Agents for Evolutionary Change

There are 5 major agents for evolutionary change. Mutation Genetic Drift Gene Flow Non-random Mating Natural Selection

A.Mutation Mutation is a change in DNA, the hereditary material of life. An organism’s DNA affects how it looks, how it behaves, and its physiology—all aspects of its life. So a change in an organism’s DNA can cause changes in all aspects of its life. CROSSING OVER during meiosis produces variety within a species but MUTATION is the ultimate source of NEW genetic varieties in a species. A process like MUTATION might SEEM TOO SMALL-scale to influence a pattern as amazing as the number of different beetles, or as large as the difference between dogs and pine trees, BUT IT’S NOT. Life on Earth has been accumulating mutations and passing them through the filter of natural selection for 3.8 billion years — more than enough time for evolutionary processes to produce its biodiversity history.

B.Genetic Drift Genetic Drift - Population Genetics KEY IDEAS 1. The study of genetic traits and changes in populations is called POPULATION GENETICS. 2. The combined genetic material of all the members of a population is its GENE POOL. 3. Genes often have two or more different forms called ALLELES. 4. ALLELE FREQUENCY is a fraction that represents the frequency of a particular

THE HARDY-WEINBERG PRINCIPLE 1.  If a population is not evolving, the allele frequencies in the population remain stable. This is called GENETIC EQUILIBRIUM. a. Under specific conditions, allele frequencies in a population REMAIN CONSTANT from generation to generation. b. If specific conditions are not met, the genetic equilibrium is disrupted and the population MAY EVOLVE.

CONDITIONS required to maintain GENETIC EQUILIBRIUM are: 2. CONDITIONS required to maintain GENETIC EQUILIBRIUM are: a. No natural selection b. Random mating c. No migration d. No mutation e. Very large population size

3. Even when all the Hardy-Weinberg conditions are met, genetic equilibrium can be affected by RANDOM CHANGES in allele frequencies. 4. The random change in allele frequencies in a population due to chance events is called GENETIC DRIFT.

C. Gene Flow Gene flow—also called migration—is any movement of genes from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries. If genes are carried to a population where those genes previously did not exist, gene flow can be a very important source of genetic variation. In the graphic below, the gene for brown coloration moves from one population to another.

The amount of gene flow that goes on between populations varies a lot depending on the type of organism. As you would expect, populations of relatively sedentary organisms are more isolated from one another than populations of very mobile organisms.   By moving genes around, it can make distant populations genetically similar to one another, hence reducing the chance of speciation. The less gene flow between two populations, the more likely that two populations will evolve into two species.

D. Non-random Mating The result of nonrandom mating is that some individuals have more opportunity to mate than others and thus produce more offspring (and more copies of their genes) than others.

E. Natural Selection Darwin’s Explanation Model of Evolution by Natural Selection (Mayer 1991). Developed as a series of 5 observations and 3 assumptions based on the observations.

Observation 1 Organisms have great potential fertility, which allows for exponential growth of populations. Assumption 1 A struggle for existence occurs among organisms in a population. Assumption 2 Varying organisms show different survival and reproduction, favouring advantageous (desirable) traits (= natural selection). Assumption 3 Natural selection, acting over many generations, gradually produces new adaptations and new species. Observation 2 Natural population normally do not increase exponentially but remain fairly constant in size. Observation 4 Variation (differences) occurs among organisms within populations. Observation 3 Natural resources are limited. Observation 5 Variation (differences) is heritable.

In other words “Natural Selection is the process in nature that results in the most fit organisms producing offspring.” (Miller & Levine 1995). Individuals whose characteristics are well-suited to their environment survive. Individuals whose characteristics are not well- suited to their environment either die or leave fewer offspring. This is called Survival of the Fittest! (Miller & Levine 1995).

Role of the Environment in Natural Selection crucial role in Natural Selection acts to select favourable variations (NOTE: it DOES NOT CAUSE VARIATION, it only filters out unfavourable traits) i.e. snowshoe hare's white coat Natural Selection does not respond to a need to change. The genetic potential must already exist in the population.  i.e. Giraffes: those born, just by chance, with longer necks had a survival advantage (reach the higher branches of trees to eat the leaves) over shorter-necked giraffes, and would therefore live longer and produce more offspring. The next generation, would then have more long-necked giraffes in the population. This process would continue till nearly all the population had long necks. i.e. Peppered Moths

Evolution Today Darwin's Theory has been expanded upon since its publication. i.e. evolutionary processes can happen more quickly than once thought (e.g. pepper moth and industrial melanism). Evolutionary research is today focussing more on genetics. Today we define fitness, adaptation, species and the process of evolutionary change in genetic terms.