Evolution at Work

Evolution can only occur when there is a change in the gene frequency within a population. Gene frequency is a term from the genetics unit and refers to the occurrence of a gene or allele within a population. For example, what percentage of your classmates has brown eyes? What is the percentage with blue eyes? Each answer would represent the gene frequency of either the brown eyed gene or the blue eyed gene of SBI 3U.

The important part to remember is that each person in this class has the ability to pass on the brown allele or the blue allele to their offspring. Let's say that the next generation of students taking SBI 3U all have brown eyes. Would you be able to explain this observation? Would this be an example of evolution?

Examine the following two cases and then decide which one is an example of evolution. Scenario 1: Beetles feast on plants and as a result of the increased levels of carbon dioxide, the amount of leaves and plants have increased. With this new abundance of plant availability, the current size of the beetle is much larger than the previous generation. Scenario 2: Most beetles in the population have the gene for bright green colouration and a few have the gene that provides the brown phenotype. A few generations later, the brown beetles are more common than the green beetles and make up the majority of the beetle population.

Solution The weight change in the beetle came about because of an environmental influence. The increase in plants determined the change in the beetle, not a change in the frequency of a gene. The second scenario represents evolution. The gene frequency of beetle colour changed dramatically between the generations over time. Why did this change occur? You should be able to provide at least one reason from the last activity, natural selection. For some reason, the brown beetle colour proved to be better suited for the living conditions of the beetle.

Genetic Variation Remember that for evolution to occur there has to be variation in the genetic make up of the organisms in a population. If all the beetles have green bodies, then genetic drift and natural selection would have no effect. Genetic variation is the key to evolutionary change and provides the beauty of diversity in nature and within your own family.

Genetic variation happens naturally in the process of meiosis. Events like crossing over and independent assortment ensure genetic diversity among the offspring produced by sexual reproduction. In addition, recall that sexual reproduction produces offspring that is genetically different from either parent. The combination of alleles produces differences among the offspring produced. Here are some more examples that explain how the gene frequency in a population can change.

1. Mutations Any change in the normal DNA sequence is a mutation. Mutations can be random or they may be influenced by external mutagens like radiation, viruses or hazardous chemicals. Mutations can be neutral, harmful, or beneficial. Harmful mutations generally reduce the fitness of the organism. Neutral mutations demonstrate no change in the overall fitness of an organism. Beneficial mutations provide the organism with an advantage that can lead to an increase in reproductive success.

Enrichment – (No need to write it down) Sickle cell anemia is an autosomal recessive hereditary condition that affects many people living in Africa. You may recall from previous units, that individuals with sickle cell anemia have irregularly shaped red blood cells and as a result the transport of oxygen throughout the body is not as efficient. Sickle cell anemia is the result of a point mutation where the nitrogen base thymine is replaced with adenine. This type of mutation alters the shape of the protein hemoglobin that carries oxygen and would be considered a harmful mutation because it reduces the fitness of people who carry the sickle cell gene. However, this gene is not selected against in regions of Africa where malaria is common. Research has shown that individuals with the heterozygous genotype for sickle cell anemia have a higher survival rate than people who are homozygous dominant with the normal red blood cell shape.

2. Genetic Drift The size of a population can influence the success of specific traits. A small population could experience change in the gene frequency as a result of a random act. For example, if you had a population of 50 drosophila fruit flies confined in a glass container in your classroom. Of those 50 fruit flies, 10 had red eyes and the remaining 40 had white eyes. By accident, you forgot to replace the lid on the container and 40 fruit flies escape.

Theoretically, of the remaining 10, only two would have red eyes and eight would have white eyes. This event significantly reduced the number of fruit flies with red eyes and if they are unable to reproduce, the red eyed gene would be eliminated from this population and essentially would be extinct.

3. Gene Flow/Migration The movement of specific genes from one population to another may increase the frequency of that gene in the new population. Pollen that is transferred to a new area or people moving to new countries are examples of migration. For example, the gene for a genetically inherited disease called Tay-Sachs is found in higher frequency in people of Ashkenazi Jewish decent. An increase in the population of Ashkenazi Jewish people within a population would also increase the frequency of the Tay-Sachs gene.

Adaptation If you think of animals that live in harsh conditions, like the artic, what physical characteristics do they possess that allow them to survive? Polar bears have their thick coat of fur that keeps them warm. Seals have layers of fat that insulate their internal organs preventing a decrease in internal temperature. Most of the plants that exist in the artic grow close together and low to the ground. This provides a shelter from the cold air and strong winds.

These are all examples of how living things have adapted to their environment. The adaptation that you observe generally represents the results of the process of natural selection over a long period of time.

Colour Adaptation There are three types of colour adaptation: Mimicry Protective Colouration Warning Colouration

1. Mimicry In this type of adaptation, a defenceless animal takes on a physical likeness to an animal that has qualities that predators know to avoid. Mimics are able to trick their predators and ward off being consumed. This viceroy butterfly looks a lot like the monarch butterfly. The monarch butterfly has defence mechanisms that enable it to ward off predators. The viceroy butterfly lacks these defence mechanisms, but because it resembles the monarch butterfly it fools its predators.

2. Protective Colouration This type of adaptation allows an animal to blend in with its environment. If the animal is not seen by a predator, it is less likely to be consumed. Also, protective colouration can also work for the predator. An animal that blends in with its environment is less likely to be detected by its prey. The white fur coat of a polar bear allows it to blend in with the snow and sneak up upon its prey.

3. Warning Colouration This type of adaptation exists in animals that produce either toxins or some other defence mechanism such as a bitter taste or odour. The colouration of these animals acts as a warning sign to predators to stay away. The monarch butterfly has an unpleasant taste and birds know to avoid this butterfly because of its distinct colouration.

Behavioural Adaptations The phrase “snow-birds” is used to describe people who travel from the cold winter months in Ontario to warm sunny areas of the United States and Mexico. These behavioural adaptations can also be seen in nature as animals react to changing seasons. Squirrels start hoarding their nuts and many other animals prepare to hibernate or migrate to warming climates.

Structural Adaptations These adaptations are physical features of an organism that enhance their survival. The fur on a polar bear or the bills on birds are examples of structural adaptation. Most living organisms contain special features that enable them to adapt to their surroundings.

Homework Read the following articles: http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_31 http://science.howstuffworks.com/dictionary/biology-terms/adaptation-info.htm