Rate of Speciation

Biology: IB Diploma Higher Level, topic 4.3.8

Speciation can both occur gradually over time in small steps or in bursts of change known as punctuated equilibrium.

In the gradual speciation model, species diverge gradually over time in small steps.
In the punctuated equilibrium model, a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time.
This early change model is called punctuated equilibrium, because it begins with a punctuated change and then remains in balance afterwards.
While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism.

The primary influencing factor on changes in speciation rate is environmental conditions.
Under some conditions, selection occurs quickly or radically.
Consider a species of snails that had been living with the same basic form for many thousands of years.
Layers of their fossils would appear similar for a long time, until there was a change in the environment.

When a change in the environment takes place—such as a drop in the water level—a small number of organisms are separated from the rest in a brief period of time.
This essentially forms one large and one tiny population.
The tiny population faces new environmental conditions.
Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.

In some cases, a population of one species disperses throughout an area, and each population finds a distinct niche.
Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species.
This is a form of divergent evolution called adaptive radiation as many adaptations evolve from a single point of origin; thus, causing the species to radiate into several new ones.
This can occur rapidly when there are many habitats available.

The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved.
Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source.
The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts.
The nectar-eating birds have long beaks to dip into flowers to reach the nectar.

Test yourself on Rate of Speciation

What is the primary influencing factor on changes in the rate of speciation?
Describe a situation in which punctuated equilibrium is more likely to take place.