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) CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR Only 5 of 10 plants leave offspring CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 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 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 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 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?