Speciation: Introduction

Abstract

Speciation is the formation of two or more new species from one ancestral species. How new species form is a complex process and a fundamental biological question. Answering this question provides insights into patterns of biodiversity and informs conservation strategies. Research on the causes and patterns of speciation is heavily influenced by species concepts – how we define species. The biological species concept is perhaps the most influential, and it groups individuals into species on the basis of the ability to interbreed. Consequently, most research on the process of speciation is driven by the search for mechanisms that affect reproductive compatibility among closely related lineages. Once formed, a new species can maintain its identity by sharing advantageous genes among dispersed and reproductively compatible populations. Conversely, distinguishing traits can be maintained between species via isolating mechanisms. These contrasting mechanisms produce discrete clusters of phenotypic variation rather than a continuum of forms.

Key Concepts:

  • Speciation tends to form clusters of biodiversity rather than continuous gradients.

  • Species concepts heavily influence conclusions about how speciation occurs.

  • Choice of species concept depends on the goal of the research and the organisms being studied.

  • Species remain distinct from each other because of various isolating mechanisms, which prevent gene flow between species.

  • Isolating mechanisms have a genetic basis.

  • Hybridisation between species pairs often occurs in nature and can yield insights into the causes of speciation.

  • Hybridisation can lead to the fusion of two species into one, to reinforcement of reproductive barriers, or even to the formation of new ‘hybrid species’ that are ecologically distinct and genetically isolated from their parent species.

Keywords: allopatry; cline; hybridisation; isolating mechanism; polyploidy; postmating barrier; premating barrier; reinforcement; sympatry

Figure 1.

Possible outcomes of contact between populations that were previously isolated by a physical barrier. Broad red arrows represent evolutionary lineages (species). Narrow horizontal arrows indicate gene flow between otherwise independently evolving lineages. The coloration of the region between diverging lineages depicts the evolution of intrinsic isolating barriers (white, no intrinsic barrier; yellow, weak intrinsic barrier; dark green, strong intrinsic barrier). (a) The populations merge and evolve as a single species; (b) reinforcement completes the formation of reproductive isolation initiated in allopatry; (c) the populations retain genetic identity, but form hybrid swarms in zones of contact; (d) the recombination of differentiated genomes results in a new, ‘hybrid’ species.

Figure 2.

When two species come into contact and hybridise, the variation in the hybrid zone can form a cline. If we plot the frequency of a trait or allele along geographic distance we should see a change as we move from positions where one species predominates to the other, with intermediate frequencies in the hybrid zone. When there is little selection against hybrids, gene flow occurs readily and the change occurs smoothly (black line). However, when hybridisation is rare or there is selection against hybrids, we expect an ‘S’ shaped curve which indicates an abrupt change, or steep cline, in trait or allele frequencies due to resistance to gene flow.

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How to Cite close
Hvala, John A, and Wood, Troy E(Jul 2012) Speciation: Introduction. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001709.pub3]