Transcript 16-1 - cloudfront.net

Ch. 16 Evolution of Populations
Page 393
A. Variation and Gene Pools
• 1. A Gene Pool is made up of all the genes
(including alleles) that are in a population.
A. Variation and Gene Pools
• 2. The Relative Frequency of an allele is how
many times it occurs in a gene pool.
B. Natural Selection on Polygenic Traits
• 1. Polygenic Traits are controlled by 2 or more
genes. (Ex: Skin color or eye color)
B. Natural Selection on Polygenic Traits
• 2. There are 3 ways Natural Selection can
affect the distribution of phenotypes.
– 1. Directional Selection
– 2. Stabilizing Selection
– 3. Disruptive Selection
B. Natural Selection on Polygenic Traits
• 3. Directional Selection- Individuals at one end
of the curve have a higher fitness than
individuals at the middle or other end.
B. Natural Selection on Polygenic Traits
• 4. Stabilizing Selection- Individuals in the
center of the curve have a higher fitness than
either end of the curve.
B. Natural Selection on Polygenic Traits
• 5. Disruptive Selection- Individuals at the
upper/lower ends of the curve have a higher
fitness than the middle.
• 1. Fossil records show that the size of the
black bears in Europe decreased during
interglacial periods of the ice ages, but
increased during each glacial period.
• 2. Babies of low weight lose heat more quickly
and get ill from infectious disease more easily,
whereas babies of large body weight are more
difficult to deliver through the pelvis and do
not survive. Infants of a more medium weight
survive much more often. For the larger or
smaller babies, the baby mortality rate is
much higher.
• 3. Suppose there is a population of rabbits. The color of
the rabbits is controlled by two incompletely dominant
traits: black fur, represented by “B”, and white fur,
represented by “b”.
• A rabbit in this population with a genotype of “BB”
would have a phenotype of black fur, a genotype of
“Bb” would have grey fur (a display of both black and
white), and a genotype of “bb” would have white fur.
• If this population of rabbits occurred in an environment
that had areas of black rocks as well as areas of white
rocks, the rabbits with black fur would be able to hide
from predators amongst the black rocks, and the
rabbits with white fur likewise amongst the white
rocks. The rabbits with grey fur, however, would stand
out in all areas of the habitat, and would thereby suffer
greater predation.
• 4. Another example is the beak size in a
population of finches. Throughout the wet years,
small seeds were more common and there was
such a large supply of the small seeds that the
finches rarely ate large seeds. During the dry
years, none of the seeds were in great
abundance, but the birds usually ate more large
seeds. The change in diet of the finches affected
the depth of the birds’ beaks in the future
generations. Their beaks range from large and
tough to small and smooth.
Create one example for each type of
selection!
• Disruptive Selection
• Stabilizing Selection
• Directional Selection