Patterns of speciation: evolutionary "trees"

The fact that existing species derive from pre-existing species, and that multiple species can derive from a common ancestral species means that the speciation has a very particular structure. This structure is often described with a metaphor called an "evolutionary tree"

We have all seen examples of these trees. Generally, the trees show multiple branches forming off of a common source. This represents speciation events which gave rise to multiple new species. Some speciation is gradual, with populations slowly growing more different over time, until they are finally clearly different species. Other speciations are more abrupt. Species can arise, sometimes many different species, over a very short geologic period

The older model of speciation said that it, and evolution in general, was a gradual process (Darwin was a "gradualist"). More recent evolutionary theory emphasizes the idea the evolution is not gradual, but that it is punctuated by periods of rapid change.

Stephen Jay Gould of Harvard University, among others, is a proponent of the idea of "punctuated equilibria" in evolution. This theory states that over most of evolutioary time species remain much the same; these periods are referred to as periods of equilibrium, emphasizing the balanced nature of the interrelations among individuals and species during the periods.

However, during certain periods rapid changes occur in large numbers of species. These events are brought about by rapid environmental changes, for example, which cause strong genetic drift (bottlenecks and founder effects) and strong directional selection in response to changes in conditions. After these events (which are often associated with events of massive mortality) stability is restored, and species change little until the next event.

The mass mortality at these events actually means that mass numbers of species become extinct at the same time. Mass extinctions have occurred repeatedly over evolutionary time, often in response to some drastic and sudden ecological change. One theory for these mass extinctions is that they are caused by the periodic collision of an asteroid with the earth. The massive explosion which would result would kill nearly all life on earth. This is one of the best examples of the random and in many ways illogical nature of evolution; surviving these extinctions by a species is usually entirely a matter of luck.

The rapid speciation which occurs at these events can also be referred to as adaptive radiations. Speciation can occur when populations are presented with unfilled "adaptive zones" which can be thought of as ways of life. The access to these adaptive zones can be physical (meaning that habitats become available for exploitation), evolutionary (meaning that a structural change in a species as a result of evolution allows it to exploit a new resource), or ecological (meaning that a species can out compete a rival species for a particular resource).