Fifty Years of Plant Evolution

Abstract

the last half-century has seen a revolution in the thinking of biologists about evolution. During the 1920s, when I became a biologist, the study of evolution as a separate discipline of the life sciences did not exist. Anatomists and systematists were using evolutionary phylogeny to interpret the facts that they observed. Paleontologists were clarifying the course of evolution, particularly in vertebrate animals, by describing successions of fossils. Most geneticists believed that the principal mechanism of evolution was in the origin of mutations having large and conspicuous effects. Darwinian natural selection was under a cloud, the prevailing belief being that it had only the negative effect of eliminating unfavorable mutations. The burgeoning field of chromosomal cytology was concerned chiefly with analyzing and understanding the mechanisms responsible for the chromosomal cycle, particularly meiosis and crossing-over, and with recording the effects of such chromosomal changes as translocations, inversions, trisomics, and polyploidy. The principal contribution of cytogenetics to evolution, before 1930, was the demonstration that new species can arise suddenly through hybridization—accompanied, preceded, or followed by doubling of the chromosome number. Population genetics did not become a viable discipline until the 1930s, and its marriage with ecology, which produced the modern, dynamic approach to the processes of evolution, was not consummated until the 1960s.