Species are fundamental units of biology, but there remains uncertainty on both the pattern and processes of species existence. Are species real evolutionary entities or not? If they exist, what are the main processes causing independent evolution and character divergence to occur? This chapter describes how systematic analyses of combined DNA and morphological data can be used to shed light on the evolutionary nature and origin of species. One widely debated test-case has been the question of whether asexual organisms diversify into species; but empirical studies are rare and discussions have often been hampered by allegiance to restrictive species concepts. We present an alternative approach, testing a set of hypotheses for what evolutionary entities might be present in a classic asexual clade, the bdelloid rotifers. Combined analyses of genetic and morphological data reveal the existence of distinct entities conforming to the predicted effects of independent evolution and isolation between sub-lineages. Interestingly, different components of what is meant by ‘species’ do not strictly coincide. We discuss the applicability of related methods to sexual lineages and to the question of whether higher taxa are real.

Introduction

Diversity is a fundamental property of the living world. However, despite long interest in the causes of diversification, we still have an incomplete understanding of patterns and processes behind the evolution of biodiversity. Most evolutionary theory has focused around the concept of species – diversity appears to be packaged in distinct units and a wealth of theory recognizes independent evolution as the cause for this phenomenon (Coyne & Orr 2004; Gavrilets 2004).