Dr. Ray L. Winstead
Professor of Biology, Indiana University of Pennsylvania

Darwin's Theory of Evolution

Definition of the term evolution:
            Evolution is any change in the combined genetic material in a POPULATION, i.e., by definition, evolution has occurred if there is any change in the POPULATION'S gene pool.  Especially note that individuals do not evolve - populations evolve.

            Evolution can easily be observed to occur WITHIN a species over time.  Darwin extended the concept to propose that, due to many different selection pressures caused by changing environments over a long period of time, enough changes in the genetic material could occur in a population over time that could result in the creation of a NEW SPECIES.  A species is defined as a group of similar organisms that share many characteristics, INTERBREED IN NATURE, and do not reproduce with organisms outside this classification.  See your notes for an elaboration on the various types of evidence for evolution and proposed mechanisms of evolution.

            Darwin's main ideas can be summarized in six basic points.  The first five are observations and the sixth point is his conclusion based on these observations.

1) A group of organisms tend to reproduce more offspring than the environment can support.

2) Most populations tend to remain fairly constant in size because of various population regulation mechanisms at work, e.g., density-dependent factors and density-independent factors regulating population size.  The population comes into an equilibrium with its present environment.  (See your notes.)

3) Competition takes place because so many individuals are introduced into an environment with limited resources.  There is a "struggle for existence."  Such a competitive struggle for existence usually includes being better adapted for obtaining the available resources in comparison to other individuals.  Especially note that physical combat is not a very important part of this concept.

4) There exists variation among individuals within any species because genetic changes occasionally occur that modify the DNA structure of chromosomes.

5) Variations caused by gene mutations are usually either harmful or useless.  However, over the course of time, beneficial mutations may occur.  Individuals that inherit beneficial mutations or beneficial gene recombinations are better adapted to survive.  This is where the phrase "survival of the fittest" comes in or the process of natural selection.  Again note that such a competitive struggle for existence usually includes being better adapted for obtaining the available resources in comparison to other individuals and that physical combat is not a very important part of this concept.

6) In a changing environment, those organisms with favorable genetic variations survive.  The surviving organisms then reproduce and transmit their DNA to their offspring.  Over a long period of time ENTIRELY NEW SPECIES EVOLVE.  Organisms that have successful genetic variations not only live longer but produce more offspring who also inherit the favorable adaptation.


Note that the Theory of Natural Selection states that:

a) organisms that are better adapted (genetically) to their environment will survive in greater
    numbers than those less well adapted, and
b) the genetic material, therefore, that controls those adaptations will become more numerous
    in the population in the next generation.
c) The ability of the entire group to survive in then increased.

Natural selection is the preservation of favorable, beneficial characteristics in the population and the elimination of the unfavorable characteristics.

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Dr. Ray L. Winstead
Direct e-mail Link: RWinstea@iup.edu