Discovering How Populations Change

1. Evolution changes populations, not individuals

A population is a group of individuals of the same species living in the same area and interbreeding. Evolution is a change in the genetic makeup — the allele frequencies — of a population across generations. This definition tells you what evolves: populations, never individuals. An individual's genes are set at conception and do not change in a directed, inherited way during its life. An individual may grow, learn, or acclimate, but those changes are not evolution because they are not passed to offspring.

2. Natural selection: the mechanism Darwin described

Darwin built his argument from observations. Species tend to overproduce offspring, yet population sizes stay roughly stable; therefore many offspring must die before reproducing — a "struggle for existence." Individuals in a population vary in their traits, and much of that variation is heritable. From this, Darwin inferred that individuals with traits better suited to their environment leave more surviving offspring — differential reproductive success — and that, over generations, advantageous traits accumulate. He called this descent with modification.

For natural selection to occur, three conditions must be met: (1) variation in traits, (2) heritability of at least some of that variation, and (3) differential reproductive success. Selection acts on observable traits (phenotypes) but, over time, changes the frequencies of the underlying alleles. Mutation supplies variation randomly; selection sorts it nonrandomly — but selection has no goal and does not create traits "because organisms need them."

3. Evidence for evolution

Evolution is supported by several independent lines of evidence. The fossil record documents change over time, including transitional forms such as Tiktaalik (between fish and four-limbed vertebrates) and Archaeopteryx (between dinosaurs and birds). Comparative anatomy reveals homologous structures — the same underlying parts reshaped for different jobs, like the forelimb bones of humans, whales, and bats — indicating common ancestry, in contrast to analogous structures, which are similar because of convergent evolution (bird and insect wings). Vestigial structures, such as whale pelvic bones, are remnants of structures that functioned in ancestors. Molecular evidence — a shared genetic code and similar DNA sequences — shows that closely related species share more DNA. Biogeography shows species are distributed according to evolutionary history. Finally, evolution has been observed directly in antibiotic-resistant bacteria and in Darwin's finches.

How it works

Darwin's chain of observation and inference:

1. Observation: Organisms produce more offspring than can survive (overproduction).
2. Observation: Population sizes tend to remain roughly stable.
3. Inference: Resources are limited, so only some offspring survive to reproduce — a struggle for existence.
4. Observation: Individuals vary in their traits, and much of that variation is inherited.
5. Inference: Individuals with heritable traits that fit the environment better survive and reproduce more (differential reproductive success).
6. Inference: Over generations, favorable traits accumulate, and the population becomes better adapted (descent with modification).

Common confusions

Memory aids

"V-H-S" — the three ingredients natural selection needs: Variation, Heritability, and Selection (differential reproductive success). If all three are present, the population will evolve.

Quick review

Topic Recap

• Evolution is a change in a population's allele frequencies over generations; populations — not individuals — evolve.
• Natural selection needs variation, heritability, and differential reproductive success; it is not goal-directed.
• Darwin's insight was descent with modification from common ancestors.
• Evidence for evolution is convergent: fossils, anatomy (homology, analogy, vestiges), molecules, biogeography, and direct observation.
• Evolution underlies antibiotic resistance, vaccine development, cancer progression, and conservation.

Knowledge Check

1. Why is it incorrect to say an individual organism evolves during its lifetime?
2. What three conditions are required for natural selection to occur?
3. A bat's wing and a bird's wing both enable flight. Are they homologous or analogous, and why?
4. How does antibiotic resistance illustrate evolution by natural selection?
5. Why does saying "it's just a theory" misrepresent the scientific meaning of the word theory?

Answers and Rationales

1. Answer: An individual's genes are set at conception and do not change in an inherited way during its life; evolution is a change in a population's allele frequencies across generations. Why: Evolution is, by definition, a population-level, multigenerational process.
2. Answer: Variation, heritability, and differential reproductive success. Why: Without variation there is nothing to select; without heritability advantages do not persist; without differential reproduction there is no sorting.
3. Answer: Analogous — bat and bird wings evolved independently in mammals and birds, which do not share a winged common ancestor. Why: Similar function with different origin means convergence, not common descent.
4. Answer: Bacterial populations vary in drug sensitivity; antibiotics kill susceptible cells while resistant ones survive and reproduce, so resistance spreads. Why: This is variation, heritability, and differential reproductive success acting in real time.
5. Answer: In science, a theory is a well-substantiated explanation supported by extensive evidence (like germ theory), not a guess. Why: Evolutionary theory is a body of tested, convergent evidence, not a casual opinion.