Branching and Unbranched Evolution

Cladogenesis applies to populations that become isolated from one another and subsequently diverge in different directions.

Anagenesis is a pattern of descent in which species form within a single, unbranched line with gene flow throughout the entire process.

Evolutionary tree diagrams show a simple way to show lines of descent from a common ancestor.

The gradual model of speciation is represented by tree diagrams with branches at slight angles to each other to show slow change over time. This fits well with evidence from certain lineages in the fossil record.

The punctuation model of speciation is drawn with short, horizontal branches that represent abrupt periods of speciation followed by stable periods. Most changes in morphology are compressed into a brief period near the onset of divergence. This is supported by fossil evidence in some lineages

Adaptive Radiations

An adaptive radiation is a burst of microevolutionary activity that results in the formation of new species in a wide range of habitats. A single lineage gives rise to many new species. New species fill a vacant adaptive zone. An adaptive zone is a "way of life". The presence of adaptive zones presents new ways of life by physical, evolutionary, or ecological access.

Extinctions

Extinction is an irrevocable loss of a species. Mass extinctions have played a major role in evolutionary history. The fossil record shows 20 or more large-scale extinctions. This reduced diversity is followed by adaptive radiation.

Background extinction is the rather inevitable loss of species as local conditions change over periods (usually long) of time.

Mass extinctions are abrupt disappearances due to catastrophic, global events. Species survival is to some extent random. For example, asteroids have repeatedly struck the Earth destroying many lineages. Changes in global temperature favor lineages that are widely distributed.