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Coevolution
• Fitness is a genotype-by-environment interaction.
• The environment for one species includes other
species
• For species that interact, they form part of each
other’s “environment”
• As one species adapts to the “environment”
defined by the other species, the other species in
turn can adapt to the changing “environment”
created by evolution in the first species.
• Interactions between species therefore can set up
an evolutionary feedback loop that causes the
species to COEVOLVE in response to one another
Coevolution
Coevolution is simply natural
selection operating within each of the
interacting species, recognizing that
each species constitutes part of the
environment of the other species.
Interspecific Interactions
Species 1
Species 2
Interaction
0
0
Neutralism
+
0
Commensalism
-
0
Amensalism
+
-
Predator-prey;
Herbivore-plant
Pathogen-host
-
-
Competition
+
+
Mutualism
True Interactions
Predator-Prey
E.g., HIV and Humans
Human Immune System and Cell Types Cause Selection On HIV v3 Region
Predator-Prey
E.g., HIV and Humans
Does HIV Induce Natural Selection in Humans? If So, This Is A Co-Evolving System.
Predator-Prey
E.g., HIV and Humans
There is much variation in the rate at which AIDS develops after
HIV infection in humans, with some people never developing
AIDS even decades after their original infection.
For HIV To Have An Impact on
Human Evolution, This Variation
Must be Inherited (& Heritable)
Predator-Prey
E.g., HIV and Humans
Can Look For Evidence of Inheritance Through Linkage
Analysis for QTL’s, With Locations Suggesting Candidate
Genes
Genome Scan For Resistance To HIV
Predator-Prey
E.g., HIV and Humans
CCR5 As A Candidate Gene For Resistance to HIV
HIV-1 initially interacts with a cell-surface receptor,
primarily CD4
Conformational changes in both the viral envelope
and the CD4 receptor permit the binding of gp120
to another cell-surface receptor, such as CCR5.
HIV then fuses with the cell.
People bearing a frameshift mutation in the CCR5
locus are resistant to HIV.
The CCR5 Frameshift Polymorphism
Under Isolation By Distance and Coalescent Theory, A New
Mutant Is Usually Most Frequent Near Its Center of Origin.
Coalescent Analysis Also Indicates Frameshift Mutant
Originated 275-1,875 Years Ago.
The Current Allele Frequencies In Europe Are Too High
To Have Evolved In This Time Period Under Genetic
Drift (drift theory implies it would take about 127,000
years to explain its current European frequencies)
Hypothesis:
•The black plague bacillus produces an
effector protein that binds CCR5 that leads
to diminished immune response
•Europe has been subjected to several
waves of black plague in the last 2000
years, including one that killed 25-33% of
the population in 1346-1352
•CCR532 arose in Europe and could have
been selected if it provides resistance to the
black plague
Subsequent Analyses
Duncan, S. R., S. Scott, and C. J.
Duncan. 2005. Journal Of Medical
Genetics 42:205-208. Pop. gen. theory
shows the plague could explain the
current frequency
Sabeti et al. 2005. PLoS Biology 3.
Bollback et al. 2008. Genetics 179:497-502. Allele may be much older,
so perhaps neutral evolution could explain it.
Zawicki, P., and H. W. Witas. 2008. Infection, Genetics and Evolution
8:146-151. Freq. of allele in bodies from 11-14th century Poland already
at freq. of 5%, so either neutral or selection prior to Black Death.
Galvani and Slatkin. 2003. PNAS 100: 15276-15279. Poxviruses also
use chemokine receptors, so perhaps evolved in response to smallpox,
another potent selective agent in Europe and Western Asia (not mutually
exclusive with role of plague as selective agent).
Other Loci Have Been Identified For HIV Resistance via Candidate Locus
Approach; e.g. Modi et al. 2006. Am J Hum Genet 79:120-128.
CCL3, CCL4, and CCL18 are potent chemoattractants produced by
macrophages, natural killer cells, fibroblasts, mast cells, CD4 + T
cells, and CD8 + T cells. CCL3 and CCL4 are natural ligands for the
primary human immunodeficiency virus type 1 (HIV-1) coreceptor
CCR5 and are also known to activate and enhance the cytotoxicity
of natural killer cells.
D in European
Americans
Other Loci Have Been Identified For HIV Resistance via
Genome Scanning and Candidate Loci Approaches, And Some
Display Epistasis and Heterozygous Effects and High Allele
Frequencies (hence, they produce heritable variation).
Current
Statistics
From
South
Africa
SDF codes for a type of protein called a
chemokine that binds to other receptors used
by HIV such as CXCR4, Causing the
Receptor To Be Taken Into the Cell and No
Longer on the Cell Surface
These differences in
resistance translate into
heritable fitness
differences, so HIV is
affecting human evolution
Studies Are Now Underway To Identify Those Genes
Subjected to Positive Selection In The Lineage Leading To
Humans. E.g., Vallender & Lahn (2004):
Function of Gene
No. of Genes
Host-Pathogen Interactions
23
Reproduction
14
Sensory Systems
7
Dietary Adaptation
4
Neurotransmission
3
Brain Size & Anatomy
3
Skin Color
1
Other
8
COMPETITION
• Sometimes, one species out-competes
competitors and drives them to extinction -ends all coevolution
• Sometimes, coevolution results in
adaptations that reduce competition such
that the species can coexist.
The Finches of the Galapagos Islands
small ground
finch
large ground
finch
warbler finch
tree finch using a
twig to fish for insects
• Darwin found several species of closely related
finches when he visited the Galapagos Islands.
– The Galapagos Islands are very isolated so it is
probable that these finches evolved from a common
ancestor.
Competition
in Darwin’s
Finches:
Beak Shape
and Size Is
An Indicator
of the Types
of Food The
Birds Can
Eat.
h2 in G. fortis
Ancestral Reconstruction of Habitat
Specialist Evolution on Two Islands
Twig
Trunk/ Crown/
Ground Giant
Trunk/
Crown
Crown
Generalist
Jamaica
Crown/ Trunk/
Twig Giant Crown
Grass/
Bush
Trunk/
Ground
Trunk/Grnd
Trunk/Grnd
Generalist
Puerto Rico
MUTUALISM
E.g., Heliconius Butterflies
Heliconius melpomene
Heliconius erato
Different
Species
In The
Same
Area
Same
Species
In
Different
Areas
Heliconius melpomene
Heliconius melpomene and H. erato comimics
Baxter, S. W. et al. Genetics 2008;180:1567-1577
Passiflora
Heliconius caterpillars feed on passiflora, which makes
them poisonous to birds. They evolve warning coloration.
Experiments show that bird predators can quickly learn
to avoid these color patterns, but this learning is more
rapid when all butterflies in a single area converge upon a
common pattern. This type of mutualistic mimicry is
called Müllerian mimicry.
Coevolutionary
interactions among
species can be complex;
e.g., Heliconius butterflies
are very long-lived as
adults and reproduce
throughout that long life.
Therefore, they need a
pollen source for proteins.
The species are involved
in intense competition for
these pollen resources.
Heliconius cydno
gathers pollen from
Psiguria flower.
(Rainforest cucumber)
Different Heliconius species lay eggs on different species of Passiflora:
are neutralists for larval food resources
Different Heliconius species use the same pollen food resources: Hence
their adult foraging behaviors are driven by competitive interactions
Different Heliconius species are selected to converge upon a common,
mutualistic warning coloration pattern by their interspecific interaction
with birds
Different traits within the same species can coevolve in
radically different directions. Mendelian genetics allows
such complex coevolution to occur.
Competition
Mutualism
Neutralism
Mendelian genetics allows such complex coevolution to occur
in part because underlying genetics is simple.
Kronforst et al. (Genetics
174:535-539, 2006) did
a genome scan using
some of the most
divergent species in
genus. Found that
different species
achieved phenotypic
convergence via
homologous genes, and
only 9 genes can explain
the amazing wing
diversity in this group.
Linkage maps comparing homologous chromosomes from H. erato, H. melpomene, and H. numata that affect
red color shifts on wings
Baxter, S. W. et al. Genetics 2008;180:1567-1577
Copyright © 2008 by the Genetics Society of America
Analysis of recombinants spanning the HmB/D region on LG18
Baxter, S. W. et al. Genetics 2008;180:1567-1577
A methionine rich storage protein (MRSP) gene was identified, and comparative genetic
mapping shows red wing color loci are in homologous regions of the genome of H. erato
and H. melpomene. Subtle differences in these convergent phenotypes imply they
evolved independently using somewhat different developmental routes, but are
nonetheless regulated by the same switch locus.
Mendelian genetics allows such complex coevolution to occur
in part because underlying genetics is simple.
Reed RD, McMillan WO,
Nagy LM (2007) Gene
expression underlying
adaptive variation in
Heliconius wing patterns:
non-modular regulation
of overlapping cinnabar
and vermilion
prepatterns.
Proceedings of the
Royal Society B:
Biological Sciences 275,
37-45.
Coevolution Can Lead to Speciation (Kronforst et al. PNAS
103:6575-6580, 2006)
0.05 Significance Threshold
H.c H.c H.p. H.p
H.c H.c H.p. H.p
Composite
Interval Map
Of Mating
Preference
Interval Map
Of Mating
Preference
Linkage map of H. melpomene LG18 (77.3 cM, log likelihood = -92.61)
Baxter, S. W. et al. Genetics 2008;180:1567-1577
Big Question: Why Do Any Species of Heliconius Have Regional
Variation in Color Patterns? It Would Seem That A Single
Universal Pattern Would Be The Most Effective Adaptive Solution.
An Early Answer To This Was Pleistocene Refuges: the idea
that climatic fluctuations in the Pleistocene created isolated
subpopulations which adaptively diverged to form the initial
“races”. Modern studies fail to support the existence of these
refuges and modern comparative analysis indicate that the
divergence times are not consistent with climatic fluctuations.
A second idea was that Heliconius species show strong
population subdivision, and genetic drift interacting with
selection (e.g., shifting balance) created differences among
local populations leading to “races”. But modern population
genetic studies indicate that most species of Heliconius are
strong dispersers marked by much gene flow and little
population subdivision.
Kronforst and Gilbert (2008, Proceedings of the Royal Society
B: Biological Sciences 275:493-500) suggest an alternative.
They examined the fine-scale population genetic structure of eight Costa
Rican Heliconius species with 1428 AFLP markers. The species were
chosen from two clades: molecular phylogenetics indicates that one
clade was the first to diversify into local “races”, and the second clade
evolved later to parallel the first group through coevolution.
Large fst,
Sign. IBD
Sign. IBD
Sign. IBD
3 of the 4 species in the
older clade show
significant isolation by
distance, and the most
abundant species (H.
erato), which should
dominate much of the
coevolution, has a large fst,
indicating a strong
potential drift and shifting
balance.
Kronforst and Gilbert (2008, Proceedings of the Royal Society
B: Biological Sciences 275:493-500) suggest an alternative.
The species in the clade that later radiated onto these patterns had intermediate
levels of genetic diversity and less genetic differentiation among populations.
Sign. IBD
Only 1 of the 4 species in
the younger clade show
significant isolation by
distance: little potential
for drift and shifting
balance.
Genetic Drift in perhaps just one, widespread, abundant but highly subdivided
species triggered adaptive local differentiation in other species through
coevolution that did have the population structure to initiate such differentiation.