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Evolution 101

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Evolution In Action?

    What evidence do you need in order to say a population of organisms is evolving?  Evolution occurs only when there is a change in gene frequency within a population over time.  Organisms do not evolve, populations do.  Only heritable genetic variations passed on through generations via reproduction constitute evolutionary change. Take for example, the following scenario: 

Beetle Diagram AScenario One:
 During several years of severe drought many of the plants that a particular species of beetles consumes begin to perish.  All the beetles have an equal chance of surviving and reproducing, but the lack of food has restricted how large a beetle can grow, and populations begin to appear physically smaller than the preceding generations.
Beetle Diagram BScenario Two:
Most beetles in a population (90%) have the genes for bright green coloration, while few (10%) have the gene that makes them more brown. Some number of generations later, things have changed: brown beetles are more common than they used to be, and make up (70%) of the population.
Photographs courtesy of the University of California Museum of Paleontology (http://evolution.berkeley.edu)
  
Which of these examples demonstrates evolution in action?

    In scenario one the change in beetle size is due to a lack of food, and is thus and example of "environmental plasticity," or the tendency for a group of organisms to change its features due to environmental stress. The genetic makeup of these smaller beetles has not changed, and if food is beneficial next year the offspring of these beetles will grow to their prior full size.  Because the change in size is not genetically determined, scenario one is not considered to be an example of evolution. However, the change in color within the species in scenario two is an example of evolution because subsequent generations of beetles demonstrate a genetic difference, due to changes in gene frequency (color) within the population over time. 

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