Topic 17. Lecture 27. Evolution of Populations and Ecosystems-II

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Topic 17. Lecture 27. Evolution of Populations
and Ecosystems-II What questions can be
addressed by considering Macroevolution of simple
phenotypes? Independently evolving individuals
Gene transmission 1.
Phenotypic plasticity
1. Mutation 2. Non-interactive
behavior 2. Maintenance of
sex 3. Semelparity and iteroparity
3. Crossing-over 4.
Clutch size
4. Systems of mating 5.
Dormancy
5. Origin of sex 6. Aging
6.
Outcomes of genetic conflicts Interactions
between individuals Complex
population-level phenomena 1. Warning
coloration 1.
Multicellularity and coloniality 2.
Dispersal
2. Anisogamy and sex allocation
3. Aggression
3. Mate choice 4. Cooperation
and altruism 4. Female
preferences and male displays

5. Conflicts between gametes and sexes

6. Conflicts between
relatives
7.
Eusociality Today, we will consider the second
half of these questions.
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Interactions between individuals 1) warning
(aposematic) coloration It is not obvious how
can warning coloration originate by natural
selection. A single mutant with conspicuous
coloration would be eaten soon, because the
predators would not have a chance to learn that
such coloration means trouble. Examples of
warning coloration
A wasp
A
salamander
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A nudibranch gastropod
A flatworm
A skunk

A frog
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Mathematical models indicate that selection on
novel warning signals is number- rather than
frequency-dependent. There exists a threshold
number of aposematic individuals below which
aposematism is selected against and above which
aposematism is selected for.
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Interactions between individuals 2)
dispersal There may be situations when the
fitness of an individual would be higher if it
does not move, but an allele that causes an
individual to move will nevertheless spread in
the population. This paradox appears because if
an individual does not migrate, it will be likely
to compete, within the same local population,
with related individuals.
One can say that migrating to another place with
some probability is an ESS (evolutionarily stable
strategy). A simple ESS is a phenotype such that,
if everyone in a population possesses it, a
different phenotype cannot be advantageous and,
thus, cannot invade.
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Interactions between individuals 3)
aggression Aggression against members of the
same population is very common. In different
species, conflicts between individuals can have
different forms and outcomes, including death of
one or both opponents.
Examples of aggressive
behavior
Betta fighting fish
Boxing" walnut flies
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However, very often conflicts are "ritualized"
neither opponent uses all the weapons available.
How could this "moderation" evolve? Can it be
explained without invoking "bad for an individual
but good of the species" group selection
arguments? Examples of ritualized conflicts
Western diamondbacks
Eastern
orynxs
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Consider only two behavioral phenotypes
("strategies") - hawk (H, always attack) and dove
(D, always be nice). When two individuals fight,
the winner gets benefit b, and the loser suffers
the cost of injury c. Two H's will fight, and the
expected payoff for each is (b-c)/2. Two D's will
not fight and will split the benefit, and the
expected payoff for each is b/2. An H and a D
will not fight, so that H gets b and D gets 0.
So, the payoff matrix is
My opponent
H
D
Me H (b-c)/2 b
D 0
b/2 So, what to expect, evolutionarily?
If everybody is a D, H is beneficial and will
invade the population. However, if everybody is
an H, D is beneficial, as long as c gt b (avoid
fighting altogether, if it is too costly). Thus,
evolutionarily stable strategy here is mixed - be
a H sometimes and a D sometimes. There may be
better strategies that simple H and D or even
their mixture start fighting, but escalate a
conflict only until some point. This probably
explains the evolution of ritualized conflicts
through individual (not group) selection.
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Still a better strategy is to persecute a weak,
and to run away from a strong. OK, but do you
want to honestly inform you opponent how
dangerous you are?
Guinea baboons appear to be honest in In
contrast, a lizard Phrynocephalus mystaceus
signaling their strengths.
pretends to be more dangerous than it
is. This is a complex subject, but,
generally, honest signaling can evolve only if
any signaling is costly - due to production cost
or social cost of the signal.
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The highly variable black facial patterns of
female paper wasps, Polistes dominulus serve as
"badges of status". In staged contests between
pairs of unfamiliar wasps, subordinate wasps with
experimentally altered facial features
('cheaters') received considerably more
aggression from the dominant. If you try
to pretend that you are stronger than you are,
they will beat you up.
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Interactions between individuals 4)
cooperation and altruism Individuals very often
help each other, even when this is costly. Under
what conditions can we expect costly cooperation
to evolve? Consider a very simple model. There
are only two phenotypes ("strategies") -
cooperate (C) and defect (D). If one partner
(prisoner) defects and another cooperates, D is
released and C gets 10 years. If both defect,
each gets 7 years, and if both cooperate, they
are released after one year. So, the payoff
matrix is
My opponent
C
D Me C
-1 -10
D 0
-7 So what is better - to cooperate with
your partner or to defect? This is a famous
Prisoner's Dilemma. If the partners encounter
each other only once, it is always better to
defect - no matter what your partner does, your
payoff would be higher if you do so.
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However, if the same two partners encounter each
other many times (repeated Prisoner's Dilemma),
this conclusion is no longer valid. Indeed, if
constant cooperation of both partners can somehow
be established, both partners will benefit, as
compared to the case of both of them constantly
defecting there will be only 1 year in jail,
instead of 7 years, for each crime. Here,
phenotypes are algorithms on the basis of what
it and its partner did in previous encounters,
and individual has to decide whether to cooperate
or defect at this time. A very simple phenotype
does very well in repeated Prisoner's Dilemma
start from cooperation, and afterwards do what
you partner did in the previous round
(tit-for-tat). TFT
individual CCCCCCDDCCCCCCDDDDCCC
Its partner CCCCCDDCCCCCCDDDDCCCC
Perhaps, it may be even better to be more
generous and to forgive occasional
defections. Thus, costly cooperation can evolve
by natural selection if a phenotype that
practices it has the highest fitness. There are
five situations when this is possible 1) kin
selection 2) direct reciprocity 3) indirect
reciprocity 4) network reciprocity 5) group
selection.
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A cooperator is someone who pays a cost, c, for
another individual to receive a benefit, b. A
defector has no cost and does not deal out
benefits. Cooperation cannot evolve in any mixed
population, because defectors have a higher
average fitness than cooperators. However, a
population of only cooperators has the highest
average fitness, whereas a population of only
defectors has the lowest. Thus, natural selection
constantly reduces the average fitness of the
population. Fisher's fundamental theorem does not
apply here because selection is
frequency-dependent.
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Five mechanisms for the evolution
of cooperation 1) Kin selection operates when
the donor and the recipient of an altruistic act
are genetic relatives. 2) Direct reciprocity
requires repeated encounters between the same two
individuals. 3) Indirect reciprocity is based
on reputation a helpful individual is more
likely to receive help. 4) Network
reciprocity means that clusters of cooperators
outcompete defectors. 5) Group selection is
the idea that competition is not only between
individuals but also between groups.
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Kin Selection "I will jump into the river
to save two brothers or eight cousins" (Haldane).
More precisely, kin selection can lead to the
evolution of cooperation if the coefficient of
relatedness between the interacting individuals,
r, exceed the cost-to-benefit ratio of the
altruistic act r gt c/b (Hamiltons' rule). Here,
relatedness is defined as the probability of
sharing an identical-by-descent allele. The
probability that two brothers share the same gene
by descent is 1/2 the same probability for
cousins is 1/8. Direct Reciprocity Repeated
Prisoner's Dilemma is an example of this
situation. Indirect Reciprocity Helping
someone establishes a good reputation.
Interacting with somebody who has a good
reputation is beneficial, thus, such individuals
can be "rewarded". Network Reciprocity A
cooperator pays a cost for each neighbor to
receive a benefit. Defectors have no costs, and
their neighbors receive no benefits. In this
setting, cooperators can prevail by forming
network clusters, where they help each other.
Group Selection If small groups of individuals
are units of selection, cooperation can evolve,
because groups of cooperators have higher
fitness. Some other possibilities There are
also some possibilities. For example, cooperators
can recognize each others ("Green beard model").
Perhaps, several of these mechanisms contribute
to evolution of cooperation in nature (J. Evol.
Biol. 20, 415-432, 2007).
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Not only complex animals can cooperate. The
social amoeba Dictyostelium discoideum is a model
for social evolution and development. When
starving, thousands of the normally solitary
amoebae aggregate to form a differentiated
multicellular organism.
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Complex population-level phenomena 1)
multicellularity and coloniality
Multicellularity evolved in red algae, brown
algae, green algae, fungi and animals (not
counting "borderline" cases). In green algae, it
evolved several times independently.
Volvox sp.
Ulva sp.
Chara sp. The origin of
multicellularity and coloniality is a complex and
murky subject. It seems that the correct way of
thinking about this subject is to consider
conflicts and cooperation among individual cells
(or organisms).
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Multicellularity opens a possibility of conflicts
between selection at different levels. A dominant
mutation causing Apert syndrome is much more
common in children of older (gt45 years) fathers.
Apparently, this is because cells that carry this
mutation have a selective advantage within male
germline.
Hi, my name is Frans Wallenberg and I have Apert
Syndrome. When I was child, maybe 3-4 years old,
I got my first surgery for my fingers...
www.apert.org/wallenberg/index.html
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Complex population-level phenomena 2)
anisogamy and sex allocation Anisogamy
(including its extreme form, oogamy) evolved many
times.
Isogamy
Anisomagy
Oogamy Why are sperm small and eggs
large? The most plausible explanation is
disruptive selection on gamete size small
gametes are favored because many can be produced,
whereas large gametes contribute to a large
zygote with consequently increased survival
chances. This model assumes that increases in
zygote size confer disproportional increases in
fitness.
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When two sexes (exogamous classes of gametes) are
present, resources are usually allocated to them,
by all individuals in the population, in equal
proportion. This 11 sex allocation evolves
owing to the simple fact that a zygote gets 50
of genes from the mother, and another 50 from
the father. As a result, 11 sex allocation is an
EES in simple cases. Suppose that sex ratio in
the population is female-biased. Consider a rare
genotype that produces more males than others.
Because a son will transmit more genes than a
daughter (there is a deficit of males), in the
2nd generation this genotype will be
overrepresented.
If there are 4 females for each male, a male, on
average, transmits 4 times more genes than a
female.
More complex situations are possible in
structured populations. For example, if matings
occurs within a sibship, an EES is to produce
just one male who will mate will all his sisters.
Melittobia digitata, a small parasitic wasp with
femalemale ratio 201. A host larva is usually
parasitized by just one female, so that only full
sibs can mate each other.
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Complex population-level phenomena 3) mate
choice Why should a female care with whom to
mate? Mate choice can exist either due to an
immediate benefit or to a delayed benefit of
producing offspring with better genotypes.
There are 2 feasible immediate benefits of mate
choice 1) Direct investment - it is
better to mate with a vigorous male,
who will help to rise
kids. 2) Reducing harm - it is better
to mate with a male who will do
you less harm.
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However, immediate benefits can hardly explain
all the instances of mate choice. Indeed, a
father often contributes nothing but genes to his
offspring and a variety of mates can be harmless.
Thus, delayed benefits are probably important.
Such benefits can be of two kinds 1)
Sexual selection - a choosy females produces
more
attractive sons, because their father
was more attractive
("Fisherian runaway"). 2) Nonsexual
selection - a choosy female produces offspring
with
generally superior genotypes,
because their father had
good genes.
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However, both these ideas are not without
problems 1) Fisherian runaway mechanism is very
fragile and does not work if there is even a
slight cost of choice for a female. 2)
Fisher's Fundamental Theorem implies that, at
equilibrium, there should be no heritable
variation in fitness and no correlation between
the quality of a father and his offspring.
However, data demonstrate that heritable
variation and parent-offspring correlations in
fitness within natural populations are often
quite large, probably, due to never-ending influx
of deleterious mutations. Thus, it seems that
the ability of good-quality fathers to sire
good-quality children is the main reason for the
evolution of female mate choice, although this
issue is not yet settled.
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Complex population-level phenomena 4)
female preferences and male displays Quite
often, females not only choose mates, but do it
according to rather bizarre criteria. Indeed, why
should anybody want to mate with a peacock?
Clearly, such exaggerated and costly sexual
displays can evolve only as a result of
coevolution with female choice.
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Costly female preferences for males with
exaggerated traits that reduce viability can
evolve when the exaggerated trait, although
maladaptive per se, indicates high overall
quality of the male's genotype. The following
evolutionary scenario appears to be plausible
1) Initially, high-quality males have slightly
longer tails. 2) Females start using long tails
as a clue for choosing high-quality mates. 3)
This females choice causes all males to evolve
exaggerated tails, that reduce their fitnesses.
However, high-quality males can tolerate longer
tails.
As a result, a stable female preference for very
long tails, and stable exaggeration of tail
length over viability optimum can evolve. This
scenario is supported by some data, but the issue
is not yet settled.
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Complex population-level phenomena 5)
conflicts between gametes and sexes An egg and a
sperm have rather different "interests". An egg
needs to be fertilized - but only once, as
otherwise it will not develop properly. A sperm
needs to fertilize an egg, and has nothing to
lose. Consequently, an arms race can occur
between the ability of a sperm to penetrate an
egg and an ability of an egg to make sure that
only one sperm will do this.
After one sperm gets in, the whole egg envelop
must instantly become resistant to all other
sperms.
Such conflicts are common and often lead to
extremely rapid coevolution, within the same
genome, of genes with egg- and sperm-specific
expression. Protein lysin in red abalone
(Haliotis rufescens) is responsible for sperm-egg
interaction and evolves extremely rapidly.
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Complex population-level phenomena 6)
Conflicts between relatives Evolutionary
interests of genes expressed in relatives can
often be very different (only if reproduction is
sexual, of course, as otherwise all relatives
have the same genotype). Such conflicts can lead
to complex phenomena if neither side is in
complete control.
The extreme form of sib-sib conflict is
siblicide. If the amount of resources is
insufficient to support all sibs, killing others
may be the only chance for an offspring to
survive. In this situation, siblicide may be also
in the evolutionary interest of parents, as
otherwise they would have no surviving offspring.
Siblicide in the brown booby
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There may also be conflicts between parents and
offspring. The evolutionary interest of a parent
is not to waste resources on weak offspring. In
contrast, the evolutionary interest of each
offspring is to survive. In organisms in which
developing embryos are independent of the mother,
under optimal conditions over 99 of embryos
develop successfully. In contrast, in mammals and
seed plants the success rate is lower, although
an embryo is protected and supplied by mother. In
humans, at least 30 of pregnancies are
spontaneously terminated at very early
stages. gt99 success
rate
70 success rate A possible reason
is that the maternal organism refuses to support
embryos that appear to be weak or abnormal.
Perhaps, this effect diminishes with the maternal
age, because of diminishing chances of having
other children.
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Complex population-level phenomena 7)
eusociality Eusociality is an extreme form of
altruism, such that many individuals do not
reproduce and, instead, help their relatives to
raise their offspring. Often, only one female
(queen) reproduces in a colony, with other
individuals being sterile workers.
Queen and workers of honey bee,
Queen and workers of an ant,
Apis mellifera
Formica fusca In
hymenopterans, workers in a single-queen colony
are her sisters and daughters.
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Naked mole rat (Heterocephalus glaber) is a
eusocial mammal. The queen is the only
reproductive female in the colony.  Other
individuals serve particular societal roles, such
as soldiers and cleaners.
Eusocial sponge-dwelling snapping shrimp,
Synalpheus regalis. They live in colonies with
tens to hundreds of members and only one
reproductive female.
Queen and workers in a termite. All modern
species of termites (order Isoptera) are eusocial.
Evolution of eusociality apparently proceeds
through kin selection. In hymenopterans, who have
haploid males, it may be aided by closer
relatedness of a female to her sisters (75 of
identical by descent genes) than to her daughters
(only 50 of such genes).
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The key factor in the evolution of eusociality
appears to be monogamy. This is to be expected in
eusociality evolved due to kin selection.
Phylogeny of eusocial Hymenoptera (ants, bees,
and wasps). Each independent origin of
eusociality is indicated by alternately colored
clades. Clades with high polyandry (gt2 effective
mates) are in solid red, those with low polyandry
(gt1 but lt2 effective mates) are in dotted red,
and monandrous genera are in black.
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Evolution of Ecosystems A ridiculously short
summary Ecosystems consist of populations of
many species. Obviously, properties of an
ecosystem are affected by the evolution of
constituent populations. Such evolution can
easily produce unexpected results. An ecosystem
consisting of just two species, a prey and a
predator, can exist either in an equilibrium
state (if the predator is inefficient) or in a
state of stable oscillations (if the predator is
more efficient). If the predator is very
efficient, the amplitude of these oscillations
can become very wide, which can lead to
extinction of the predator, due to lower critical
density phenomenon (Allee effect). Thus, adaptive
evolution of a predator can first destabilize the
ecosystem and later even lead to the predator's
extinction.
Possible consequences of slow evolution of more
efficient predators.
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Quiz What observations and experiments can be
used to establish the mechanisms of evolution of
cooperation?