Lecture 8 - Macroevolution rates and dates

Macroevolution definition

Evolutionary change occurring at or above the species level, including speciation, extinction, and large‑scale patterns of diversification.

Rates of evolution

The speed at which evolutionary change accumulates, varying across lineages, traits, and geological time.

Gradualism model

Evolution proceeds slowly and continuously, with small incremental changes accumulating over long periods.

Punctuated equilibrium model

Long periods of evolutionary stasis are interrupted by short bursts of rapid change, often associated with speciation events.

Stasis

A period where species show little or no morphological change despite long geological durations.

Adaptive radiation

Rapid diversification of a lineage into many species, often following ecological opportunity or key innovations.

Key innovation

A novel trait that allows access to new ecological niches, triggering rapid diversification (e.g., wings in insects).

Mass extinction

A global event causing widespread species loss, resetting ecosystems and opening niches for surviving lineages.

Background extinction rate

The normal, low rate of species extinction outside of mass extinction events.

Dating evolutionary events

Uses molecular clocks, fossils, and phylogenetic methods to estimate when lineages diverged.

Molecular clock assumption

Genetic mutations accumulate at an approximately constant rate over time, allowing divergence time estimation.

Relaxed molecular clocks

Models that allow mutation rates to vary across lineages, improving accuracy when rates are not constant.

Calibration points

Fossils or geological events used to anchor molecular clock estimates to absolute time.

Fossil record limitations

Fossils are incomplete, biased toward hard‑bodied organisms, and rarely preserve soft tissues or DNA.

Stratigraphy

Dating fossils based on rock layers; older layers lie beneath younger ones unless disturbed.

Radiometric dating

Uses decay of radioactive isotopes (e.g., carbon‑14, uranium‑lead) to determine absolute ages of rocks and fossils.

Phylogenetic dating

Uses genetic divergence and molecular clocks to estimate when species last shared a common ancestor.

Crown group

All living members of a clade plus their most recent common ancestor.

Stem group

Extinct lineages more closely related to a crown group than to any other group, but not part of the crown group itself.

Diversification rate

The balance between speciation and extinction rates within a lineage.

Speciation rate

The frequency at which new species arise within a lineage.

Extinction rate

The frequency at which species disappear from a lineage.

Rate heterogeneity

Evolutionary rates differ across lineages due to ecological, genetic, or environmental factors.

Long branch attraction

Fast‑evolving lineages appear falsely related due to high substitution rates, distorting macroevolutionary inference.

Saturation effect

Over long timescales, multiple mutations at the same site obscure true evolutionary distances.

Using multiple genes

Reduces error in dating and rate estimation by averaging across different evolutionary histories.

Comparative methods

Statistical tools that use phylogenies to test hypotheses about trait evolution, rates, and correlations.

Evolutionary tempo

The pattern or speed of evolutionary change (e.g., gradual, episodic, punctuated).

Evolutionary mode

The type of evolutionary change, such as directional selection, stabilising selection, or random drift.