Evolution Day 4
Name:______________________________________
Use the Variation in Populations presentation and the links to 2 short videos to answers the following questions.
1. Differentiate between heritable and nonheritable variation. (slide 2)
2. The word “poly” means many. The word “morph” refers to form or type. Using these definitions and the
presentation define polymorphism. (slide 3)
3. Explain how snakes exhibit polymorphism. (slide 3)
4. Define a cline. (slide 4)
5. Explain how mammals demonstrate a cline when moving from the equator to the poles. Why does this cline
occur? (slide 4)
6. Explain the variation created by mutations. (slide 5)
7. Explain the variation created by sexual reproduction. (slide 5)
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 30,00060,000 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.
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 video at the following website:
http://www.pbs.org/wgbh/evolution/library/01/2/quicktime/l_012_02.html
• 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?
Recombination is usually the most immediate cause of variability. Recombination can occur in at least three ways:
1. Crossing over of chromosomes during meiosis
2. Random assortment of maternal and paternal chromosomes during the production of egg and sperm
3. Random combination of egg and sperm at fertilization.
For over 25 years Robert Vrijenhoek has studied unique populations of minnows in the small hillside pools of the
Sonoran desert in Mexico. Here he found two different species of minnows living side-by-side, one an asexual
reproducer and the other a sexual reproducer. Vrijenhoek has been trying to understand which conditions might favor
the sexual minnows and which favor the asexual ones. To find out what he discovered about the value of sexual
reproduction, view the Evolution Show Five video segment "Red Queen."
http://www.pbs.org/wgbh/evolution/library/01/5/quicktime/l_015_03.html
Describe the advantages of sexual reproduction for the minnow populations.