Ever since Aristotle, or perhaps even before, we’ve felt the need to separate everything into separate categories, especially living beings. While this sometimes creates the problem of things that do not fit into certain categories, it makes communication and identification easier. It can, for instance, be helpful to make a distinction between a wolf and a chihuahua. We also cannot over categorize, simply defining individual with a new name (look at how quickly we ran through names for hurricanes.) So, we are left with a need to distinguish individual species.
Species can arise in several ways. First, if a group is isolated geographically from the rest of the population, it may follow a distinct evolutionary path. This is referred to as allopatric speciation. Whether separated by a volcanic eruption, glacial advance, or even a meandering river, the interruption in gene flow can cause two distinct groups to arise.
Different species can also arise in the same area, a process called sympatric speciation. Barriers to reproduction, environmental, genetic, or opportunistic, can separate groups within a population. When these groups can no longer interbreed, they are considered distinct species.
This traditional definition of species is referred to as the biological species concept. This was popularized by Ernst Mayr, who wrote “species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” He left this definition pretty broad; any factor could “isolate” groups. He simply modified this definition by Theodosius Dobzhansky from five years earlier: “a species is a group of individuals fully fertile… but barred from interbreeding with other similar groups by its physiological properties.”*
Both gentlemen left the theory fairly incomplete, however. How do you isolate a species by fertility if the organisms reproduce asexually? Sometimes, more specific definitions for “species” may be necessary in different kingdoms. On the other hand, definitions can be too specific: sometimes when species shouldn’t be able to hybridize, they can. Peter and Rosemary Grant, while studying the Galapagos finches, discovered that up to two percent of some species of finch mated successfully with other species. I once saw a dog that was a cross between a golden retriever and a Chihuahua. It shouldn’t have been… but it was. The different breeds may be on their way to speciation, but they aren’t exactly there yet, if you ask the dogs and/or Darwin’s finches.
Species: My hesitant definition
- A species is a group of individuals that have become genetically distinct enough to eventually isolate themselves from another group, whether through reproductive barriers or environmental or geographical separation.
This definition allows for allopatric or sympatric speciation, can apply to organisms that reproduce asexually, but allows for generalizations to focus on groups, rather than individuals. “Through reproductive barriers or environmental or geographical separation” is as broad, if not more so, than Mayr’s definition, but adds a new ambiguity: “genetically distinct enough.” I feel as if I’m opening a whole can of worms here, as I do not define what percentage constitutes “enough”. Two very different flowers may turn out to have more similarities than the 1.5% separating chimpanzees and humans.
At the same time, separating a species by genetic distinctiveness allows one type of bacteria to be distinguished from another, without bringing reproductive factors into the mix. Bacteria have smaller amounts of DNA than sexually reproducing species. The DNA is also more compact; no space is wasted on introns (unreadable code.) Because of this, a single point mutation will have a more drastic effect on the development of the bacteria, making it quickly distinct from others of its kind. If it survives, it has the opportunity to invade a new niche in the ecosystem, and become a distinct species. In the end, the two species of bacteria will have a greater percentage of genetic difference, even though they have few actual differences.
In the case of sexually reproducing species, the percentage of genetic difference may be low, but other factors can lead to distinct species. This is why my definition allows for small genetic changes that reproductively isolate one group from another. In these cases, the percentage of difference need not be great, only enough to limit reproduction.
If different groups are able to fit in the available ecological niches, they will continue to survive. One changing group of primates (chimps) hangs in the jungle near the trees, while another (humans) moves to the flatlands and grows larger brains. One changing group of flowers likes to bloom in the sun and gets fertilized by butterflies, while another group starts blooming at night, and gets fertilized by moths. One group of bacteria likes it warm, and stays in one place, while another group discovers the cold bathroom floor is all right, and spreads like wildfire. As long as they fill the niche and remain distinct, they will survive and speciate.
Notes: My knowledge of viruses is lacking; I can’t say if they qualify as alive or not. So, I’m not certain if my definition of speciation can apply to their class. I hope to find out.*Quotes from Mayr and Dobzhansky borrowed from “Speciation Conclusion” by John Wilkins in Evolving Thoughts, 2005. Chihuahua and Great Dane photo borrowed from PBS’s Nova