirical support for the theory covered here. It also underpins Phylogenetics, where the patterns of descent with modification are reconstructed as branching trees. The concept of heritable variation connects forward to genetics and molecular biology later in the course.
Evolution, natural selection, Darwin, Darwin's postulates, fitness, adaptation, heritable variation, descent with modification, scientific theory, theory vs hypothesis, theory vs law, theory vs guess, finch beak, Galapagos finches, beak morphology, differential survival, reproductive success, population change, allele frequency, BIO I, Introduction to Biology, University of Washington, midterm review