By Donna M. Recktenwalt
With all the scientific names in the killifish world, and the fairly constant rate of change as more is learned about the relationship of species, understanding this complex subject becomes progressively more confusing. Yet the concept of species is integral to our understanding of killifish relationships and the natural world.
Before further discussion, a few definitions may be in order.
Species: a group of animals that usually share morphological similarities (that is, they look pretty much alike), are able to breed, and produce offspring that are viable through numerous generations.
Subspecies: a group of individuals belonging to the same species, but that are 1) usually somewhat morphologically different than the “main” form, and 2) have an allopatric distribution (their range is not coincidental with the “main” type).
Population: a group genetically belonging to a species, but that is reproductively completely isolated from other members of that species. The “population” designation rarely has any taxonomic value.
“The concept of species seems intuitively easy to grasp,” says Jeff Lehman, “although it’s not too hard to find problems with, especially when considering the Fundulopanchax gardneri or Aphyosemion bittatum species complexes.”
“These concepts (species, subspecies, and population) are all artificial,” points out Rui Carniero Martins. “For the fish, it makes no difference if its partner belongs to its species or not – as long as the right conditions and courtship signals are met, they’ll try to breed.”
“We can find examples of the controversy of how to delineate a species … in Cyprinodon,” says Ted Klotz. “Most crossing experiments reveal these ‘species’ to be completely able to produce viable offspring, yet they are considered different species. Their separation has been based on geographical barriers.”
Such “species complexes” share a number of characteristics (morphological, physiological, and even genetic) but usually don’t intercross in nature. Geographical barriers are just one of many possible reasons: differences in courtship, circadian behavior, water chemistry, etc. In artificial habitats (aquaria) these natural conditions are rarely met; consequently the natural barriers to breeding usually disappear and interbreeding can occur.
“Preserving species/subspecies/populations ought to be a common goal for all aquarists,” says Rui Martins. “Now, we all know that any organism is the result of what is encoded on its genes. Change that information and you get a different organism.” Members of a species, although different individually, share common genetic information that allows for some compatibility when reproducing. However, when crosses occur between different species, genetic incompatibility arises and the offspring are rarely viable. This rule is somewhat fluid: Offspring of such hybrids might be fertile, partially fertile, or infertile, but they still remain hybrids and their distribution is strictly forbidden, except for scientific purposes and with adequate documentation.
“Usually, subspecies and populations are segments of the species that are beginning to take an evolutionary step towards speciation,” adds Rui. Unfortunately, it’s hard to tell beforehand how far apart they are; you really don’t know if you’ll get infertile species or not. It might take two or three generations before the incompatibility is established and fertility drops.
Keeping track of the correct information on the fish we keep is no minor task. However, “the complications imposed by keeping track of location names is trivial compared to the problems introduced by the accidental ‘hybrids’ created because the location names were ignored and the fish were really not the same species,” argues Lee Harper.
Unfortunately, points out Wright Huntley, “the scientific (taxonomic) community is years behind” hobbyists, and genetic difference does not mean morphological difference – two species might appear nearly identical and be completely incompatible
“Aquarium strains” can also be considered populations, since they, too, are evolving and can become extinct. They also can become quite robust and establish themselves. A. australe is a good example of this: some fish produce very few eggs (if at all), while others produce large spawns. For this reason, you should never look down on aquarium strains.
“Subspecies are all guesswork,” says Richard Sexton, “and especially hard for freshwater populations. There typically should be a geographical separation. If two populations can mix geographically but still retain their identity, they are usually separate species.” – G.C.K.A. Newsletter, June 1999
“The subspecies concept was (is) all too subjective and contentious,” contends Jeff Lehman. “The use of populations is much more clearly defined, although one is left with establishing the degree of relatedness of different populations of the same species.” The use of collection data to identify populations allows a more or less easy way for the hobbyist to keep different “strains” separate, with obvious benefit to the hobby.
“Locality codes are useful to the purists who wish to maintain the integrity of the genetics of the small individual populations. They have no official standing in the biological literature,” observes Ralph Taylor. “As long as we use morphomological characters or chromosome numbers as a way of defining a species we will always have problems of determining what a species is. The current rush to DNA analysis and molecular genetics studies will hopefully answer some questions.”
– G.C.K.A. Newsletter, June 1999