Download APBIO Evolution (22 and 23) 2014 15

Chapters 22 and 23
Darwin and
Mechanisms of Evolution
Evolutionary change is observed in laboratory experiments,
in natural populations, and in the fossil record.
Evolution is genetic change occurring in
a population over time
Evolution Is Both Factual and
the Basis of Broader Theory
• Charles Darwin
was interested in
geology and
natural history.
• In 1831, Darwin
began a 5-year
voyage around
the world on a
Navy survey
vessel, the HMS
Beagle.
The Voyage that Changed Everything
Many of Darwin’s observations came from
artificial selection of domesticated plants and
animals.
Selection on different characters in a single
species of wild mustard produced many
crop plants.
Essence of Darwin’s ideas
(1) Variation exists in natural populations
(2) Many more offspring are born each season than can
possibly survive to maturity
(3) As a result, there is a struggle for existence
- competition
(4) Characteristics beneficial in the struggle
for existence will tend to become more common in the
population, changing the average characteristics of the
population
- adaptations
(5) Over long periods of time, and given a steady input of new variation into a
population, these processes lead to the emergence of new species
Milestones in the Development of
Evolutionary Theory
Competition and phylogeny
determine community
structure in Müllerian comimics. Alexandrou, et al.
2011 Nature 469:84
Natural selection acts on individuals, but only populations evolve
Average beak depth (mm)
Natural selection acts on individuals, but only populations evolve
10
9
8
0
1978
1976
(similar to the (after
prior 3 years) drought)
Gene pool—sum of all copies of all
alleles at all loci in a population.
Allele frequency—proportion of
each allele in the gene pool.
Genotype frequency—proportion
of each genotype among
individuals in the population.
Microevolution is a change in allele frequencies
in a population over generations
5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
Non-random mating
Selection
1. Mutation & Variation
• Mutation creates variation
– new mutations are constantly appearing
• Mutation changes DNA sequence
– changes amino acid sequence
– changes protein
– changes in protein may
change phenotype &
therefore change fitness
2. Gene Flow
• Movement of individuals &
alleles in & out of populations
– seed & pollen distribution by
wind & insect
– migration of animals
• sub-populations may have
different allele frequencies
• causes genetic mixing
across regions
• reduce differences
between populations
3. Non-random mating
• Sexual selection
4. Genetic drift
• Effect of chance events
– founder effect
• small group splinters off & starts a new colony
– bottleneck
• some factor (disaster) reduces population to
small number & then population recovers & expands
again
Founder effect
• When a new population is started
by only a few individuals
– some rare alleles may be
at high frequency;
others may be missing
– skew the gene pool of
new population
• human populations that
started from a small group
of colonists
• example:
colonization of New World
Ken Kidd’s model showing
marked founder effect
associated with human
expansion out of Africa
Bottleneck effect
• When large population is drastically reduced
by a disaster
– famine, natural disaster, loss of habitat…
– loss of variation by chance event
• alleles lost from gene pool
– not due to fitness
• narrows the gene pool
Cheetahs
• All cheetahs share a small number of alleles
– less than 1% diversity
– as if all cheetahs are
identical twins
• 2 bottlenecks
– 10,000 years ago
• Ice Age
– last 100 years
• poaching & loss of habitat
Case Study:
Impact of Genetic
Drift on the Greater
Prairie Chicken
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken
(a)
Location
Illinois
1930–1960s
1993
Population
size
Percentage
Number
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
Conservation issues
Peregrine Falcon
• Bottlenecking is an important
concept in conservation biology
of endangered species
– loss of alleles from gene pool
– reduces variation
– reduces adaptability
Golden Lion
Tamarin
5. Natural selection
• Differential survival & reproduction due to
changing environmental conditions
•
•
•
•
climate change
food source availability
predators, parasites, diseases
toxins
– combinations of alleles
that provide “fitness”
increase in the population
• adaptive evolutionary change
Evolution Can Be Measured by Changes in
Allele Frequencies
• Allele frequency =
number of copies of allele in population
total number of copies of all alleles in population
Hardy-Weinberg equation can be used to test
whether a population is evolving
 Hypothetical, non-evolving population
 preserves allele frequencies
 Serves as a model (null hypothesis)
 natural populations rarely in H-W equilibrium
 useful model to measure if forces are acting on
a population
 measuring evolutionary change
G.H. Hardy
mathematician
W. Weinberg
physician
The Hardy-Weinberg Theorem
 Deviation from Hardy-Weinberg
equilibrium usually results in
evolution
 Understanding a non-evolving
population, helps us to understand
how evolution occurs
Hardy-Weinberg theorem
 Counting Alleles
 assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q
 frequencies must add to 1 (100%), so:
p+q=1
BB
Bb
bb
Hardy-Weinberg theorem
 Counting Individuals
 frequency of homozygous dominant: p x p = p2
 frequency of homozygous recessive: q x q = q2
 frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
Bb
bb
Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium
2pq=.48
q2=.16
BB
Bb
bb
2=.74
pp2=.20
BB
2pq=.10
2pq=.64
Bb
=.16
qq22=.16
bb
Null hypothesis
Sampled data
How do you explain
the data?
Hardy Weinberg Application
 We can use the Hardy-Weinberg theorem to
estimate the percentage of the human
population that carries the allele for a
particular inherited disease, phenylketonuria
(PKU) in this case.
 About 1 in 10,000 babies born in the United States
is born with PKU, which results in mental
retardation and other problems if left untreated.
The disease is caused by a recessive allele.
Natural Selection Can Be
Stabilizing, Directional, or
Disruptive
• Quantitative traits—influenced by alleles
at more than one locus; likely to show
continuous variation (body size of
individuals).
Stabilizing selection
27-33
Human Birth Weight Is Influenced by Stabilizing Selection
Directional selection
27-35
Disruptive selection
27-36
Heterozygote Advantage
The sickle-cell
allele causes
mutations in
hemoglobin but
also confers
malaria
resistance
Key
Frequencies of the
sickle-cell allele
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
What about selection at the
level of the genome?
• Nucleotide substitution—change in
one nucleotide in a DNA sequence (a
point mutation).
• Synonymous substitution—most don’t
affect phenotype because most amino
acids are specified by more than one
codon.
• Nonsynonymous substitution—
deleterious or selectively neutral.
Rates of Substitution Differ
Genomes Reveal Both Neutral
and Selective Processes of Evolution
• Neutral theory—at the molecular level,
the majority of variants in most
populations are selectively neutral.
• Neutral variants must accumulate
through genetic drift rather than positive
selection.
Outdated AdaptationExample of Neutral Variation
Hypothesis: Large, hard-shelled fruit of the
calabash tree is actually an adaptation for seed
distribution by large mammals such as the extinct
gomphothere
If the hypothesis is correct, these fruit characteristics can
no longer be considered adaptations for seed distribution.
Janzen, D.H., and P.S. Martin. 1982. Neotropical anachronisms: The fruits the gomphotheres
ate. Science 215:19–27.
Convergent Molecular Evolution of Lysozyme
Lysozymes of langurs
and cattle are
convergent
for 5 amino acids,
indicating
independent evolution
of
foregut fermentation in
these two species.
GHOSTS
EXPERIMENT
Predator: Killifish; preys
mainly on juvenile
guppies (which do not
express the color genes)
Experimental
transplant of
guppies
Pools with
killifish,
but no
guppies prior
to transplant
Guppies: Adult males have
brighter colors than those
in “pike-cichlid pools”
Predation and
Coloration in
Guppies
Predator: Pike-cichlid; preys mainly on adult guppies
Guppies: Adult males are more drab in color
than those in “killifish pools”
RESULTS
12
Number of
colored spots
12
10
8
6
4
2
0
Source
population
Transplanted
population
10
8
6
4
2
0
Source
population
Transplanted
population
A Large Proportion of DNA Is Noncoding
Rate of Speciation
 Current debate:
Does speciation happen gradually or rapidly
 Gradualism
 Charles Darwin
 Charles Lyell
 Punctuated equilibrium
 Stephen Jay Gould
 Niles Eldredge
Niles Eldredge
Curator
American Museum of Natural History
Gradualism
 Gradual divergence
over long spans of
time
 assume that big
changes occur as the
accumulation of many
small ones
2005-2006
Punctuated Equilibrium
 Rate of speciation is not
constant
 rapid bursts of change
 long periods of little or
no change
 species undergo rapid
change when they 1st
bud from parent
population
Time
Primates
• The mammalian order Primates includes lemurs,
tarsiers, monkeys, and apes
• Humans are members of the ape group
Derived Characters of Primates
Most primates have hands
and feet adapted for grasping
Early Primates - Traits
 Common physical primate traits:
 Dense hair or fur covering
 Warm-blooded
 Live young
 Suckle
 Infant dependence
 Common social primate traits:
 Social life
 Play
 Observation and imitation
 Pecking order
Common Primate Traits
2005-2006
Fig. 34-38
(a) New World monkey
(b) Old World monkey
(a) Gibbon
(b) Orangutan
Apes diverged from
Old World monkeys
about 20–25 million
years ago
(c) Gorilla
(d) Chimpanzees
(e) Bonobos
Did Man evolve from Apes?
 No!!
 Similar ancestor
 Both:
Primates
 Different Genus & Species
Derived Characters of Humans
• A number of characters distinguish humans from
other apes:
– Upright posture and bipedal locomotion
– Larger brains
– Language capabilities and symbolic thought
– The manufacture and use of complex tools
– Shortened jaw
– Shorter digestive tract
Homo erectus, Australopithecus,& Human
Hominins originated in
Africa about 6–7 million
years ago
Early hominins had a
small brain but
probably
walked
upright
2005-2006
Walking upright: Bipedalism
 Hominins began to
walk long distances
on two legs about
1.9 million years
ago
Hominid Evolution
 Homo habilis (2.0 – 1.6mya)
 H. rudolfensis (2.4-1.6mya)
 H. heidelbergensis (800-100kyBP)
 H. neanderthalensis (300-30kyBP)
 H. sapiens (130kyBP – present)
Scale: Millions of Years BP
 H. erectus (1.9-270kyBP)
Hominid Evolution
 Major Homo advances:








Brain size
Better bipedalism
Hunting
Fire (H. erectus)
Tools (H. habilis)
Built shelters (H. heidelbergensis)
Clothing (H. neandertalensis)
Language (Neandertals?)
Homo habilis








Artist’s representation of a
Homo habilis band as it might
2005-2006
have
existed two million years
612 cc brain
2.3 - 1.6 mya
first toolmaker
prognathic face, brow ridge
probable meat-eater
possibly arboreal
discovered in 1960 by Leakeys
no speech
H. habilis v. H. erectus
 Finds in east Africa indicate that Homo habilis
was not very different from the
australopithecines in terms of body size and
shape.
 The earliest Homo erectus remains indicate
rapid biological change.
 The fossil record for the transition from H. habilis
to H. erectus supports the punctuated equilibrium
model of evolution.
 H. erectus was considerably taller and had a larger
brain than H. habilis.
Homo erectus
 1891 - Eugene Dubois discovers





H. erectus in Java
Dubois calls it Pithecanthropus
erectus initially, also dubbed
“Java Man”
finds in China called
Sinanthropus
dates from 1.9 mya to 27,000
years B.P.
994 cc brain size (compare to 612
for H. habilis)
Acheulean tool industry
Photograph of Nariokotome boy, an
early Homo erectus found near Lake
Turkana, Kenya.
Homo neanderthalensis
 discovered in the Neander Valley (Tal)
near Dusseldorf, 1856
 massive brain--about 1,400cc on average
 large torso, short limbs, broad nasal
passages
 later remains show decrease in
robustness of the front teeth and face,
suggesting use of tools replaced teeth
 retained occipital torus, some mid-facial
prognathism
The skull of the classic Neandertal
found in 1908 at La Chapelle-auxSaints.
Human Ancestors
What happened to Neandertals?
 H. neanderthalensis coexisted with H. sapiens for at least 20,000
years, perhaps as long as 60,000 years
 What happened?
 Neandertals interbred with H. sapiens
 Neandertals were killed off by H. sapiens
 H. sapiens drove Neandertals into extinction by competition
Fig. 34-43
EXPERIMENT
Hypothesis: Neanderthals gave rise to European humans.
Expected
phylogeny:
Chimpanzees
Neanderthals
Living Europeans
Other living humans
RESULTS
Chimpanzees
Neanderthal 1
Neanderthal 2
European and other
living humans
Homo Sapiens
• Homo sapiens appeared in Africa by 195,000 years
ago
• All living humans are descended from these African
ancestors
Homo sapiens
 Archaic – 100,000 to 35,000
years BP
 Sometimes called Homo
sapiens and Homo sapiens
neanderthalensis
 Modern – 35,000 years BP to
present
 Anatomically modern
 Sometimes called Homo
sapiens sapiens
Cro-Magnon Man
 Cro-Magnon humans




35,000 years B.P. in western Europe to
17,000 years B.P.
1,600 cc cranial capacity
Name comes from a hotel in France
Not a different species, just old Homo
sapiens from Europe
Artist’s reconstruction of a Cro-Magnon man
• Rapid expansion of our species may have been
preceded by changes to the brain that made
cognitive innovations possible
– For example, the FOXP2 gene is essential for human
language, and underwent intense natural selection
during the last 200,000 years
• Homo sapiens were the first group to show
evidence of symbolic and sophisticated thought