Difficulty: Introductory | Prerequisites: DNA Structure and Genetics study notes (companion document)
Big picture: Evolution is the unifying principle of biology. It explains why there are millions of different species, why organisms are well suited to their environments, and why species that look nothing alike can share deep structural similarities. This topic builds directly on DNA and genetics: mutations in DNA create genetic variation, and natural selection acts on that variation over generations. If you have not reviewed the DNA notes, start there.
TL;DR: Evolution is the change in inherited traits within a population over successive generations. It is driven primarily by natural selection, the process by which organisms with traits better suited to their environment are more likely to survive and reproduce. Evidence for evolution comes from the fossil record, comparative anatomy, and molecular biology (DNA comparisons).
Evolution
The process by which populations of living organisms change over successive generations through changes in inherited traits. Think of it as the gradual "editing" of a species' genetic makeup over time.
Natural selection
The mechanism by which organisms with traits better suited to their environment tend to survive and reproduce more than those without. In simple terms, nature "selects" which traits get passed on by rewarding survival and reproduction.
Fitness (biological fitness)
An organism's ability to survive and reproduce in its environment. This is not about physical strength; it is about how many offspring an organism successfully raises.
Adaptation
A trait that has evolved through natural selection because it improves an organism's fitness in its environment. For example, the thick fur of an arctic fox is an adaptation to cold climates.
Common ancestry
The idea that all living organisms descend from a shared ancestor, far back in time. The further back you go, the more species converge on the same branch of the tree of life.
Speciation
The formation of new, distinct species from an existing one. This typically happens when a population is split into isolated groups that evolve independently over a long period.
Fossil record
The collection of all known fossils, the preserved remains or traces of ancient organisms found in rock layers. It provides a physical timeline of how life on Earth has changed.
Comparative anatomy
The study of similarities and differences in the body structures of different species. Structures that share a common origin (homologous structures) suggest common ancestry.
Molecular biology (as evidence for evolution)
The comparison of DNA, RNA, and protein sequences across species. Greater similarity in these sequences indicates a more recent common ancestor.
Natural selection requires four conditions:
Variation: individuals in a population differ in their traits.
Inheritance: those traits are at least partly heritable (passed from parent to offspring via DNA).
Differential survival: some trait variants make an organism more likely to survive in a given environment.
Differential reproduction: organisms that survive longer or better tend to leave more offspring.
Over many generations, traits that improve survival and reproduction become more common in the population. Traits that reduce survival become rarer.
Natural selection does not act on individuals. It acts on populations over time. An individual organism does not evolve; a population does.
Example from the source material: animals in a forest with visual predators. Those better camouflaged are more likely to survive and reproduce, so camouflage becomes more common in the population over generations.
Three major lines of evidence support the theory of evolution:
The fossil record
Fossils are the preserved remains or impressions of ancient organisms, found in layers of sedimentary rock.
Older rock layers contain simpler organisms; younger layers show progressively more complex forms.
Transitional fossils show intermediate forms between major groups (e.g., fossils with features of both fish and amphibians).
The fossil record documents the existence of species that no longer exist (extinction).
Comparative anatomy
Homologous structures: body parts in different species that share a common underlying structure but may serve different functions (e.g., the forelimb bones of a human arm, a whale flipper, and a bat wing).
Homologous structures point to a shared ancestor from which those species diverged.
Vestigial structures: reduced or functionless remnants of structures that were functional in ancestors (e.g., the human appendix, hind-limb bones in some snakes).
Molecular biology
DNA and protein sequences can be compared across species.
Species that share a more recent common ancestor have more similar DNA sequences.
This molecular evidence independently confirms the relationships suggested by anatomy and fossils.
Speciation is the process by which one species splits into two or more distinct species.
The most common mechanism is geographic isolation (allopatric speciation):
A physical barrier (a river, a mountain range, a glacier) separates a population into two or more groups.
The separated groups experience different environments and different selection pressures.
Over time, mutations accumulate independently in each group, and natural selection pushes them in different directions.
Eventually the groups become so different that they can no longer interbreed, even if the barrier is removed. At that point, they are separate species.
Speciation requires time, isolation, and sustained differences in selection pressure.
Antibiotic resistance in bacteria is evolution in action: when antibiotics kill most bacteria in an infection, the few with mutations conferring resistance survive and reproduce, producing a resistant population. This is why doctors urge patients to complete their full course of antibiotics. Selective breeding in agriculture applies the same principle deliberately, choosing plants or animals with desirable traits and breeding them over generations to improve crop yields, disease resistance, or livestock productivity.
Students often think evolution means an individual organism changes during its lifetime. It does not. Evolution is a population-level process that occurs across generations.
"Survival of the fittest" does not mean survival of the strongest or fastest. Fitness, in biology, means reproductive success: how many viable offspring an organism leaves behind.
Students sometimes believe evolution is goal-directed, that organisms evolve "towards" something or "try" to adapt. Evolution has no direction or intention. It is the result of random mutations filtered by environmental pressures.
A common exam error is treating "theory" as meaning "guess." In science, a theory is a well-supported explanatory framework backed by extensive evidence. The theory of evolution has the same standing as the theory of gravity.
⚠️ Be able to list the four conditions required for natural selection (variation, inheritance, differential survival, differential reproduction).
⚠️ Know the three lines of evidence for evolution (fossil record, comparative anatomy, molecular biology) and be ready to explain each in a few sentences.
⚠️ Understand what speciation is and the role of geographic isolation in allopatric speciation.
⚠️ Be prepared to distinguish between evolution (change in a population) and individual development or learning. Exams frequently test this distinction.
⚠️ Do not confuse "theory" (well-supported scientific framework) with "hypothesis" (a testable prediction). This comes up in multiple-choice questions.
True or false: Evolution acts on individual organisms.
Answer: False. Evolution acts on populations over generations.
Fill in the blank: The process by which a new species forms from an existing one is called ____.
Answer: Speciation.
True or false: In biology, "fitness" refers to an organism's physical strength.
Answer: False. Fitness refers to reproductive success.
Fill in the blank: The three major lines of evidence for evolution are the fossil record, comparative anatomy, and ____.
Answer: Molecular biology.
True or false: A scientific theory is the same as an everyday "guess."
Answer: False. A scientific theory is a well-supported explanation backed by extensive evidence.
Q: Define natural selection and list the four conditions it requires.
A: Natural selection is the process by which traits that improve survival and reproduction become more common in a population over generations. It requires: (1) variation in traits, (2) heritability of those traits, (3) differential survival, and (4) differential reproduction.
Q: A population of beetles lives on a dark-coloured rock. Some beetles are dark and some are light. Birds eat the beetles they can see. Over many generations, what would you expect to happen to the proportion of dark vs. light beetles? Explain why.
A: The proportion of dark beetles would increase. Dark beetles are harder for birds to spot against the dark rock, so they survive and reproduce at higher rates. Over generations, the dark-colour trait becomes more common through natural selection.
Q: Name and briefly describe three lines of evidence that support the theory of evolution.
A: (1) The fossil record: preserved remains of ancient organisms in rock layers show how life has changed over time. (2) Comparative anatomy: shared body structures (homologous structures) across species point to common ancestry. (3) Molecular biology: similarities in DNA sequences indicate how closely related species are.
Q: Explain how geographic isolation can lead to speciation.
A: A physical barrier splits a population into isolated groups. Each group faces different environmental pressures. Over time, different mutations accumulate and natural selection drives each group in different directions. Eventually the groups diverge enough that they can no longer interbreed, forming separate species.
Q: A classmate says "Evolution is just a theory, so it might be wrong." How would you respond?
A: In science, "theory" does not mean "guess." A theory is a well-tested explanatory framework supported by a large body of evidence from multiple fields. The theory of evolution is supported by the fossil record, comparative anatomy, molecular biology, and direct observation of natural selection in the field.
This material connects directly to DNA and genetics (covered in the companion study notes): DNA mutations are the source of the variation that natural selection acts upon. It also links forward to ecology and population biology, where natural selection plays out in real ecosystems, and to taxonomy and classification, where evolutionary relationships determine how organisms are grouped.
Related Terms / Search Tags
Evolution, natural selection, survival of the fittest, biological fitness, adaptation, common ancestry, speciation, allopatric speciation, geographic isolation, fossil record, paleontology, comparative anatomy, homologous structures, vestigial structures, molecular biology, DNA comparison, theory of evolution, Charles Darwin, descent with modification, BIO 101, intro biology, University of Washington