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Chapter 23~ Chapter 23~ The Evolution of Populations

Chapter 23~ Chapter 23~ The Evolution of Populations

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Chapter 23~

• Chapter 23~ The Evolution of Populations

Population genetics• Population: a localized group of

individuals belonging to the same species

• Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring

• Gene pool: the total aggregate of genes in a population at any one time

• Population genetics: the study of genetic changes

in populations• Modern synthesis/neo-Darwinism• “Individuals are selected, but

populations evolve.”

Hardy-Weinberg Theorem• Serves as a model for the genetic

structure of a nonevolving population (equilibrium)

• Evolution = change in allele frequencies in a population

– hypothetical: what conditions not would cause allele frequencies to change?

– non-evolving populationREMOVE all agents of

evolutionary change1. very large population size

(no genetic drift)2. no migration (no gene

flow in or out)3. no mutation (no genetic

change)4. random mating (no sexual

selection)5. no natural selection

(everyone is equally fit)

Hardy-Weinberg Equation• p=frequency of one allele (A); q=frequency of the

other allele (a); p+q=1.0 (p=1-q & q=1-p)

• p2=frequency of AA genotype; 2pq=frequency of Aa genotype; q2=frequency of aa genotype;

• frequencies of all individuals must add to 1 (100%), so:

p2 + 2pq + q2 = 1

G.H. Hardymathematician W. Weinberg

physician

What are the genotype frequencies?What are the genotype frequencies?

Using Hardy-Weinberg equation

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

q2 (bb): 16/100 = .16

q (b): √.16 = 0.40.4

p (B): 1 - 0.4 = 0.60.6

population: 100 cats84 black, 16 whiteHow many of each genotype?

population: 100 cats84 black, 16 whiteHow many of each genotype?

bbBbBB

p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16

Must assume population is in H-W equilibrium!Must assume population is in H-W equilibrium!

5 Agents of evolutionary change

Mutation

Gene FlowGenetic Drift

Selection

Non-random mating

Microevolution, I

• A change in the gene pool of a population over a succession of generations

• 1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

Microevolution, II: type of genetic drift• The Bottleneck

Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

Conservation issues• Bottlenecking is an

important concept in conservation biology of endangered species– loss of alleles from gene pool– reduces variation– reduces adaptability

Breeding programs must consciously outcrossBreeding programs must consciously outcross

Peregrine Falcon

Golden Lion Tamarin

Microevolution, III type of genetic drift• Founder Effect:

a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population– just by chance 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

Microevolution, IV• 2- Gene Flow:

genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

• seed & pollen distribution by wind & insect

• migration of animals

Microevolution, V• 3- Mutations: • Mutation

creates variation a change in an

organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)

Microevolution, VI

• 4- Nonrandom mating:

• Sexual selection• inbreeding and

assortive mating (both shift frequencies of different genotypes)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Sexual selection

It’s FEMALE CHOICE, baby!

Microevolution, VII5.NaturalSelectio

n• differential success

in reproduction; • climate change• food source availability• predators, parasites,

diseases• toxins

• only form of microevolution that adapts a population to its environment

• combinations of alleles that provide “fitness” increase in the population

Natural Selection• Selection acts on any trait that

affects survival or reproduction– predation selection– physiological selection– sexual selection

Variation & natural selection

• Variation is the raw material for natural selection– there have to be differences within population

– some individuals must be more fit than others

Mean beak depth of parents (mm)

Medium ground finch8

8 9 10 11

9

10

11

1977 1980 1982 1984

Dry yearDry year

Dry year

Wet year

Bea

k de

pth

Bea

k de

pth

ofof

fspr

ing

(mm

)

Where does Variation come from?• Mutation

– random changes to DNA• errors in mitosis & meiosis• environmental damage

• Sex – mixing of alleles

• recombination of alleles– new arrangements in every

offspring

• new combinations = new phenotypes

– spreads variation• offspring inherit traits from parent

Population variation• Polymorphism:

coexistence of 2 or more distinct forms of individuals (morphs) within the same population

• Geographical variation: differences in genetic structure between populations (cline)

Variation preservation• Prevention of natural

selection’s reduction of variation

• Diploidy 2nd set of chromosomes hides variation in the heterozygote

• Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

Natural selection

• Fitness: contribution an individual makes to the gene pool of the next generation

• 3 types:• A. Directional• B. Diversifying• C. Stabilizing

Effects of Selection• Changes in the average trait of a population

DIRECTIONALSELECTION

STABILIZINGSELECTION

DISRUPTIVESELECTION

giraffe neckhorse size human birth weight rock pocket mice

Sexual selection

• Sexual dimorphism: secondary sex characteristic distinction

• Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

Any Questions??