Few biologists have been willing to explore non-Darwinian explanations of evolution, but some have been highly critical of commonly accepted assumptions. Some systematists, known as cladists (phylogenetic systematists) generally did not offer any alternatives to neo-Darwinian theory, but they were highly critical of the use of “adaptations” when deciding on which traits were most ancient or more recent (known as character polarity). The cladists preferred that character states for the group of organisms being studied (the ingroup) be assessed by comparison to “outgroups,” more distantly related taxa or groups of species. Deciding that a particular character was “advanced” before looking at the entire dataset (including the ingroup of interest plus the outgroups) could lead to erroneous conclusions, they argued, and this constituted poor scientific methodology. One example (Nelson & Platnick?) has to do with phylogenetic relationships among spiders. To us humans, the webs of the orb-weaving spiders seem like such nice pieces of engineering (and so photogenic when covered with dew) that they must be advanced when compared with the messier webs of the cob-weavers. However, when all of the relevant evidence is considered and no character is given extra weight because of its good looks, the orb-weavers are identified as the more ancient group, while cob-spinners appear to be the most recently evolved. In reality, either style of web must surely be capable of capturing insects, but the theory-laden view of some systematists compelled them to consider an orb type web as better adapted, therefore more “advanced.”
Some cladists argued that they needed no theories of evolution in order to accurately describe relationships among species and higher groups – all they needed were good datasets. In fact, to approach any study of relationships by looking for assumed adaptations, they argued, was so theory-laden that it would lead to invalid, biased studies. Many evolutionary biologists and ecologists were horrified – if these cladists weren’t believers in neo-Darwinian theory, then they must be creationists! There are no other options, apparently. In creating this false dichotomy, neo-Darwinians acted to “protect their turf” of evolutionary biology, but they closed off the search for other possible sources of evolutionary causality. This reaction serves nobody.
The cladists had another serious complaint with the selectionist worldview, this one having to do with the definition of species. The classic definition of species, a la Ernst Mayr (a couple of refs…), is the one still offered consistently (and mistakenly) in biology classes. In the classic version, species are defined exclusively by sexual reproduction, as a group of organisms capable of producing fertile offspring by mating with each other, but not with individuals of other species. The problem with this definition is that it excludes species that either don’t reproduce sexually or are able to hybridize (matings between different species) and still produce fertile offspring.
Many plants hybridize readily, given the opportunity, and the hybrid offspring are often quite fertile. In nature one can find populations of the two “parent” species, usually in somewhat different habitats, with a population of hybrids growing somewhere between the two parent populations. If the parents are not species, then what are they?
Other plant species do not engage in sex, but are apomictic (parthenogenetic) where a diploid cell of the parent plant in the proper location within a flower simply develops into an embryo without meiosis, eggs or sperm. Some other plants may have sex with themselves, when the pollen from a flower’s anthers fertilizes the ovules of the same flower. Still other plant species, though capable of sex, tend to engage in vegetative reproduction. For example, a population of lovely quaking aspens (Populus tremuloides) in the Rocky Mountains is likely to be a single, huge individual with all of the trees connected by underground stems. In aspens and related trees (members of the willow family), each plant is one sex only, male or female, an uncommon situation for a flowering plant (most flowering plants are hermaphrodites, both male and female). So, an aspen population that covers many acres may consist entirely of a single male or female individual. In this case there will be little sex. occurring, but reproduction continues.
Are plants that hybridize, self-fertilize, or reproduce by apomixis or by clones not species? What do we call these entities if not species? They exist. We can’t ignore them. There is only one logical conclusion: if there are natural groups that don’t fit within the standard definition, then that definition must be wrong. There must be more to species than sex. Some cladists (Kluge…add ref) argued that, until we have some better understanding of what species are, we ought to settle for an operational definition based on characters shared uniquely by all of the individuals making up the species (autapomorphies). We may not have a full definition, but at least we’ll have a reliable way to recognize the entities we’re calling species. (More on species in a later section.)