Plant evolution in disturbed habitats: An introduction

Abstract

In this era with a loss of biodiversity to an unprecedented extent, one would expect that disturbance, responsible for considerable habitat destruction and fragmentation, causes the extinction of taxa and prevents evolutionary processes. Many recent studies address the spatiallyand ecologically-disturbed landscape infrastructure, which has many negative effects on the evolutionary and ecological perspectives of many species. In several reported cases, population complexes are broken up, ultimately resulting in the breakdown of the metapopulation. This causes losses of genetic variation through drift and inbreeding, which are not compensated for by gene flow. In combination with reduced reproductive success this genetic erosion clearly lowers the population viability of such taxa. This process ultimately may lead to regional or total extinction (ELLSTRAND & ELAM 1993, YOUNG et al. 1996). Human activities do not only destruct habitats, but they lead together with climate change to the spread of species far beyond their natural ranges. Alien species may threaten the indigenous flora, completely change the character of the place they invade, cause diseases and be pest organisms. The problem of biological invasions is growing in severity as global trade and travel accelerate. Habitat disturbance increases, and global climatic change is to be expected. Habitat disturbances and biological invasions create contact zones between conand heterospecific populations which were isolated by distance and/or by the environment. Natural hybridization might be the outcome. This has been viewed as deleterious for biodiversity. Numerous examples have been cited where rarer species are considered "threatened" by consequences of hybridizations with a more common, related taxon, as for instance outbreeding depression and genetic assimilation. On the other hand, hybridization is undoubtedly part of the evolutionary history of plants which is especially documented by molecular markers. It becomes more and more obvious that hybrid zones are focal points of genetic interactions between plant species, and may provide the raw material for adaptive evolution in rapidly changing environments (ARNOLD 1997, RiESEBERG & CARNEY 1998). Many years ago, the principle of hybridization of the habitat was introduced to describe situations where, due to human influence, two ecologicallyand spatially-separated habitats