Download Organic Evolution

EVOLUTION AND ITS
PROCESSES
Chapter 11
Discovering How Populations Change:
Observations of Nature Changed our Thinking
 Aristotle
 Described nature as a continuum of organization
 Lifeless matter through complex plants and animals
 Influenced later European thinkers
 Modified the continuum based on their own beliefs
 By the 1400th century they believed in a “great chain of being”
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Lowest form (snakes)  humans  spiritual beings
Each link in the chain = a species
Each was forged at the same time, same place, in a perfect state
Complete and continuous
Because everything that needed to exist already did, there was no
room for change
Discovering How Populations Change:
Observations of Nature Changed our Thinking
 European naturalists
 Globe-spanning expeditions
 Asia, Africa, North and South America
 Catalogued each new species found in the chain of being
 1800s
 The naturalists found patterns that raised questions that
could not be answered within the framework of the chain
of being
 Where species live
 Similarities in body plans
 Prompted questions about the natural forces that shape
life
Discovering How Populations Change:
Observations of Nature Changed our Thinking
 European naturalists
 Biogeography
 Study of patterns in the geographic distribution of species
 Naturalists found plants and animals living in extremely
isolated places that looked suspiciously similar to species
living across oceans or over mountains
 “links”?
 “perfect state”?
Discovering How Populations Change:
Observations of Nature Changed our Thinking
 European naturalists
 Biogeography
 Study of patterns in the geographic distribution of species
 Flightless birds
 Look alike (and are closely related), on different continents
 If related, how could they have ended up on different continents?
 How would they be placed in the “great chain of being”?
Change over Time
 Georges Cuvier (1769-1832)
 Proposed that many species that had once existed
were now extinct
 Catastrophic geological events
"Why has not anyone seen that fossils alone gave birth to
a theory about the formation of the earth, that without
them, no one would have ever dreamed that there were
successive epochs in the formation of the globe." Georges
Cuvier, Discourse on the Revolutions of the Surface of the
Globe
http://www.ucmp.berkeley.edu/history/cuvier.html
Change over Time
 Jean-Baptiste Lamark (1744-1829)
 Proposed an idea about what
processes might drive change
 Environmental pressures cause an
internal “need” for change
 The resulting change is seen in offspring
 Inherent drive toward perfection, up the
chain of being
Change over Time
 Charles Darwin (1809-1882)
 Influenced by
 5-year expedition on the Beagle as the naturalist
 Charles Lyell’s Principles of Geology
 Challenged the prevailing belief about the age of the Earth
 Thomas Malthus
 Correlated human population sizes with famine, disease, and
war
 Humans have the capacity to produce more individuals than
their environment can support = competition to survive
 Selective breeding of pigeons, dogs, and horses
Change over Time
 Alfred Wallace (1823-1913)
 Influenced by
 Existing studies indicating “change” in populations
 Studying wildlife in the Amazon basin and the Malay
Archipelago
Natural Selection
 Charles Darwin and Alfred Wallace both proposed
that natural selection could be a mechanism by
which evolution (change) occurs
 Many naturalists/scientists had accepted the idea of
descent with modification (evolution)
 Darwin and Wallace suggested a mechanism
Natural Selection
 Natural Selection
 Observations about populations
 All organisms have a far greater reproductive potential than is ever realized
 As a population expands, resources that are used by its individuals are limited
 Limited resources results in competition
 Observations about genetics
 Species share traits
 Inherited variations exist (alleles)
 Inferences
 A certain form of a shared trait may make its bearer better able to survive
 Those that survive tend to leave more offspring
 Thus, an allele associated with an adaptive trait tends to become more common
in a population over time
Natural Selection
 Natural selection example in sea stars
 Can produce 2,500,000 eggs/year!
 A given population of sea stars has various traits (alleles)
 Body wall thickness: T = thick t = thin
(Simplified)
 Crashing waves and predators make life a constant struggle and
many sea stars die
 A thick body wall might help protect from crashing waves and
predators
 Those with adaptive traits (TT and Tt) are more likely to survive
and reproduce
 Thus a larger frequency/percentage of the offspring will inherit the
T allele than was present in the parent generation
Natural Selection
 Vocabulary
 Change/Evolution/Modification
 Change in the line of descent
 Adaptations
 An adaptation is a structure or a process that increases an
animal’s potential to survive and reproduce in specific
environmental conditions
 Can be manifested as behavioral, physiological, or
morphological traits
 Fitness
 Degree of adaptation to an environment
Evolution
 Organic evolution or microevolution is the change in
allele frequency of a population over time
 A population is a group of the same species
 Populations share morphological and
physiological traits
 However, within the shared traits there are allelic
variations
 These variations are created by
 Mutation and changes in chromosome number or
structure
 Crossing over at meiosis I
 Independent assortment at meiosis I
 Fertilization
Evolution
 Organic evolution is not…
Evolution
 Allele frequencies
 All of the alleles of all the genes of a population are
referred to as the gene pool
 The abundance of any particular allele in the gene
pool of a population is the allele frequency
Evolution
 Allele frequencies
 Population level phenotype frequency
 Total # of phenotypes = 20
 15 blue/20 total = 0.75
 5 white/20 total = 0.25
Evolution
 Allele frequencies
 Population level genotype frequency
 Total # of genotypes = 20
 Homozygous dominant
 5/20 = 0.25
 Heterozygous
 10/20 = 0.5
 Homozygous recessive
 5/20 = 0.25
Evolution
 Allele frequencies
 Population level allele frequency
 Total # of alleles = 20 x 2 = 40
 20 A (blue)/40 = 0.5
 20 a (white)/40 = 0.5
Hardy-Weinberg and Population Genetics
 To follow the allele frequency in a population we
have to change our perspective from individuals to
populations
 Previously: crossed 1 purple flowering plant with 1
white flowering plant
 In a population there might be 75 purple flowering
plants and 25 white flowering plants
 Any of them could end up mating with any other plant
 Must know the frequency of each possible allele to
determine the possible offspring genotype and phenotype
ratios
Hardy-Weinberg and Population Genetics
 Two parents
 1:2:1 genotype
 3:1 phenotype
 Population
 AA = .56 = 56%
 Aa = .19 + .19 = .38 (38%)
 Aa = .06 = 6%
Hardy-Weinberg and Population Genetics
 Hardy and Weinberg derived an equation to
determine allele and genotype frequencies of a
population
 Based on the results of a Punnett square for a
population that has a given gene with two alleles (A
and a)
 p = one allele frequency (A)
 q = the other allele frequency (a)
Hardy-Weinberg and Population Genetics
 Hardy and Weinberg derived an equation …
 p2 = homozygous dominant genotype frequency (AA)
 2pq = heterozygous genotype frequency (Aa)
 q2 = homozygous recessive genotype frequency (aa)
Hardy-Weinberg and Population Genetics
 Hardy and Weinberg derived an equation …
 The frequencies of homozygous dominant (AA),
heterozygous (Aa) and homozygous recesive (aa)
should add up to 1 (100% of the population)
 AA + Aa + aa = 100%  p2 + 2pq + q2 = 1
Hardy-Weinberg and Population Genetics
 Hardy and Weinberg derived an equation …
 Because there are only two alleles the following
equation is also true
 p+q=1
Hardy-Weinberg
 Example
 Daisies with a gene for flower color
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490 dark blue flowers (BB)
420 light blue flowers (incomplete dominance) (Bb)
90 white flowers (bb)
1000 total
 How many copies of flower color alleles are there?
 How many of these copies are B?
 How many copies are b?
 What are the allele frequencies (divide the number of B and b
alleles by the total number of alleles) ?
Hardy-Weinberg
 Example
 Daisies with a gene for flower color
 How many copies of flower color alleles are there?
 2000 (2 x 1000)
 Two alleles per individual
 How many of these copies are B?
 (490 x 2) + (420) = 1400
 How many copies are b?
 (420) + (90 x 2) = 600
 What are the allele frequencies?
 B
 b
1400/2000= 0.70
600/2000= 0.30
Hardy-Weinberg
 Example
 Daisies with a gene for flower color
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490 dark blue flowers (BB)
420 light blue flowers (Bb)
90 white flowers (bb)
1000 total
 What are the genotype frequencies?
 (divide the number of each genotype by the total number
of flowers)
Hardy-Weinberg
 Example
 Daisies with a gene for flower color
 Genotype frequencies
 Homozygous dominant (BB) 490/1000 = 0.49
 Heterozygous (Bb)
420/1000 = 0.42
 Homozygous recessive (bb) 90/1000 = 0.09
Hardy-Weinberg and Population Genetics
 The Hardy-Weinberg equation can theoretically predict the
genotype proportions of the next generation if the population
meets five specific conditions
 No mutations
 No new alleles introduced to the population
 Large population
 Ensures change doesn’t occur by chance alone
 No migration
 Prevents alleles from entering or exiting the population
 Random mating
 Each individual has an equal chance of mating
 No selection
 All individuals survive and produce the same number of offspring
Hardy-Weinberg and Population Genetics
 If a population meets all five specific conditions it is
said to be at genetic equilibrium
 Equilibrium occurs when the allele frequency of a
population does not change (not evolving)
 This is a theoretical reference point for a population
1st generation
2nd generation
3rd generation
Hardy-Weinberg and Population Genetics
 If a population meets all five specific conditions it is
said to be at genetic equilibrium
 These conditions never occur all at once in nature
 When the equilibrium is disturbed then the frequency
of alleles changes
 Populations evolve when the frequency of alleles
changes
Evolutionary Mechanisms
 How do allele frequencies change?
 Effects of evolutionary mechanisms
 When a population does not meet one of the five criteria
for genetic equilibrium, the allele frequency can change
Evolutionary Mechanisms
 Mutation
 Changes in the genetic information results in different
proteins creating different traits
 Mutations are random and may result in beneficial,
neutral, or detrimental changes
 Not all mutations matter to evolution
 Can add new alleles to a population
Evolutionary Mechanisms
 Genetic Drift
 Chance events that change the allele frequency
 More likely to occur in small populations
 Similar to flipping a coin
 Should get equal numbers of heads and tails
 In a small population (10 flips) you might get 7 heads and 3
tails
 In a large population (1000 flips) you will get closer to 50%
heads and 50% tails
Evolutionary Mechanisms
 Genetic Drift
 Founder Effect
 A few individuals (founders) from a population colonize a
new habitat
 Founders may bring a different frequency of alleles
Evolutionary Mechanisms
 Genetic Drift
 Bottleneck effect
 Occurs when the number of individuals in a population is
drastically reduced
 Disease, starvation, over-hunting, etc.
 Population is left with only a remnant of the original gene
pool
Evolutionary Mechanisms
 Gene Flow
 Also called migration
 Any movement of genes from one population to another
 Changes the relative allele frequency
 Adds new alleles or remove alleles
Evolutionary Mechanisms
 Natural Selection
 Occurs when some phenotypes are more successful at
leaving offspring than others
Evolutionary Mechanisms
 Natural Selection
 Modes of selection
 Directional selection occurs when one extreme
Number of individuals
in population
Number of individuals
in population
Number of individuals
in population
phenotype is at a disadvantage
Range of values for the trait at time 2
Range of values for the trait at time 1
Range of values for the trait at time 3
Evolutionary Mechanisms
 Natural Selection
 Modes of selection
 Stabilizing selection occurs when both extreme
Range of values for wing-color trait at time 1
Number of individuals
in population
Number of individuals
in population
Number of individuals
in population
phenotypes are at a disadvantage
Range
Range
of values
of values
for wing-color
for the trait
trait
at time
at time
22
Range of values for wing-color trait at time 3
• Sociable weavers living in
the African savanna
•
Too small
•
•
Don’t store enough fat
to avoid starvation
Too large
•
More attractive to
predators and not as
agile when escaping
Evolutionary Mechanisms
 Natural Selection
 Modes of selection
 Disruptive selection occurs when the intermediate
Range of values for wing-color trait at time 1
Number of individuals
in population
Number of individuals
in population
Number of individuals
in population
phenotype is at a disadvantage
Range of values for wing-color trait at time 2
Range of values for wing-color trait at time 3
Type I
Type II
Female
• Males of the plainfin midshipman have two body forms
• Type I males are large and territorial
• Type II males are small and can dart into the nest during spawning to
fertilize eggs
• The medium size male fish can not compete successfully
African finches: Large bills are better with hard seeds
Small bills are better with soft seeds
lower bill 12 mm wide
lower bill 15 mm wide
Evolutionary Mechanisms
 Natural Selection
 Sexual selection occurs when individuals have varying
success obtaining mates
 Structures used for competition for territory and mates
 Horns and antlers
 Structures used to attract a mate
 Brightly colored peacock tail feathers
Evidence of Evolution
 Evolution of a population (change in allele
frequency) is observable in the field and in the
laboratory
 Rise of super rats
 Warfarin
 Rat poison
 Interferes with blood clotting
 Some rats had an allele that coded for a warfarin resistant trait
 Bacteria gaining resistance to antibiotics