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)

CRCR

CRCW

CRCR

CWCW CRCR

CRCW

CRCW

CRCWCRCR

CRCR

Only 5 of10 plantsleaveoffspring

CWCW CRCR

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Only 2 of10 plantsleaveoffspring

CRCR

CRCR CRCR

CRCRCRCR

CRCR

CRCR

CRCR

CRCRCRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3

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)

• 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 diversityEx: 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.

Founder effect (Genetic drift)

albino deer Seneca Army Depot

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 FitnessThe 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 homozygotesEx: sickle cell

3. Frequency-dependent: fitness of a phenotype depends on how common it is in the populationEx: 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?