Evolutionary species and chronospecies

(A) Evolutionary species. Forms a, b, & c are representatives of distinct lineages of ancestor-descendent populations (grey shading) that undergo morphological change over time. Forms a & c are contemporary, form b is a fossil species that disappeared through extinction. Form b arose by speciation at t₁ and persisted unchanged after speciation of form c at t₂. Notice that the shaded ranges of variation in each species are all distinct on the "morphology" axis.

(B) Chronospecies. A single lineage a evolves over time into a morphologically distinct form b (left). If the connection is recognized, both forms will be assigned to a single evolutionary species. Especially if the fossil record of the lineage is incomplete (right), forms a & b may be regarded and named as separate biological species. Form a undergoes a "pseudoextinction" when it evolves into form b.

Homework: What are  the similarities and differences between the situations of forms a & b in part (B) versus those of forms b & c in part (A)? (after Futuyma 1997).

Text material © 2005 by Steven M. Carr