16-2 Evolution as Genetic Change
•Natural selection does not directly act on genes, but
instead upon the phenotypes they create.
•Natural selection on single-gene traits can change
allele frequencies and cause evolution to occur.
•EX. If a population of red lizards migrated to an area
with black soils, more would be eaten. If a mutation
occurred for black coloration, this phenotype would be
favored for survival and the original red phenotypes
would be selected against.
For polygenic traits, natural selection can have three
effects on phenotypes. Each of these can be
represented by the type of curve that would result from
a graph.
1. Directional selection- individuals at one end have
higher fitness than those in the middle or at the
opposite end.
2. Stabilizing selection- individuals in the center have
greater fitness than those at the ends of the curve.
3. Disruptive selection- the individuals at the upper
and lower extremes have greater fitness than those
in the middle.
•Directional Selection occurs toward
one end. The mean shifts right or left.
•Stabilizing Selection occurs in the
middle. The mean is more
concentrated for selection.
•Disruptive Selection occurs at the
two extremes. The mean is selected
against.
•In this example, a population of seed-eating birds
experiences directional selection when a food shortage
causes the supply of small seeds to run low.
•In this case, birds with larger beaks have higher fitness.
• Therefore, the average beak size increases.
In this example of stabilizing selection, human babies
born at an average mass are more likely to survive
than babies born either much smaller or much larger
than average.
•In this example of disruptive selection, average-sized seeds
become less common, and larger and smaller seeds become
more common.
•As a result, the bird population splits into two subgroups
specializing in eating different-sized seeds.
Alleles can become more common just by chance.
Random changes in allele frequency are called genetic
drift and typically only occur in small populations.
•A situation in which allele
frequencies change as a
result of the migration of a
small subgroup of a
population is known as the
founder effect.
•One example of the
founder effect is the
presence of various finch
species on the Galapagos
islands. All of those
species descended from
the same original
mainland population that
flew to the islands millions
of years ago when the
islands were young.
•When a population remains constant with no change
in allele frequencies, a unique phenomenon occurs
called genetic equilibrium.
•The Hardy-Weinberg Principle provides a
mathematical formula to account for the gene
frequencies within a population so that we can tell if the
population is evolving or locked in genetic equilibrium.
Five conditions that must be met for HWP to be
accurate.
1. Random mating within the population
2. Very large population size
3. No immigration or emigration
4. No mutations
5. No natural selection
p and q are variables used to represent alleles.
(Ex. p = A, q = a, pq = Aa)
(p + q)2 = p2 + 2pq + q2
Because the total alleles represent 100% of the
population, (p + q)2 must equal 1.00
Exit Slip
• How are disruptive selective and directional
selection alike? How are they different?