Lecture 12: Evolution

1
Lecture 12 Evolution

• Key terms
• Reading
• Ch16 Microevolution
• Ch17Speciation
• Ch18Macroevolution

2
Biological Change Over Time

• Microevolution
• Changes with in species
• Well defined mechanism
• Easily observed
• Based on selection

• Macroevolution
• Change from one species to another
• Undefined mechanism
• Interpretation of
• Cladistics
• Fossil record
• Geological data

3
Microevolutionary Processes

• Drive a population away from genetic equilibrium
• Small-scale changes in allele frequencies brought
  about by
• Natural selection
• Gene flow
• Genetic drift

4
Microevolution

• Genetics
• Microevolution changes a population not
  individuals
• Traits in a population vary among individuals
• Microevolution is change in frequency of traits

• Natural Selection
• Reproductive success for winning phenotypes
• Acts directly on phenotypes and indirectly on
  genotypes
• The first changed individual has no advantage

5

• The Gene Pool
• All of the genes in the population
• Genetic resource that is shared (in theory) by
  all members of population

• Phenotype Variation
• Two copies of each gene (2 alleles)
• Inherit different allele combinations
• Different combinations different phenotypes
• Inherit genotype, NOT phenotypes
• Variation is inherited

6
Genotypes, Phenotypes and Environmental Effects

• Himalayan rabbit experiment
• Pluck hare
• Grow hair with cold pack
• Rabbits share genotype but phenotype is dependent
  on environmental conditions

Fig. 10.18, p. 166
7
Genetic Equilibrium Allele frequencies at a
locus are not changing

• 5 Rules for Equilibrium
• No mutation
• No immigration/ emigration
• Gene doesnt affect survival or reproduction
• Large population
• Random mating

• Interpreted
• No Variation
• No Variation
• No selection
• No selection
• No selection

8
What happens when the rules are broken?
9
Rule 1 No Mutation

• Biological information changes
• Each gene has own mutation rate
• What determines rates?
• Effect of mutations on selection
• Lethal
• Neutral
• Advantageous

10
Variation in the gene pool?

• Recombination
• Crossing over at meiosis I
• Independent assortment
• Meiosis II (haploid germ cells)
• Fertilization
• Haploid haploid diploid
• Changes in chromosome number or structure
• Mutations

11
Rule 2 No Immigration

• Immigration from a separate, segregated
  populations
• New variation
• Alleles
• Mutations
• Effects of immigration
• Shifts allele frequency
• Introduces new mutations through breeding

12
Gene Flow

• Physical flow of alleles into a population
• Tends to keep the gene pools of populations
  similar
• Counters the differences between two populations
  that result from mutation, natural selection, and
  genetic drift

13
Rule 3 Survival or Reproductive Advantage

• What does selection do for a population?
• Survival advantage or Reproductive advantage

14
Pillars of Natural Selection

• Individuals of all populations have the capacity
  to produce more offspring than the environment is
  able to support, so individuals must compete for
  resources.
• Individuals of a population vary in size, form,
  and other traits. The variant forms of a trait
  may be more or less adaptive under prevailing
  conditions.
• When a form of a trait is adaptive under
  prevailing conditions, and when it has a
  heritable basis, its bearers tend to survive and
  reproduce more frequently than individuals with
  less adaptive forms of the trait. Over
  generations, the adaptive version becomes more
  common in the population.
• Natural selection is the result of differences in
  survival and reproduction among individuals of a
  population that differ from one another in one or
  more traits.
• Natural selection results in modifications of
  traits within a line of descent. Over time, it
  may bring about the evolution of a new species,
  with an array of traits uniquely its own.

15
Basics of Natural SelectionCapacity and
Competition

• All populations have the capacity to increase in
  numbers
• No population can increase indefinitely
• Eventually, the individuals of a population will
  end up competing for resources

16
Basics of Natural SelectionCapacity and
Competition

• The alleles that produce the most successful
  phenotypes will increase in the population
• Less successful alleles will become less common
• Change leads to increased fitness
• Increased adaptation to a specific environment

17
Results of Natural Selection

• Three possible outcomes
• Directional selection
• Decreases variation in favor of an extreme.
• Stabilizing selection
• Selects most average/ common form of a trait
• Disruptive selection
• Selects against intermediate forms

18
Directional Selection
Number of individuals in the population
Range of values for the trait at time 1

• Allele frequencies shift in one direction

Number of individuals in the population
Range of values for the trait at time 2
Number of individuals in the population
Range of values for the trait at time 3
19
Stabilizing Selection
Number of individuals in the population

• Intermediate forms are favored and extremes are
  eliminated

Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3
20
Disruptive Selection
Number of individuals in the population

• Forms at both ends of the range of variation are
  favored
• Intermediate forms are selected against

Range of values for the trait at time 1
Number of individuals in the population
Range of values for the trait at time 2
Number of individuals in the population
Range of values for the trait at time 3
21
Resistance

• Antibiotic Resistance
• Bacteria
• Antiviral Resistance
• HIV
• Pesticide Resistance
• Insects

• Chemical kills susceptible individuals
• Resistant individuals survive
• If resistance is heritable, following generations
  exhibit the same trait.

22
Example Pesticide Resistance
Evolution in Action The DDT Paradigm
23
Pre-adapted to survive
99 Non-resistant die
Spray Pesticide
100 resistant survive
24
Second generation
Second generation survivors
25
Third generation
Third generation survivors
26
Mutation rate 1 x 10-4 or 1 in 10,000
100 butterflies
27
1 million butterflies
Beneficial mutation 1 x 10-9 or 1 in
1,000,000,000
28
Insects Evolve at a High Rate
Breeding super-bugs in the home?
29
African Finches
60

• Selection favors birds with very large or very
  small bills
• Birds with intermediate-sized bill are less
  effective feeders

50
40
Number of individuals
30
20
10
10
12.8
15.7
18.5
Widest part of lower bill (millimeters)
30
Sexual Selection

• Selection favors certain secondary sexual
  characteristics
• Through nonrandom mating, alleles for preferred
  traits increase
• Leads to increased sexual dimorphism

31
Balanced Polymorphism

• Polymorphism - having many forms
• Occurs when two or more alleles are maintained at
  frequencies greater than 1 percent

32
Sickle-Cell Trait Heterozygote Advantage

• Allele HbS causes sickle-cell anemia when
  heterozygous
• Heterozygotes are more resistant to malaria than
  homozygotes

Malaria case
Sickle cell trait
less than 1 in 1,600
1 in 400-1,600
1 in 180-400
1 in 100-180
1 in 64-100
more than 1 in 64
33
Rule 4 Large Population

• What happens if the population or allele
  frequency gets wacked?

34
Genetic Drift

• Random change in allele frequencies
• Most pronounced in small populations
• Sampling error - Fewer times an event occurs,
  greater the variance in outcome
• Fixation one allele is established in a
  population

35

• Founder Effect
• Small number of individuals start a new
  population
• Low probability that allele frequencies are the
  same as original population
• Effect is pronounced on isolated islands

• Bottleneck
• A severe reduction in population size
• Causes pronounced drift
• Results
• All progeny will be very similar.
• Gene pool very shallow

36
Large Population Simulation
100
Gene Frequency
50
allele A neither lost nor fixed
0
Generation (500 stoneflies at the start of each)
37
Bottleneck Simulation
100
AA in five populations
Gene Frequency
50
allele A lost from four populations
0
Generation (25 stoneflies at the start of each)
38
Rule 5 Random Mating
39
Inbreeding

• Nonrandom mating between related individuals
• Leads to increased homozygosity
• Can lower fitness when deleterious recessive
  alleles are expressed

40
Genetic Equilibrium Allele frequencies at a
locus are not changing

• 5 Rules for Equilibrium
• No mutation
• No immigration/ emigration
• Gene doesnt affect survival or reproduction
• Large population
• Random mating

• Interpreted
• No Variation
• No Variation
• No selection
• No selection
• No selection

41
Macroevolution and Speciation

• Biological evolution is the theory that all
  living things are modified descendants of a
  common ancestor that lived in the distant past,
  or descent with modification.
• Evolution simply means change over time.
• Descent with modification occurs because all
  organisms within a single species are related
  through descent with modification

42
Biological Species Concept

• Species are groups of interbreeding natural
  populations that are reproductively isolated from
  other such groups.
• Ernst Mayr

43
Morphology Species

• Morphological traits may not be useful in
  distinguishing species
• Members of same species may appear different
  because of environmental conditions
• Morphology can vary with age and sex
• Different species can appear identical

44
Variable Morphology
Grown in water
Grown on land
45
Isolation and Divergence

• Reproductive Isolation
• Cornerstone of the biological species concept
• Speciation is the attainment of reproductive
  isolation
• Reproductive isolation arises as a by-product of
  genetic change

• Genetic Divergence
• Gradual accumulation of differences in the gene
  pools of populations
• Natural selection, genetic drift, and mutation
  can contribute to divergence
• Gene flow counters divergence

46
Reproductive IsolationCant allow gene flow

• Prezygotic Isolation
• Ecological Isolation
• Temporal Isolation
• Behavioral Isolation
• Mechanical Isolation
• Gametic Mortality

• Postzygotic Isolation
• Zygotic mortality
• Hybrid inviability
• Hybrid sterility

Zygote is a fertilized egg
47
Speciation

• Allopatric
• Different lands, (physical barrier)
• Sympatric
• Same lands (no physical or ecological barrier
• Parapatric
• Same border (small hybrid zone)

48
Allopatric Effect

• Speciation in geographically isolated populations
  
• Probably most common mechanism
• Some sort of barrier arises and prevents gene
  flow
• Effectiveness of barrier varies with species

49
Extensive Divergence Prevents Inbreeding

• Species separated by geographic barriers will
  diverge genetically
• If divergence is great enough it will prevent
  inbreeding even if the barrier later disappears

50
Hawaiian Islands

• Volcanic origins, variety of habitats
• Adaptive radiations
• Honeycreepers - In absence of other bird species,
  they radiated to fill numerous niches
• Fruit flies (Drosophila) - 40 of fruit fly
  species are found in Hawaii

51
Hawaiian Honeycreepers
FOUNDER SPECIES
52
Reproductive IsolationCant allow gene flow

• Prezygotic Isolation
• Ecological Isolation
• Temporal Isolation
• Behavioral Isolation
• Mechanical Isolation
• Gametic Mortality

• Postzygotic Isolation
• Zygotic mortality
• Hybrid inviability
• Hybrid sterility

Zygote is a fertilized egg
53
Speciation without a Barrier

• Sympatric speciation
• Species forms within the home range of the parent
  species
• Parapatric speciation
• Neighboring populations become distinct species
  while maintaining contact along a common border

54
Speciation by Polyploidy

• Change in chromosome number (3n, 4n, etc.)
• Offspring with altered chromosome number cannot
  breed with parent population
• Common mechanism of speciation in flowering plants

55
Possible Evolution of Wheat
Triticum monococcum (einkorn)
T. aestivum (one of the common bread wheats)
Unknown species of wild wheat
T. turgidum (wild emmer)
T. tauschii (a wild relative)
CROSS-FERTILIZATION, FOLLOWED BY A SPONTANEOUS
CHROMOSOME DOUBLING
X
X
42AABBDD
14AA
14BB
14AB
28AABB
14DD
56
Parapatric Speciation

• Adjacent populations evolve into distinct
  species while maintaining contact along a common
  border

BULLOCKS ORIOLE
BALTIMORE ORIOLE
HYBRID ZONE
57
Are We All Related?

• Are all species are related by descent?
• Do we share genetic connections that extend back
  in time to the first prototypical cell?

58
Patterns of Change in a Lineage

• Cladogenesis
• Branching pattern
• Lineage splits, isolated populations diverge
• Homology and morphology
• Anagenesis
• No branching
• Changes occur within single lineage
• Gene flow throughout process

59
Evolutionary Trees
extinction (branch ended before present)
new species
branch point (a time of divergence, speciation)
a new species
branch point (a time of divergence, speciation)
dashed line (only sketchy evidence of presumed
evolutionary relationship)
a single lineage
a single lineage
60
Gradual Model

• Speciation model in which species emerge through
  many small morphological changes that accumulate
  over a long time period
• Fits well with evidence from certain lineages in
  fossil record

61
Punctuation Model

• Speciation model in which most changes in
  morphology are compressed into brief period near
  onset of divergence
• Supported by fossil evidence in some lineages

62
Adaptive Radiation

• Burst of divergence
• Single lineage gives rise to many new species
• New species fill vacant adaptive zone
• Adaptive zone is way of life

63
Adaptive Radiation
64
Extinction

• Irrevocable loss of a species
• Mass extinctions have played a major role in
  evolutionary history
• Fossil record shows 20 or more large-scale
  extinctions
• Reduced diversity is followed by adaptive
  radiation

65
Who Survives?

• Species survival is to some extent random
• Asteroids have repeatedly struck Earth destroying
  many lineages
• Changes in global temperature favor lineages that
  are widely distributed

66
Critics of Evolution

1. Critics of Evolution do not propose any
   alternative hypotheses that can be tested by
   evidence.
2. The critics selectively use evidence as the basis
   of their alternative hypotheses.
3. Science is not democratic, the majority of the
   scientific community rejects the critics
   regardless of their evidence.
4. There is no controversy

67
Jones vs. SmithReturning a cracked kettle

1. Smith never borrowed the kettle
2. When Smith returned the kettle it wasnt broken
3. The kettle was already cracked when Smith
   borrowed it
4. There is no kettle

68
extinction (branch ended before present)
new species
branch point (a time of divergence, speciation)
a new species
branch point (a time of divergence, speciation)
dashed line (only sketchy evidence of presumed
evolutionary relationship)
a single lineage
a single lineage
Fig. 17.11 p. 268
69
Fig. 17.12 p. 269
70
Mechanism of Evolution
Progeny Large Populations Genetic Variability
Parental Generation
Selection
Genetic Variability
71
Mechanism of Evolution
72
Factors that cause change

• Mutations- new alleles
• Genetic Drift- unselected random change in allele
  frequencies
• Genetic Bottlenecks
• Founder effect
• Inbreeding
• Gene Flow- moving alleles with mating
• Natural Selection

Evolution changes allele frequencies in
populations not individuals
73
Mechanism of Evolution

• Variation
• Mutations- new alleles
• Natural Selection
• Genetic Drift
• Gene Flow
• Selection
• Directional Selection
• Stabilizing Selection
• Disruptive Selection

• Survival
• Selective forces
• Abiotic- weather, nature
• Biotic- diseases
• Competition
• Reproduction
• Advantageous traits must be passed to progeny
• Ability to pass on the genotype to the next
  generation is the measure of success