Download Chapter 2 - Green Resistance

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Effects of climate change on
evolution and distribution of species
As climates change -> species populations
advance, retreat
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From cold climate…


For most of the past 2-3 million years, the Earth has been
quite cold
Evidence from the distribution of oxygen isotopes in cores
taken from deep ocean floor  as many as 16 glacial
cycles, each lasting up to 125,000 years with intervals of
only 10,000 to 20,000 years
… to a warmer climate
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During the 20,000 years since the peak of the last
glaciation, global temperatures have risen by ~ 8 C
Analysis of buried pollen can show how vegetation has
changed during this period
Migrations of trees in eastern North America from 18,000
years ago to present are known from pollen grains
deposited in bogs and lakes:

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the compositions of communities shifted as species migrated
across the landscape
in particular, the composition of forests during the past 18,000
years has:
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included combinations of species that do not occur today
lacked combinations of species that do occur at present
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Changes in Climate 1
(c) 2001 by W. H. Freeman and Company
Climate change…
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

Naturally – with the change in plants  change in animals
Even in regions never glaciated, pollen deposits record complex
changes in distribution
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What about the tropics?
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In the mountains of Nevada – woody species show different patterns of
change in elevational range
Species composition of vegetation is continually changing – and is still
changing
One theory: during cooler, drier glacial periods the tropical forests
retreated to smaller patches, ‘hotspots’ of species diversity
So what could happen in the next 100 years?
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Temperatures predicted to rise between 2 to 7 C in 100 years
Postglacial warming of 8 C occurred over 20,000 years
Now: trees will have to move at 300-500 km/100 years
Typically: trees move 20 – 40 km/100 years
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Effects of continental drift on
ecology of evolution
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The history of life can be gauged by the geological
time scale.

Ecologists recognize key features of the geological
record:
earth formed 4.5 billion years ago
 life arose within the first billion years
 life remained primitive for most of earth’s history
 ancient physical environments were quite different from
those of the present:



the early atmosphere had little oxygen and early microbes used
anaerobic metabolism
increased oxygen led to diversification of complex life forms
The Geologic Record

About 590 Mya, most of the modern phyla of
invertebrates appeared in the fossil record:
these early animals began to protect themselves with hard
shells, which make excellent fossils
 the Paleozoic era is thus the first of three major divisions of
geologic time reflecting diversification of animals:


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Paleozoic: 590 Mya to 248 Mya
Mesozoic: 248 Mya to 65 Mya
Cenozoic: 65 Mya to present
Continental Drift

The continents are islands of low-density rock floating
on the denser material of the earth’s interior and
carried along by convection currents:


the movements of the continents over time are called
continental drift
These movements have two important ecological
consequences:
positions of continents, ocean basins influence climate
 continental drift creates and breaks barriers to dispersal

Continental Drift: Mesozoic to Present


In the early Mesozoic era, 200 Mya, continents
formed a single giant landmass called Pangaea
By 144 Mya (beginning of the Cretaceous period) the
northern continents (Laurasia) had separated from the
southern continents (Gondwana)


at this time Gondwana itself was also breaking apart
By the end of the Mesozoic era (65 Mya), South
America and Africa were widely separated, and
many other patterns were emerging.
Continental drift
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Consequences of Continental Drift

Details of continental drift have yet to be resolved,
but implications for evolution of animals and plants
are clear; for example:

the distributions of the flightless ratite birds (such as
ostriches) are the results of connection between the southern
continents that made up Gondwana:

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these birds are descended from a common Gondwanan ancestor
splitting of a widely distributed ancestral population by continental
drift is called vicariance
Lineages of ratite birds were separated by the fragmentation of Gondwana – 80
mya. Evolutionary relationships of these birds reconstructed from DNA sequences
(A) Large
flightless
birds
(B) the
phylogenetic
tree of the
flightles
birds
(C) Ostrich (left)
is African;
rhea
(middle) is
found in
similar
grasslands in
South
America; the
emu (right) in
Australia
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Evolution: Convergent and parallels
Analogous: similar in superficial form or function
Homologous: derived from an equivalent
structure in a common ancestry
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Convergence
Convergence is the process whereby unrelated species
living under similar ecological conditions come to
resemble one another more than their ancestors did:
 Analagous but not homologous
 there are numerous examples of convergence:
woodpecker-like birds that fill the woodpecker niche in
many systems lacking woodpeckers
 similarities of plants and animals of North and South
American deserts
 similar body forms of dolphins and penguins, which both
resemble tuna, whose swimming lifestyle they share

Convergent evolution
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
Wings of bats and birds – similar in form/function
but not from a common ancestry; structurally
different
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Pairs of unrelated
African and South
American rain forest
mammals: similar
lifestyles and
adaptations; striking
convergence
Parallel evolution
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

Ancestrally related groups – then isolated from
each other
In contrast to convergent evolution, diversified from
a common ancestral line and both inherited a
common set of potentials and constraints
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Parallel evolution
of marsupial
and placental
mammals:
Pairs of species
are similar in
both
appearance
and habit and
usually (not
always) in
lifestyle
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To recap
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Convergent Evolution
 Convergent evolution takes place when
species of different ancestry begin to
share analogous traits because of a
shared environment or other selection
pressure. For example, whales and fish
have some similar characteristics since
both had to evolve methods of moving
through the same medium: water.
Parallel Evolution
 Parallel evolution occurs when two
species evolve independently of each
other, maintaining the same level of
similarity. Parallel evolution usually
occurs between unrelated species that
do not occupy the same or similar
niches in a given habitat.



(a) divergent
(b) convergent
(c) parallel
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End of chapter 2
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Test 1 (Chapters 1 and 2)
Next Friday – October15
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Additional information
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Genetics review

The phenotype is the outward expression of an
individual's genotype
 Genotype:
unique genetic constitution
 Phenotype: outward expression of that genotype
 A genotype = set of genetic instructions; blueprints
 Phenotype = the expression of that genotype in the
form of an organism
 (is that enough? Are there external factors?)
 Effects
of environmental influences are like details in a
blueprint that are left to the discretion of the building
contractor.. What does that mean?
More genetics

All phenotypic traits have:
 Genetic
basis + influence by variations in the
environment
 What

kind of environmental variations?
Phenotypic plasticity
 Capacity
of an individual to exhibit different responses
to its environment
 How the individual responds to environmetnal variation
Genetic variation (review, right?)

Alleles
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Heterozygous
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Two different alleles for a particular gene
Homozygous
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Different forms of a particular gene
Both copies of a gene are the same
Dominant… Recessive…
Gene pool

All the alleles of the genes of every individual in a
population
Sources of genetic variation
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How does genetic variation arise?
Mutation

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Any change in the sequence of the nucleotides that make up
a gene or in regions of the DNA that control the expression
of a gene
Consequence?
Drastic – maybe lethal – changes in the phenotype
 No detectable effect – silent mutations
 New phenotypes produced  better suited to the local
environment  phenotypes increase
 Multiple effects  pleiotropy (effects of a single gene on multiple
traits)

Genetic basis of continuously varying
phenotypic traits

Many phenotypic traits with ecological
relevance vary continuously over a range of
values (eg: body size)
Adaptations result from natural selection on
heritable variation in traits that effect
evolutionary fitness
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The most important consequence of genetic variation for
the study of ecology is evolution by natural selection
Evolution
Any change in a population’s gene pool (what is a gene
pool?)
 Individuals whose traits enable them to have higher rates of
reproduction have more offspring  alleles increase
 Adaptations or evolutionary adaptation
 Process = adaptation

Adaptation (process of evolution by
natural selection)
Variation among individuals
1.

Eg – bird beaks; different individuals have differentsized beaks
Inheritance of that variation
2.

Size of bird’s beak has an existence of its own in a
population; individual is borrowing that trait
Differences in survival and reproductive success (or
fitness) related to that variation
3.

Fitness: production of descendants over an individual’s
lifetime.
+
Evolutionary change
Change in a California citrus pest
Cyanide fumigation no longer
effective
Stabilizing, directional, and disruptive
selection

Stabilizing selection
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Directional
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Individuals with intermediate (average) phenotypes have higher reproductive success
Population moved towards an optimum point
Maintains a single fittest phenotype
When the environment of a population is relatively unchanging: dominant mode; little
evolutionary change
Fittest individual have a more extreme phenotype;
When new optimum reached – becomes stabilizing selection
Disruptive

Increase genetic and phenotypic variation within a population and in the extreme case creates a
bimodal distribution of phenotypes; relatively uncommon; eg: individuals specializing on one of
a small number of food resources; strong competition among individuals
+
Example of
Disruptive selection
Population genetics and the prediction
of evolutionary change

Population genetics

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Study of the dynamics of natural selection and genetic change in
populations
Populations are continually engaged in dynamic evolutionary
relationships with their environment that shape their ecological
interactions
(one) Goal of population genetics  to develop methods for
predicting changes in gene frequencies in response to selection
Why?
Ability to predict them can tell us whether the genetic changes we
observe are consistent with our understanding of evolution
(check out the ‘more on the web’ links)
Population genetics and ecologists
every population harbors some genetic variation that influences
fitness .. Potential for evolution exists in all populations
1.

Except?
Changes in the environment will almost always be met by an
evolutionary response that shifts the frequencies of genotypes
within the population. (translate?)
2.

Magnitude of the evolutionary response depends on genetic
variation present in the pop at a given time
Rapid environmental changes brought about by the
appearance of new adaptations in populations of enemies or
by human-caused changes in the environment (eg?) can exceed
the capacity of a population to respond by evolution
3.

So?