4.3.8

Rate of Speciation

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Rate of Speciation

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

Varying rates of speciation

Varying rates of speciation

  • 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.
Factors affecting speciation

Factors affecting speciation

  • 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.
Factors affecting speciation - 2

Factors affecting speciation - 2

  • 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.
Adaptive radiation

Adaptive radiation

  • 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.
Adaptive radiation - example

Adaptive radiation - example

  • 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.
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Practice questions on Rate of Speciation

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  1. 1
    What is the primary influencing factor on changes in the rate of speciation? Multiple choice
  2. 2
    Describe a situation in which punctuated equilibrium is more likely to take place. Multiple choice
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Speciation

Allopatric & Sympatric Speciation