Evolution and Natural Selection continued

Stabilizing Selection The most common form of selection involves favouring the average condition. This type of selection favours the species that show very little divergence from the norm. An example of stabilizing selection can be seen in the Siberian Husky.

The Siberian Husky is a medium sized dog that weighs between 16-27kg. These dogs have strong pectoral and leg muscles, allowing them to move quickly in dense snow. If the Siberian Husky had heavier muscles, it would sink deeper into the snow. This would slow them down and cause them to sink and get stuck in the snow. However, if the Siberian Husky had lighter muscles, they would not be strong enough to pull sleds and equipment and therefore have little value as a working dog.

Directional Selection In this type of selection, average individuals are not favoured but rather one of the extremes is best adapted for survival. This occurs as a result of a change in the environment. Antibiotic resistant bacteria developed when antibiotics became widely used. Insect resistance to pesticides occurred when the use of pesticides became popular to increase food production. Insects that have built a resistance to pesticides are able to survive and produce more insects that have the same resistance.

Insects that lack the resistance are unable to stay “fit” and therefore cannot survive and reproduce. Directional selection is influential in the evolution and formation of new species.

Disruptive Selection This type of selection process favours the extreme traits in a population. Changing environmental conditions or the presence of a natural predator creates a change in the species that favours the extremes and excludes the average. Darwin observed this type of selection in the finch population that he studied in the Galapagos Islands. Darwin noted that the finches shared similarities that indicated that they derived from a common ancestor. Each of the finches had a different beak size that showed an adaptation to the type of seeds found on the island.

The big beaks were suited to retrieve bigger seeds and the small beaks were able to utilize the small seeds. Medium sized beaks were too large to retrieve the small seeds and the beaks were not strong enough to crush the large seeds. Disruptive selection is influential in the evolution and formation of new species.

Review Question Determine and explain the type of selection that appears in figure 1. Determine and explain the type of selection that appears in figure 2. Determine and explain the type of selection that appears in figure 3. Note in all the images, the red line represents before selection and the blue line represents after selection.

Answers Figure 1 is an example of stabilizing selection. The average tail length is beneficial for the survival and reproduction of lizards. Lizards with small tails and long tails have a lower survival rate. Figure 2 is an example of disruptive selection. Each of the extreme colours of shells, white or brown, is selected over the average yellow. This means that the yellow shell has a lower survival and reproductive success as compared to the white and brown shell. Figure 3 is an example of directional selection. Only one extreme is selected. Elephants with the longer trunk have a higher success of reproduction and survival compared with the other elephants

Sexual Selection What features do people possess that attract you? Is it the colour of their eyes, their hairstyle, or the fancy clothes that they wear? What will you look for in a mate? Believe it or not, most organisms possess some physical feature that is geared to attract the opposite sex partner for reproduction. Think about the bright fancy feathers that some birds possess.

Even though plants cannot select their mate, they do need the help of birds and insects for pollination. Therefore the fragrant odour and colourful petals are purposely exposed to attract organisms to help spread the pollen from one plant to another. Another feature of sexual selection pits males against males in terms of their ability to overpower and dominate their territory. By governing an area, a male can attract and attain a female for a mate. These qualities only exist in males and therefore are not affected by the environment conditions.

Artificial Selection All the previous types of natural selection occurred naturally in an undisturbed fashion. Artificial selection, as the name implies, is the manipulation of species due to the involvement of humans. Even Charles Darwin noted this as the domestication of plants and animals became popular. Today, artificial selection is much more prevalent and sophisticated than in Darwin's era. Horse and dog breeding is a perfect example of how humans can choose to selectively breed certain species together to create a desired product.

Scientists have even taken this one step further with the use of recombinant DNA in genetic engineering and also in the process of nuclear cloning. Researchers in the field of genetic engineering are able to manipulate the DNA of an organism so as to alter the gene. When this is done, a desired phenotype is now expressed in the organism. Artificial selection accelerates the rate of evolution as it eliminates the randomness of mating. Essentially, humans are the ones that choose which species are the fittest and which species will be eliminated from the population.

A genetically modified (GM) plant, like corn that produces a gene that acts as a pesticide, is an example of artificial selection using recombinant DNA. The GM corn has an advantage over corn that does not produce the pesticide and is therefore less likely to be affected by insects or birds. This will reduce the non-modified corns' ability to survive and reproduce.

Further Reading http://www.biology-online.org/2/10_natural_selection.htm http://www.biology-online.org/2/11_natural_selection.htm Natural Selection Activity (available on the Student-Teacher share drive: Mr. White Biology / Unit 5)

Homework There are many hypotheses about creation and evolution including those proposed by Aristotle, Charles Lyell, and Baron Georges Cuvier. Research the theory and explain the contributions that each of the following individuals have made to creation and/or evolution: Thomas Malthus Jean-Baptiste Lamarck Stephen Jay Gould Niles Eldredge Charles Darwin is known as the father of evolution for his research while on the Galapagos Islands. Darwin made several conclusions about the process of evolution. Using research skills determine the main conclusions that Darwin made about evolution.