AP Biology Unit 7 Notes: Speciation, Phylogenetic Trees, and Extinction

Phylogeny

The evolutionary history and inferred relationships among organisms; a hypothesis about common ancestry and “who is related to whom.”

Phylogenetic tree (cladogram)

A branching diagram that represents inferred evolutionary relationships among taxa; shows lineage splitting from common ancestors (not a “ladder of progress”).

Clade (monophyletic group)

A group consisting of a common ancestor and all of its descendants.

Sister taxa

Two lineages that share an immediate (most recent) common ancestor.

Homologous structures (homologous traits)

Features that are similar because of shared ancestry (e.g., the same underlying forelimb bones in humans, bats, and whales).

Analogous traits

Features that are similar due to convergent evolution rather than shared ancestry (e.g., bird wings vs. insect wings); can mislead phylogeny if treated as evidence of close relatedness.

Convergent evolution

Independent evolution of similar traits in different lineages, often because they experience similar environments or selection pressures.

Cladistics

A phylogenetic approach that groups organisms based on shared derived characters to identify clades.

Shared derived character (synapomorphy)

A derived trait shared by members of a clade; especially informative for identifying that clade and its common ancestor.

Outgroup

A taxon outside the group of interest used to infer whether a trait is ancestral or derived (if the outgroup has the trait, it is more likely ancestral).

Maximum parsimony

A method for choosing among possible phylogenetic trees by favoring the tree that requires the fewest evolutionary changes.

Molecular clock

The idea that some DNA/protein sequences accumulate changes at a roughly predictable rate, allowing estimates of divergence times (often requiring calibration, e.g., with fossils).

Speciation

The process by which one ancestral population splits into two or more species, increasing biodiversity by creating separate evolutionary lineages.

Biological Species Concept (BSC)

Defines a species as populations whose members can interbreed in nature to produce viable, fertile offspring and are reproductively isolated from other such groups.

Gene flow

Movement of alleles between populations; tends to make populations more similar and can slow or prevent divergence.

Reproductive isolation

Any feature that reduces or prevents gene flow between populations, allowing them to diverge via selection, drift, and mutation.

Prezygotic barriers

Reproductive barriers that prevent mating or fertilization (e.g., habitat, temporal, behavioral, mechanical, or gametic isolation).

Postzygotic barriers

Reproductive barriers that act after fertilization, reducing gene flow by lowering hybrid fitness (e.g., reduced hybrid viability, reduced hybrid fertility, hybrid breakdown).

Allopatric speciation

Speciation caused by geographic separation that reduces gene flow; populations diverge over time and evolve reproductive barriers.

Sympatric speciation

Speciation without geographic separation; gene flow is reduced by mechanisms such as polyploidy, disruptive selection with assortative mating, or host/habitat shifts.

Polyploidy

A change that increases chromosome number (common in plants) that can create immediate reproductive isolation because polyploids may not produce fertile offspring with the original diploid population.

Adaptive radiation

Rapid evolution of many species from a common ancestor when new ecological opportunities arise (e.g., new habitats, reduced competition, or after extinctions open niches).

Background extinction

The ongoing, “normal” rate of extinction caused by typical ecological and evolutionary processes (e.g., environmental change, competition, disease, small population vulnerability).

Mass extinction

A relatively short geologic interval when extinction rates rise dramatically and many lineages are lost, often opening niches that can promote diversification among survivors.

Coextinction

Extinction of one species leading to extinction of another species that depends on it (e.g., a specialized parasite losing its host, or a plant losing a specific pollinator).