Evolution

Basic Principles of Evolutionary Theory

Dec 26, 2009
Elizabeth Humphreys

How the Peppered Moth Adapted - http://www.truthinscience.org.uk/site/images/stori

Evolution is the basis of much of modern science and medicine. An understanding of evolution is important to understand life itself.

The theory of evolution through natural selection was first put into concrete form by Charles Darwin in his 1859 book On the Origin of Species. Since then evolution has become the leading and most influential concept in modern science and medicine. Certain basic principles are key to understanding evolution. Before trying to learn more about evolution it is advisable to understand these.

Basic Terms

Gene – A gene refers to a portion of an organism’s genome that codes for a specific trait

Allele – An allele is one form of a gene

Trait – A trait is an expression of a gene or a series of genes

Variation in Heritable Traits

In order for a species to evolve there must be variation in heritable traits. A heritable trait is one that is genetically determined and inherited – that is to say, one that is passed from one generation to the next and is not influenced by the environment. An example of a heritable trait is eye color.

Variation is caused by the mutation of genes or by an error in the replication of genes from one generation to the next. In any given environmental condition some traits will be better suited to survive in that environment than others.

Basic Genetics

Every organism has two alleles for every trait. Alleles can be recessive or dominant. As the name implies when a dominant allele is paired with a recessive allele the dominant allele is expressed. Recessive alleles are only expressed when paired with the same recessive allele.

That is a very simplified explanation; alleles can also be co-dominant and share expression. A species can have more than two alleles for a trait, but each individual can only have two.

Selective Pressures

A selective pressure is an environmental factor that acts upon an organism. This can be caused directly by the environment or as an interaction between organisms. An example of a selective pressure that is caused directly by the environment is an organism’s ability to survive in extreme heat. An example that is an interaction between organisms is a prey animal’s ability to hide from a predator.

Genetic Drift

Genetic drift occurs by the random mating and changes in gene recombination. Over time when one allele is even slightly more common than the other alleles for that trait it will gradually become even more common until it is fixed within a population. When a trait is fixed in a population, there is no longer any genetic variation for that trait in that population – for example a population with only brown eyes can be said to be fixed for that trait.

Evolution through Natural Selection

Evolution occurs when selective pressures act upon variable traits. This normally happens very, very gradually, but can happen quickly when the environment changes dramatically within a short period of time.

If a selective pressure affects the survival of some members of a population then those not negatively affected will have greater success at breeding and replicating their genes. This causes the percentage of alleles to shift in favor of the alleles of the more successful individuals. Over time this may cause a permanent change and the allele that is favored may become fixed.

One modern day example of evolution through natural selection is that of the Peppered Moth. There are two basic alleles in this species which attribute to the color of the moth; pale with speckles and black. The pale moth was by far the more common of the two forms (‘morphs’) and it was once perfectly camouflaged against the moss and lichen on tree bark. After industrialization the trees near large cities and industrial plants became dark with soot and the moss died. In these areas the dark colored moth is now much more common. It is believed that because the light colored moths were no longer camouflaged they were more easily seen and eaten by predators such as birds. This meant that the black moths were better camouflaged and were more successful at reproducing. In less polluted areas the pale moth is still more common, further supporting the theory that the change was caused by adaptation.

With this basic knowledge of evolutionary theory reading and researching evolution and natural selection will become easier.