What is a Species?
The notion of what a ‘species’ is is widely understood at some level. Man has always distinguished between and named the plants and animals around him but a formal naming system with rules and objectives was first established in the last decades of the 17th and the first half of the 18th centuries.
Taxonomy is the science of naming and cataloging living things, the father of which is Carl Linnaeus. Such a reference system is the foundation of the study of natural history. His particular innovation was the binomial system where a species is represented by both a Genus and a species name [like Homo sapiens]. By convention the Genus is capitalized, the species is in lower case and both are underlined or italicised.
A Genus is a group comprising one or more species that share some salient ‘morphological’ features but differ in some others. ‘Morphology’ is the study of form and has provided a very sound basis for distinguishing between species and defining Genera [plural of Genus] in a meaningful way but now biochemical and genetic characteristics can also be important. Above genus there are Families, containing one or more Genera and Orders containing one or more Families and so forth.
It all seems pretty straightforward, you use pictures or description to match a specimen with a name in a book but this conceals what is really going on with taxonomy. Imagine you have collected a number of similar-looking plants. Now, no two individuals are identical so how do you decide how different two individuals need to be to be called different species? Originally species barriers were confirmed by whether a cross could produce fertile offspring; an unambiguous test. However, some things that are different species by all other criteria can cross and not all organisms even have a sexual cycle.
There are many attributes that can be observed, measured, recorded. But what attributes are the most reliable for separating similar individuals into different species? Primarily, attributes are chosen if they are ‘conservative’, that they show minimum differences in a population of a given species. Conservative features are considered to be important to the success of the organism based on the idea that they are conservative due to the selective pressure of natural selection. So the taxonomy of higher plants, for example, is based on the detail of flower structure. That said, it is often very unclear how some feature could possibly be important.
Taken together these form what is referred to as the ‘species concept’; it’s like a definition. Think of a wolf and coyote, two species that are similar but one is larger and more heavily built. Now think of a Toy Poodle and a Great Dane, the same species but apparently very different. The taxonomy of mammals is based on teeth and certain other features of the skull, not hairiness or size although in other groups these features could be useful markers to aid identification.
The binomial name serves as a place marker for any data about that species. In an ecological connection, you want to know the name because that will give you some information about what the organism does. This would certainly include life-cycle and structural information, most probably biochemical information, it may give you a hint about the environmental and edaphic conditions where it is growing and may give you information about what other organisms are likely to be present. The real test of such a system is whether it ‘works’, whether by describing and naming a specimen yields extra information; that it predicts something that before the identification you didn’t know.
Other notes:
Although we usually think of the ‘species’ as the basic unit of ecosystems and evolution, in my view this is mistaken. The concept of ‘fitness’ has no meaning outside of the associated organisms. In short, no species would pass the ‘natural selection’ test [of surviving] without the surrounding community of organisms. Perhaps we can return to this huge subject in future.
There are categories below ‘species’ level as well. If a property is observed to be consistent but minor a ‘sub-species’ is sometimes erected. For horticulture where breeding programs have brought out specific characters of a species, the terms ‘variety’ or ‘cultivar’ is applied. Varieties do not ‘breed true’ when they are the result of a hybrid cross - their progeny will ‘revert to parental type’ and the features of the variety are lost. Even fussier but also useful, fungal plant pathogens often have attached to the binomial a ‘formae speciales’ which refers to the plant that it infects since within a species, indistinguishable varieties will infect only a specific plant species.
Most species have been described by two or more workers independently of each other so there has to be ‘rules of nomenclature’ to determine the official name. The oldest name takes precedence except very occasionally when a more recent name has become entrenched through years of use before an older report is re-discovered or if it isn’t absolutely clear the older report describes the same species.
Taxonomists inhabit one of two tribes: the splitters and the lumpers. A splitter might consider a genus to contain, say 20 good species and a lumper might find 8 or 10 on the basis that some of the splitters’ species can’t be reliably separated in practice or the differences are too insignificant. Often this comes down to how much a given diagnostic character varies with growth conditions, in other words is not conservative enough to be strictly interpreted. Genus boundaries can also be disputed but this is usually about whether some shared trait becomes to be interpreted as more important than one used to split the group, a hierarchical dispute.
Hybridization is where different species cross and also where these hybrids subsequently cross. In certain plant groups especially, this can erode the species concept and cloud interpretation of species boundaries.
Mick Frank
