Transcript Slide 1

Topic 17. Lecture 27. Evolution of Populations and
Ecosystems-II
What questions can be addressed by considering Macroevolution of simple phenotypes?
Independently evolving individuals:
1. Phenotypic plasticity
2. Non-interactive behavior
3. Semelparity and iteroparity
4. Clutch size
5. Dormancy
6. Aging
Gene transmission:
1. Mutation
2. Maintenance of sex
3. Crossing-over
4. Systems of mating
5. Origin of sex
6. Outcomes of genetic conflicts
Interactions between individuals:
1. Warning coloration
2. Dispersal
3. Aggression
4. Cooperation and altruism
Complex population-level phenomena:
1. Multicellularity and coloniality
2. Anisogamy and sex allocation
3. Mate choice
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.
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
A nudibranch gastropod
A skunk
A flatworm
A frog
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.
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.
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:
Boxing" walnut flies
Betta fighting fish
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
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
Me: H
D
(b-c)/2
0
D
b
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 > 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.
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
signaling their strengths.
In contrast, a lizard Phrynocephalus mystaceus
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.
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.
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.
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.
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.
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.
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
> 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).
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.
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).
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 (>45
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
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.
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 1:1 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, 1:1 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 female:male
ratio 20:1. A host larva is usually parasitized by just one
female, so that only full sibs can mate each other.
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.
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.
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.
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.
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.
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.
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
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.
>99% 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.
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,
Apis mellifera
Queen and workers of an ant,
Formica fusca
In hymenopterans, workers in a single-queen colony are her sisters and daughters.
Queen and workers in a
termite. All modern
species of termites
(order Isoptera) are
eusocial.
Eusocial sponge-dwelling
snapping shrimp,
Synalpheus regalis. They
live in colonies with tens
to hundreds of members
and only one reproductive
female.
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).
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.
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 (>2 effective
mates) are in solid red, those with low
polyandry (>1 but <2 effective mates)
are in dotted red, and monandrous
genera are in black.
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.
Quiz:
What observations and experiments can be used to establish the mechanisms of evolution
of cooperation?