Genetic Variation
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.
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
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?
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-byside, 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.
Optional: Go the following website and complete the simulation “Sex and the Single Guppy.”
http://www.pbs.org/wgbh/evolution/sex/guppy/ed_pop.html
As you complete the simulation, answer the following questions…
What hypotheses did you test?
• What conclusions did you reach based on your simulation results?
• How do sexual selection and natural selection interact in this guppy simulation? In nature?
• What other questions did the guppy simulation raise? How might you test for answers?
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