Rethinking classic examples of recent speciation in plants

This paper reviews the evidence that three pairs of diploid annual plant species are related as progenitor and recent derivative. The species pairs are Layia glandulosa and Layia discoidea, Clarkia biloba and Clarkia lingulata, and Stephanomeria exigua ssp. coronaria and Stephanomeria malheurensis. The three cases are examples of Verne Grant's model of 'Quantum Speciation', in which a derived species is budded off and acquires new traits while the parental species continues more or less as before. The derived species differ from their progenitors in different ways and show different modes of reproductive isolation. However, the number of differences between each derivative and its progenitor appears to be few and relatively simple in genetic terms. Comparison of a recently evolved species with its progenitor can reveal what happens during speciation because overall divergence is minimal and the direction of evolution is clear. Evidence from DNA sequences may be particularly useful to recognize additional examples of progenitor and derivative relationship. (C) New Phytologist (2003).