# Biological Evolution Practice

This practice covers three key topics from the Biological Evolution unit. Each section includes a brief review of core concepts followed by questions to test your understanding.

Use this practice to identify what you know well and where you may need to review.

# Section 1: Reading the Rock Record

Earth's history is recorded in layers of sedimentary rock. The Law of Superposition tells us that in undisturbed rock layers, the oldest layers are on the bottom and the youngest are on top. Scientists use relative dating to determine which rocks are older or younger without knowing exact ages, and absolute dating (radiometric dating) to calculate a rock's actual age in years by measuring radioactive decay and half-lives.

Index fossils are remains of organisms that lived for a short time but were widespread geographically. Finding the same index fossil in rock layers at different locations means those layers are approximately the same age, a process called correlation. The geologic time scale organizes Earth's 4.6-billion-year history into eons, eras, periods, and epochs, with boundaries defined by major events like mass extinctions. Additional geological evidence comes from ice cores (which preserve ancient atmospheric samples), faults (cracks where rock shifted), and igneous intrusions (hardened magma that cuts through existing rock).

# Section 2: Natural Selection

Natural selection is the mechanism that drives evolution. It requires four conditions working together. Overproduction: organisms produce far more offspring than can survive, creating competition for limited resources. Variation: individuals in a population differ in their traits. Selection: individuals with traits better suited to the environment survive and reproduce at higher rates (differential survival). Inheritance: advantageous traits are passed to offspring through genes.

Over many generations, beneficial traits become more common and the population evolves. Genetic variation, the raw material for natural selection, comes from three sources: mutations (random DNA changes, the only source of entirely new genetic material), sexual reproduction (meiosis shuffles existing genes into new combinations), and gene flow (migration introduces new alleles between populations). In biology, fitness does not mean physical strength. It means how well an organism is suited to survive and reproduce in its specific environment.

# Section 3: Adaptation and Speciation

An adaptation is an inherited trait that increases an organism's ability to survive and reproduce. Adaptations come in three types. Structural adaptations are physical body features (polar bear fur, cactus spines, webbed feet). Behavioral adaptations are inherited actions (migration, hibernation, pack hunting). Physiological adaptations are internal body processes (venom production, antifreeze proteins, efficient kidneys).

Natural selection reshapes populations in three patterns. Directional selection favors one extreme of a trait, shifting the population in that direction (peppered moths darkening during industrial pollution). Stabilizing selection favors the average and reduces variation (human birth weight clustering around a healthy middle). Disruptive selection favors both extremes over the middle, potentially splitting the population (seed-cracker finch beaks diverging into large and small sizes). Speciation most commonly occurs through geographic isolation: a physical barrier divides a population, gene flow stops, different selection pressures cause each group to diverge genetically, and eventually the groups can no longer interbreed. They have become separate species.