Part of the ideas behind the Drosophila project is an examination of what exactly constitutes a species, and when it is possible to say that an organism has evolved into a new species. Biologists are conscious of what is referred to as ‘the species problem’, a phrase which points to the difficulties in deciding exactly what species an organism belongs to or when exactly to recognise a new species. There is also the case that sometimes we find that a previously recognised species is in fact two or more separately evolving groups, and that we don’t even actually have a term for that yet (subspecies doesn’t quite do it).
“I was much struck how entirely vague and arbitrary is the distinction between species and varieties”
Charles Darwin. Origen of the Species.
Its usually important to start from the basics, so this is an overview of what we mean by the various taxonomic ranks such as family, genus, species, subspecies, strain, phenotype, etc etc – the various ways in which we distinguish ‘this’ organism from ‘that one’.
Family
Family is actually quite difficult to define specifically, mostly referring to a group of related organisms within a particular order. Usually they will be grouped into a family because of obviously shared characteristics. For example, humans belong to the order of primates (primatae) along with chimps, monkeys and lemurs. We are in the family of hominids (hominidae) due to our shared characteristics with chimps, gorillas and orang utans of being able to walk more or less upright, having no tail, similar dental structure and so on.
Drosophila belong to the family of Drosophilidae within the order of Diptera (generally speaking, flies with 2 wings).
Genus
Once again there is dispute about exactly what is meant by genus. The genus will generally be used as the principle scientific name (in binomial nomenclature) for the organism – such as Drosophila for fruit flies, Canis for dogs and their relatives and Homo for humans.
According to F B Gill the rules of thumb for delimiting a genus are:
a) monophyly – all descendants of an ancestral taxon are grouped together;
b) reasonable compactness – a genus should not be expanded needlessly; and
c) distinctness – in regards of evolutionarily relevant criteria, i.e. ecology, morphology, or biogeography
Species
One of the easiest ways of defining a species within a Genus is that the organisms are capable of breeding and producing fertile offspring. In addition other factors such as similarity of DNA, ecological niche and morphology are also taken into account. Species within a genus will have the same ancestor.
Many different kinds of species have been described, each with their own criteria for defining the term ‘species’ – for example; ecological species, Biological / reproductive species, Biological / Isolation species, Genetic species, Morphological species and so on.
Some biologists may view species as statistical phenomena, as opposed to the traditional idea, with a species seen as a class of organisms. In that case, a species is defined as a separately evolving lineage that forms a single gene pool. Darwin concluded that the term ‘species’ is provisionally useful for naming groups of interacting individuals.
The most commonly species of the Genus Drosophila, and the one I am working with is Drosophila melanogaster. The species name, as well as often the genus name, will often have quite poetic roots – in our case ‘the dark bellied dew lover’.
Subspecies
A species can have none or two or more subspecies but never just one. Organisms that belong to different subspecies of the same species are capable of interbreeding and producing fertile offspring, but they often do not interbreed in nature due to geographic isolation or other factors. The differences between subspecies are usually less distinct than the differences between species.
Phenotype
A phenotype is an organism’s observable characteristics or traits: such as its morphology, development, biochemical or physiological properties and behavior. Phenotypes result from the expression of an organism’s genes as well as the influence of environmental factors and the interactions between the two. Over the course of time a species may modify its phenotype to succeed in or exploit their environment. All phenotypes of a particular organism will belong to the same species. Phenotypic variation is caused by underlying heritable genetic variation and is a fundamental prerequisite for evolution by natural selection.
Strain
A strain is a genetic variant or subtype of an organism which can be produced by inbreeding, artificial selection or genetic engineering. Strains are generally artificially produced for scientific or economical (specifically agricultural) purposes.
