MULTIVARIATE ANALYSIS OF THE INTROGRESSIVE REPLACEMENT OF CLARKIA NITENS BY CLARKIA SPECIOSA POLYANTHA (ONAGRACEAE).

Introgressive hybridization has received considerable attention following the initial delineation of the concept by Anderson and Hubricht (1938). Introgression was viewed as ". . . the introduction of a comparatively few genes from one species into the germplasm of another . . ." (Anderson, 1949, p. 18). As such, the process offers the recipient species the unique opportunity of testing the adaptive value of genes which have already survived the gauntlet of selection in a closely related donor species. However, no consensus has been reached as to the evolutionary importance of introgression. At one extreme the hybridization required for introgression is viewed as merely "evolutionary noise" (Wagner, 1970) with no biological significance while at the other extreme it is considered of importance in producing totally new genetic combinations, even capable of yielding new genes (cf. Watt, 1972). Further, there is no agreement on the frequency of introgression in natural populations. Certainly the number of strongly documented cases for introgression is very low (Heiser, 1973). It is not clear whether this deficiency is due to a rarity of introgression or rather due to the difficulty of recognizing this process in natural populations. The study reported here indicates the latter may be the case. Clarkia speciosa and C. nitens are outcrossing annuals of central C'alifornia. C. speciosa consists of several ecogeographic races in the Coast Ranges and one in the foothills of the southern Sierra Nevada, C. s. polyantha. C. s. polyantha is replaced northward in the sierra foothills by C. nitens, which was originally recognized as distinct from C. speciosa because of numerous floral and vegetative differences and the very low fertility of artificial hybrids (Lewis and Lewis, 1955). The low fertility was due to the fact that C. nitens and C. speciosa differ in chromosome arrangement. The former species has the "North" arrangement and the latter the "South" arrangement, and these differ by seven reciprocal translocations involving eight of the nine haploid chromosomes. However, additional work revealed one major exception to this simple correspondence between the morphological and chromosomal differentiation in the two species. The populations immediately south of C. nitens for a distance of about 100 miles appear to resemble C. s. polyantha most closely but have the North chromosome arrangement characteristic of C. nitens (Mosquin Lnd Lewis, 1959). The polyantha phenotype, therefore, in the southern half of its range is associated with the South chromosome arrangement (typical of C. speciosa) but the northern populations of C. s. polyantha have the North chromosome arrangement which is otherwise restricted to C. nitens. The North and South chromosome arrangements have been found to be separated by a 20 mile wide boundary (within the C. s. polyantha phenotype) in which populations have a minimum of ten different chromosome arrangements (Bloom and Lewis, 1972). Each of these boundary arrangements appears to be distributed quite locally; they differ from the North and South arrangements by as many as six translocations. Chromosomal studies of