CHROMOSOME PHYLOGENY AND HABITAT PREFERENCE OF CLARKIA

Phylogenies based upon morphological traits are often used to reconstruct the history or movements of taxonomic groups. But these phylogenies are limited by the fact that trends of only the most general sort can be established, because of the numerous factors involved. Phylogenies based upon chromosomal distinctions are more certain because the mechanisms which alter the structure and number of chromosomes are more simple and often result in linear relationships which can be determined with considerable rigor. For example, the chromosomal relationships between a number of species of Drosophila have been demonstrated (Patterson and Stone, 1952). Another outstanding example of a phylogeny based at least in part upon chromosomal consideration is that constructed by Babcock (1947) for the genus Crepis. Clarkia,' a genus of annual plants, offers an unusual opportunity to examine phylogenetic changes in chromosome number at the diploid level in relation to ecological adaptation. The chromosome number of every known species has now been determined (Hakansson, 1941; Hiorth, 1941; Lewis, 1949; Lewis and Lewis, 1953; Lewis and Ernst, in press). Twentythree species are diploid with basic chromosome numbers of 5, 7, 8, and 9. All of these are endemic to western temperate North America and the geographical distribution of each has been determined from extensive field observation and herbarium studies. It has been possible in the course of the field studies to compare the gross ecology of each species throughout most of their distribution areas. The breeding habit, population structure, and mechanisms of evolution in this genus have been discussed elsewhere (Lewis, 1953). The original basic chromosome number of Clarkia is almost certainly 7, not only because its closest relatives have this number but also because cytological evidence indicates that genomes with 5 and 6 chromosomes have been derived from a higher number while those with 8 and 9 have resulted from the addition of the equivalent of whole chromosomes. Clarkia is very similar morphologically to Oenothera and many species have been included in the latter by some taxonomists. Clarkia may have been derived from Oenothera. If not, the two genera have probably had a common ancestor. The basic number of all species of Qenothera that have been determined is 7 (Gates, 1928; Darlington and Janaki Ammal, 1945; Cleland, 1950; etc.) including representatives of at least 6 subgenera that I myself have determined. Furthermore, the species of Clarkia that most closely resemble Oenothera (e.g., C. amoena and C. rubicunda) also have 7 chromosomes. Genomes consisting of fewer than 7 chromosomes are known in two species. One of these, C. amoena, normally has 7 chromosomes but genomes with 6 are sometimes found in wild populations (Ha'kansson, 1942; Hiorth, 1942-as Godetia whitneyi). These genomes of 6 are apparently deficient and occur only as heterozygotes. However, Hiorth (1948a, 1948b) and H'akansson (1943, 1945, 1946) have shown beyond doubt that the genomes of 6 have been derived from the normal 7 through translocation followed by the loss of a centromere region, together, probably, with other small chromosome parts. 1 Clarkia (Onagraceae) includes many species referred to Godetia by other authors.