Developing Complexity

The Theory of Evolution is a notion that all life descended (or evolved) from a common ancestor, which arose from nonliving material approximately 3.8 billion years ago. [1] There are four major mechanisms of evolution. Here is a generalized overview of each.

 

1. Natural Selection

     In this mechanism, the differences in how successful different organisms are at reproducing leads to changes in the frequency of the traits that those organisms carry. For example, a taller giraffe has access to a greater food supply than a shorter giraffe, and therefore is able to produce more offspring than the shorter giraffe. The result is that there is a greater percentage of taller giraffes in the next generation. If certain organisms end up having no offspring, this process can continue until some traits (or species) no longer exist. To continue the illustration, if the shorter giraffes in the world could not reproduce due to lack of food, then the next generation would contain no short giraffes. Essentially, natural competition causes more successful traits to become more common. [2]

 

2. Genetic Drift

     In this mechanism, only a portion of a population reproduces due to random events (i.e. independent of fitness). For example, a group of 5 white flowers and 5 red flowers exists in a field, and each flower always produces two offspring of its own color each generation. Then, a random mudslide kills half of the flowers—4 of the white and 1 of the red—leaving us with 1 white flower and 4 red ones. Now the next generation will have 2 white flowers and 8 red flowers. As you can see, random events, regardless of fitness, can alter the prevalence of traits in a population. [2]

     This has a greater effect in smaller populations as compared to larger ones; sheer numbers tend to balance out, or dilute the effects of, any cases of genetic drift. To continue the illustration, let us say that there are 100 flowers, 50 white and 50 red. Another unforeseeable mudslide once more kills half of the flowers, 27 of which were white, and 23 of which were red, leaving us with 23 white flowers and 27 red ones. Following the same pattern of reproduction, our next generation would have 46 white flowers and 54 red flowers. Remember, in the first example, there were three more red flowers than white ones that survived, and we ended up with 20% white and 80% red in the next generation. However, in this second example, there were four more red flowers than white ones that survived, but we still ended up with a pretty even-keel 46% white and 54% red. As you can see, the bigger the population, the less of an effect random events have on it. [2]

 

3. Geneflow

     In this mechanism, the frequency of a population's traits is altered by the introduction of new individuals who subsequently mate with members of the original population to create a new combination of genes in the second population. The act of mating is critical in geneflow because it is the only way new individuals could alter the frequency of a population's traits. If these new individuals did not mate, then their traits would only exist in the population until they died. Afterward, the original population and its traits would continue interacting as they would have otherwise, and the new individuals would ultimately have had no impact. For example, a group of red and yellow penguins exists on an icy southern island. Then, a catastrophic storm washes a foreign group of red, green, and blue penguins onto the same island. Subsequently, all the red, yellow, and green penguins proceed to intermate, but the blue penguins do not. Now, the second generation of the original population of red and yellow penguins has a higher frequency of red penguins, as well as a new type—green penguins—but no blue penguins. Essentially, adding new individuals to a population changes the frequency of traits in that population, as long as the new individuals pass on their genes. [2]

 

4. Mutation

     In this mechanism, environmental factors alter the makeup of DNA. Things such as high-energy radiation, replication errors, and chemical carcinogens can cause small bits of the DNA to be replaced with others, changing the overall genetic message. Essentially, random events can cause the descendant generations of a population to have slightly different genes. [2]