Download Handout #13

Whale Evolution:
Fossil Record of Evolution
Modern toothed whales
Rodhocetus kasrani
reduced hind limbs
could not walk;
swam with up-down motion
like modern whales
Pakicetus attocki
lived on land;
skull had whale
characteristics
Ambulocetus natans
walked on land
…like sea lions
swam
by flexing & paddling
… like otters
IV. Comparative Anatomy
Organisms can be grouped (classified) by
unique anatomical characteristics
Similar anatomy is found in organisms with
greatly divergent functions
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Similar anatomy is found in organisms with
greatly divergent functions
Homologous structures
! structures that may have different appearances
and functions, yet all derived from a common
ancestor
Evolution helps us understand patterns in the diversity of life
Vestigial structures are Evolutionary relicts. They
present a strong argument for common ancestry.
Vestigial structures
! No apparent purpose
! Resemble similar functional
structures in other, closely related
species
! also human appendix
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V. Embryology and Comparative Development
Embryological stages
pig
cow
rabbit
human
All vertebrate embryos look similar early in
their development
evidence of common ancestry
! similar developmental “instructions” in
DNA
V. Embryology and Comparative Development
Comparative Development Reveals
Descent from a Common Ancestor
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VI. Genetic Analysis
Modern technology reveals relatedness among diverse
organisms
Essentially all organisms have DNA as genetic material
With very few exceptions, all organisms use the same genetic
code
Similarities in genes and proteins exist in predictable ways
(based on morphological similarities)
Molecular Record – Independent test
! Distantly related organisms are expected to accumulate a greater
number of evolutionary differences than closely related species.
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VII. Biogeographical Evidence
Islands often have unique (endemic) species
Species on islands are most similar to those on
nearest continent (or nearest island)
Species of Gallotia lizards on
the Canary Islands
I. Populations, Genes, & Evolution
• Evolution is a property of
populations - NOT individuals
• Genes and environment interact to
determine traits (P = G + E)
• Genes:
– functional segments of DNA
– located on chromosomes
• Alleles
– alternative forms of genes
• Genotype
– combination of an individual’s
alleles
Chromosomes
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I. Populations, Genes, & Evolution
• Gene pool: All the possible alleles for a particular
gene (or all the genes) within a given population
EVOLUTION =
Changes in:
• allele frequencies (% of total)
• in a population’s gene pool
• over time
EVOLUTION = Descent with Modification
Charles Darwin, 1859:
On the Origin of the Species
Charles Darwin proposed
Natural Selection as the
mechanism of evolution
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Darwin proposed natural selection as the mechanism of
evolution:
– Darwin observed that organisms
•
•
Produce more offspring than the environment can support
Vary in many characteristics that can be inherited
– Darwin reasoned that natural selection
•
Results in favored traits being represented more and more
and unfavored ones less and less in ensuing generations
of organisms
– Darwin found convincing evidence for his ideas in the results of
artificial selection
•
The selective breeding of domesticated plants and animals
II. Hardy-Weinberg Principle
Under certain conditions:
• allele and genotype frequencies in a population:
– will remain constant over time
– No evolution
! equilibrium population
(This is a hypothetical population!)
How Many Natural Populations are in Genetic Equilibrium?
VERY FEW, if ANY!
So why use Hardy-Weinberg?
Provides a useful starting point for studying the mechanisms
of Evolution by establishing a baseline to compare change
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II. Hardy-Weinberg Principle
Hardy-Weinberg Equilibrium
Under several assumptions, evolution will not occur
i.e., Allele frequencies will not change
1. no mutation
2. no gene flow (no immigration/emigration)
3. large population size (no genetic drift)
4. random mating
5. no natural selection
If these assumptions are met, sexual reproduction alone
will not change allele frequencies in the population
III. What Causes Evolutionary Change?
(1) Mutation
– changes in DNA sequence
– ultimate source of new alleles
– mutations are fairly rare occurrences
(2) Gene flow (migration between populations)
– populations exchange genetic material
– can change allele frequencies by altering the gene pool
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(3) Genetic drift in small populations: Northern
Two cases
elephant seal: ~ 20 animals ~1900, now
A. Population bottlenecks
•
populations reduced to small #
•
then recover
•
but lose genetic variation
•
reduced capacity to evolve
~130,000 animals
(3) Genetic drift in small populations: Two cases
A. Population bottlenecks
• populations reduced to small #
• then recover
• but lose genetic variation
• reduced capacity to evolve
B. Founder effect:
• population “founded” by only a few individuals
• founders not likely to have all the
alleles present in the source population
• rare alleles can be a significant
fraction of the allele pool
• leads to lowered genetic diversity
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Agents of Evolutionary Change, or the Mechanisms of
Evolution. (All of these can change allelic frequencies!)
(4) Nonrandom mating
•
assortative mating
When phenotypically similar individuals mate.
Increases the proportions of homozygotes.
•
•
•
dominant males
females can be picky
inbreeding is a type of
nonrandom mating
(5) Selection
• Not all genotypes are equal
A. Artificial Selection: the breeder selects for the desired
characteristics
B. Natural Selection: environmental conditions determine
which individuals in a population produce more offspring
! Those that are more fit will leave more progeny
IV. Natural Selection
A. Important points
(1) Natural selection does not cause genetic
changes in individuals
(2) Change in allele frequency occurs in
populations
(3) Fitness "! Reproductive Success
= survival to reproductive maturity,
passing on genes, > # of offspring
(4) Evolutionary changes are not goal
oriented
mutations are random
environments/selective forces constantly
change>>
(5) Of the 5 agents of Evolutionary Change,
ONLY selection produces adaptive
evolutionary change
Example:
Penicillin Resistant
Staphylococcus bacteria
•
Antibiotic 1st used in
WWII
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As enters body - kills
majority of bacteria
•
A few survive - have rare
allele, not susceptible
•
They reproduce
•
A few bacterial
generations later…
•
Frequency of rare
“resistant” allele !100%
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C. Components of Fitness
(1) Competition:
•
scarce resources favor the
best-adapted individuals
(2) Predator/Prey Interactions
•
one organism (the predator) eats another (the prey)
– coevolution of predator and prey
(3) Symbiosis
•
leads to the most intricate
coevolutionary adaptations
(4) Sexual selection (access to sexual partners)
! Favors traits that help an organism attract a mate
! Usually, all females get to breed, but only some males
! Result is sexual dimorphism
2 mechanisms:
Male-male combat (large structures for fighting; large
male size)
Female choice (males have attractive structures to lure
females)
***Under both scenarios – females get “good genes”
for offspring
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C. Components of Fitness (cont.)
(5) Kin Selection
• Actions of an individual increase survival and reproductive success
of relatives
• Who have some of the same alleles!
Prairie Dogs
Spadefoot toad tadpoles
V. Forms of Selection (Natural or Artificial)
(1) Directional selection
•
shifts traits in specific direction
•
favors individuals at one end of trait distribution range
•
(ex: Giraffe neck length)
(2) Stabilizing selection
•
favors average individuals
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selects against individuals with extreme values
•
due to opposing pressures (ex: human birth weights)
(3) Disruptive selection
•
favors individuals at both ends of the distribution of a trait
•
selects against average individuals
•
adapts individuals within a population to different habitats:
•
(ex: African seed cracker finches)
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Favors one
extreme
Favors average
Favors both
extremes
V. Forms of Selection: What type of Selection?
100
70
50
30
20
15
10
10
5
7
5
3
2
Percent infant mortality
Percent of births in population
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2 3 4 5 6 7 8 9 10
Birth weight in pounds
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Adaptation and Natural Selection:
Peppered Moths and Industrialized Melanism
! Until the mid-nineteenth century, peppered moths, Biston betularia,
had predominately light-colored wings.
! Subsequently, dark individuals became predominant
! Industrial smog helped turn lichens on tree trunks dark.
! Contrasting colors between trunk color and moth color led to
differential predation by birds.
Natural Selection and Adaptation
Peppered Moths
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Peppered Moths and Industrialized Melanism
! The second half of the twentieth century saw widespread
implementation of pollution controls, thus trends have
reversed and light colored moths again are the dominate
form.
VI. Adaptation (cont.)
Camouflage –
a result of
natural selection
Flower mantid
Leaf mantid in Costa Rica
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VII. Constraints on Evolutionary Perfection
! Historical constraints
• Evolution tinkers with existing structures
! Adaptations are compromises
• Seal flippers on land and water
! Chance and selection interact
• No prediction of future conditions
! Selection can edit only existing variation
• Mutation is random
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