Hamrick and Godt, 1990
The level of genetic diversity in plant populations is generally positively correlated with population size (e.g., Hamrick and Godt, 1990; Leimu et al., 2006); however, this is not always the case. Populations can become small and/or species rare for different historical reasons (Rabinowitz, 1981; Gitzendanner and Soltis, 2000), leaving populations with varying levels of genetic diversity in the long-or short-term. The study of Eastwood et al. (2004) is an example documenting a positive relationship between genetic diversity and population size. They found low levels of genetic diversity in several species of Elaphoglossum, which are restricted to the small island of St. Helena, comparable to what is often found in island species of seed plants (Table 4.1). By contrast, Ranker (1994) conducted an allozyme survey of one population of the rare Hawaiian endemic epiphyte, Adenophorus periens, that occurred in a forest covering a lava flow only 300-400 years old. Adenophorus periens had been collected in historical times on all of the main Hawaiian Islands, but in the early 1990s was only known from two populations on the island of Hawaii, with a few scattered individuals known from Kauai and Molokai (Ranker, 1994 and personal observations). The levels of allozymic diversity observed are among the highest ever recorded for any fern (A = 2.8, P = 80.0, and He = 0.213; Table 4.1). Apparently, the reduction in population size of this species has occurred too recently to impact levels of genetic diversity. A similar example of a recently restricted species, albeit heterosporous, still harboring high levels of genetic variation is the rare Isoetes sinensis from China (Kang et al, 2005).
A long evolutionary history coupled with a formerly large population size can allow population lineages to maintain high levels of genetic diversity and to exhibit high levels of divergence across intraspecific lineages. Su et al. (2004) employed cpDNA sequence data from atpfi-rbcl intergenic spacers to explore population genetic structure and phylogeographic patterns among modern relictual populations of Alsophila spinulosa. This species was distributed worldwide during the Jurassic Period (180 million years ago), became much more restricted during the Quaternary Period, and is now extremely rare in China due to the continued loss and fragmentation of habitat because of human destruction. Nevertheless, extant populations possess high levels of haplotype and nucleotide diversity and populations from different regions of China are extremely divergent from each other (Fst = 0.95). Su et al. (2004) suggest that the high levels of genetic diversity within geographic regions may be caused by the accumulation of new mutations over the long evolutionary history of the species. They further suggest that the divergence of populations across different regions has been facilitated because different lineages accumulate different mutations, which has been maintained by low levels of inter-regional gene flow owing to a primarily inbreeding mating system.
Schneller and Holderegger (1996) discuss how a variety of historical and life-history phenomena could have impacted levels and patterns of genetic diversity in small populations of several species of Asplenium and Polypodium vulgare in the lowlands of Switzerland. For example, Holderegger and Schneller (1994) studied three small populations (9, 15, and 30 individuals each) of the autotetraploid Asplenium septentrionale in the vicinity of Lake Zurich. The populations were isolated from each other by 3-7 km and were ca. 40 km from the nearest larger populations in the Swiss Alps. Two populations were genetically uniform, one exhibited some genetic diversity, but each possessed at least one unique isozyme phenotype. The patterns of variation within and across populations suggested that there was zero gene flow among them and that each population primarily reproduced via intragametophytic selfing. The authors suggested that each population might have been the result of independent long-distance dispersal events from larger populations in the Alps.
What are the predominant mating systems operating in natural populations of ferns and lycophytes? Prior to the application of enzyme electrophoretic studies for estimating allelic frequencies and heterozygosities of natural populations of ferns and lycophytes, inferences of mating systems were made by studying gametophyte sexuality in the laboratory (see Lloyd, 1974) and, rarely, in nature (see Cousens et al., 1985, and discussion above on genetic load). As summarized by Haufler (1987) and Soltis and Soltis (1987b), such gametophytic studies are useful for showing the potential of what could be occurring in nature, but do not necessarily reflect what is really happening (e.g., see Chapter 13; Ranker and Houston, 2002). Allozymes have proven to be a powerful tool for inferring mating systems because they represent nuclear-encoded enzymes that are biparentally inherited and co-dominantly expressed. They can be used, therefore, to estimate the observed (Ho) and the expected (He) heterozygosities and, thus, the fixation index (Wright, 1943):
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