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ECOLOGICAL GENETICS OF THE COLONIZING ABILITY OF ROSE CLOVER (TRIFOLIUM HIRTUM ALL.)
Thirteen populations sampled from among the oldest plantings of rose clover (Trifolium hirtum All.), introduced into the California range durinig the late 1940’s, and 12 roadside populations established through natural colonization were compared for their genetic and demographic features. Roadside colonies showed a greater amount of reproductive effort in terms of a larger number of heads per plant and larger calyx, lower rate of seed carryover, higher and more stable plant density, lower seedling survivorship, and earlier flowering. The calyx was more hirsute in roadside collections and remained attached with the seed, a feature accounting for higher germination probabilities on the soil surface or in litter along the roadsides, in contrast to the range where grazing animals would often work the seed into the soil. Outcrossing rates are slightly higher in roadside colonies in which genetic polymorphisms at three marker loci represented as high levels of genetic variability as in the range populations. The colonizing success of rose clover seems to be largely determined by a few and rapid morphological changes and by the retention of some outbreeding and genetic variation. Such joint analyses of genetic and demographic features of colonizing species are needed to support various deductions about the characteristics of “ideal’ weeds and colonizers. POPULATION biology of colonizing species has been a subject of great interest to man because of its many useful agricultural applications as well as its usefulness in the study of undesirable weed and pest problems. The genetic and ecological characteristics of successful colonizers were explored theoretically by Lewontin (1965) and MacArthur and Wilson (1967) in relation to the life history features. Numerous discussions in the past 15 years have centered on the concepts of r-vs.K-selection, a topic well explored by Stebbins (1958) from his studies in Compositae (also see Salisbury, 1961). A symposium organized by Baker and Stebbins (1965) focused on various features of genetic systems such as polyploidy, breeding system, and genetic polymorphisms. Self-compatibility (Baker, 1955), phenotypic plasticity (Baker, 1974), dispersability (Carlquist, 1974), and combinations of several genetic system features (Ehrendorfer, 1965), among others, have established many excellent plant examples for further experimental work. Harper (1977) extensively reviewed the population biology of plants and showed that many weedy species have been the prime research or’ Received for publication 4 April 1978; revision accepted 20 December 1978. We are grateful to Dr. R. Merton Love for his continued interest in this project, and to Dr. P. A. Werner for her useful suggestions in rewriting the manuscript. 2 Present address: Instituto de Genetica, E.S.A. “Luiz de Queiroz,” 13400 Piracicaba, S.P., Brazil. ganisms. Most deductions about the colonizing characteristics are based on correlations derived from surveys of related plant species. However, very few attempts have been explicitly made in simultaneously (a) following the colonizing episodes of a species in terms of the fate of colonies through census data, (b) analyzing the old and new colonies for the dynamics of survivorship and reproductive rates (demography), and (c) determining the genetic changes through their comparative experimental studies. Several introduced species in California grasslands offer excellent material for experimental studies on colonizing plants (Jain, 1969, 1975). Rose clover (Trifolium hirtum All.), a native annual forage legume of the Mediterranean region and introduced into California in 1946 through extensive plantings, was recently (1966-69) observed actively colonizing certain roadside areas in several counties (e.g., Placer, Nevada, El Dorado). Some of the oldest plantings in Shasta, Madera and Glenn counties were extensive on the range but apparently not successful in expansion to the adjacent roadsides. These observations raised the following issues: How much genetic differentiation among populations of different regions has occurred? In what ways are successful roadside colonies different from the range populations that are in the vicinity? Can we derive certain common colonizing characteristics from the comparisons of demographic and genetic variables in range versus roadside pop
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