Forces of evolutionary change
• Natural selection
– traits that improve survival
or reproduction accumulate
in the population
• ADAPTIVE change (survival and
reproduction)
• Genetic drift
– frequency of traits changes
in a population due to
chance events
• RANDOM change
Natural Selection
• Selection acts on any trait that affects survival
or reproduction(alleles being passed down)
– predation selection (speed, camouflage, defenses)
– physiological selection (disease resistance, protection from
injury)
– sexual selection (attractiveness, fertility)
Genetic Drift
• Chance events changing frequency of allele
frequencies in a population
– not adaptation to environmental conditions
– Unpredictable changes
Two types:
– Founder effect
– Bottleneck
Genetic drift (can greatly affect small populations)
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCR
CRCW
Only 5 of
10 plants
leave
offspring
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
Effects of Genetic Drift (summary)
• Genetic drift is significant in small populations
• Genetic drift can cause allele frequencies to
change at random
• Genetic drift can lead to a loss of genetic
variation in a population
• Genetic drift can cause harmful alleles to
become fixed. (Greater Prairie Chickens decreased
because prairies were converted to farmland, LT 50% of eggs
hatched)
Founder effect (Genetic drift)
• A new population is started
by a small group of individuals or are isolated
from other population
– just by chance some rare traits may
be at high frequency;
others may be missing
– skews the gene pool of
new population
– less genetic diversity
Ex: Albino deer: Several dozen wild white-tailed
deer were probably caught within the fence that was built to
surround the Seneca Army Depot in 1941. Isolated from
predators and hunters, the deer population grew quickly.
albino deer Seneca Army
Distribution of blood types
• Distribution of the O type blood allele in native populations of the
world reflects original settlement
Bottleneck effect
• When large population is drastically reduced by a
disaster
– famine, natural disaster, loss of habitat…
– loss of variation by chance event
• narrows the gene pool
Cheetahs
• All cheetahs share a small number of alleles
– less than 1% diversity
– as if all cheetahs are
identical twins
• 2 bottlenecks
– 10,000 years ago
• Ice Age
– last 100 years
• poaching & loss of habitat
Relative Fitness
The contribution an individual makes to the
gene pool of the NEXT generation is relative to
the contributions of other individuals.
• Fitness subjected to natural selection is the
whole organism not an underlying genotype
• Fitness acts indirectly on the genotype
depending on how it affects the phenotype
Modes of Selection
(depends on which phenotype
is favored.
Directional: Darker
mice are favored
because they live in
dark rocks.
Disruptive:
Intermediate color at
a disadvantage. Mice
live in both light and
dark rocks.
Stabilizing:
Intermediate color is
advantaged, extremes
are not favored.
How is genetic variation preserved?
1. Diploidy: pair of chromosomes
• Recessive alleles less favorable
• Natural selection occurs only when both parents carry
the same recessive allele
• Frequency is very rare
2. Heterozygote advantage: advantage lies in the
heterozygous rather than both homozygotes
Ex: sickle cell
3. Frequency-dependent: fitness of a phenotype
depends on how common it is in the population
Ex: scale eating fish page: 414
Question???
• What do you think is the definition of a
species?
• Pair up and write down your definition of a
species and how do we get new species?