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RS - AP Biology - Chapter 23-3


Chapter 23

The Evolution of Populations
Causes of Evolution of a Population

Topic 5:

Case 1: How insects become resistant to pesticides
? Adaptive

evolution: evolution that results in a better match between organisms and their environment.

1. Natural selection
? Differential survival & reproduction due to changing environmental conditions
? ? ? ? climate change food source availability predators, parasites, diseases toxins

– combinations of alleles that provide “fitness” increase in the population
? adaptive evolutionary change

Case 2: How genetic drift affect a small population

2. Genetic drift
? Genetic drift : Chance events cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations.
Founder Effect Bottleneck Effect

2.1 - Founder effect
? When a new population is started by only a few individuals
– some rare alleles may be at high frequency; others may be missing – skew the gene pool of new population
? human populations that started from small group of colonists ? example: colonization of New World

Distribution of blood types
? Distribution of the O type blood allele in native populations of the world reflects original settlement

Distribution of blood types
? Distribution of the B type blood allele in native populations of the world reflects original migration

Out of Africa
Likely migration paths of humans out of Africa
10-20,000ya

10-20,000ya

50,000ya

Many patterns of human traits reflect this migration

2.2 - Bottleneck effect
? When large population is drastically reduced by a disaster
– famine, natural disaster, loss of habitat… – loss of variation by chance event
? alleles lost from gene pool
– not due to fitness

? narrows the gene pool

Case 3: Impact of Genetic Drift on the Greater Prairie Chicken

Case 3: Impact of Genetic Drift on the Greater Prairie Chicken

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

Conservation issues
? Bottlenecking is an important concept in conservation biology of endangered species
– loss of alleles from gene pool – reduces variation – reduces adaptability

Peregrine Falcon

Breeding programs must consciously outcross

Golden Lion Tamarin

2. Genetic drift
? Results of chance events
– founder effect ? small group splinters off & starts a new colony – bottleneck ? some factor (disaster) reduces population to small number & then population recovers & expands again

Effects of Genetic Drift: A Summary

Case 4: Gene flow and local adaptation

3. Gene Flow
? Movement of individuals & alleles in & out of populations
– seed & pollen distribution by wind & insect – migration of animals
? sub-populations may have different allele frequencies ? causes genetic mixing across regions ? reduce differences between populations

Human evolution today
? Gene flow in human populations is increasing today
– transferring alleles between populations

Are we moving towards a blended world?

4. Non-random mating
? Sexual selection

5 Agents of evolutionary change
Mutation Gene Flow Non-random mating

Genetic Drift

Natural Selection

Barron’s

Causes of Evolution of a Population
? Natural Selection ? Genetic Drift: Founder Effect & Bottleneck Effect ? Gene Flow ? Nonrandom Mating ? Mutation
Worksheet Q20~22


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