2. Evolution and Classification

2.1.  Phylogeny and Classification

There are different ways in which objects can be classified and the human mind is very good at generating criteria for classification. This is why the following list, assembled by the Argentinean author Jorge Luis Borges, and purportedly extracted from an ancient Chinese encyclopedia (Lakoff 1987), strikes us as funny: “[ … ] it is written that animals are divided into:

· those that belong to the Emperor;

· embalmed ones;

· those that are trained;

· suckling pigs;

· mermaids;

· fabulous ones;

· stray dogs;

· those that are included in this classification;

· those that tremble as if they were mad;

· innumerable ones;

· those drawn with a very fine camel's hair brush;

· others;

· those that have just broken a flower vase;

· those that resemble flies from a distance.”

The two major criteria that are used to classify things (neither met by Borges' list), are utility or affinity:

•  Utility generates classifications whose objects are easy to find. An example of such a classification would be a dictionary, whose entries are arranged alphabetically;

•  Affinity, on the other hand generates classification wherein adjacent objects are straightforward to compare (because adjacent entries share important features).

In the European Middle Ages, animal books ('Bestiarum') were usually ordered alphabetically. However, such ordering eventually struck people as odd, especially as people realized, in the course of long debates on 'universals' (on whether names are 'natural' attributes of things, or not), that names are arbitrary labels.

Thus, authors gradually began seeking for natural classifications, wherein organisms are ordered by affinities, these affinities being initially conceived as reflective of the general rules which god used when creating these organisms.

The work of Linnaeus, whose Systema Naturae, the tenth edition (1758) of which still marks the beginning of zoological nomenclature, is an example of such attempts to identify the underlying affinities among plants and animals. The resulting 'natural' classifications have started to make sense, however, only since Darwin, in The Origin of Species (1859), provided a rationale for affinities, that is, shared ancestry. However, Darwin not only provided a basis for the affinities between organisms. He also provided a mechanism by which new species and higher taxa emerged out of common ancestors. This mechanism he called natural selection.

2.2. Darwin and Natural selection

Natural selection is the core of Charles Darwin's work and is best defined in his own terms: “many of every species are destroyed either in egg or [young or mature (the former state the more common)]. In the course of thousand generations infinitesimally small differences must inevitably tell; when unusually cold winter, or hot or dry summer comes, then out of the whole body of individuals of any species, if there be the smallest differences in their structure, habits, instincts [senses], health, etc., <it> will on an average tell; as conditions change a rather larger proportion will be preserved: so if the chief check to increase falls on seeds or eggs, so will, in the course of 1,000 generations, or ten thousand, those seeds (like one with down to fly) which fly furthest and get scattered most ultimately rear most plants, and such small differences tend to be hereditary like shades of expression in human countenance” (Darwin 1842).

Natural selection, thus, consists of three elements:

•  organisms usually produce far more progeny than their habitat can accommodate;

•  each member of the progeny differs in some inheritable attributes or properties;

•  there is a tendency for those progeny with attributes or properties that are more suitable for the habitat in question to suffer a lower rate of mortality and thus for more of them to reach reproductive age than their sibling.

These three features jointly cause animals and plants, over evolutionary time, to 'track' fluctuation of their environment. In this process, and in conjunction with other mechanisms such as the 'founder effect' and the effect of neutral selection, isolated populations can become so different from the mother species that their members will not be able to cross-mate if the barrier that once separated them disappears.

2.3. The species concept

Species are “groups of actually (or potentially) interbreeding natural populations which are reproductively isolated from other such groups” (Mayr 1942, p. 120).

2.3.1. What's in a name?

Since species are the basic rank of biological nomenclature, naming species is very important and we now follow for this a model proposed by Linnaeus (see above), wherein the species is defined by a so-called binomen consisting of a unique genus name, which always starts with a capital letter, and a species epithet, which is never capitalized; usually, both are written in italics. With regard to the capitalization rule, simply recall that the binomen is the short version of an earlier mode of description wherein a whole paragraph was used to describe, and thereby define, a species. The binomen, thus, was the start of a sentence.

Important additions to a species name are the name of the author who first described a species and the year of that description; as in, for example, the Linnaean species Salmo trutta Linnaeus, 1758. At times you will encounter a species, e.g., Oncorhynchus mykiss, with an author's name and year in brackets, e.g., (Walbaum, 1792). This means that the species whose epithet is mykiss was originally described as a member of another genus, in this case, Salmo. However, due to better understanding of its relationships with other trout and salmon species, it was subsequently moved into the genus Oncorhynchus.

Also, many species have been described and named more then once. In that case, the oldest description takes preference, and the names given in later descriptions become 'junior synonyms'.

Another rule important to animal species names is that the genus part of the name must be unique to the animal kingdom. From the year 2000 on, it must also be unique among all organisms. Thus, when a generic name is coined, the author must verify that this name has never been used by any other zoologist, and, from 2000 on, by any botanist, bacteriologist, etc. This seemingly daunting task is not impossible. Global catalogues of organism names are now being created; the most important of these is the Catalogue of Life (see www.catalogueoflife.org). Exercise

•  In www.fishbase.org, go to 'Information by Country' and select your country and 'All fishes.' Look at the scientific names of ten species whose author name is in brackets and identify for each the original name and several synonyms. List and define the different kinds of synonyms.

Example: Oreochromis niloticus (Linnaeus, 1758) [new combination, valid]

Perca nilotica Linnaeus, 1758 [original combination, not valid]

Tilapia nilotica (Linnaeus, 1758) [new combination, not valid]

Tilapia nilotious (Linnaeus, 1758) [misspelling of new combination, not valid]

Tilapia calciati Gianferrari, 1924 [original combination, junior synonym, not valid]

2.3.2. Subspecies vs. populations

Given the mechanism of natural selection, every fish population can be conceived as being a potential new species. All one needs to imagine is that populations become isolated from others long enough for their members to lose the ability to mate with those of other populations. However, as long as some members of each population continue to mate with members of other populations of the same species, a mating barrier will not emerge (only a small gene flow is required to prevent the emergence of a mating barrier). Thus populations, though they may be easy to define in terms of attributes such as number of scales or spines or body proportions, should not be given full taxonomic status, because (contrary to species) they usually do not maintain themselves over a long period. Not having taxonomic status also means they should not have formal names, such as the trinomen that are still frequently used today, e.g., Oreochromis niloticus baringoensis Trewawas, 1983. The third part of the trinomen refers to a subspecies, which is, in fact, a population, or, to use a term much used in earlier times, a 'race'. Fish taxonomists gradually do away with subspecies by either giving them species rank or making their names a synonym of the respective species. In our example, a taxonomist has to review the case and decide whether the individuals referred to as Oreochromis baringoensis Trewawas, 1983 are different enough to be recognized as a valid species, or if the population is well connected with others, in which case Oreochromis niloticus baringoensis Trewawas, 1983 becomes a junior synonym of Oreochromis niloticus (Linnaeus, 1758).

2.3.3. Within-species diversity

Species differ as to the extent of their diversity. Some species consist of a single population of a few individuals – these are often endangered species. Others have wide ranges and a rich population structure. This situation tempted authors to name subspecies. However, it is often not objectively defined within-species diversity which motivated authors to define subspecies, but national or local research traditions, and the resources available for sampling specimens over large areas, and curate them. Thus, Berg (1965) established numerous subspecies and even lower taxa for the fishes of adjacent lakes and rivers of the former Soviet Union, while subspecies are rarely proposed by taxonomists working on the many coral reef species of the Indo-Pacific, although their distribution spans thousands of kilometres with many populations and limited gene-flow.

2.3.4. Common names

The common names of fish are what most people know about most fish. Thus, capturing the common names of fish in various languages captures most of what people who speak these languages know about fish. For this reason, FishBase includes over 260,000 names of fish in over 200 languages, ranging from widespread languages such as English or Spanish, to languages spoken by few speakers, such as Haida in Haida Gwaii, British Columbia. Anthropologists, notably Berlin (1965), have established that essentially all ethnic groups in the world spontaneously differentiate a similar number (about 500) of 'kinds' of organisms, the kinds roughly corresponding to genera, with important species being named, as well as some of their life history stages.

The sounds in fish names also generate interesting patterns. Thus, small fishes tend to have names containing high pitch sounds such as 'i' or 'ee', while large fish tend to have names with lower pitch sounds, such as 'a', or 'aa' (Berlin 1992; Palomares et al. 1999). Exercises

•  Identify a language with at least 50 different common names in FishBase. Relate the number of species with i/ee sounds in their names against the maximum length reported for those species, i.e., test the occurrence of a sound-size association for fish in the language in question. [Hints: use the Information by country/island search to get a list of common names (and the corresponding scientific names) by language; get maximum size information from the Species Summary page and see item (5) of www.f ishbase.org/Tips.htm on how to export data to a spreadsheet (Excel format) for further analysis.]

•  Find from FishBase, using search by Common Name, some blind species and some species that have the ability to 'fly' out of the water and the ability to 'walk' on land. [Hint: common names of species often contain the word that describes special abilities, characters or traits].