|
|
 |
 |
SESSION 4: What Are the Processes for Evolution? |
|
|
|
|
|
Natural selection acts upon two major sources of genetic variation: mutations and recombination of genes through sexual reproduction.
Most mutations do not affect the reproductive fitness of individuals -- some may be beneficial, some may be harmful, and many may be neutral. Mutation rates per gene are generally low. However, because there are so many genes in organisms (current estimates are 30,000-60,000 genes in humans), about 10 percent may be phenotypically expressed (i.e., they affect the anatomy and physiology of the organism) and acted upon by selection. Rates of mutation in genes that are phenotypically detectable can vary by 500-fold among genes within species and by as much as 100,000-fold between species. So it is probable that all offspring carry at least one new allele in their genome.
(Adapted from Freeman, Scott, and Jon Herron. Evolutionary Analysis. 2d ed. Upper Saddle River, NJ: Prentice Hall, 2001, pp. 83-84)
 |
|
A mutation might be advantageous in some environments, yet it might be harmful in others. To see how this can occur, view the Evolution Library video segment "A Mutation Story," about sickle cell genes and malaria.
For more information on the heredity of sickle cell anemia, see the Palomar College Web site.
|
|
A Mutation Story
View in:
QuickTime | RealPlayer |
|
Now answer these questions:
|
| • | What does natural selection act on: the phenotype or genotype of individuals? |
|
| • | Why do deleterious genes for traits such as sickle cell anemia, Tay-Sachs, and Huntington's disease remain in a population even though natural selection is in operation? |
|
| • | Describe what "fitness" means in a population affected by natural selection. |
|
| • | What are some examples of fitness in plants and animals? |
|
| • | Why doesn't variation decrease in populations where certain variants are selected for/against? |
|
|
| |
| |
 |
|
|
|
Natural selection can move a population far beyond its initial variation. How is that possible? Here are two examples of how artificial selection has influenced the variation within a population:
|
1. | Evolution of the Dog: Dogs have evolved over many generations from ancestral wolves. For example, in the case of the Pekinese, hidden genes for small size were present in the wolves. Dog breeders selected genes leading to the small size, funny faces, and long hair of this breed. (From Boyd and Silk, How Humans Evolved, 2d ed. [New York: W.W. Norton and Company, Inc., 2000] p. 75). |
|
2. | Oil Content in Corn Plants: In 1896, researchers at the Illinois Experiment Station began an experiment to increase the oil content in corn. The initial population of 163 ears of corn had an oil content ranging from 4 percent to 6 percent. After about 80 generations of selecting and growing the corn with the highest oil content, oil content increased to 19 percent, far beyond the initial range of variation. (From Boyd and Silk, ibid, p. 74) |
|
|
|
 |
|
|
Facilitator Note 2 |
| |
Next: Explore Part D: Genetic Variation By Recombination |
| |
|
