World Builders™                                                                    Session Four  --  Microbiology             

                      
                                             The Gene Pool     

Living things hold a balance between innovation and conservation in their gene pools.


     As we start to study reproduction in life forms, we see two very important needs:

  • The need to conserve and pass down the information in the DNA with no errors

  • The need to provide variety in the performance and capabilities of the organisms

     The DNA of all the living organisms in a species is called the gene pool.  Look around you.  The people that you see are all recognizably different from each other -- but you have no difficulty telling a human from a tree or a dog from a cat.  Each species has its own characteristics, and those characteristics are conserved in the gene pool.  Differences are a sort of insurance policy, and can help some members of  the species to survive if  the community is threatened by disease or environmental changes.

     Many endangered animals are suffering from a loss of genetic diversity.  Scientists suggest that when a species drops to below 500 animals, it is likely to be doomed.  One disease could kill them all.

     We already see examples of this.  Cheetahs, for instance, have little genetic diversity due to some sort of catastrophic event in the past.  In a recent incident, a group of cheetahs in a zoo picked up a disease and a third of them died.  Only two thirds had whatever genes it took to survive the disease.

     Dr Viau is more concerned about cloning fruit trees and raising crops with little genetic diversity.  During the Irish Potato Famine in the 1800s all the potatoes in Ireland were from the same stock, and the Irish farmers depended on them.  When the potatoes became diseased and slimy, a lot of people starved to death.  Could something similar happen in our huge orchards and immense grain fields?

   Now let's think about how genetic changes come about.

         We have learned that organisms inherit characteristics which are encoded on the chromosomes in their cells.  These chromosomes are passed from parent to child, and are reshuffled in each generation through sexual reproduction.

     Some of the genes that code for a specific characteristic come in "different flavors".  For example, this diagram shows some genes that determine eye color. They are all found on the same place on the same chromosome. The organism will have two chromosomes, and the eye color will be determined by the genes on those two chromosomes. There is a variety of eye colors in this population.

     Genes that are variants of one another are called alleles. They have the same function, they are found in the same location, but what they determine (eye color here) is not expressed identically in all individuals.

     Alleles probably started out as one gene with one mode of expression. Over thousands of reproductive copyings, a mistake was made, and the gene was changed a little. Such a change is called a mutation, and the individual whose phenotype (physical appearance or functioning) is different is called a mutant.

     Many mutations cause changes that are not really noticeable, but that may turn out to be useful someday. For instance, there appears to be a mutation that allows people to become infected with HIV but not get sick or die. This mutation was not valued or noticed, but perhaps it will turn out to be really important. It is rare in our population, but could become more common over the next few centuries.

    Some mutations are lethal. These cause the organism which inherits them to die prematurely. Some cause death in infancy or childhood, so these mutations are not passed on. Some cause death in mid-adulthood or later, and may  persist in a population because they are passed on before the individual affected dies.

     Nature is always looking for the life forms that are most successful.  If there were no mutations, organisms would reach a high state of fitness and then there would be very little change in them.  This has worked out well in extremely stable environments, such as the abyssal plains in the deep ocean and ancient tropical rain forests.  However, if the ice age comes, or a new predator is introduced, or a new disease appears, mutations may hold the very key to the survival of their species. 

     Sometimes a group of mutations works together to allow new potentialities to develop.  For instance, it took a number of mutations to allow humans  to become bipedal, walking on two legs.  As the species walked more and more on two legs, the forepaws could become hands and carry things.  This would favor brain development as the creatures figured out what they wanted to carry, and learned to make tools and weapons.  Increasing uses for the hands would benefit from better thinking in the brain.  Useful mutations are retained, and life forms change in response to changes in themselves and the environment.

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Photos from archives at Biology Department, University of Bowling Green, Ohio
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net .