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Chapter 16 - Evolution of Populations and Speciation
I. Genetic Equilibrium
A. Variations in a Population
1. population genetics - study of evolution from a genetic point
of view
a. population is group of individuals of same species that
interbreed
b. evolution is a gradual change in the genetic material of a
pop.
2. individuals in a population vary in observable traits
a. when graphed, it is a bell curve
3. heredity and environment influence variations
4. variations in genotype due to:
a. mutation
b. recombination of genes (independ. assort. and crossing
over)
c. random joining of gametes
B. Allele Frequencies and The Gene Pool
1. gene pool - total genetic information in a population
2. allele frequency - measure of the relative occurrence of
alleles in a population
a. determined by dividing the number of a certain allele by
the total # of alleles in population
C. Predicting Phenotype and Genotype Frequencies
1. phenotype frequency - # of individuals with certain
phenotype divided by tot. # individuals in pop.
D. Hardy-Weinberg Genetic Equilibrium
1. allele frequencies in a population tend to remain the same
from gen. to gen. unless acted on by outside influences
(based on a hypothetical non-evolving pop.)
a. no mutations occur
b. individuals don’t enter or leave a pop.
c. pop. is large
d. individuals mate randomly
e. natural selection does not occur
2. p = freq. of dominant allele
q = freq. of recessive allele
p+q=1
p2 + 2pq + q2 = 1
p(.6)
q(.4)
p2 = freq. of homozygous dominant
2pq = freq. of heterozygous
p(.6) .36pp .24pq
q2 = freq. homozygous recessive
q(.4) .24pq .16qq
3. Solving gene freq. problems
the allele for black fur (B) is dominant to the allele for white
fur (b) 9% of the pop. has white fur
What % is heterozygous black?
Given: q2 = .09
q = .09 = .3
Since p + q = 1 then:
p=1-q
p = 1- .3 = .7
% heterozygous = 2pq= 2 X .7 X .3 = .42 X 100 = 42%
II. Disruption of Genetic Equilibrium
A. Mutation
1. spontaneous mutations occur all the time
2. organisms are subjected to mutagens
B. Migration
1. immigration - movement of individuals into a population
2. emigration - movement of individuals out of a population
3. gene flow - genes moving from one pop. to another
C. Genetic Drift
1. allele frequencies change due to random events or chance in
a smaller pop.
a. failure of an individual to reproduce or one individual
mating often
b. disease
c. individual dies
2. freq. of an allele is 2% how many have it in a pop. of: 50,
100, 200, 500, 1000, 2000, 10000
D. Nonrandom Mating
1. many species do not mate randomly
E. Natural Selection
1. stabilizing selection - extremes of trait reduced, average is
favored
a. rabbit leg length, lizard size (new predator), human birth
weight
2. directional selection - one of the extremes of a trait favored
a. anteater tongue length, giraffe neck length
3. disruptive selection - both extremes of a trait favored
a. limpets (white shell favored on rocks with goose
barnacles, dark tan favored on bare dark colored rocks,
intermediate color seen more easily in both situations),
acorn size
III. Formation of Species
A. What is a species?
1. speciation - process of species formation
2. based on morphology - internal and external appearance of
an organism
3. biological concept
a. members of a species are morphologically similar and
can interbreed and produce fully fertile offspring
B. Isolating Mechanisms
1. geographic isolation - physical separation of members of a
pop.
a. deep canyon, river change course, valley dries and
becomes desert-like, stretch of ocean
b. subpopulations isolated - gene flow stops - each gene
pool evolves - nat. selection, mutation, gen. drift cause
them to diverge making them incompatible for mating
(pupfish p. 310) - Kaibab and Albert squirrel
2. reproductive isolation - barriers to successful breeding
between members of pop. in same area
a. sometimes caused by disruptive selection
b. incompatible behavior
1) mating call not recognized
2) difference in mating times
3) courtship patterns differ
3. adaptive radiation - many related species evolve from a
single ancestral species (referring to form and structure)
a. Galapagos (Darwin’s) finches
1) ancestor on mainland - some blown off course to
Galapagos islands (600 mi. of ocean) - geog.
isolation from mainland and island to island
2) no land birds, food plentiful, no predators - pop.
grew fast
3) competition for food later
4) variations in beak size
5) started feeding on different items
a) diff. size seeds, cactus pulp, insects,
woodpecker finch
6) reproductive isolation also
7) 13 species of finches (none on mainland)
C. Rate of Speciation
1. gradual evolution - over millions of years, continual change
2. punctuated evolution - sudden shifts in form over few
thousand or less yrs.