BIO102 Evolution Part2 Ch.20

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Genes Within
Populations
Chapter 20
1
Genetic Variation and Evolution
• Evolution: change in a species through time
1. Species accumulate differences
2. Descendants differ from their ancestors
3. New species arise from existing ones
2
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
3
Lamarck’s theory of how giraffes’ long necks
4
evolved
Darwin’s Theory of Evolution
•
•
Observed land & organisms
everywhere they went
Began to notice connections
between species
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
6
Voyage of the Beagle
7
http://etext.library.adelaide.edu.au/d/darwin/charles/
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
9
Tortoises on different
islands…
“normal” tortoise
Saddleback allows tortoise to reach
higher leaves on drier islands
Darwin’s Evidence
Similarity of related species
- Darwin noticed variations in related
species living in different locations
11
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 selection
1. Variation: individuals in a population differ
from one another
2. Heritability: variations are inherited from
parents
3. 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
Darwin’s
theory for
how long
necks
evolved in
giraffes
15
Post-Darwin Evolution Evidence
Fossil record
Mechanisms of heredity
Comparative anatomy
Molecular evidence
16
Why are albinos rare?
Why does a peacock have a large tail?
Hardy-Weinberg Principle
-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)
18
Hardy-Weinberg Principle
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
19
Hardy-Weinberg Principle
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
20
Hardy-Weinberg Principle
21
Hardy-Weinberg Principle
Using Hardy-Weinberg equation to predict
frequencies in subsequent generations
22
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)
23
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
24
Agents of Evolutionary Change
2. Gene flow: A movement of
alleles from one population
to another
-Powerful agent of change
-Tends to homogenize allele
frequencies between
populations
25
Agents of Evolutionary Change
3. Nonrandom Mating: mating with specific
genotypes
– E.G. Sexual Selection – “Peacocks”
– Shifts genotype frequencies
26
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
27
3b. Disassortative Mating:
•phenotypically different individuals mate
•Stabilizing selection: intermediate
(heterozygous) forms are favored and
extremes are eliminated
28
Genetic Drift
4. 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
29
Founder Effect
30
Genetic Drift: A bottleneck effect
31
Selection
5. 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)
32
Selection
33
http://www.pbs.org/wgbh/evolution/library/01/1/l_011_03.html
Evolution of the eye: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html
34
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
35
Body size and egg-laying in water striders
36
• 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
37
Maintenance of Variation
• Heterozygotes may exhibit greater fitness
than homozygotes
• Heterozygote advantage: keep
deleterious alleles in a population
38
Maintenance of Variation
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
39
Maintenance of Variation
Frequency of sickle cell allele
40
Maintenance of Variation
Disruptive selection acts to eliminate
intermediate types
41
Maintenance of Variation
Disruptive selection for large and small
beaks in black-bellied seedcracker finch of
west Africa
42
Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes
43
44
Maintenance of Variation
Stabilizing selection: acts to eliminate
both extremes
45
Maintenance of Variation
Stabilizing selection for birth weight in
humans
46
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