Ecological Barriers

Abstract

Isolating mechanisms or barriers to hybridization were first clearly shown by Moritz Wagner and were studied widely during the last half of the 19th century and again during the past 15 years. There is considerable confusion in the terminology of isolating mechanisms because some of the more important terms have been used with different meanings by different writers. A classification is proposed which omits some of the terms which have been used in several different senses. Since ecological barriers are known to be broken down with the development of hybrid swarms, it is suggested that perhaps a search through the taxonomic literature would show that natural hybridization and hybrid swarms are more common than is generally realized and might indicate that certain plant groups, in different regions of the world, can productively be studied for natural barriers. It is emphasized that certain groups of plants badly need taxonomic revision since hybridization has been extensive in these groups and many of the designated spp. were named and described years ago from single specimens growing in botanical gardens far from their native habitat. This is especially true of the Aloe and Gasteria group.