The Definition of a Species
What is the definition of a species? How are species determined? What causes species to change? These questions are a bit more complicated than they look at first glance. The definition of species and the processes by which they change are not exactly straight forward for all organisms on Earth.
The usual basic definition of species is a group of individuals who can interbreed in nature. However, this is not completely accurate. For instance, a donkey and a horse are two different species but are able to interbreed to produce a mule. The definition of species can be amended to say that the individuals that are produced by the interbreeding must be viable, or fertile. Since mules are sterile and cannot produce offspring, it would stand to reason that horses and donkeys are two different species.
There is more to the controversy than just horses and donkeys, though. What about species that are asexual? They don't interbreed at all and instead just split through binary fission or another type of asexual reproduction. Since they don't interbreed, each individual prokaryote or other asexual individual would be a different species.
Since technology has improved dramatically over the past century and DNA has been discovered and studied, defining a species has become a bit less subjective. Comparing the genome of individuals can show similarities and differences to other individuals. Even if the individual organisms do not look the same, the DNA evidence can pinpoint them as the same species. Conversely, individuals that were once thought to be the same species may indeed not be after the DNA is examined. It is generally accepted that the closer the DNA pattern of species, the more closely related they are on the evolutionary tree of life.
Speciation: How Species Change Over Time
The macroevolution of a species happens as a result of speciation. Speciation is the branching off of individuals from the species they originally were. Over time, as natural selection occurs, individuals may build up adaptations that are no longer compatible with others in their species. This is most often due to geographic isolation or reproductive isolation from other individuals within the species.
Once the DNA is different enough, or they are no longer able to interbreed, they are now a different species that has branched off from the main species. At this point, they would be a new entry on a cladogram. How quickly does this process happen? If evolution for most species takes a very long time, it would be logical to think the same would happen for speciation. However, there are two major accepted different hypotheses on the pace of evolution due to speciation. Arguments for both hypotheses are sound and there has been no evidence found so far to disprove either one.
The first of these hypotheses is called gradualism, or phyletic gradualism. Just as the name implies, this is a gradual accumulation of adaptations over a long period of time. The Earth is around 4.6 billion years old. Even though it seems like speciation happens rapidly when we see it, gradualism proposes that there were many adaptations that slowly added up over a steady, long period of time. We did not notice those small changes until they led to a clearly new species. Therefore, speciation and subsequently evolution, happens because of those steady changes over long periods of time.
The other main hypothesis about the pace of speciation and evolution is called punctuated equilibrium. Its name is also descriptive of the hypothesis. This hypothesis claims that species stay in a state of stasis for very long periods of time and then a relatively short period of quick, successive changes occur to cause speciation and the new species evolves suddenly. The idea of punctuated equilibrium assumes that transitional forms between the species were only around for such a short period of time that none were preserved as fossils and may be a "missing link" forever.