Natural Selection Image Map

Gene Frequencies 

A given population of a species has a certain distribution of genes, called gene frequencies. If these frequencies are changing, it could suggest that the species has a lot of capacity for evolution, if selection forces act more on one set of genes than on others.

Two scientists, named Hardy and Weinberg, tackled the problem of describing how alleles in a population change over time.  They determined that allele frequencies remain constant over time if these characteristics of the population were met:

        The population is large

        Mates are chosen at random

        Mutation is not present

        Migration does not occur

        Natural selection forces are absent

Are these conditions ever met in nature?  If so, in what situations?  Sharks and turtles, for example, come very close to fulfilling these requirements.  So, would we expect sharks to exhibit relatively constant allele frequencies?  Sure!  In fact, researchers have determined that the genetic information in mammals changes about 3% every 1 million years, while the genetic information of sharks change at a rate seven to eight times slower than this. 

Do any populations ever fulfill all of the conditions perfectly?  No. The Hardy-Weinberg law describes an imaginary situation in which no selection forces (natural selection, migration, …) were acting.  However, in reality, some combination of selection forces is always present.  Go to The Hardy-Weinberg Calculator and experiment with this principle.

Creating New Species

Now that we have a grasp of species and populations, we can begin to see how different species arise.  As you learned in the Information Coding and Translation section of the Cells are Us module, genes carry the genetic information codes for the various traits of organisms.  The gene pool refers to all the genes found within a population. Differences in the gene pool separate populations from each other.  In another sense, a significant change in the gene pool of an existing population could mark the formation of a new population and even a new species.  Why would this be important from an ecological perspective?

How does this happen?  A species may arise whenever there is a major change in its gene pool.  To change the gene pool, you must alter the normal process that maintains it

Geographical isolation can isolate gene pools.  In the Grand Canyon, for example, centuries of erosion gradually separated populations from each other.  Now, different species of squirrels are found on opposite rims of the canyon. Of course, at one point in the past, these squirrels shared a common ancestor.  Only when the canyon began to form did they become physically separate and develop into two new, but very similar, populations.  Can you think of any physical barriers besides canyons that might lead to the formation of a new species? 

Other ways exist to alter gene flow internally. Let’s use plants as an example.  Once again, in Cells are Us you learned that the genetic material of an organism is carried on chromosomes.  Sometimes, new species of plants arise from mistakes during the cell division process.  The result is a plant with an abnormal number of chromosomes.  This new plant can only produce fertile offspring with plants that have the same number of chromosomes.  Why would chromosome number be significant in reproduction?

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