1 Genes Within Populations Chapter 20. 2 Evolution: change in a species through time 1.Species...

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Genes Within Populations

Chapter 20

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• Evolution: change in a species through time

1. Species accumulate differences

2. Descendants differ from their ancestors

3. New species arise from existing ones

Genetic Variation and Evolution

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Mechanism of evolutionary change

Lamarck’s theory of evolution

Inheritance of acquired characteristics:

• Individuals passed on physical and behavioral changes to their offspring

• Variation by experience…not genetic

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Lamarck’s theory of how giraffes’ long necks evolved

Darwin’s Theory of Evolution

• Observed land & organisms everywhere they went

• Began to notice connections between species

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Charles Darwin

Served as naturalist on 5 year mapping expedition(HMS BEAGLE) around coastal South America.

Used many observations to develop his ideas

Proposed that evolution occurs by

natural selection

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Voyage of the Beagle

http://etext.library.adelaide.edu.au/d/darwin/charles/

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Charles Darwin

evolution: modification of a species over generations

-“descent with modification”

natural selection: individuals with superior characteristics are more likely to survive and reproduce than those without such characteristics

Tortoises on different islands…

Saddleback allows tortoise to reach higher leaves on drier islands

“normal” tortoise

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Darwin’s Evidence

Similarity of related species

- Darwin noticed variations in related species living in different locations

Most famous for his observations of Galapagos finches

• Some islands much drier than others• Different islands had their own, slightly

different varieties of animals• Darwin hypothesized that new species

could gradually appear, much like animal breeders can artificially develop new varieties through selective breeding

Explaining his observations: natural selection1. Variation: individuals in a population differ

from one another2. Heritability: variations are inherited from

parents3. Overproduction: organisms produce more

offspring than can survive (survival of the fittest)

4. Reproductive advantage: some variations allow the organism that possesses them to have more offspring – those variations become more common, and the population changes over time

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Darwin’s theory for how long necks evolved in giraffes

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Post-Darwin Evolution Evidence

Fossil record

Mechanisms of heredity

Comparative anatomy

Molecular evidence

Why are albinos rare? Why does a peacock have a large tail?

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-Genetic equilibrium: Allele (and genotype) frequencies in a population will remain constant from generation to generation

-if equilibrium is upset evolution

(punctuated equilibrium chp 22)

Hardy-Weinberg Principle

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Requirements to maintain genetic equilibrium:

1. No mutation

2. No genes are transferred to or from other sources

3. Random mating

4. Very large population

5. No selection


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Calculate genotype frequencies(p+q)2 = p2 + 2pq + q2

• p = frequency of the 1st allele• q = frequency of the 2nd allele

• p2 = individuals homozygous for 1st allele• 2pq = heterozygous individuals• q2 = individuals homozygous for 2nd allele

• because there are only two alleles: p plus q must always equal 1


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Using Hardy-Weinberg equation to predict frequencies in subsequent generations


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A population not in Hardy-Weinberg equilibrium indicates an agent of

evolutionary change is operating in a population

(one or more of the 5 conditions are not being met)

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Agents of Evolutionary Change

1. Mutation: A change in a cell’s DNA– Mutation rates are generally

so low they have little effect on Hardy-Weinberg proportions of common alleles.

– Ultimate source of genetic variation

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2. Gene flow: A movement of alleles from one population to another-Powerful agent of change-Tends to homogenize allele frequencies between populations


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3. Nonrandom Mating: mating with specific genotypes– E.G. Sexual Selection – “Peacocks”– Shifts genotype frequencies

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3a. Assortative Mating: •mates that are phenotypically similar•does not change frequency of individual alleles•Disruptive selection: forms at both ends of the range of variation are favored over intermediate forms

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3b. Disassortative Mating: •phenotypically different individuals mate•Stabilizing selection: intermediate (heterozygous) forms are favored and extremes are eliminated

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Genetic Drift4. Small populations

Genetic drift: Random fluctuation in allele frequencies over time by chance

• important in small populations–founder effect – When a few individuals

start a population (small allelic pool)»Amish

–bottleneck effect - drastic reduction in population, and gene pool size

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Founder Effect

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Genetic Drift: A bottleneck effect

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Selection5. Natural selection: environmental

conditions determine which individuals in a population produce the most offspring

• This is the only agent that produces adaptive evolutionary change (selects individuals that are more fit)

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Selection

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Evolution of the eye: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

http://www.pbs.org/wgbh/evolution/library/01/1/l_011_03.html



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Fitness and Its Measurement

• Fitness: A phenotype with greater fitness usually increases in frequency

• Fitness is a combination of:–Survival: how long does an

organism live–Mating success: how often it mates–Number of offspring per mating that

survive

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Body size and egg-laying in water striders

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• Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time

• Galápagos Islands ground finches

– Wet conditions favor big bills (abundant seeds)

– Dry conditions favor small bills

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• Heterozygotes may exhibit greater fitness than homozygotes

• Heterozygote advantage: keep deleterious alleles in a population

Maintenance of Variation

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Example: Sickle cell anemia

•Homozygous recessive phenotype: exhibit severe anemia

•Homozygous dominant phenotype: no anemia; susceptible to malaria

•Heterozygous phenotype: no anemia; less susceptible to malaria


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Frequency of sickle cell allele

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Disruptive selection acts to eliminate intermediate types


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Disruptive selection for large and small beaks in black-bellied seedcracker finch of

west Africa


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Directional selection: acts to eliminate one extreme from an array of phenotypes


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Stabilizing selection: acts to eliminate both extremes


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Stabilizing selection for birth weight in humans


1 Genes Within Populations Chapter 20. 2 Evolution: change in a species through time 1.Species...

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