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
remain constant over time if these
characteristics of the population were met:
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
Coding and Translation section of the Cells are Us module, genes
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.
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?