Issues with classification based on experimentation
As the modern synthesis coalesced, the biological species concept (BSC) (Dobzhansky, 1935; Mayr, 1942) also gained wide acceptance. Although the BSC created an experimentally verifiable mechanistic definition of species (Coyne & Orr, 2004), the concept did not help to resolve the issue of the nature of intermediates in the process of speciation.
Classic taxonomists felt threatened by the idea of classification below the species level as well as the biological species concepts because both were predicated on experimentation being necessary for classification. Peter Raven was adamantly opposed to the idea that experiments might be used as a method to classify groups of populations and declared: ‘The period of 1935–1960 in particular was marked by a “conflict of categories” in which some workers attempted to substitute experimental criteria for morphological–ecological ones in plant classification, and we are not yet completely free of the effects of this confusing and naïve effort’ (Raven, 1976: 288). In other words, the biological species concept and therefore any categorization scheme based on gene flow and reproductive isolation threatened the established taxonomic rules dictating the categorization of plant species.
Empirical results reveal complexity of the ecotype concept
More importantly than issues of taxonomy, other plant evolutionary biologists found results inconsistent with the concept of ecotype. Over the second half of the 20th Century, it became clear that the distribution of adaptive genetic variation within a plant species could range from a smooth distribution along an environmental gradient to extremely discrete. The classic works by Gregor (1930, 1931, 1938, 1939) on the distribution of phenotypic variation of Plantago maritima along the coastline of Great Britain, by Langlet (1936, 1971) on the Scots Pine (Pinus sylvestris), and by McMillan (1959, 1965, 1967, 1969) on grasses across the Great Plains of North America contrasted with that of Clausen, Keck, and Hiesey. In these three systems, phenotypic variation was not associated with discernable ecotypic groups but, rather, was distributed continuously. Huxley (1938) was concerned that this type of variation was overlooked by classification systems of his era and thus introduced the term ‘cline’ to describe graduation of variation in traits over space. Langlet (1963, 1971) argued that all intraspecific plant variation should be described by clines and that Turesson had received far too much credit for what he believed to be the false concept of ecotype: ‘a term automatically and unavoidably gives an impression of uniformity within and disparity between the groups to which it is applied’ (Langlet, 1971: 706). Just as Langlet found gentle clines in traits in pine trees, Calvin McMillan (1969) found that trait variation of the grass species he studied across the Great Plains of North America was distributed gently (Fig. 1B). However, McMillan (1967) argued that this was a function of the distribution of environmental variation, and that Clausen, Keck, and Hiesey were conducting research over much steeper ecological gradients, which were more likely to drive ecotype formation (Fig. 1C). Notably, recent research on one of McMillan's grasses, Panicum virgatum, has revealed that, although many traits are distributed clinally across Eastern North America, there are also phenotypically divergent upland and lowland ecotypes that are interspersed across much of this gradient (Casler, 2005; Zhang et al., 2011). Thus, even in regions such as the Great Plains of North American, there may be sufficient topographical variation to drive the formation of discernable ecotypes.
G. Ledyard Stebbins, in his grand review Variation and Evolution in Plants, appears to make a similar argument to that of McMillan in that widespread distinct ecotypes could evolve under highly heterogeneous conditions but not across gentle environmental gradients:
‘In species occupying an area like the eastern United States, which is comparatively uniform in many climatic characteristics and where a single set of factors, such as temperature and length of the growing season, varies gradually and continuously, continuous or clinal ecotypic variation will be particularly prevalent. On the other hand, diversity and discontinuity of the available habitats will promote the differentiation of more distinct, easily recognizable groups of biotypes within the species, and therefore distinct ecotypes’ (Stebbins, 1950: 47).
Stebbins concluded that: ‘clines and ecotypes are not mutually exclusive concepts, but merely express different ways of approaching the same problem’ (Stebbins, 1950: 48). However, three decades later, Stebbins expressed deep doubts about ecotypes: ‘as Mayr has commented, Turesson was definitely a typologist’ and ‘where the gradients were continuous he [Langlet] had continuous clines, and where the gradients were abrupt, as between central Sweden and Lapland, he had sudden change. Langlet's careful, more complete survey showed that Turesson was incorrect’ (Stebbins, 1980: 141). Stebbins makes no caveat about wind pollination promoting clinal instead of ecotypic variation here as he did in 1950. Similarly, Stebbins criticized the Carnegie group: ‘Clausen found clines that he would not recognize because he saw discontinuities in them’ (Stebbins, 1980: 143). Here, Stebbins failed to recognize that clines describe single characters, whereas ecotypes reflect multidimensional trait variation across space as Clausen had argued: ‘The term cline can be used only for individual characters and not for an assemblage of characters of group’ and ‘clines are therefore not commensurate with natural entities, and are oversimplified abstractions’ (Clausen, 1951: 28).
Stebbins' rejection of intermediates in the stages of speciation may have had considerable influence on how others perceived the views of botanists because he was the primary spokesperson of plant evolution biologists for much of the 20th Century. It is thus unfortunate that neither Turesson, nor Clausen were still alive to respond to Stebbins in the 1980s. Both Keck and Hiesey far out lived both of them and their views were unwavering. In a set of interviews conducted by Joel Hagen in 1981, they made their support clear:
Keck: ‘Turesson did great work. He was a keen observer with good imagination. His ecotype, ecospecies, cenospecies distinctions were indeed valid and extremely helpful. They could be applied in nature to genus after genus’
Hiesey: ‘Perhaps the greatest impact of Experimental Taxonomy on orthodox taxonomy in the 1930s and 1940s was (1) to bring an increasing awareness of the importance of variation within species in the description and delimitation of species and (2) a realization of the significance of cytology and genetics in throwing light on species relationships’
At the opposite extreme from Langlet and McMillan, another set of researchers discovered adaptation of plants to extremely local edaphic conditions, such as mine-tailings (Jain & Bradshaw, 1966; Antonovics & Bradshaw, 1970; Snaydon, 1970) and serpentine outcrops (Kruckeberg, 1951). Kruckeberg (1951) found evidence for discrete serpentine-adapted ecotypes of Achillea within the range of ecological races previously documented by Clausen et al. (1948). Given this result, Kruckeberg argued: ‘In light of the case of Achillea borealis where edaphic races appear to be superimposed upon climatic races … ecotype seems appropriate only when a single environmental factor is under scrutiny’ (Kruckeberg, 1951: 415) because analysis under multiple environmental conditions ‘would render the term “ecotype” synonymous with either a local population or a small segment of a population’ (Kruckeberg, 1951: 416). He concludes: ‘Natural populations might best be visualized as consisting of a continuous or discontinuous array of ecotypic variation in response to the sum total of environmental factors in an area’ (Kruckeberg, 1951: 416). Although sets of serpentine and mine adapted populations are ecotypes in the sense that they share composite of many similar traits (heavy metal tolerance, drought tolerance, flowering time, etc.), they do not negate the more regionally widespread ecotypes within which they occur.
Gene flow and the cohesiveness of species
Given the mounting evidence that functional genetic variation within plant species could be distributed continuously or extremely discretely, multiple reviews (Heywood, 1959; Langlet, 1963, 1971; Quinn, 1978) dismissed the utility of the term ecotype. Quinn's (1978) main contention with ecotypes was rooted in his disbelief that widespread ecotypes could ever form because: (1) Quinn believed that near uniform environments were necessary for ecotype formation and argued that such environments are never geographically widespread and (2) gene flow is too low among plant populations to maintain the cohesiveness of widespread ecotypes or species. The second argument is at least partially rooted in the views of Ehrlich & Raven (1969), who, upon reviewing the data showing patterns of restricted gene flow among populations, argued that gene flow was insufficient to hold species together and, thus, the biological species concept itself was flawed. The proliferation of species concepts emerging during that era that followed has also been attributed to Ehrlich & Raven's viewpoints on this issue (Morjan & Rieseberg, 2004).
In response to the arguments that widespread ecotypes or species cannot persist as a result of low levels of cohesive gene flow, Verne Grant reasoned that ‘extensive interbreeding within the population system is not an essential property of biological species; non-interbreeding with other population systems is’ (Grant, 1981: 91). In other words, it does not make sense to make arguments about what holds a species together when it will continue on as a species unless reproductive isolation breaks it apart. Grant then goes on to write (Grant, 1981: 92), ‘Biological species represents a stage in divergence … and other stages of uncompleted speciation and secondary refusion of species also exist. Consequently the array of population systems at any given time consists of both biological species and semispecies’. Thus, Grant, similar to Keck and Hiesey, was a holdout supporter of stages in the formation of plant species.
Recent discussions of clines and ecotypes
During the 1980s and 1990s, there was less discussion in the literature regarding ecotypes and stages in speciation. Briggs & Walters (1997), in the final edition of Plant Variation and Evolution, presented a brief summary of Clausen's ideas on stages in the process of speciation, although they did not take any particular stand on validity of Turesson and Clausen ideas. Briggs and Walters nonpartisan treatment of the subject viewed variation as being distributed in different ways depending on the characteristics of a species and geographical features of its range, and noted that ‘with hindsight one can see in Turesson's own results the possibility that, in common species, variation patterns were more complex than the ecotype concept implied’ (Briggs & Walters, 1997: 190). Linhart & Grant (1996), who conducted the most comprehensive review of local adaptation in the 1990s, suggested that ‘the cline versus ecotype controversy has not proved particularly useful and it has mostly faded’ because ‘some characters can vary gradually, others discontinuously, depending on, for example, gene flow, intensity of selection, number of genes involved, and terrain configuration’ (Linhart & Grant, 1996: 241). It is true that individual traits may vary in different ways but, as mentioned above, ecotypes reflect the composite response of multiple traits to the common selection pressures of ecoregions.
Local speciation and chromosomal rearrangements
One of the few plant botanists of the 1990s to take a strong stand regarding the question of intermediate stages in the process of speciation was Donald Levin. Levin (1993, 1995, 2000) argued that species formation occurs almost exclusively at the level of the local population or meta-population (Barrett, 2001; Wilson & Kimball, 2001). His arguments against geographical widespread stages in the formation of species were almost exactly the same as those of Quinn (1978) regarding his doubts about the existence of widespread uniform environmental conditions and that sufficient gene flow could occur within widespread ecotypes to facilitate their conversion to species. Levin's (1993, 2000) viewpoints also have deep roots in peripatric founder effect speciation (Mayr, 1954; Coyne, 1994) and quantum speciation (Lewis, 1962; Grant, 1981).
Levin's argument for local speciation are based on the assumption that underdominant chromosomal rearrangements are the most significant source of reproductive isolation among species and that massive ecogenetic reorganizations occur rapidly in bottlenecked populations to facilitate this process. Furthermore, Levin (1993) argued that widespread ecotypes could not be converted to good species because it would be difficult for underdominant chromosomal rearrangements to spread and complete speciation over wide geographical areas. However, recent studies suggest a more limited role for the involvement of underdominant rearrangements, at least in the early stages of speciation (Rieseberg, 2001; Gottlieb, 2004; Rieseberg & Willis, 2007; Lexer & Widmer, 2008; Lowry et al., 2008a; Bomblies, 2010; Lowry & Willis, 2010; Rieseberg & Blackman, 2010). By contrast to underdominant chromosomal rearrangements, there is mounting evidence that chromosomal rearrangements frequently capture adaptive loci, which facilitate the spread of those rearrangements (Kirkpatrick, 2010). Thus, genome repatterning may at least in part be driven by geographically widespread natural selection, which is the same selection as that responsible for the evolution of widespread ecotypes. The question that remains is whether natural selection or drift processes are more often responsible for the evolution of genic incompatibilities and chromosomal repatterning that Clausen considered to be so important for preventing the reversal of speciation at later stages in the process.