1. AP Biology Big Idea #1 - Evolution
Adapted from Rebecca Rehder Wingerden ©

2. Simply put…
Evolution is simply a change in the genetic makeup of a population over time.

3. Natural Selection is a major mechanism of evolution
a. According to Darwin’s theory of natural selection, competition for limited resources results in differential survival.
- Individuals with more
favorable traits
are more likely to survive
and produce more
offspring,
- Thus passing
traits to subsequent
generations (Descent
With Modification).
“I think” Darwin’s 1837 sketch

4. Natural Selection is a major mechanism of evolution
b. Evolutionary fitness is measured by reproductive success (aka: Darwinian Fitness).
(a) Cactus-eater
(c) Seed-eater
Beak Variation in
Galapagos Finches
(b) Insect-eater

5. Natural Selection is a major mechanism of evolution
c. Genetic variation and mutation play roles in natural selection.
A diverse gene pool is important for the survival of a species in a changing environment.
Variation in a population

6. Natural Selection is a major mechanism of evolution
d. Environments can be more or less stable or fluctuating, – and this affects evolutionary rate and direction; – different genetic variations can be selected in each generation.

7. Natural Selection is a major mechanism of evolution
e. An adaptation is: – a genetic variation that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment. f. In addition to natural selection, chance and random events can influence the evolutionary process, especially for small populations.
PBS Evolving Ideas - How Does Evolution Really Work?(7:00 min.) -
Bozeman Biology: Natural Selection (11:00 min.) -

8. In the absence of evolutionary influence…
a. …a population is said to be in Hardy-Weinberg equilibrium.
Conditions for a population or an allele to be in Hardy-Weinberg equilibrium are:
- 1. A large population size (No Genetic Drift – PopGen Sim)
- 2. Absence of migration (No Gene Flow)
- 3. No net Mutations
- 4. Random mating (No Sexual Selection)
- 5. Absence of environmental selection (No Natural Selection)
• These conditions are seldom met in real populations.

9. Hardy-Weinberg
h. Mathematical approaches are used to calculate changes in allele frequency, providing evidence for the occurrence of evolution in a population. – The Hardy-Weinberg principle describes a population that is not evolving. – If a population does not meet the criteria of the Hardy- Weinberg principle, then it can be concluded that the population is evolving.

10. Hardy-Weinberg
• By convention, IF there are only 2 alleles at a locus, p and q are used to represent their frequencies. - p = A, and q = a • The frequency of all alleles in a population will add up to 1 (= 100%) – For example, p + q = 1 • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p2 + 2pq + q2 = 1 – where p2 (AA) and q2 (aa) represent the frequencies of the homozygous genotypes and 2pq (Aa) represents the frequency of the heterozygous genotype.

11. Selecting alleles at random from a gene pool
2) If all of these alleles
could be placed in a
large bin, 80% would
be CR and 20% would
be CW.
3) Assuming mating is
random, each time two
gametes come together,
there is an 80% chance
the egg carries a CR
allele and a 20% chance
it carries a CW allele.
1) The allele
frequencies of
the population
are 0.8 (80%)
and 0.2 (20%).
Alleles in a population
Frequencies of alleles
Gametes produced
p = frequency of
CR allele = 0.8
CW allele = 0.2
Each egg: Each sperm:
80% % % %
chance chance chance chance

12. Selecting alleles at random from a gene pool
80% CR ( p = 0.8) % CW (q = 0.2)
Gametes for each
generation are
drawn at random
from the gene pool
of the previous
generation.
Sperm CR
(80%) CW
(20%) CR
(80%)
Egg
64% -p2
CRCR
16% -pq
CRCW
If the gametes come
together at random,
then the genotype
frequencies of this
generation are in
Hardy-Weinberg
equilibrium: CW
(20%)
16% -pq
CRCW
4% -q2
CWCW
64% CRCR / 32% CRCW / 4% CW CW
Gametes of this generation:
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
With random mating,
these gametes will
result in the same mix
of genotypes in the
next generation
64% CRCR / 32% CRCW / 4% CWCW plants

13. Applying the Hardy-Weinberg Principle
We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:
– The PKU gene mutation rate is low
– Mate selection is random with respect to whether or not an
individual is a carrier for the PKU allele!
• PKU metabolic disorder resulting from homozygosity for a recessive
allele:
– left untreated leads to mental retardation
– newborns are tested for PKU at birth
– symptoms can be lessened with a phenylalanine-free diet

14. Applying the Hardy-Weinberg Principle
• If the occurrence of PKU is 1 per 10,000 births…what is the frequency of heterozygote carriers? • The frequency of the recessive PKU allele is: – q2 = – q = 0.01 • The frequency of the normal allele is: – p = 1 – q = 1 – 0.01 = 0.99 • The frequency of heterozygous carriers is: – 2pq = 2 x 0.99 x 0.01 = – or approximately 2% of the U.S. population
Bozeman Biology: Solving Hardy Weinberg Problems (11:00 min.)
Bozeman Biology: Population Genetics and Evolution (6:00 min.)
Bozeman Biology: Microevolution (7:00 min.)