Location: Outside U.S.
Date: Spring 2011
I've been doing some reading on evolutionary biology and
taxonomy, and I have a question regarding speciation. I understand
this is a topic that elicits much disagreement among the scientific
community, however I was hoping that someone here would be able to
provide at least some clarity. My question is simple: When and how
does speciation occur? Evolutionary changes are often small, and in
most cases the differences between generations only add up after
tens of millions of years. Isn't the term "species" a bit arbitrary,
if every organism on Earth is constantly evolving little by little?
You’re right, the term species is somewhat ill-defined. Nature frequently throws us anomalies that don’t quite align with expectations. As a consequence, there is no “unified theory” that can accommodate the vast differences observed in nature when it comes to sexual and asexual reproduction, horizontal gene transfer (a major headache for taxonomists), and hybrids.
Most authorities subscribe to the biological species concept - this comprises a group of individuals that can potentially breed and produce viable offspring. Another variant is the recognition species concept, where individuals in a population identify each other as prospective mates.
Here’s a scenario to demonstrate the shortcomings of the current system: consider a liger (a male lion x female tiger hybrid). In captivity they have been known to mate.
If lions and tigers are reproductively compatible, can one intimate that lions and tigers are the same species? Under the biological species concept, arguably yes, because fertile offspring were produced. Under the recognition species concept, arguably no, because they would preferentially mate with their own type if given the choice. They also would never ordinarily encounter each other in the wild for an amorous rendezvous.
Here are some other scenarios:
It has been proposed (unsuccessfully) that the HeLa cell lines used in cancer research constitute a new species! These cervical cancer cells are a composite of a human’s genes (Henrietta Lacks) and those of an HPV virus. The virus apparently deposited some DNA horizontally into this individual’s genome. Is this worthy of a species title? The resultant organism is fully functional and can reproduce asexually.
What about endosymbiotic theory? Organelles like mitochondria and chlorophyll have their own DNA. How would a taxonomist denote a species within a species using traditional Linnaean nomenclature?
All of these scenarios pose a genuine dilemma for taxonomists, biologists, and geneticists alike.
Geographic isolation is generally used to describe why speciation takes place. Every organism on earth is evolving little by little, but continued geneflow (through interbreeding) ensures that compatibility within the population is maintained. Close the spigot, and things get a little provincial.
What can prompt this isolation? Usually factors like topographical changes (i.e. formation of a mountain range), although migration patterns can also be a contributing factor. If an organism has a particularly expansive range, sheer distance can also functionally transect a population into two or more populations. In their diverse environments, each discrete population experiences a different array of selective pressures. New traits develop (via random mutations) and are quickly fixed in the population. The effects are cumulative. The original ancestral lineage diverges, and each has just enough subtle differences (i.e. behavioral and/or genetic incompatibilities) to constitute a new freestanding species.
One could argue that speciation is also an outcome of infection with microorganisms like Wolbachia. This is a parasitic microbe that infects arthropods. The impact is profound – typically only infected individuals can mate with each other, and only when they are infected with the same strain. Technically this would constitute a biological isolating mechanism and the formation of new species. In practice, populations are simply subdivided into Wolbachia positive and negative.
A species is a decidedly human invention, and one continually ripe for refinement. Although the term seems largely interpretative, it does serve an important role; namely as an information storage and retrieval system with predictive power. If you say Felis catus, I know it’s a mammal, has four legs, is a predator, and most likely enjoys catnip.
Dr. Tim Durham
Instructor, Office of Curriculum and Instruction
Department of Biological Sciences
Florida Gulf Coast University
Yes, it is a bit arbitrary, but not as much as you might suspect. The working definition of "species" is an interbreeding population. If one population is suddenly isolated from another, for a while they COULD interbreed if they could meet. So in that situation, whether the two populations constitute a single population or two is arbitrary. But over time, the two populations will genetically diverge by drift and by adaptations to the different selection pressures they face. So, the longer the populations are separate, the less likely they would be to produce viable offspring in cross-breeding.
How long would that take? Well, here's a calibration: a study of Icelanders found that the most fertile pairings are those between fourth or fifth cousins. In other words, fertility declines with separation beyond only five generations!
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
I always ask my students-who made up the definition of species, and of course the answer is US. Organisms don't always obey our scientific rules, however. There are many different definitions of species, but generally speaking, a species is a population of organisms that can mate and have fertile offspring. So are dogs and wolves the same species because they can mate and have offspring? Are we witnessing a speciation event in progress? Or are they really still the same species? Anyway-speciation is generally a minor change that just prevents mating-the reasons can be physical ("parts" don't fit together anymore), seasonal, behavioral, etc. But for some reason they can't mate, or their babies are sterile. But if we are talking about Darwin's finches, their beak shapes determine what their food source is, which may be on different islands, preventing coming together to mate. Some are living together on the same island. But, they are all still finches. I think what many people think of when they think of different species is the difference between a frog and and lizard; how did that happen? It happens the same way-random mutation, which natural selection then acts upon, but over a longer period of time. Over time evolution is POSSIBLE, it doesn't necessarily happen. It depends on the environment the organisms find themselves in and the selective pressures that act upon them. Some will be able to survive, others won't, and the population changes over time.
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