Chapter 20 How Populations Evolve

1
Chapter 20 How Populations Evolve

• Hardy-Weinberg and Beyond

2
Evidence of Evolution
Fossils suggested that life forms change
-embraced by Lamarck in the early 1800s

• Hominid skull

• Petrified trees

3

• The voyage of the HMS Beagle in the 1830s

Great Britain
NorthAmerica
Europe
PacificOcean
AtlanticOcean
Africa
GalápagosIslands
SouthAmerica
Equator
Australia
Andes
Cape ofGood Hope
Tasmania
Cape Horn
NewZealand
4

• Charles Darwin observed

• similarities between living and fossil organisms
• the diversity of life on the Galápagos Islands,
  blue-footed boobies, finches, and giant tortoises

5

• Darwin

• concluded that living things also change, or
  evolve over generations
• stated that living species descended from earlier
  life-forms descent with modification

6

• Ammonite casts

• Scorpion in amber

• Ice Man

7

• organisms have appeared in historical sequence

• Many fossils link early extinct species with
  species today
• hind leg bones link living whales with their
  land-dwelling ancestors

8

• Comparative anatomy

Human
Cat
Bat
Whale
9
Darwin proposed natural selection as the
mechanism of evolution

• Darwin observed
• organisms produce more offspring than the
  environment can support
• organisms vary in many characteristics
• these variations are inherited

10

• Darwin individuals best suited for an
  environment are more likely to survive and
  reproduce

• Natural selection basic mechanism of
  evolution
• individuals with favorable characteristics
  increase
• Populations gradually change in response to the
  environment

11

• Examples of Evolutionary Adaptations camouflage
  adaptations of mantids in different environments

12
Populations the units of evolution

• A species is a group of populations whose
  individuals can interbreed and produce fertile
  offspring

A gene pool is the total collection of genes in a
population at any one time Microevolution is a
change in the relative frequencies of alleles in
a gene pool
13
The gene pool of a nonevolving population remains
constant over the generations

• Hardy-Weinberg Law- the shuffling of genes during
  sexual reproduction does not alter the
  proportions of different alleles in a gene pool

14
(No Transcript)
15
Five conditions are required for Hardy-Weinberg
equilibrium

• The population is very large
• The population is isolated
• Mutations do not alter the gene pool
• Mating is random
• All individuals are equal in reproductive success
  that is, no selection

16
(No Transcript)
17
(No Transcript)
18
p q 1 p2 2pq q2 1
19
(No Transcript)
20
(No Transcript)
21
Allele Frequency The proportion of all alleles in
all individuals in the group in question which
are of a particular type. (often referred to as
"gene frequency") e.g. 40 individuals which are
AA 47 individuals which are Aa 13 individuals
which are aa Genotype
22
Allele frequency of A 127/200
0.635 pA0.635 pa 73/200 0.365 1- pA
23
1 q 0
24

• Health scientists use the Hardy-Weinberg equation
  to estimate frequencies of disease-causing
  alleles in the human population
• Example Problem phenylketonuria (PKU), a
  recessive allele

1 in every 10,000 babies in US born with PKU
25
What is the frequency of people with the
disease? With a population of 280 million, how
many people will not be carriers of the disease?
26
Testing for Hardy-Weinberg Equilibrium within a
population Determine frequencies of genotypes
directly through phenotypes analyzing
protein or DNA sequences Determine allele
frequencies from the genotypes Use parental
allele frequencies to predict offsprings
frequencies Compare with observed genotype
frequencies in the offspring using Chi-square
analysis
27
Example Population 283 individuals 223 have
genotype AA 57 genotype Aa 3 genotype
aa Determine genotype frequencies AA 223/283
0.788 Aa 57/283 0.201 aa 3/283
0.011 Determine allele frequencies of parents A
p 0.89 a q 0.11
28
Predicted genotype frequencies of offspring AA
p2 (0.89) 2 0.792 Aa 2pq 2(0.89)(0.11)
0.196 aa q2 (0.11) 2 0.012 Observed
genotype frequencies AA 0.782 Aa 0.201 aa
0.017
29
Chi-square analysis has failed to prove any
Hardy-Weinberg assumptions are violated
30
Factors that alter allele frequencies in
populations Natural Selection
31
(No Transcript)
32
Changes in Allele Frequency Forces that Cause
Change Mutation Genetic Drift Random loss of
alleles due to chance events Implies --gt
small population Founder effect - unusual
(changed) allele frequency caused by
small initial population Bottlenecks -
unusual (changed) allele frequency in
population caused by sudden great reduction in
population Migration - difficult
to evaluate quantitatively Non-random mating
33
Mutation Insect resistance
34
Founder Effect
35
(No Transcript)
36
Nonrandom Mating
37
An individuals fitness contribution it makes
to the gene pool of the next generation Productio
n of fertile offspring is the only score that
counts in natural selection
38
Quantitative Traits (Polygenic) Height, yield,
coat color
39
Natural Selection of quantitative traits leads to
these outcomes
40
(No Transcript)
41
Measuring heritability of quantitative traits
42
Heritability Interest is in estimating how much
the characteristics of the offspring are
dependent on the parent. This is referred to as
heritability. Heritability is the ratio of the
genetic variance over the phenotypic variance and
includes all components of genetic variance
h2 VG/VG VE
Since VP VG VE
h2 VG/VP
43
Measuring Heritability
44
MECHANISMS OF SPECIATION
Geographic isolation can lead to speciation

• When a population is cut off from its parent
  stock, species evolution may occur
• An isolated population may become genetically
  unique as its gene pool is changed by natural
  selection, genetic drift, or mutation
• This is called allopatric speciation

45
(No Transcript)
46
New species can also arise within the same
geographic area as the parent species

• In sympatric speciation, a new species may arise
  without geographic isolation
• A failure in meiosis can produce diploid gametes
• Self-fertilization can then produce a tetraploid
  zygote

Parent species
Zygote
Offspring maybe viable andself-fertile
Self-fertilization
Meioticerror
2n 6Diploid
4n 12Tetraploid
Unreduced diploid gametes
47

• Many plants are polyploid
• They are the products of hybridization
• The modern bread wheat is an example

48

• The evolution of wheat

AA
BB
WildTriticum(14 chromo-somes)
Triticummonococcum(14 chromosomes)
AB
Sterile hybrid(14 chromosomes)
Meiotic error andself-fertilization
AABB
DD
T. turgidumEMMER WHEAT(28 chromosomes)
T. tauschii(wild)(14 chromosomes)
ABD
Sterile hybrid
Meiotic error andself-fertilization
AA BB DD
T. aestivumBREAD WHEAT(42 chromosomes)
49
(No Transcript)
50
(No Transcript)
51
(No Transcript)