GED Science Practice Test: Models for Evolution

In this lesson, we have talked about what evolution is, the mechanisms by which it happens (random and non-random), and the sources of genetic diversity in a population.  We can also talk about models for evolution that describe the rate and how evolution happens.

Gradualism is selection and variation that happens gradually. Over a short period of time it is hard to notice. Small variations that fit an organism slightly better to its environment are selected for: a few more individuals with more of the helpful trait survive, and a few more with less of the helpful trait die. Very gradually, over a long time, the population changes, and change is slow, constant, and consistent.

In punctuated equilibrium, change comes in spurts. There is a period of very little change, and then one or a few huge changes occur, often through mutations in the genes of a few individuals. Remember that mutations are random changes in the DNA that are not inherited from the previous generation, but are passed on to generations that follow. Though mutations are often harmful, the mutations that result in punctuated equilibrium are very helpful to the individuals in their environments. Because these mutations are so different and so helpful to the survival of those that have them, the proportion of individuals in the population who have the mutation/trait and those who don’t changes a lot over a very short period of time. The species changes very rapidly over a few generations, then settles down again to a period of little change.


In the above image showing both gradualism and punctuated equilibrium, you can see that the end result is similar….increased variety of colors in butterfly wings.  However, the changes happen slowly over time in gradualism, while they happen in quick bursts in punctuated equilibrium.

Adaptive Radiation: In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges and opens environmental niches. Starting with a recent single ancestor, this process results in evolution of a wide array of species exhibiting different traits with which they can adapt to and exploit a range of divergent environments.



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