Download Chapter 15: Temporal and Spatial Dynamics of Populations

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Finished chapters 10 and
11. Skip to Chapter 16 to
cover evolution
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Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Chapter 16: Population
Genetics and Evolution
Evolutionary responses to climate changes
+(drought: 1975-1978) (mortality+selection)
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Background: Molecular Basis for
Genetic Variation
 Genetic
information is encoded by DNA.
 Genetic
variation is caused by changes in the
nucleotide sequence of DNA.
 DNA
serves as a template for the
manufacturing of proteins and other nucleic
acids:
each amino acid in a protein is encoded by a
sequence of 3 nucleotides, called a codon
 the genetic code contains redundancy because only
20 amino acids need be encoded from 64 possible
codons

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The source of genetic variation is
mutation and recombination.
 Mutations
are errors in the nucleotide
sequence of DNA:
 substitutions
(most common)
 deletions, additions, and rearrangements also
may occur
 Causes
of mutations:
 random
copying errors
 highly reactive chemical agents
 ionizing radiation
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Can mutations be beneficial?

Most mutations are neither
harmful nor beneficial:
 the altered properties of
proteins resulting from
mutations are not likely to be
beneficial
 natural selection weeds out
most deleterious genes,
leaving only those that suit
organisms to their
environments
 an example is the sickle-cell
mutation, which alters the
structure of the hemoglobin
molecule with deleterious
effects for its carriers
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More on Mutation
 Mutations
are likely to be beneficial when the
relationship of the organism to its environment
changes:

selection for beneficial mutations is the basis for
evolutionary change, enabling organisms to exploit
new environmental conditions
 Processes
that cause mutations are blind to
selective pressures -- mutation is a random force
in evolution, producing genetic variation
independently of its fitness consequences.
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Mutation Rates
 The
rate of mutation for any nucleotide is low, 1 in
100 million per generation.
 (Contextualize
it and it changes ….)
 Because
a complex individual has a trillion or so
nucleotides, each individual is likely to sustain one
or more mutations.
 Rates
of expressed gene mutations average about 1
per 100,000 to 1 per million:

rates of expression of phenotypic effects are often higher
because they are controlled by many genes.
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Recombination
 Variation
is introduced during meiosis when
parts of the genetic material inherited by an
individual from its mother and father
recombine with each other:
recombination is the exchange of homologous
sections of maternal and paternal chromosomes
 recombination produces new genetic variation
rapidly
 to know: ‘evolution of body size in Galapagos marine
iguanas. Natural and sexual selection have opposing
influences on the size of males. Read ‘more on the
web’ (page 314 – go to the website of the textbook)

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Sources of Genetic Variation
 While
mutation is the ultimate source of genetic
variation:



recombination multiplies this variation
sexual reproduction produces further novel combinations of
genetic material
the result is abundant variation upon which natural selection
can operate
+The genotypes of all individuals
make up the gene pool.
 The
gene pool represents the total genetic
variation within the population. (all the genes in
all the individuals in a population)
 Not
all combinations of alleles for a given gene
will be represented in the gene pool,
especially those with low probability.
 If
a rare combination of alleles confers high
fitness, individuals with this combination will
produce more offspring, and these alleles will
increase in frequency.
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The Hardy-Weinberg Law
 In
1908, Hardy and Weinberg independently
described this fundamental law: the frequencies
of both alleles and genotypes will remain
constant from generation to generation in a
population with:
a large number of individuals
 random mating
 no selection
 no mutation
 no migration between populations

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Wait! Too many assumptions!


Natural populations rarely meet all of these conditions.
Large populations rarely occur in isolation

all populations experience some degree of random mutation

mating is seldom random, but rather is the result of careful
selection of mates.

Most importantly, selective pressures favor individuals whose
alleles give them the greatest fitness, so survival and
reproductive success are never random.

Because of these factors inherent in natural selection, allelic
frequencies do not remain constant and evolution occurs.
So
–
why
study
Hardy-Weinberg
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law?
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
(1) the law proves that natural selection is necessary for evolution to
occur. Darwin's theory of evolution states that for evolution to occur,
populations must be variable, there must be inheritance between
generations, and natural selection must make survival and reproductive
success non-random. The conditions set up by the Hardy-Weinberg
Law allow for variability (the existence of different alleles) and
inheritance, but they eliminate natural selection. The fact that no
evolution occurs in a population meeting these conditions proves that
evolution can only occur through natural selection.

(2) the Hardy-Weinberg Law allows us to estimate the effect of
selection pressures by measuring the difference between actual and
expected allelic frequencies or phenotypes.
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Consequences of Hardy- Weinberg
Law (what does it mean?)
 No
evolutionary change occurs through the
process of sexual reproduction itself.
 Changes
in allele and genotype frequencies can
result only from additional forces on the gene pool
of a species.
 Understanding
the nature of these forces is one of
the goals of evolutionary biology.
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Deviations from Hardy-Weinberg
Equilibrium 1
 For
a gene with two alleles, A1 and A2, that occur in
proportions p and q, the proportions of the 3
possible genotypes in the gene pool will be:
 A1A1: p2
 A1A2: 2pq
 A2A2: q2
 Deviations
from these proportions are evidence for
the presence of selection, nonrandom mating, or
other factors that influence the genetic makeup of
a population.
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Deviations from Hardy-Weinberg
Equilibrium 2
 Most
natural populations deviate from HardyWeinberg equilibrium.
 We
thus consider some of the forces responsible
for such deviations (setting aside mutation and
selection):



effects of small population size
nonrandom mating
migration
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Want more?

http://bcs.whfreeman.com/ricklefs6e/content/cat_010/hw01
.html

After completing this module you will understand how the
Hardy-Weinberg equilibrium provides a foundation for
studying changes in gene frequencies over time. In addition,
you will understand how to statistically test whether
populations are or are not in equilibrium.
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Genetic Drift
 Genetic
drift is a change in allele
frequencies due to random variations in
fecundity and mortality in a a population:
 genetic
drift has its greatest effects in small
populations
 when all but one allele for a particular gene
disappears from a population because of genetic
drift, the remaining allele is said to be fixed
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Founder Events
 When
a small number of individuals found
a new population, they carry only a partial
sample of the gene pool of the parent
population:
 this
phenomenon is called a founder event
 founding of a population by ten or fewer
individuals results in a substantially reduced
sample of the total genetic variation
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Population Bottlenecks
 Continued
existence at low population size
of a recently founded population results in
further loss of genetic variation by genetic
drift, referred to as a population
bottleneck:
 such
a situation may have occurred in the recent
past for the population of cheetahs in East Africa
 fragmentation of populations into small
subpopulations may eventually reduce their
genetic responsiveness to selective pressures of
changing environments
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Saharan Cheetah,
Algeria
Estimates put the numbers of
the animal, also known as
the Northwest African
cheetah (Acinonyx jubatus
hecki) as low as 250, but,
says Sarah Durant of the
Zoological Society of
London, this is guesswork.
"Virtually nothing is known
about the population," she
says.
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The Cheetah
“The cheetah is unusual among felids in exhibiting near
genetic uniformity at a variety of loci previously screened to
measure population genetic diversity. It has been
hypothesized that a demographic crash or population
bottleneck in the recent history of the species is causal to the
observed monomorphic profiles for nuclear coding loci. The
timing of a bottleneck is difficult to assess, but certain
aspects of the cheetah's natural history suggest it may have
occurred near the end of the last ice age (late Pleistocene,
approximately 10,000 years ago), when a remarkable
extinction of large vertebrates occurred on several
continents. To further define the timing of such a bottleneck,
the character of genetic diversity for two rapidly evolving
DNA sequences, mitochondrial DNA and hypervariable
minisatellite loci, was examined” (Dating the genetic
bottleneck of the African cheetah, 1993)
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Effective Population Size: The bottleneck effect
“Alleles” in original population
“Alleles” remaining after bottleneck
+ Founder effect
The
 Outlying populations
founded by a small
number of individuals
from source population
 Analogous to bottleneck
 Expect higher drift,
lower diversity in
outlying populations
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The significance…

… of founder events and bottlenecks for natural populations
is that the fragmentation of populations into small
subpopulations may eventually restrict the evolutionary
responsiveness of those subpopulations to the selective
pressures of changing environments, making them MORE
vulnerable to extinction
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Assortative Mating
 Assortative
mating occurs when individuals
select mates nonrandomly with respect to their
own genotypes:



positive assortative mating pairs like with like
negative assortative mating pairs mates that differ
genetically (opposites attract)
assortative mating does not change allele frequencies
but does affect frequencies of genotypes
+ Positive assortative mating leads to
inbreeding.
 Positive
assortative mating can lead to an
overabundance of homozygotes:
 one
result is the unmasking of deleterious recessive
alleles not expressed in heterozygous condition
(inbreeding depression)
 most species have mechanisms that assist them in
avoiding matings with close relatives:
 dispersal, recognition of close relatives, negative
assortative mating, genetic self-incompatibility
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Is inbreeding always undesirable?
 Inbreeding
creates genetic problems, particularly
loss of heterozygosity.
 But
- In some cases inbreeding may be
beneficial:
 plants
that can self-pollinate are capable of sexual
reproduction even when suitable pollinators are absent
 when organisms are adapted to local conditions,
matings with distant individuals may reduce fitness of
progeny
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
Inbreeding: Can you marry your first-cousin?
To marry or not to marry your 1st
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cousin? (illegal in 24 US states)
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
a team of scientists led by Robin L. Bennett, a genetic counselor at the
University of Washington and the president of the National Society of
Genetic Counselors, announced that cousin marriages are not significantly
riskier than any other marriage.

The study, published in the Journal of Genetic Counseling in 2002,
determined that children of first cousins face about a 2 to 3 percent higher
risk of birth defects than the population at large

first-cousin marriages entail roughly the same increased risk of
abnormality that a woman undertakes when she gives birth at 41 rather
than at 30. Banning cousin marriages makes about as much sense, critics
argue, as trying to ban childbearing by older women.

A closer look reveals that moderate inbreeding has always been the rule,
not the exception, for humans. Inbreeding is also commonplace in the
natural world, and contrary to our expectations, this can be a very good
thing. It depends in part on the degree of inbreeding.
+ Global inbreeding
Researchers who study inbreeding track consanguineous
marriages—those between second cousins or closer. In
green countries, at least 20 percent and, in some cases, more
than 50 percent of marriages fall into this category. Pink
countries report 1 to 10 percent consanguinity; peachcolored countries, less than 1 percent. Data is unavailable for
white countries.
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Optimal Outcrossing Distance
 Mating
with individuals located at
intermediate distances (optimal
outcrossing distance) may be desirable:
 nearby
individuals are likely to be close relatives,
resulting in inbreeding
 distant individuals may be adapted to different
conditions:

in controlled matings in larkspur plants, crosses
between individuals 10 m apart enhanced seed set
and seedling survival, compared to selfing and
mating with distant individuals
+Optimal
outcrossing
distance
balances the
risks of
inbreeding and
of mating with
individuals
adapted to
different
conditions
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Inbreeding / outbreeding

The consequences of inbreeding are unpredictable and depend
largely on what biologists call the founder effect

Field biologists have often observed that animals reared together
from an early age become imprinted on one another and lack
mutual sexual interest as adults; they have an innate aversion to
homegrown romance – to avoid incest

normal patterns of dispersal actually encourage inbreeding. When
young birds leave the nest, for instance, they typically move four
or five home ranges away, not 10 or 100; that is, they stay within
breeding distance of their cousins. Intense loyalty to a home
territory helps keep a population healthy because it encourages
"optimal inbreeding." This elusive ideal is the point at which a
population gets the benefit of adaptations to local habitat—the
coadapted gene complexes—without the hazardous unmasking of
recessive disorders.
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Inbreeding / outbreeding

In some cases, outbreeding can be the real hazard. A study
conducted by E. L. Brannon, an ecologist at the University of
Idaho, looked at two separate populations of sockeye salmon,
one breeding where a river entered a lake, the other where it
exited. Salmon fry at the inlet evolved to swim downstream to
the lake. The ones at the outlet evolved to swim upstream.
When researchers crossed the populations, they ended up
with salmon young too confused to know which way to go. In
the wild, such a hybrid population might lose half or more of
its fry and soon vanish.

What about humans?
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Humans vs salmon?

Patrick Bateson, a professor of ethology at Cambridge University,
argues that outbreeding has at times been hazardous for humans
too. For instance, the size and shape of our teeth is a strongly
inherited trait. So is jaw size and shape. But the two traits aren't
inherited together. If a woman with small jaws and small teeth
marries a man with big jaws and big teeth, their grandchildren
may end up with a mouthful of gnashers in a Tinkertoy jaw. Before
dentistry was commonplace, Bateson adds, "ill-fitting teeth were
probably a serious cause of mortality because it increased the
likelihood of abscesses in the mouth." Marrying a cousin was one
way to avoid a potentially lethal mismatch.

Studies have shown that people overwhelmingly choose spouses
similar to themselves (assortative mating). The similarities are
social, psychological, and physical, even down to traits like
earlobe length. Cousins, Bateson says, perfectly fit this human
preference for "slight novelty."
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Migration and Deviations from
Hardy-Weinberg Equilibrium
 Mixing
individuals from populations with different
allele frequencies can result in deviations from
genotypic frequencies under the Hardy-Weinberg
equilibrium:



such movement of genes is called gene flow
mixing results in under-representation of heterozygotes
this phenomenon is called the Wahlund effect
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Genotypes vary geographically.
 Differences
in allelic frequencies between
populations can result from:
 random
changes (genetic drift, founder events)
 differences in selective factors
 Such
differences are particularly evident
when there are substantial geographic
barriers to gene flow.
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Ecotypes
 The
Swedish botanist Göte Turreson used a
common garden experiment to show that
differences among plants from different
localities had a genetic basis:
 under
identical conditions (in the common
garden) plants retained different forms seen in
their original habitats
 Turreson called these different forms ecotypes
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Ecotypes may be close to one
another or distant.
 Although
ecotypes may be geographically
isolated and found some distance apart,
this is not always the case:
 if
selective pressures between nearby localities
are strong relative to the rate of gene flow,
ecotypic differences may arise:
 plants on mine tailings and uncontaminated
soils nearby may differ greatly in their
tolerance to toxic metals (copper, lead, zinc,
arsenic)
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Clines and Other Geographic
Patterns
 Some
traits may exhibit patterns of gradual
change over distance:
 such
patterns are referred to as clines
 clinal variation usually represents adaptation to
gradually changing conditions of the environment
 Other


genetic patterns may be found:
geographic variation related to random founder effects
differentiation related to abrupt geographic barriers and
spatial/temporal variation
+Clinal variation may occur over distance with
variations in the environment: duration of
nymphal development and width of the adult
head in males of the field cricket show a cline
from north to south; genetic basis…
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No genetic basis
for these
variations (the
differences
geographically
bear no
relationship to
habitat or
locality)
These traits (for
the land snail)
vary over
distance
independently
of variations in
the
environment
Geographic isolation: 2 pop. ->
+
genetically distinct
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Natural Selection
 Natural
selection occurs when genetic factors
influence survival and fecundity:

individuals with the highest reproductive rate are said to be
selected, and the proportion of their genotypes increases
over time
 Natural
selection can take various forms depending
on the heterogeneity of, and rate of change in, the
environment.
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Stabilizing Selection
 Stabilizing
selection occurs when
individuals with intermediate, or average,
phenotypes have higher reproductive
success than those with extreme
phenotypes:
 favors
an optimum or intermediate phenotype,
counteracting tendency of phenotypic variation to
increase from mutation and gene flow
 this is the prevailing mode of selection in
unchanging environments
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Directional Selection
 Under
directional selection, the fittest
individuals have more extreme phenotypes
than the average for the population:
 individuals
producing the most progeny are to
one extreme of the population’s distribution of
phenotypes
 the distribution of phenotypes in succeeding
generations shifts toward a new optimum
 runaway sexual selection is an excellent example
of this phenomenon
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3 forms of natural selection
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Disruptive Selection
 When
individuals at either extreme of the
range of phenotypic variation have greater
fitness than those near the mean,
disruptive selection can take place:
 tends
to increase phenotypic variation in the
population
 may lead to bimodal distribution of phenotypes
 uncommon, but could result from availability of
diverse resources, benefits associated with
alternative life histories, or strong competition
for a preferred resource
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Disruptive
selection –
increases
phenotypic
variation in a
population:
dimorphism in
beak size in a
Cameroon
population of the
African estrildid
finch
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Directional selection changes
allele frequencies.
 Selection
changes the makeup of the gene pool.
 Selection
has several important aspects:
 directional
selection against a deleterious allele
results in a decrease in frequency of that allele,
coupled with an increase in frequencies of favorable
alleles
 the rate of change in the frequencies of alleles is
proportional to the selective pressure
 evolution stops only when there is no longer any
genetic variation to act upon; directional selection
thus removes genetic variation from populations
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Maintenance of Genetic Variation 1
A
paradox:
 natural
selection cannot produce evolutionary
change without genetic variation
 however, both stabilizing and directional
selection tend to reduce genetic variation:
 how does evolution continue under such
circumstances?
 does availability of genetic variation ever limit
the rate of evolutionary change?
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Maintenance of Genetic Variation 2
 Mutation
and migration supply populations with
new genetic variation.
 Spatial
and temporal variation tend to maintain
variation by favoring different alleles at different
times and places.
 When
heterozygotes have a higher fitness than
homozygotes, the relative fitness of each allele
depends on its frequency in the population
(frequency-dependent selection):


alleles are selected for when at low frequency and against
when at high frequency
heterozygote superiority is also called heterosis
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How much genetic variation?
 About
1/3 of genes that encode enzymes
involved in cellular metabolism show
variation in most species:
 about
10% of these are heterozygous in any
given individual
 however, most genetic variation is apparently
neutral or has negative effects when expressed
 thus most variation has no fitness consequences
or is subject to stabilizing selection
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Genetic Variation is Important
 Under
changing environmental conditions,
the reserve of genetic variation may take on
positive survival value.
 There
seems to be enough genetic
variation in most populations so that
evolutionary change is a constant
presence.
+ Evolutionary Changes in Natural
Populations
 Evolutionary
changes have been widely
documented, particularly in species that
have evolved rapidly in the face of
environmental changes caused by humans:
 cyanide
resistance in scale insects (Chapter 9)
 pesticide and herbicide resistance among
agricultural pests and disease vectors
 increasing resistance of bacteria to antibiotics
 In
each case, genetic variation in the gene
pool allowed these populations to respond to
changed conditions.
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Useful Conclusions from
Population Genetics Studies
 Every
population harbors some genetic
variation that influences fitness.
 Changes
in selective factors in the
environment are usually met by
evolutionary responses.
 Rapid
environmental changes caused by
humans will often exceed the capacity of a
population to respond by evolution; the
consequence is extinction.
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More interesting genetic news

Schizophrenia Linked to Large Genetic Alterations

Patients with schizophrenia frequently have large chunks of
DNA added to or missing from their genomes. Researchers
checked the genomes of 150 patients with schizophrenia and
those of 268 healthy people, looking for large duplications
and deletions of genetic material that disrupted the function
of a gene. About 15 percent of the schizophrenics had such
mutations, compared with only 5 percent of the people
without the disease. In people whose onset of schizophrenia
occurred before age 19, the proportion was 20 percent.
+Really interesting: Lizardlike
Tuatara Sets a Speed Record
for DNA Change
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
The tuatara reptile looks as if it has hardly changed in the more than 200
million years since it shared habitat with the early dinosaurs. A report
published in March in the journal Trends in Genetics revealed that the
lizardlike native of New Zealand has undergone the fastest rate of
molecular evolution of any vertebrate animal studied thus far.

Evolutionary biologist David Lambert of Griffith University in Australia and
his team analyzed DNA samples taken from ancient tuatara bones and from
living specimens. Over the past 9,000 years, parts of the tuatara genetic
code have changed 50 % more quickly than those of any other vertebrate
tested by the same method.

The finding shows that changes in the genetic code don’t always dictate
changes in the appearance, function, or behavior of an organism.
“Evolution is multidimensional,” Lambert says. “It’s not just about DNA.”
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Evolution for MCAT

After this chapter the only section that will remain is the
‘Origin of Life’

I will post material to assist you all with learning that section
on the website