Mechanisms of Evolutiom

Mechanisms of Evolution

· Evolution does not occur in individual but in populations.

· A population is an interbreeding group of individuals of one species in a given geographic area.

· A population evolves because the population contains the collection of genes called gene pool. As changes in the gene pool occur, a population evolves.

                    

                        1. Mutation

· is a random change in the gene pool of a population. It is a change in the nature of the DNA in one or more chromosomes.

· Gives rise to new alleles; therefore, they are the source of variation in a population.

· It may be harmful but may be also beneficial. For example, mutation may permit organismsin a population to produce enzymes that will allow them to use certain food materials. Over time, these types of individuals survive, while those not having the mutations perish. Therefore, natural selection tends to remove the less fit individuals, allowing more fit individuals to survive and form a population of fit individuals.

2. Gene Flow

· Refers to change in the allele frequencies due individuals entering or leaving a population

· When migrating individuals interbreed with the new population, they contribute to the gene pool of the local population. This establishes gene flow in the population.

· Gene flow occurs, for example, when wind carries seeds far beyond the bounds of the parent plant population. As another example, animals may be driven off from the herd. This forces them to migrate to a new population, thereby bringing new genes to a gene pool. Gene flow tends to increase the similarity between remaining populations of the same species because it makes gene pools more similar to one another.

                      

                        3. Genetic Drift

· Occurs when a small group of individuals leaves a population and establishes a new one in a geographically isolated region.

· Example, when a small population of fish is placed in a lake, the fish population will evolve into one that is different from the original. Fitness of a population is not considered in genetic drift, nor does genetic drift occur in a very large population.

4. Natural Selection

· The most important influence on evolution is natural selection.

· In natural selection, an organism is subject to its environment. The fittest survive and contribute their genes to their offspring , thereby producing a population that is better adapted to the environment. The genes of less-fit individuals are eventually lost. The important selective force in natural selection is the environment.

· Environmental fitness may be expressed in several ways. For example, it may involve an individual’s ability to avoid predators, it may imply a greater resistance to disease, it may enhance ability to obtain food, or it may mean resistance to drought. Fitness may also be measured as enhanced reproductive ability, such as in the ability to attract a mate. Better-adapted individuals produce relatively more offspring and pass on their genes more efficiently than less-adapted individuals.

· Types of natural selection:

1. Stabilizing Selection – occurs when he environment continually eliminates individuals at extremes of a population.

2. Disruptive Selection – here, the environment favors extreme types in a population at the expense of intermediate forms, thereby splitting the population into two or more populations.

3. Directional Selection – The environment acts for or against an extreme characteristic, and they likely result is the replacement of one gene group with another gene group. The development of antibiotic resistant bacteria in the modern era is an example of directional selection.

Species Development

· The evolution of species is called speciation.

· Speciation can occur when a population is isolated by geographic barriers, such as occurred in the isolation of Australia, New Zealand, and the Galapagos Islands. The variety of life forms found in Australia but nowhere else is the characteristic result of speciation by geographic barriers.

· Speciation can also occur when reproductive barriers develop. For example, when members of the population develop anatomical barriers that make mating with other members of the population difficult, a new species can develop. The timing of sexual activity is another example of a reproductive barrier. Spatial difference, such as one species inhabiting treetops while another species occurs at ground level, is another reason why species develop.