2
• 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
3
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
Darwin’s Theory of Evolution
• Observed land & organisms everywhere they went
• Began to notice connections between species
6
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
9
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
11
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
16
Post-Darwin Evolution Evidence
Fossil record
Mechanisms of heredity
Comparative anatomy
Molecular evidence
18
-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
19
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
Hardy-Weinberg Principle
20
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
Hardy-Weinberg Principle
22
Using Hardy-Weinberg equation to predict frequencies in subsequent generations
Hardy-Weinberg Principle
23
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)
24
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
25
2. Gene flow: A movement of alleles from one population to another-Powerful agent of change-Tends to homogenize allele frequencies between populations
Agents of Evolutionary Change
26
Agents of Evolutionary Change
3. Nonrandom Mating: mating with specific genotypes– E.G. Sexual Selection – “Peacocks”– Shifts genotype frequencies
27
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
28
3b. Disassortative Mating: •phenotypically different individuals mate•Stabilizing selection: intermediate (heterozygous) forms are favored and extremes are eliminated
29
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
32
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)
34
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
35
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
37
• 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
38
• Heterozygotes may exhibit greater fitness than homozygotes
• Heterozygote advantage: keep deleterious alleles in a population
Maintenance of Variation
39
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
Maintenance of Variation
42
Disruptive selection for large and small beaks in black-bellied seedcracker finch of
west Africa
Maintenance of Variation
43
Directional selection: acts to eliminate one extreme from an array of phenotypes
Maintenance of Variation