mechanisms for evolution
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Mechanisms for Evolution. Natural selection traits that improve survival or reproduction accumulate in the population ADAPTIVE change Genetic drift frequency of traits changes in a population due to chance events RANDOM change. Natural Selection. - PowerPoint PPT PresentationTRANSCRIPT
Mechanisms for Evolution• Natural selection
– traits that improve survival or reproduction accumulate in the population• ADAPTIVE change
• 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– predation selection (speed, camouflage, defenses)
– physiological selection (disease resistance, protection from injury)
– sexual selection (attractiveness, fertility)
Modes of Selection
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.
Genetic Drift • Chance events change frequency of
traits in a population– not adaptation to environmental
conditions– 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
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
albino deer Seneca Army Depot
Example: Distribution of blood types• Distribution of the O type blood allele in native
populations of the world reflects original settlement
Bottleneck effect (genetic drift)
• A large population drastically reduced by a disaster– famine, natural disaster, loss of habitat…– loss of variation by chance event• narrows the gene pool
Example: 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
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?