Expert: Dana Krempels, Ph.D. - 10/13/2008

Question
QUESTION: Hi there! I have a question about the taxonomy and genetics of the Betta genus of fish, aka 'Siamese Fighting Fish'. Sorry, the introduction to my question will be a little long!

I was doing some studying on the genetic variations of the fish (which results in the various colors and tail variations), and I was surprised to learn that some of the variations were not mutations of the species Betta Splendens, but rather inter-breeding with Bettas of other species, specifically Imbellis, Smaragdina, and Mahachai (all 4 of these species are native different areas of Thailand, Splendens and Mahachai in the central region, Imbellis in the south, and Smaragdins in the northeast).

I remember something from my high-school biology class, something like 'breeding can occur between 2 animals from different species but same genus, which can result in an animal that is infertile, therefore not a new species', like a mule or a liger. However, offspring from breedings of different Betta species result in fertile offspring, with often spectacular features.

So my question is, are the various kinds of Bettas really different species that have enough in common to inter-breed successfully? Or are they just a single species that is geographically differentiated and given incorrect taxonomy?

I hope to have your insight to this question! Sorry again for the long intro at a relatively short question!

Ted

ANSWER: Dear Ted,

There are several different definitions of "species."  The one you cite is the biological species definition (coined by Ernst Mayr):  a group of similar organisms that can interbreed in nature to produce fertile, viable offspring.

But living organisms don't always adhere to our narrow human definitions.  There are several other working definitions of "species," and an excellent overview can be found here, at our old pal Wikipedia:

http://en.wikipedia.org/wiki/Species#Definitions_of_species

So whether or not the different genera of Betta-relatives are truly different species depends on your definition.  In nature, they are reproductively isolated by geographic barriers, so in a sense, they do fit Ernst Mayr's definition of species unless humans intervene.

There are many species like this.  The endangered Black-footed Ferret, for example, can apparently interbreed with the European Ferret if the two are brought together artificially.  But does that make them the same species?  It's arguable.

Taxonomy is populated by "splitters"--those who tend to create a new taxon for every new organism they find--and "lumpers"--those more likely group organisms together taxonomically until there is good evidence to do otherwise.  I'm suspecting that the fish genera you describe above are the work of splitters.  It's obvious that they all share a relatively recent common ancestor, or they would not be able to produce fertile, viable offspring.  Why these fish have been accorded separate genus status is known only to those who described and named them.  :)

Hope that helps a little!

Dana



---------- FOLLOW-UP ----------

QUESTION: Thank you for your quick answer!

After doing all that reading, I guess I now understand why biologists still have such a hard time classifying different organisms.

So, with the way evolution and natural selection etc. works, if left to their own devices, is it safe to assume that these 4 different species would eventually acquire enough genetic differentiation to not be able to produce offspring?

And also, even though I've tried reading about it, I still don't understand the definition of a sub-species. Is it a population of a certain species that is in the process of becoming a separate species? I hope you can explain this to me in layman's terms! Also, could this be applied to the fighting fish?

Thanks in advance!

Ted

ANSWER: Dear Ted,

I'll take each question in turn:

"With the way evolution and natural selection etc. works, if left to their own devices, is it safe to assume that these 4 different species would eventually acquire enough genetic differentiation to not be able to produce offspring?"

It's impossible to predict this, but it *could* happen.  This is where another term used by evolutionary biologists comes into play:  cohesive species.  This is defined as a species that tends *not* to undergo reproductive isolation among its populations, even when they are separated geographically and in other ways.  Other species tend to become reproductively isolated at the drop of a hat.

A few interesting examples:

   * Red-winged Blackbird Agelaius phoeniceus) has populations in California and Florida which are physically indistinguishable, have the same mating behaviors, etc. and there is no gene flow between them. Despite lack of gene flow, they have not become separate species. (cohesive)

   * Drosophila spp. in Hawaii have huge variation in appearance and behavior, and many are reproductively isolated from one another. However, they are genetically almost indistinguishable. (NOT cohesive!)

   * Coyote (Canis latrans) and Timber Wolf (Canis lupus) occasionally hybridize, producing fertile offspring--yet their lineages have remained separate for two million years.

   * Black Cottonwood (Populus trichocarpa) and Balsam Poplar (Populus balsamifera) are separate species, physically distinguishable. They have existed as such for 12 million years. Yet they occasionally hybridize to produce fertile offspring.

No one really understands what makes one species cohesive and another one not.  Lifespan, generation time, reproductive cycle etc. may play a role.  But not in a predictable way.

So what will happen to the various closely related genera that contributed to the genome of Betta splendens isn't easy to predict.  Unless, of course, we drive them all extinct, too.

Given our species' penchant for doing that, maybe *that* is the most likely scenario.  :(

Hope that helps.

Dana



---------- FOLLOW-UP ----------

QUESTION: Hi again!

Sorry for so many questions! But you're such a great help, and I'm enjoying all this new information! Your explanation of cohesive species was very informative.

However, I understand you must be busy...but you forgot about my sub-species question, and how it could be applied to the Betta fishes?

Answer
Dear Ted,

Sorry for overlooking the subspecies question.

A "subspecies" is a bit like a "race" of a particular species.  Members of different subspecies are usually geographically separate and they almost always have somewhat different physical appearance, but they are not considered sufficiently different from one another (in terms of potential reproductive isolation) to warrant separate species status.

I don't think the term "subspecies" could be applied to Betta splendens, since it's apparently a "man made" hybrid species, somewhat akin to an orchid that has been developed by cross-pollinating different (but related) genera of orchids.  

I'm not sure how the rules of nomenclature were applied to Betta splendens, but here they are today, species or not!

Hope that helps.

Dana