Exam 4 Slide 1

1
Evolution of Vertebrate Limbs

• 1. Remarkable evolutionary diversification in
  structure and function.
• 2. Based on an evolutionarily conserved common
  design.

2
Tetrapod Limbs
3
Evolution of Vertebrate Limbs

• 1. Remarkable evolutionary diversification in
  structure and function.
• 2. Based on an evolutionarily conserved common
  design.
• How can we explain this mix of evolutionary
  diversification and evolutionary conservation?

4
Evolution of Vertebrate Limbs

• Late Silurian (410 million years ago)
• First fishes with paired pectoral and pelvic
  fins.
• Middle Devonian (380 million years ago)
• First lobe-finned fishes with
• 1. proximal elements homologous to those of
  tetrapod limbs.
• 2. distal elements (digits) either lacking or not
  clearly homologous to those of tetrapod limbs.
• Example Eusthenopteron and Panderichthys

5
Evolution of Vertebrate Limbs

• Late Silurian (410 million years ago)
• First fishes with paired pectoral and pelvic
  fins.
• Middle Devonian (380 million years ago)
• First lobe-finned fishes with
• 1. proximal elements homologous to those of
  tetrapod limbs.
• 2. distal elements (digits) either lacking or not
  clearly homologous to those of tetrapod limbs.
• Examples Eusthenopteron, Panderichthys,
  Tiktaalik
• Eusthenopteron

6

• Late Devonian (360 million years ago)
• First tetrapods (primitive amphibians) four
  limbs, with digits in later forms, always five.
• Example Acanthostega, Ichthyostega

Eusthenopteron Panderichthys Tiktaalik
Acanthostega
tetrapod
lobe-finned fish
transitional forms
7
Evolution of Vertebrate Limbs

• Question How did digits evolve?
• Answer By a new distal pattern of Hox gene
  expression late in limb-bud development!

8
Development of Limbs and Fins
9
Pattern of Hox Gene Expression in fin/limb buds
of Zebrafish and Mice
Zebrafish
Mouse
Zebrafish
Mouse
10
Morphological Diversity of Tetrapod Limbs

• 1. Remarkable evolutionary diversification in
  structure and function.
• Attributable to evolutionary changes in timing or
  pattern of expression of critical pattern-forming
  genes (Hox genes, etc.).
• 2. Based on an evolutionarily conserved common
  design.
• Attributable to conserved underlying
  developmental genetic pathways.
• Example Evolutionary origin and development of
  the hindlimb of birds.

11
Evolution of the Hindlimb of Birds

• Hindlimb of adult birds characterized by
• fusion of tibia and some tarsal bones into
  tibiotarsus.
• fusion of tarsal and metatarsals into
  tarsometatarsus.
• Hindlimb of chicks much more similar to that of
  other tetrapods, with much less fusion of
  separate elements.

12
The Hindlimb of Crocodiles and Birds
13
Evolution of the Hindlimb of Birds Classic
Experiment of Hampé
Mica shield placed between fibula and tibia of
developing chick.
Result
14
Evolutionary Reduction of Vertebrate Limbs

• Many tetrapods have reduced limbs or fewer than
  five digits.
• What is the developmental basis of these
  evolutionary changes?

15
Evolutionary Reduction of Vertebrate Limbs

• Evolution of limb reduction follows predictable
  evolutionary rules.
• Late-developing distal elements (e.g., digits)
  are more likely to be lost than early-developing
  proximal elements (e.g., humerus, ulna, radius).
• Late-developing digits (I, V) are more likely to
  be lost than early-developing digits (II, III,
  IV).
• Examples limb reduction in lizards and horses.

16
Limb Reduction in Lizards
17
Evolution of the Forelimb in Horses
18
Evolution of Limb Loss in Snakes

• In pythons
• 1. No forelimbs buds at all!
• Altered pattern of Hox gene expression.
• Expanded region of thoracic identity.
• 2. Hindlimb buds initiated, but
• No AER (apical ectodermal ridge) develops, so no
  FGF (fibroblast growth factor) signalling to
  promote limb elongation.

19
Quantitative Genetics and Polygenic Traits

• Up to now, we have focused on genetics of
• Qualitative traits genetic variants fall into
  discrete, easily detectable classes.
• 1. Seed shape in peas (round or wrinkled)
• 2. Eye color in Drosophila (red or white)
• 3. Blood types in humans (A, B, AB, or O)
• But what about
• Quantitative traits phenotypic variation
  continuous, and individuals do not fall into
  discrete classes.
• 1. Height in humans
• 2. Bill depth in Galápagos finches
• 3. Seed production by milkweed plants

20
Phenotypic variation in quantitative traits often
approximates a bell-shaped curve the normal
distribution!Example distribution of height in
humans
1914 Class of the Connecticut Agricultural
College (fig. 13.17 from text)
21
What is the genetic basis of variation in
quantitative traits?

• Characteristics of Quantitative Traits
• 1. Polygenic affected by genetic variation at
  many different gene loci.
• 2. Phenotypic effects of allelic substitution are
  usually small and additive.
• Each allelic substitution results in an
  incremental change in overall phenotype.
• 3. Phenotypic variation in quantitative traits
  usually influenced by environmental variation as
  well as by genetic variation.

22
Simplified model for polygenic inheritance of
human height

• Assume
• A variable number (N) of polymorphic loci
  contribute to variation in human height.
• Each locus has two alleles, and .
• At each locus, the two alleles are equally common
  in the human population (allele frequencies
  0.5).
• At each locus, alleles are incompletely dominant
  and act to either decrease () or increase ()
  height by one unit thus, effects of allelic
  substitution are additive.
• Then

23
1. As the number of polymorphic loci increases,
phenotypic variation approaches a continuous
normal distribution!
24
2. Additional phenotypic variation introduced by
environmental variation will further blur
distinctions between genotypic classes!
25
Example of Polygenic Inheritance Insecticide
Resistance in Drosophila
Resistant Strain
F1 Hybrids
Control Strain
26
Question For a quantitative trait, how much of
the total phenotypic variation is attributable to
genetic variation, and how much is attributable
to environmental variation?

• Phenotypic variation (VP) can be partitioned into
  its genetic (VG) and environmental (VE)
  components
• VP VG VE

27
Heritability

• The heritability of a quantitative trait is the
  proportion of the total phenotypic variation (VP)
  that is attributable to genetic variation (VG ).
• Given VP VG VE , then
• Heritability (h2) VG / VP VG / (VG
  VE)
• Heritability is a proportion and varies between 0
  and 1.
• If h2 0, none of the phenotypic variation is
  attributable to underlying genetic variation.
• If h2 1, all of the phenotypic variation is
  attributable to underlying genetic variation.

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Two Methods for Estimating Heritability1.
Resemblance Between Parents and Offspring
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Two Methods for Estimating Heritability2.
Response to Artificial Selection and Realized
Heritability

• Using artificial selection, phenotypic extremes
  are selected and bred.
• Comparison of the progeny with the selected
  parents provides an estimate of realized
  heritability.
• Example heritability of litter size in hamsters

30
Realized heritability of litter size in hamsters
Mean of base stock 8.4 young
Mean of offspring of
selected females 10.1
Frequency
Mean of selected
females 13.6
S
R
Litter Size
Selection differential
S
13.6 - 8.4 5.2
Response to selection
R
10.1 - 8.4 1.7
Realized heritability
R/S
1.7/5.2
0.33
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Two Cautions About Heritability

• Heritability is NOT a measure of the extent to
  which a trait is genetically fixed or
  genetically determined (whatever that means!).
• Heritability of a trait depends on environment in
  which heritability is measured.

32
Sources of Genetic Variation

• A population of organisms will evolve only when
  it contains heritable variation.
• 1. Heritability Offspring resemble their
  parents.
• 2. Variability Offspring do not always resemble
  their parents!
• If either ingredient missing, evolution will not
  occur. How can we have both heritability and
  variation?
• Mutation DNA replication machinery is nearly
  perfect, but not 100 perfect!

33
Sources of Genetic Variation Mutation

• Rare mistakes during DNA replication create
  genetic variation on which natural selection and
  other evolutionary processes can act.
• Rates of mutation are highly variable, but
  usually around 1 x 10-5 per gene per gamete.
• Mutation is the ultimate source of all genetic
  variation.

34
Sources of Genetic Variation Sex and
Recombination I

• Creates new combinations (teams) of alleles
  from pre-existing genetic variation.
• Sexual organisms Two processes
• A. Sex fusion of haploid gametes to form a
  diploid zygote.
• B. Recombination during meiosis.
• 1. Independent assortment of homologous
  chromosomes.
• 2. Crossing over and genetic exchange between
  homologous chromosomes.

35
Sources of Genetic Variation Sex and
Recombination II

• Asexual organisms
• No sex or sexual reproduction, but usually some
  sort of genetic recombination.
• Example Conjugation in bacteria and protists.
• Mutation, sex and recombination create genetic
  variation in natural populations.
• How do we measure the extent of that variation?

36
Population Genetics Essential Jargon I

• Population An interbreeding group of
  individuals of a single species that occupy a
  more-or-less well defined geographic region.
• Effective Population Size The number of
  individuals in a population that are actively
  reproducing and contributing gametes to the gene
  pool of the next generation usually abbreviated
  as Ne.
• Gene Pool The set of all copies of all alleles
  in a population that potentially could be
  contributed by members of one generation to the
  next.

37
Population Genetics Essential Jargon II

• Polymorphic Locus A gene locus at which two or
  more alleles are present in a single population.
• Allele Frequency The relative frequency of a
  particular allele in the gene pool of a
  population, expressed as a proportion between 0
  and 1.
• Genotype Frequency The relative frequency of a
  particular genotype among the individuals of a
  population, expressed as a proportion between 0
  and 1.
• Evolution Genetic change in a population over
  time.

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Measuring the Extent of Genetic Variation in
Natural Populations

• I. Measuring Genetic Variation in Quantitative
  Traits
• Estimate heritability by either
• 1. Parent-offspring resemblance.
• 2. Response to artificial selection.

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Artificial Selection for Abdominal Bristle Number
in Drosophila
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II. Measuring Genetic Variation at Individual
Gene Loci

• A. Protein electrophoresis Identifies
  alternative allelic forms of a protein based on
  electrophoretic mobility.
• Mutations that alter the charge and/or size of a
  protein can alter its mobility in an electric
  field.

41
Protein Electrophoresis
42
II. Measuring Genetic Variation at Individual
Gene Loci (cont.)

• A. Protein Electrophoresis (cont.)
• A very conservative technique only about 25 of
  amino acid substitutions alter protein size or
  charge sufficiently to be detectable.
• Can detect genetic variation only in
  protein-coding regions of genes.
• B. DNA-based techniques.
• Search for restriction fragment-length
  polymorphisms (RFLPs).
• Direct DNA sequencing.

43
Bottom Line Lots of Genetic Variation in Natural
Populations!
44
Natural Populations Contain Extensive Genetic
Variation!

• Question What will happen to all this genetic
  variation?
• General model worked out in 1908 by G. H. Hardy
  and W. Weinberg The Hardy-Weinberg Theorem!

45
The Hardy-Weinberg Equilibrium Model

• In a diploid, sexually reproducing population,
  assume
• 1. Population size is very large.
• Thus, no genetic drift population must be large
  enough so that changes in allele frequencies will
  not occur simply because of chance events
  (sampling errors).
• 2. Individuals mate at random within the entire
  population.
• Thus, no assortative mating, inbreeding, or
  subpopulation structure.
• 3. No input of new alleles from other sources.
• Thus, no mutation or gene flow.
• 4. No natural selection.
• Thus, all alleles are equally competent at
  replicating themselves and entering the gene pool
  as gametes.

46
If the assumptions of the Hardy-Weinberg
equilibrium model are met

• 1. After one generation of random mating,
  genotype frequencies will achieve Hardy-Weinberg
  Equilibrium.
• Genotype frequencies will depend only upon the
  allele frequencies.
• 2. No evolutionary change will occur.
• Allele frequencies will remain constant from one
  generation to the next.

47
Derivation of the Hardy-Weinberg Model

• 1. Consider a single locus with two alleles, A
  and a.
• 2. The frequency of A in the population is p, and
  the frequency of a in the population is q.
• 3. p q 1.
• 4. If Hardy-Weinberg assumptions hold, random
  mating will produce offspring as follows

48
After one generation of random mating

• I. Genotype frequencies of the next generation
  will be
• AA p2
• Aa 2pq
• aa q2
• II. Allele frequencies will remain unchanged.
• Frequency of A allele (2p2 2pq)/2 p2 pq
  p2 p(1-p) p(p(1-p)) p
• Frequency of a allele (2q2 2pq)/2 q2 pq
  q2 q(1-q) q(q(1-q)) q

49
Question Is the Hardy-Weinberg Theorem Useful?

• Given that its assumptions will almost never be
  met in nature, what good is it?
• Answer
• 1. Genotype frequencies observed in natural
  populations can be compared to those expected
  under Hardy-Weinberg conditions!
• 2. Clearly prescribes the conditions that can
  cause either
• Genotype frequencies to depart from
  Hardy-Weinberg expectation.
• Allele frequencies to change over time.

50
Hardy-Weinberg Example

• Variation in abdomen color in fruit fly
  Drosophila polymorpha of Brazil is determined by
  a single locus with two alleles, E and e
• Genotype frequencies EE (dark) 3969
• Ee (intermediate) 3174
• ee (light) 927
• Total 8070
• Step 1 Estimate allele frequencies from sample
• Freq (E) p (2 3969) 3174 / (2 8070)
  0.6885
• Freq (e) q (2 927) 3174 / (2 8070)
  0.3115
• Total p q 1.0000

51
Hardy-Weinberg Example (cont)

• Step 2 Calculate expected genotype frequencies
• Given p 0.6885, q 0.3115,
• Rel. freq. Frequency
• Exp (EE) p2 0.68852 0.4740 (x8070)
  3825.44
• Exp (Ee) 2pq 2(0.6885)(0.3115) 0.4289
  (x8070) 3461.51
• Exp (ee) q2 0.31152 0.0971 (x8070)
  783.05
• Total 1.0000 8070.00

52
Hardy-Weinberg Example (cont)

• Step 3 Compare observed and expected genotype
  frequencies with Chi-square statistic
• Observed (O) Expected (E) (O - E)2
  / E
• EE (Dark) 3969 3825.44 5.39
• Ee (Intermediate) 3174 3461.51 23.88
• ee (Light) 927 783.05 26.46
• Total 8070 8070.00 55.73
• Chi-square statistic (c2) ? (O - E)2 / E
  55.73
• c2 gt 3.84 (critical value) Not in H-W
  equilibrium!

53
The Hardy-Weinberg Equilibrium ModelWhat
Happens When The Assumptions Are Violated?

• In a diploid, sexually reproducing population,
  If
• 1. Population size is very large.
• 2. Individuals mate at random within the entire
  population.
• 3. No input of new alleles from other sources.
• 4. No natural selection.
• Then
• 1. Genotype frequencies will achieve
  Hardy-Weinberg Equilibrium after one generation
  of random mating!
• 2. Allele frequencies will remain constant from
  one generation to the next no evolutionary
  change will occur!

54
The Hardy-Weinberg Equilibrium ModelWhat
Happens When The Assumptions Are Violated?

• Assumption 1 Population size is very large.
• In small populations, allele frequencies can
  change simply because of chance events Genetic
  Drift!
• Allele frequencies drift toward fixation (0.0 or
  1.0) in small populations.
• Two consequences
• 1. Decrease in genetic variation within
  populations.
• 2. Increase in genetic variation among
  populations.
• Evolutionary significance of drift increases as
  effective population size (Ne) decreases.

55
The Hardy-Weinberg Equilibrium ModelWhat
Happens When The Assumptions Are Violated?

• Assumption 2 Individuals mate at random within
  the entire population.
• Three major types of non-random mating
• 1. Inbreeding Matings more likely between
  relatives than between non-relatives.
• 2. Assortative mating like mates with like.
• Inbreeding and assortative mating normally wont
  affect allele frequencies, but will profoundly
  affect genotype frequencies.
• Both lead to a deficiency of heterozygotes and an
  increase in homozygosity!
• 3. Population substructure Limited dispersal
  means than matings are more likely to occur
  between neighbors.
• Effectively decreases Ne, increasing the
  potential importance of genetic drift.
• Produces a local deficiency of heterozygotes.

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Effect of Complete Inbreeding on Genotype
Frequencies
One locus, two alleles (A and A), p0.4 and q0.6
57
Increase in Homozygosity Under Inbreeding(give
n one locus, two alleles, p0.5 and q0.5)
58
The Hardy-Weinberg Equilibrium ModelWhat
Happens When The Assumptions Are Violated?

• Assumption 2 Individuals mate at random within
  the entire population.
• Three major types of non-random mating
• 1. Inbreeding Matings more likely between
  relatives than between non-relatives.
• 2. Assortative mating like mates with like.
• Inbreeding and assortative mating normally wont
  affect allele frequencies, but will profoundly
  affect genotype frequencies.
• Both lead to a deficiency of heterozygotes and an
  increase in homozygosity!
• 3. Population substructure Limited dispersal
  means than matings are more likely to occur
  between neighbors.
• Effectively decreases Ne, increasing the
  potential importance of genetic drift.
• Produces a local deficiency of heterozygotes.

59
The Hardy-Weinberg Equilibrium ModelWhat
Happens When The Assumptions Are Violated?

• Assumption 3 No input of new alleles.
• Two possible sources of new alleles
• 1. Mutation
• Although critical as the ultimate source of
  genetic variation, mutation occurs too rarely to
  produce significant evolutionary change by
  itself.
• 2. Gene flow the movement of alleles into or
  out of a population by dispersal.
• Introduces new alleles into populations.
• Increases Ne and counters the effects of genetic
  drift.

60
The Hardy-Weinberg Equilibrium ModelWhat
Happens When The Assumptions Are Violated?

• Assumption 4 No natural selection.
• Natural selection Differential survival and/or
  reproduction of individuals that differ in one or
  more characters.
• Impact of natural selection on the two
  conclusions of the Hardy-Weinberg theorem
• Depending upon the form it takes, natural
  selection may cause
• 1. genotype frequencies in a population to depart
  from Hardy-Weinberg expectations.
• 2. allele frequencies in a population to change
  over time.

61
Evolution by Natural Selection

• 1. Organisms have tremendous reproductive
  potential (produce many more young than can
  possibly survive).
• 2. Individuals vary in traits that affect their
  ability to survive and reproduce.
• 3.Some variants are fitter and more likely to
  survive and reproduce.
• 4.Characteristics of fitter individuals are
  heritable (passed on through offspring to
  subsequent generations).
• Result EVOLUTIONARY CHANGE

62
Evolution by Natural Selection
Unlimited Reproductive Potential Limited
Resources
Struggle for Existence Variation
Natural Selection Heritability
Evolution
Struggle for existence opportunity for
selection.
Variation in traits that affect survival and
reproduction.
63
Common Misconceptions about Natural Selection

• 1. Natural selection is a force (NOT!)
• Natural selection is an outcome an inevitable
  consequence of unlimited reproductive potential,
  limited resources, and variation in traits
  affecting survival and reproduction.
• 2. Natural selection is directed toward an
  ultimate goal (NOT!)
• Natural selection is non-random survival and
  reproduction, and thats all!
• 3. Natural selection favors traits that promote
  the survival of the species (NOT!)
• In almost all cases, natural selection favors
  traits that enhance survival and reproduction of
  individuals.

64
Lemming Suicide?
65
Types of Natural Selection
Stabilizing Directional Disruptive
or
Fitness
Before selection
or
After selection
Phenotype of Quantitative Trait
66
Types of Natural Selection

• 1. Directional selection selection favors
  individuals at one extreme of a phenotypic
  distribution.
• Example Directional selection for large bills in
  the Galápagos finch Geospiza fortis
• 2. Stabilizing selection selection favors
  individuals of intermediate (average) phenotype.
• Example Stabilizing selection on human
  birthweight.
• 3. Disruptive selection selection favors
  individuals at both extremes of a phenotypic
  distribution can lead to polymorphisms!
• Example Disruptive selection for bill-size
  polymorphism in the black-bellied seedcracker
  (Pyrenestes ostrinus).

67
The Galápagos Islands
68
Darwins Finches
Medium Ground Finch (Geospiza fortis)
14 Species, all restricted to the Galápagos and
Cocos Island
69
A. Population Decline During 1977 Drought
No measurable rain!
70
B. Decrease in Seed Abundance During Drought
Drought
71
C. Increase in Seed Hardness During Drought
Drought
72
D. Increase in Body Size Bill Size During
Drought
Drought
73
Evolution by Natural Selection
Unlimited Reproductive Potential Limited
Resources
Struggle for Existence Variation
Natural Selection Heritability
Evolution
Struggle for existence opportunity for
selection.
Variation in traits that affect survival and
reproduction.
74
Heritability of Bill Depth
Mean after drought
Mean before drought
75
Darwins Finches
14 Species, all restricted to the Galápagos and
Cocos Island
76
Types of Natural Selection
Stabilizing Directional Disruptive
or
Fitness
Before selection
or
After selection
Phenotype of Quantitative Trait
77
Types of Natural Selection

• 1. Directional selection selection favors
  individuals at one extreme of a phenotypic
  distribution.
• Example Directional selection for large bills in
  the Galápagos finch Geospiza fortis
• 2. Stabilizing selection selection favors
  individuals of intermediate (average) phenotype.
• Example Stabilizing selection on human
  birthweight.
• 3. Disruptive selection selection favors
  individuals at both extremes of a phenotypic
  distribution can lead to polymorphisms!
• Example Disruptive selection for bill-size
  polymorphism in the black-bellied seedcracker
  (Pyrenestes ostrinus).

78
Stabilizing Selection on Human Birthweight
79
Types of Natural Selection
Stabilizing Directional Disruptive
or
Fitness
Before selection
or
After selection
Phenotype of Quantitative Trait
80
Types of Natural Selection

• 1. Directional selection selection favors
  individuals at one extreme of a phenotypic
  distribution.
• Example Directional selection for large bills in
  the Galápagos finch Geospiza fortis
• 2. Stabilizing selection selection favors
  individuals of intermediate (average) phenotype.
• Example Stabilizing selection on human
  birthweight.
• 3. Disruptive selection selection favors
  individuals at both extremes of a phenotypic
  distribution can lead to polymorphisms!
• Example Disruptive selection for bill-size
  polymorphism in the black-bellied seedcracker
  (Pyrenestes ostrinus).

81
Bill-size polymorphism in the black-bellied
seedcracker (Pyrenestes ostrinus)
Small-billed Morph Prefers soft seeds of Scleria
goossensii (seed hardness 13 Newtons)
Large-billed Morph Prefers hard seeds of Scleria
verrucosa (seed hardness 153 Newtons)
82
Disruptive Selection in the Black-bellied
Seedcracker
Survival probability

• Bill size largely determined by a single locus
  with two alleles ("large bill" dominant, "small
  bill" recessive), but...
• Continuous (polygenic?) variation exists within
  each bill-size class.
• Juvenile survival during dry season is highest
  for either small or large morphs, but low for
  intermediates!

Juvenile survival Died Survived
Adult Population
83
Types of Natural Selection

• 4. Frequency-dependent selection fitness of a
  particular phenotype depends on its frequency in
  the population.
• Usually occurs when a particular phenotype is
  favored by natural selection when rare, but
  selected against when common!
• Example Handedness in African scale-eating
  cichlid fish.

84
Right- and Left-Handed Mouth Polymorphism in a
Scale-eating Cichlid Fish
Right-Handed (Dextral)
Left-Handed (Sinistral)

• Genetic Basis Probably one locus, two alleles,
  dextral (right) dominant

85
Dietary specialization of the two forms
86
Attack Behavior
87
Frequency of dextral and sinistral morphs
oscillates around 0.5!
88
Foraging success of each form is related to its
frequency in the population!
Even
Predicted
Actual
89
Types of Natural Selection

• 5. Balancing selection a specific form of
  stabilizing selection in which heterozygotes are
  at a selective advantage relative to homozygotes
  heterozygote superiority.
• Along with disruptive selection and
  frequency-dependent selection, balancing
  selection tends to increase or maintain genetic
  variation within populations.
• Example Sickle-cell anemia in humans.

90
Sickle-cell Anemia

• A genetic disease cause by a single amino-acid
  substitution (valine for glutamic acid in 6th
  position) in b-hemoglobin.

91
Sickle-cell Anemia and Balancing Selection

• Sickle-cell historically a lethal or nearly
  lethal recessive homozygotes suffer from severe
  anemia and die prematurely.
• Frequency of sickle-cell allele (S) is as high as
  10-20 in some regions of equatorial Africa.
• Question Why hasnt natural selection eliminated
  the S allele? Why is its frequency so high in
  equatorial Africa?
• Answer Heterozygotes are resistant to malaria
  and are at a selective advantage in areas where
  malaria is prevalent. Thus, balancing selection!

92
Association Between Sickle Cell Allele and Malaria
93
Types of Natural Selection

• 6. Sexual Selection selection for traits that
  improve mating success.
• Sexual selection usually acts more strongly on
  males than on females. Why?
• What is the difference between males and females?
• Female Mating type that produces large,
  resource-rich gametes (eggs).
• Male Mating type that produces small, mobile,
  resource-poor gametes (sperm).

94
Gamete Types and Sex in Algae

• Isogamy two mating types, but neither female
  nor male!
• Anisogamy and Oogamy clear distinction between
  gamete types.

Female
Male
95
Life Cycle of the Alga Ulva Isogamy!
96
Life Cycle of the Alga Halicystis Anisogamy!
Female
Male
97
Consequences of Female-Male Differences in Gamete
Size

• Given ...
• Female Large, resource-rich gametes (eggs).
• Male Small, resource-poor gametes (sperm).
• Therefore
• Female reproductive success is usually limited by
  the amount of resources that the female can
  obtain and invest in eggs and young!
• Male reproductive success is usually limited by
  the number of matings that the male can obtain!

98
Consequences of Female-Male Differences in Gamete
Size

• Thus, in a species without male parental care...
• sexual selection (selection for traits that
  improve mating success) is usually stronger in
  males than in females!

99
Two Forms of Sexual Selection

• 1. Intrasexual selection (within-sex sexual
  selection) Selection for traits that improve
  fighting ability and are advantageous in direct
  competition for mates.
• Examples Large body size in males, antlers in
  deer, large canine teeth in raccoons, etc.
• 2. Intersexual selection (between-sex sexual
  selection) Selection for traits that are more
  attractive to members of the opposite sex.
• Example Elaborate male plumage of the
  Long-tailed Widowbird, Euplectes progne.

100
Intersexual Selection in the Long-tailed
WidowbirdWhy do Males Have Such Long Tails?
101
Results of Field Experiment Show that Females
Prefer Males with Long Tails!
Before Treatment
After Treatment
102
Natural Selection vs. Sexual Selection

• Because traits that improve mating success may
  decrease survival, sexual selection and natural
  selection are often in conflict!
• Example variation in color patterns of male
  guppies (Poecila reticulata) in Trinidad.
• Upstream populations males brightly colored.
• Downstream populations males drab, even though
  females prefer bright males.
• Presence of a predator (pike cichlid, Crenicichla
  alta) in downstream populations selects against
  brightly colored males!

103
Male Coloration Evolves Rapidly in Experimental
Populations Exposed to Different Levels of
Predation
104
Evolution of Female Mating Preferences

• OK, so...
• female widowbirds mate preferentially with
  long-tailed males, and...
• female guppies mate preferentially with brightly
  colored males.
• Question Why are females so selective in these
  species (and many others!) where males provide
  neither critical resources nor parental care?

105
Evolution of Female Mating Preferences in Peafowl

• Female peafowl prefer males with elaborate trains
  with many large eyespots. Why?

106
Offspring of attractive males grow faster, and ...
Male offspring
Female offspring
107
have higher survival than offspring of less
attractive males!

• Thus, elaborate plumage of male peafowl appears
  to be an indicator of overall genetic quality!

108
Two Fundamental Problemsof Evolutionary Biology

• 1. Adaptation and apparent design
• A product of the cumulative effect of natural
  selection!
• 2. Variation and biological diversity
• A product of the cumulative effect of natural
  selection, genetic drift, and other evolutionary
  processes acting on genetic variation created by
  mutation and recombination.
• But...
• Biological diversity is not continuous it
  usually takes the form of discrete units called
  species!
• How can we explain the origin of new species?

109
Questions About Speciation

• 1. What processes lead to the formation of new
  species?
• 2. What processes maintain the integrity and
  distinctiveness of existing species?
• 3. Can the microevolutionary processes
  responsible for speciation also account for
  macroevolutionary change the evolution of major
  groups of organisms?

110
What Is a Species?

• Several definitions available, all of which are
  unsatisfactory for one reason or another.
• Most popular is the Biological Species Concept
  A species is a group of organisms that actually
  (or potentially) interbreed in nature and are
  reproductively isolated from other such groups
  (Ernst Mayr, 1942).
• reproductive isolation can be caused by either
  pre-zygotic or post-zygotic barriers to
  hybridization.

111
Problems with the Biological Species Concept

• 1. Difficult to apply to organisms that are
  either extinct (fossil forms) or asexual (do not
  reproduce sexually).
• 2. Difficult to determine if allopatric
  (geographically separated) populations would
  potentially interbreed in nature.
• Despite definitional problems, species do exist!
  How are new ones formed?

112
Origin of New Species

• A single ancestral species splits into two
  separately evolving lineages that ultimately
  diverge into two distinct species.
• Something must prevent gene flow (genetic
  exchange) between the two lineages so that
  natural selection and genetic drift can produce
  evolutionary divergence.
• What provides initial barriers to gene flow?

113
Two Models of Speciation

• 1. Allopatric Speciation (allopatric different
  range) Populations diverge (via natural
  selection and genetic drift) under geographic
  isolation.
• Geographic isolation provides the initial barrier
  to gene flow.
• Over time, genetic divergence in allopatric
  populations will lead to the origin of
  reproductive isolation gene exchange and
  hybridization cannot occur even if/when
  geographic barriers are eliminated.

114
Allopatric Speciation
Time
115
Reproductive isolation can be caused by either
pre-zygotic or post-zygotic bariers to
hybridization!
116
Evidence for Allopatric Speciation I

• 1. Geographic variation.
• A. Within a group of closely-related species.
• Example Geographic variation in the scrub-jay
  species complex

117
The Scrub-Jay Species Complex
Dispersal
118
Evidence for Allopatric Speciation II

• 1. Geographic variation.
• A. Within a group of closely-related species.
• Example Geographic variation in the scrub-jay
  species complex
• B. Within a single species.
• Example Geographic variation in the rat snake
  Elaphe obsoleta
• Example Geographic variation within two
  California species of the plant Achillea

119
Geographic Variation in the Rat Snake (Elaphe
obsoleta)
120
Evidence for Allopatric Speciation II

• 1. Geographic variation.
• A. Within a group of closely-related species.
• Example Geographic variation in the scrub-jay
  species complex
• B. Within a single species.
• Example Geographic variation in the rat snake
  Elaphe obsoleta
• Example Geographic variation within two
  California species of the plant Achillea

121
Geographic Variation in Two Species of Plants in
the Genus Achillea
Achillea borealis
Achillea lanulosa
122
Geographic Variation in Achillea is Based on
Underlying Genetic Variation
High Sierra
Sierra Foothills
Pacific Coast
123
Evidence for Allopatric Speciation III

• 1. Geographic variation.
• A. Within a group of closely-related species.
• B. Within a single species.
• 2. Adaptive radiation in isolated oceanic
  archipelagos.
• Isolated, biologically impoverished island chains
  provide outstanding opportunities for rapid
  speciation and evolutionary divergence adaptive
  radiation!

124
Rapid Speciation and Adaptive Radiation on
Isolated Oceanic Archipelagos

• Biologically impoverished many vacant niches
  with few competitors and predators.
• Strong diversifying selection favors
  individuals that move into unexploited niches.
• Strong genetic drift new populations often
  founded by just a few individuals.
• Geographic barriers to gene flow promotes
  repeated episodes of evolutionary divergence in
  allopatry followed by occasional dispersal to
  other islands.
• Examples Galápagos Finches and Hawaiian
  Drosophila.

125
The Galápagos Islands
126
Adaptive Radiation in the Finches of the
Galápagos Archipelago
127
Adaptive Radiation in Oceanic Archipelagos
128
Adaptive Radiation in Hawaiian Drosophila

• Over 500 endemic species of fruit flies in
  Hawaiian Islands!
• Tremendous diversity in preferred habitat, larval
  food source, morphology, etc.
• Analysis of DNA sequence similarity suggests that
  most recently derived species are found on SE
  islands, while older species found on NW islands.
  Why?

129
Evolutionary Relationships Within the Drosophila
heteroneura Species Complex
Time
Phylogeny based on sequence similarity of mtDNA
130
Adaptive Radiation in Hawaiian Drosophila

• Over 500 endemic species of fruit flies in
  Hawaiian Islands!
• Tremendous diversity in preferred habitat, larval
  food source, morphology, etc.
• Analysis of DNA sequence similarity suggests that
  most recently derived species are found on SE
  islands, while older species found on NW islands.
  Why?
• Speciation pattern related to timing of island
  formation!

131
Hawaiian Islands formed by northwestward movement
of Pacific plate over a volcanic hot spot in
underlying mantle
Loihi Seamount
132
The Hawaiian Islands, Now and Then
A. Present
B. 5 Million years ago
133
Interisland Colonizations and the Evolution of
the Picture Wing Drosophila of Hawaii

134
Two Models of Speciation

• 1. Allopatric Speciation
• 2. Sympatric Speciation (sympatric same range)
  Evolutionary divergence occurs in two
  populations that occupy the same geographic
  range.
• Ecological specializations provide the initial
  barrier to gene flow and select for evolutionary
  divergence in sympatry.

135
Sympatric Speciation
(a) Single Interbreeding Population
Time
136
Sympatric Speciation in Progress Host Race
Evolution in the Apple Maggot Fly Rhagoletis
pomonella

• Pre-1864 R. pomonella known only from fruits of
  hawthorn trees.
• Females lay eggs on fruits.
• Males seek mates on fruits.
• 1864 a host shift recorded in upstate New York!
• Some flies shifted to apples.
• Present Now many genetic differences between the
  two ecologically isolated host races, and little
  hybridization between them!

137
Range of Apple and Hawthorn Host Races in
Rhagoletis pomonella
138
Types of Reproductive Barriers

• What is the genetic basis of pre-zygotic and
  post-zygotic barriers to hybridization?

139
Genetic Basis of Pre-zygotic Barriers to
Hybridization

• Example California abalones.
• Seven species of abalone (marine mollusks in
  genus Haliotis) found along coast of California.
• All spawn by releasing eggs and sperm into sea
  water.
• Despite external fertilization, natural hybrids
  among the seven species are rare. Why?
• Between-species differences in the amino-acid
  sequence of both lysin (sperm protein) and the
  lysin-receptor (egg glycoprotein) create genetic
  incompatibities!

140
Genetics of Reproductive Isolation
Genetics of pre-zygotic barriers in Pacific
abalones
Genetics of post-zygotic barriers in Drosophila
141
Genetic Basis of Post-zygotic Barriers to
Hybridization

• Example Sterility of male hybrids in Drosophila.
• In hybrids between two closely related species of
  Drosophila (D. mauritiana and D. simulans), males
  are often sterile. Why?
• Genetic incompatibilities between the X-linked
  transcription factor OdsH of mauritiana and the
  autosomal genes of simulans!
• OdsH codes for a DNA-binding protein that
  regulates genes involved in the production of
  sperm.
• OdsH has evolved rapidly amino acid sequence of
  OdsH in mauritiana is very different than that of
  simulans.
• OdsH protein is over-expressed in testes of
  sterile hybrids.

142
Genetics of Reproductive Isolation
Genetics of pre-zygotic barriers in Pacific
abalones
Genetics of post-zygotic barriers in Drosophila
143
General Summary of Speciation
144
Speciation and Macroevolution

• Speciation generates biodiversity the
  proliferation over time of unique and genetically
  distinct species. But
• What generates morphological diversity the wide
  array of body forms and plans found in
  multicellular organisms?
• Answer Morphological diversity generally arises
  by evolutionary change in the regulation of
  development!
• Example 1 Between-species differences in
  pigmentation patterns on wings and abdomens of
  Drosophila.

145
Interspecific Variation in Pigmentation Patterns
on Wings and Abdomens of Drosophila
D. melanogaster
D. kikkawai
146
Drosophila Pigmentation Determined by Expression
of Gene Yellow
Expression of Yellow
Abdomen Pigmentation
D. melanogaster
D. kikkawai
147
Evolution of Drosophila Pigmentation

• Question What is genetic basis for evolutionary
  change in expression of Yellow in different
  Drosophila species?
• Answer Evolutionary gain and loss of binding
  sites for transcription factors in regulatory
  sequences flanking the Yellow gene!
• Gain or loss of binding sites alters pattern of
  Yellow transcription in different embryonic
  regions.

148
Abd-B expression
Yellow expression
Abdomen pigmentation
Evolution of Abdomen Pigmentation in Drosophila
D. willistoni
D. melanogaster
D. kikkawai
Evolutionary Time
149
Evolution of Wing and Abdomen Pigmentation in
Drosophila
D. willistoni
D. melanogaster
D. biarmipes
Evolutionary Time
150
Evolutionary Change in Regulation of Development

• Example 2 Loss of spiny pelvic fin in the
  three-spined stickleback (Gasterosteus aculeatus)
• Two forms in waters of western Canada and Alaska
• Deep-water marine form with pelvic spines.
• Shallow-water lake form without pelvic spines.
• Spine loss has evolved multiple times as marine
  sticklebacks have invaded newly-formed glacial
  lakes in last 15,000 years. Why and How?

Deep-water marine form
Shallow-water lake form
151
Evolutionary Loss of Pelvic Spines Why?

• Deep-water sticklebacks Pelvic spines protect
  sticklebacks from larger predator fish.
• Shallow-water sticklebacks Few or no large
  predatory fish in recent glacial lakes.
• Pelvic spines are a liability in shallow water,
  as predatory dragonfly larvae can grasp them.
• Thus, natural selection has favored loss of
  pelvic spines in glacial lakes!

152
Evolutionary Loss of Pelvic Spines How?

• In shallow-water sticklebacks, mutations have
  disabled an enhancer site for the developmental
  gene Pitx1.
• Disabled enhancer responsible only for
  development of the pelvic fin other crucial
  Pitx1 functions are unaffected!

153
Evolutionary Change in Regulation of Development
Example 3 Diversity in vertebrae number. Highly
variable among vertebrates, but snakes are the
champs!
154
Evolutionary Change in Vertebrae Number

• Vertebrae arise from presomatic mesoderm which
  develops into somites.
• Somites paired blocks of tissue along the A/P
  axis.
• Presomitic mesoderm (PSM) has clock and
  wavefront gene expression pattern.
• Clock gene ticks with cyclical on/off
  expression pattern at posterior end of PSM.
• Pairs of somites bud anteriorly with each clock
  cycle.

155
Evolutionary Change in Vertebrae Number
Question Why do snakes have so many
vertebrae? Answer Somite formation clock ticks
about 4 times faster (relative to growth rate) in
snakes than in other vertebrates!
156
Evolutionary Change in Vertebrae Number in Snakes
Evolution of vertebrae number in snakes is an
example of heterochrony evolutionary change in
the timing of development. A relatively small
change in the timing of a clock gene results
in major morphological differences!