Microevolution

1
Microevolution

• Chapter 16

2
Selective Breeding Evolution

• Evolution is genetic change in a line of descent
  through successive generations
• Selective breeding practices yield evidence that
  heritable changes do occur

3
Domestication of Dogs

• Began about 50,000 years ago
• 14,000 years ago - artificial selection
• Dogs with desired forms of traits were bred
• Modern breeds are the result

4
Results of Artificial Selection

• Extremes in size
• Great Dane and Chihuahua
• Extremes in form
• Short-legged dachshunds
• English bulldog
• Short snout and compressed face
• Extreme traits lead to health problems

5
Evolutionary Theories

• Widely used to interpret the past and present,
  and even to predict the future
• Reveal connections between the geological record,
  fossil record, and organismal diversity

6
Early Scientific Theories

• Hippocrates - All aspects of nature can be traced
  to their underlying causes
• Aristotle - Each organism is distinct from all
  the rest and nature is a continuum or organization

7
Confounding Evidence

• Biogeography
• Comparative anatomy
• Geologic discoveries

8
Biogeography

• Size of the known world expanded enormously in
  the 15th century
• Discovery of new organisms in previously unknown
  places could not be explained by accepted beliefs
• How did species get from center of creation to
  all these places?

9
Comparative Morphology

• Study of similarities and differences in body
  plans of major groups
• Puzzling patterns
• Animals as different as whales and bats have
  similar bones in forelimbs
• Some parts seem to have no function

10
Comparative Anatomy
Human
Python
11
Geological Discoveries

• Similar rock layers throughout world
• Certain layers contain fossils
• Deeper layers contain simpler fossils than
  shallow layers
• Some fossils seem to be related to known species

12
19th Century - New Theories

• Scientists attempt to reconcile evidence of
  change with traditional belief in a single
  creation event
• Two examples
• Georges Cuvier - multiple catastrophes
• Jean Lamark - inheritance of acquired
  characteristics

13
The Theory of Uniformity

• Lyells Principles of Geology
• Subtle, repetitive processes of change, had
  shaped Earth
• Challenged the view that Earth was only 6,000
  years old

14
Darwins Voyage

• At age 22, Charles Darwin began a five-year,
  round-the-world voyage aboard the Beagle
• In his role as ships naturalist he collected and
  examined the species that inhabited the regions
  the ship visited

15
Voyage of the Beagle
EQUATOR
Galapagos Islands
16
GalapagosIslands
Volcanic islands far off coast of Ecuador All
inhabitants are descended from species that
arrived on islands from elsewhere
Isabela
17
Glyptodonts Armadillos

• In Argentina, Darwin observed fossils of extinct
  glyptodonts
• Animals resembled living armadillos

18
Malthus - Struggle to Survive

• Thomas Malthus, a clergyman and economist, wrote
  essay that Darwin read on his return to England
• Argued that as population size increases,
  resources dwindle, the struggle to live
  intensifies and conflict increases

19
Galapagos Finches

• Darwin observed finches with a variety of
  lifestyles and body forms
• On his return he learned that there were 13
  species
• He attempted to correlate variations in their
  traits with environmental challenges

20
Darwins Theory

• A population can change over time when
  individuals differ in one or more heritable
  traits that are responsible for differences in
  the ability to survive and reproduce

21
Alfred Wallace

• Naturalist who arrived at the same conclusions
  Darwin did
• Wrote to Darwin describing his views
• Prompted Darwin to finally present his ideas in a
  formal paper

22
On the Origin of Species

• Darwins book
• Published in 1859
• Laid out in great detail his evidence in support
  of the theory of evolution by natural selection

23
Missing Links

• If one species can evolve into another, there
  should be transitional forms
• When Darwin published his work, no such forms
  were known
• First fossil Archaeopteryx found in 1860

24
Populations Evolve

• Biological evolution does not change individuals
• It changes a population
• Traits in a population vary among individuals
• Evolution is change in frequency of traits

25
The Gene Pool

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

26
Variation in Phenotype

• Each kind of gene in gene pool may have two or
  more alleles
• Individuals inherit different allele combinations
• This leads to variation in phenotype
• Offspring inherit genes, NOT phenotypes

27
What Determines Alleles in New Individual?

• Mutation
• Crossing over at meiosis I
• Independent assortment
• Fertilization
• Change in chromosome number or structure

28
Genetic Equilibrium

• Allele frequencies at a locus are not changing
• Population is not evolving

29
Five Conditions

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

30
Microevolutionary Processes

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

31
Gene Mutations

• Infrequent but inevitable
• Each gene has own mutation rate
• Lethal mutations
• Neutral mutations
• Advantageous mutations

32
Hardy-Weinberg Rule

• At genetic equilibrium, proportions of genotypes
  at a locus with two alleles are given by the
  equation
• p2 AA 2pq Aa q2 aa 1
• Frequency of allele A p
• Frequency of allele a q

33
Punnett Square
34
Frequencies in Gametes
F1 genotypes
Gametes
35
No Change Through Generations
STARTING POPULATION
490 AA butterflies Dark-blue wings
420 Aa butterflies Medium-blue wings
90 aa butterflies White wings
THE NEXT GENERATION
490 AA butterflies
420 Aa butterflies
90 aa butterflies
NO CHANGE
THE NEXT GENERATION
490 AA butterflies
420 Aa butterflies
90 aa butterflies
NO CHANGE
36
Natural Selection

• A difference in the survival and reproductive
  success of different phenotypes
• Acts directly on phenotypes and indirectly on
  genotypes

37
Reproductive Capacity 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

38
Variation in Populations

• All individuals have the same genes that specify
  the same assortment of traits
• Most genes occur in different forms (alleles)
  that produce different phenotypes
• Some phenotypes compete better than others

39
Change Over Time

• Over time, 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 environment

40
Results of Natural Selection

• Three possible outcomes
• A shift in the range of values for a given trait
  in some direction
• Stabilization of an existing range of values
• Disruption of an existing range of values

41
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
42
Peppered Moths

• Prior to industrial revolution, most common
  phenotype was light colored
• After industrial revolution, dark phenotype
  became more common

43
Pesticide Resistance

• Pesticides kill susceptible insects
• Resistant insects survive and reproduce
• If resistance has heritable basis, it becomes
  more common with each generation

44
Antibiotic Resistance

• First came into use in the 1940s
• Overuse has led to increase in resistant forms
• Most susceptible cells died out and were replaced
  by resistant forms

45
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
46
Selection for Gall Size

• Gall-making fly has two major predators
• Wasps prey on larvae in small galls
• Birds eat larvae in large galls
• Flies that cause intermediate-sized galls have
  the highest fitness

47
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
48
African Finches

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

60
50
40
Number of individuals
30
20
10
10
12.8
15.7
18.5
Widest part of lower bill (millimeters)
49
Sexual Selection

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

50
Balanced Polymorphism

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

51
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
52
Gene Flow

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

53
Genetic Drift

• Random change in allele frequencies brought about
  by chance
• Effect is most pronounced in small populations
• Sampling error - Fewer times an event occurs,
  greater the variance in outcome

54
Computer Simulation
1.0
AA in five populations
0.5
allele A lost from four populations
0
Generation (25 stoneflies at the start of each)
55
Computer Simulation
1.0
0.5
allele A neither lost nor fixed
0
Generation (500 stoneflies at the start of each)
56
Bottleneck

• A severe reduction in population size
• Causes pronounced drift
• Example
• Elephant seal population hunted down to just 20
  individuals
• Population rebounded to 30,000
• Electrophoresis revealed there is now no allele
  variation at 24 genes

57
Founder Effect

• Effect of drift when a small number of
  individuals start a new population
• By chance, allele frequencies of founders may not
  be same as those in original population
• Effect is pronounced on isolated islands

58
Inbreeding

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