Unit 7 Evolutionary Reasoning: How Biologists Know Life Changes Over Time

Evolution

Change in the genetic makeup (allele frequencies) of populations over generations.

Fossil record

Preserved history of past life in materials like rock, amber, or ice that provides time-ordered evidence of change through time.

Transitional features

Combinations of ancestral and derived traits in fossils that fit predictions of descent with modification; not required to be direct ancestors of modern species.

Homologous structures

Features in different species that are similar due to inheritance from a common ancestor; functions can differ.

Analogous structures

Features that are similar due to similar selective pressures (similar function/environment), not due to recent common ancestry; evidence of convergent evolution.

Vestigial structures

Reduced or unused remnants of features that were functional in ancestors; may retain minor or secondary functions.

Comparative embryology

Study of similarities in early development across species, including shared structures, timing, and gene expression patterns, supporting shared ancestry.

Biogeography

Study of how species are distributed across Earth; geographic patterns can reflect isolation, divergence, and Earth history (e.g., islands, continental drift).

Sequence similarity

Molecular pattern where closely related species tend to have more similar DNA/protein sequences than distantly related species, implying a more recent common ancestor.

Pseudogene

A nonfunctional gene copy (often produced after gene duplication) that can accumulate mutations without being selected to maintain function.

Shared retroviral insertion

A shared viral DNA insertion at the same genomic location in different species; unlikely to occur independently in the same way, so it strongly suggests common ancestry.

Common ancestry

The idea that two or more species share an ancestral population in the past.

Descent with modification

Core evolutionary concept that lineages split (speciation) and branches accumulate genetic changes over time, producing both unity and diversity of life.

Phylogenetic tree

Branching diagram representing hypotheses about evolutionary relationships among taxa based on evidence (traits, DNA, etc.).

Clade

Group that includes a common ancestor and all of its descendants.

Most recent common ancestor (MRCA)

The most recent ancestral node shared by two taxa on a phylogenetic tree; a more recent MRCA indicates closer relatedness.

Synapomorphy (shared derived character)

A trait that evolved in the common ancestor of a group and is present in its descendants; used to define clades.

Gene duplication

Creation of an extra gene copy; one copy can keep the original function while the other is free to accumulate mutations and possibly gain new/specialized function or become nonfunctional.

Endosymbiotic theory

Hypothesis that mitochondria (and chloroplasts) originated when an ancestral cell formed a symbiosis with bacteria that became organelles; supported by their own DNA, binary-fission-like replication, and double membranes.

Allele frequency

Proportion of a particular allele in a population; changes in allele frequencies over time constitute evolution in population genetics.

Hardy-Weinberg equilibrium

Null model predicting genotype frequencies (p^2, 2pq, q^2) when no evolution occurs at a locus under ideal conditions (large population, random mating, no mutation, migration, or selection).

Natural selection

Mechanism where individuals with certain heritable traits leave more offspring than others, causing associated alleles to increase in frequency.

Miller-Urey experiment

Classic experiment showing that amino acids and other organic molecules can form abiotically under simulated early-Earth conditions; it did not create life.

Protocells

Simple cell-like compartments (often lipid vesicles) that can form spontaneously and help concentrate molecules, separate internal chemistry, and allow selection on protocell-level traits.

RNA world hypothesis

Hypothesis that early life relied on RNA to both store genetic information and catalyze reactions, helping address the need for heredity and catalysis before modern DNA/protein systems.