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Reciprocal Altruism
• What about cooperation among
unrelated individuals?
• Trivers proposed individuals will act
altruistically if favor is later returned
• Two conditions for reciprocal alturism
to evolve:
– Cost to actor must be smaller than or
equal to benefit to recipient
– Individuals that fail to reciprocate must be
Reciprocal Altruism
• Most likely to evolve when:
– Each individual repeatedly interacts with
same set of individuals
– Many opportunities for altruism in an
individual’s lifetime
– Individuals have good memories
– Potential altruists interact in symmetrical
• Roughly equal benefits and costs
Reciprocal Altruism
• Will evolve in long-lived, intelligent,
social species with small group size,
low dispersal rates, and mutual
dependence in activities
• Less likely to evolve in species with
dominance hierarchies
• Difficult to observe and quantify in
Reciprocal Altruism
• Blood-Sharing in Vampire Bats
– Social group of 8-12 females and their
dependent offspring
– Roost together and associate with each
other daily
– Average r between individuals in study
group in Costa Rica was 0.11
– Vampire bats share blood meals
– Hunting is difficult and individuals are only
successful 67-93% of time
• Prey are wary
Reciprocal Altruism
• Blood-Sharing in Vampire Bats
– Without eating for three nights, a bat will
starve to death
– Both degree of relatedness and degree of
association were significantly related to
probability of regurgitating blood
– Blood-sharing is not random but based on
relatedness and hope of future reciprocity
Interactions Among
1. Two species that interact
affecting the genetic structure
of one another. Each one acts
as a selective force on the
other (lineages change in
Co-speciation – Do 2 lineages
speciate in the same pattern?
Perhaps like a lichinous fungi
and its algae symbiont.
Concepts of Coevolution
• Coevolution as a process of reciprocal adaptive response
1. Specific coevolution: Coevolution of two (or few) species
2. Guild Coevolution (diffuse, or multispecific): Coevolution among
sets of ecologically similar species
3. Escape-and-radiate coevolution
4. Cospeciation (introduced by interaction)
• Coevolution as a pattern, detected by phylogenetic
1. Cospecieation (coincident speciation)
2. Parallel cladogenesis
Co-evolution VS. Co-adaptation
• Co-evolution is when genetic composition of
both species changes, each affecting the
We assume co-evolution leads to coadaptation.
But you can have co-adaptation without coevolution (birds on same island with different
bill shapes may have evolved in allopatry
before sympatric overlap)
So, co-evolution should lead to coadaptation but co-adaptation is not
necessarily the result of co-evolution
Using Phylogenies to Answer
• Coevolution
– Leaf-cutting ants and fungi they farm
• Leaf cutters grow fungus on leaves that
they cut for food
• 200 ant species of tribe Attini each farm a
different fungus species
• Did the two groups cospeciate?
– Phylogenies should be congruent
• Hinkle found congruence on all branches
but one
• Fungi were domesticated more than once
Leaf Cutting Ants
Evolution of Mimicry
• Complex interaction among multiple
species believed to arise from coevolution, though it has not been
proven so. For sure, at least, this is coadaptation.
• Major Types of Mimicry:
– Mullerian Mimicry
– Batesian Mimicry
– Mertensian Mimicry
Mullerian Mimicry
• When a group of species that are distasteful,
poisonous, or otherwise noxious, resemble
each other in morphology or behavior
Often brightly colored and have some kind of
warning system  Aposematic Trait
They call attention to themselves and warn of
danger. This warning is assumed to ward off
potential predators or increase fitness in
some way.
The more these species look alike, the easier
it is for a predator to remember that one
warning pattern (eg. coral snakes bands)
Examples of Mullerian Mimicry
• Coral snakes all coral snakes
are venomous. There are around
70 species in the new world. Over
90% of them look extremely similar,
especially with respect to color and pattern
We assume that their similarity in
appearance allows predators to evolve the
ability to identify them as poisonous and
leave them along
Thus, there is an advantage that all share
from looking similar
Batesian Mimicry
• A non-noxious or non-poisonous mimic
looks like a noxious model
• For our coral snake example, the
venomous coral snakes would be the
model and non-venomous snakes
looking like coral snakes are the
Examples of Batesian Mimicry
In each picture, the snake to
the right is the Venomous Coral
Snake, while those to the
left are the mimics (harmless)
Mertensian Mimicry
• We have seen some examples of deadly
poisonous snakes such as Micrurus
(Elapidae) and non-poisonous snakes such
as Lampropeltus and Pliocercus
But there are other snakes that are
moderately poisonous, such as members of
the genera Rhinobothryum, Erthrolammprus
and Pseudoboa.
Mertens suggests that the moderately
poisonous snakes could be the model, not
the poisonous snakes.
Mertensian Mimicry
• If the moderately poisonous snakes bite a
predator, it would get sick and therefore would
learn to avoid those and similar snakes in the
But, if a deadly poisonous snake bites a predator,
it would die and never have a chance to learn.
So, Mertens proposes that the moderately
poisonous snakes are the model and both the
poisonous and non-poisonous snakes are the
mimics. This situation is termed Mertensian
Evolution of Life
History Characters
Reproduction Strategies
• Mice mature early and reproduce
quickly whereas bears mature late and
reproduce late
• Some plants live and flower for only
one season, others live and flower for
• Some bivalves produce millions of tiny
eggs at once, others less than 100 large
eggs at a time
Life History Analysis
• The branch of evolutionary biology that
tries to sort our reproductive strategies
• A “perfect” organism would mature at
birth and produce many high quality
offspring throughout life
• No organism can do this because there
are tradeoffs in time, size of offspring,
and parental investment
Life History Analysis
• Life history extremes
– Thrip egg mites are born already inseminated by
mating with brothers inside mother’s body
• Adults have short lives
• The offspring eat there way out of their mother
when she is four days old
– Brown kiwis lay eggs 1/6 of their body weight
• Chicks are self-reliant within a week
• Takes one month for female to produce each egg
Life History Analysis
• Organisms may grow to a large size to
make large offspring or reproduce
earlier at a smaller size to make smaller
• For organisms that wait, chance of
dying before reproducing is high
• Environmental variation creates life
history variation
Life History Analysis
• Questions to Consider
– Why do organisms age and die?
– How many offspring should an individual
produce in a year?
– How big should each offspring be?
• Must balance among fitness aspects
• Conflicts arise between male and
female parents
Life History Analysis
• Female Virginia opossum
– Nursed for three months and then weaned
– Continued to grow for several months until
reaching sexual maturity
– Had first litter of 8 offspring
– Months later had second litter of 7
– At 20 months was killed by a predator
– Energy allocation changed through life
Life History Analysis
• Differences among life history concern
differences in energy allocation
• Other female opossums could mature
earlier and reproduce earlier
– Or devote less energy to reproduction and
more to maintenance
• Natural selection optimizes energy
allocation in a way that maximizes total
lifetime reproduction
Why Do Organisms
Age and Die?
• Senescence = late life decline of fertility
and probability of survival
• Aging reduces an individual’s fitness
and should be opposed by natural
• Two theories on why aging persists
Why Do Organisms
Age and Die?
• Rate-of-Living Theory
– Senescence is caused by accumulation of
irreparable damage to cells and tissues
– Damage caused by errors during
replication, transcription, and translation,
and by accumulation of poisonous
metabolic by products
– All organisms have been selected to resist
and repair damage as much as
physiologically possible
– Have reached limit of possible repair
Why Do Organisms
Age and Die?
• Rate-of-Living Theory
– Populations lack genetic variation needed to
enable more effective repair mechanisms
– Two predictions of theory:
• Because damage is partially caused by metabolic
by products, aging rate should be correlated to
metabolic rate
Because organisms have been selected to repair
the maximum possible, species should not be able
to evolve longer life spans
Why Do Organisms
Age and Die?
• Rate-of-Living Theory
– Austad and Fischer tested first prediction
• Calculated amount of energy expended per
gram of tissue per lifetime for 164 mammal
• Should expend same amount regardless of
length of life
• Found great variation in energy
• Found that bats expend three times the
energy of other mammals their size
Why Do Organisms
Age and Die?
• Rate-of-Living Theory
– Luckinbill tested second prediction
– Artificially selected for longevity in fruit
– Increased life span from 35 days to 60
– These long-lived fruit flies had lower
metabolic rates during first 15 days of life
Why Do Organisms
Age and Die?
• Rate-of-Living Theory
– Both of the predictions of the theory have
been falsified
– Examine energy expenditure on cells and
chromosomes, not whole organism
• Normal animal cells are capable of a finite
number of divisions before death
• All cells except cancer cells, germ line
cells, and stem cells
• Senescence may result from chromosome
Why Do Organisms
Age and Die?
• Rate-of-Living Theory
– Telomeres of chromosomes consist of
tandem repeats
– Added by enzyme telomerase
• Overactive in cancer cells
– During each replication pieces are lost
– Progressive telomere loss is associated
with senescence and death
– Cells die because chromosomes are too
damaged to function
Why Do Organisms
Age and Die?
• Rate-of-Living Theory
– Life spans of mammals are correlated with
life spans of skin and blood cells
– These results consistent with rate-of-living
– Why doesn’t natural selection activate
telomerase to add more telomeres?
– Could be tradeoff between extending cell
life and proliferating cancer
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– If genetic variation for extending life spans
does exist, why hasn’t natural selection
produced this result in all species?
– Aging is not caused by damage itself but
the failure to repair the damage
– Damage is not repaired because of
deleterious mutations or tradeoffs between
repair and reproduction
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– Hypothetical life history of individual with
wild-type genotype
• Mature at age 3
• Die at age 16
• Probability of survival from one year to the
next is 0.8
• Expected lifetime reproductive success =
– Will consider two mutations that alter life
history strategy
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– Mutation Accumulation Hypothesis
– Mutation cause death at age 14
– Deleterious mutation, but how
– Expected lifetime reproductive success
reduced to 2.340
• Still 96% of original
– Weakly selected against
• May persist in mutation-selection balance
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– Deleterious mutations that affect
individuals late in life can accumulate in
populations and be the cause of aging
– Cancers that usually occur late in life only
slightly affect fitness of the individual
– Not strongly selected against and can
accumulate rapidly
– Can cause senescence and death with few
fitness consequences
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– Mutation of two different life history characters
with pleiotropic action
– Matures at 2 years
– Dies at 10 years
– Tradeoff between early reproduction and survival
late in life
• Antagonistic pleiotropic effects
– Expected lifetime reproductive success is 2.66
• Mutation is beneficial
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– Reproduce so much early that early death is not
selected against
– Mutation devotes less to repair and more to
– Heat-shock protein hsp70
– Prevents damage due to denaturation
– Heat-shock binding interferes with normal cellular
– Heat-shock genes only expressed during
environmental stress
• Proteins removed after stress passes
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– Expression of hsp70 in Drosophila causes
longer life span but lower reproduction
early in life
– Tradeoff between early fecundity and late
survival is mediated by hsp70
– Heat-shock proteins may mediate this
tradeoff in many organisms
Why Do Organisms
Age and Die?
• Evolutionary Theory of Aging
– Other examples of tradeoffs
– Collared flycatchers are polymorphic in
when they begin reproduction
• Birds that begin at age 1 have smaller
clutch sizes throughout life
• Birds that begin at age 2 have larger
clutches throughout life
• First year breeders still had higher
reproductive success
– 1.24 vs. 0.9
Why Do Organisms
Age and Die?
• Natural Experiment in Aging
– High adult mortality rates should lead to
earlier maturation
– Austad compared Virginia opossums on
mainland southeastern US and on a barrier
island off of Georgia
– In mainland, opossums have high
ecological mortality rates
– On island they have no predators
– Two habitats and populations are similar
Why Do Organisms
Age and Die?
• Natural Experiment in Aging
– Populations separated 4000-5000 years
– Island population should have evolved
delayed senescence
– Island females showed delayed senescence
in month-to-month probability of survival
– Island females showed delayed senescence
in reproductive performance
– Island females showed delayed senescence
in connective tissue physiology
Why Do Organisms
Age and Die?
• Natural Experiment in Aging
– Results consistent with Evolutionary
Theory of Senescence
• Evolutionary Theory of Senescence
successful in explaining life history
How Many Offspring to Have?
• The more offspring a parent attempts to
raise at once, the less time and energy
the parent can spend on each of them
• How many eggs should a bird lay per
• David Lack’s Hypothesis
– Selection will favor clutch size that
produces the most surviving offspring
– When researchers added eggs to nests,
survival of all chicks decreased
How Many Offspring to Have?
• David Lack’s
– Number of surviving
offspring reaches a
maximum at
intermediate clutch
How Many Offspring to Have?
• Boyce and Perrins tested Lack’s
hypothesis in a long-term great tit
– Mean clutch size was 8.53
– Greatest number of survivors came from
clutches of 12
– Experimentally larger nests still had 12
– All females could have lain 12 eggs and
had higher fitness
How Many Offspring to Have?
• Boyce and Perrins’ test of Lack’s
• Not consistent with Lack’s hypothesis
• Many other studies have shown that
birds have smaller clutches than
• Why isn’t Lack’s hypothesis correct?
– Must be a violation of one of the
underlying assumptions
How Many Offspring to Have?
• Assumptions of Lack’s hypothesis
– No tradeoff between parent’s reproductive
effort in one year and survival and
reproduction in the future
– Linden and Moller reviewed 60 bird studies
and found that 26 showed a tradeoff
between current and future reproductive
– 4 of 16 studies found tradeoff between
current reproduction and future survival
– Optimal clutch size may be less than most
productive clutch size
How Many Offspring to Have?
• Assumptions of Lack’s hypothesis
– Only effect of clutch size on offspring is
determining whether offspring survive
– Schluter and Gustafsson added or
removed eggs from collared flycatcher
– When those offspring had their own
clutches, the ones from clutches that had
been augmented produced smaller
clutches and vice versa
– Effects of clutch size extend further into
future than thought
How Many Offspring to Have?
• Assumptions of Lack’s hypothesis
– Clutch size is fixed by a particular
– Phenotypic plasticity of clutch size has
been shown in many bird species
– If birds predict good or bad environmental
conditions, they can adjust their clutch
size to the optimal value for that season
How Many Offspring to Have?
• Lack’s hypothesis applied to parasitoid
– Although Lack’s hypothesis appears to be
too simple to accurately predict clutch
size, it serves as a valuable null model
– Parasitoid wasps inject their eggs into a
host insect
• The larvae eat the host, pupate, and emerge
– Host insect is analogous to a nest
How Many Offspring to Have?
• Lack’s hypothesis for parasitoid wasps
– Trichogramma embryophagum deposits
eggs in many different host species
– Charnov and Skinner calculated optimal
clutch sizes for three insect hosts
How Many Offspring to Have?
• Lack’s hypothesis for parasitoid wasps
– Females shift behavior in response to
different hosts
– Lay smaller clutches than predicted by
Lack’s hypothesis
How Many Offspring to Have?
• Lack’s hypothesis for parasitoid wasps
– Why are clutches smaller than optimal?
– Larger clutches may reduce female fitness
in unknown ways
– There may be a tradeoff in current and
future reproduction and survival
– Parasitoids may lay two clutches in
• Fitness may be related to survival of all
clutches, not just one
How Many Offspring to Have?
• Lack’s hypothesis for parasitoid wasps
– While female searches for a host her
fitness is 0
– Must incorporate fitness gained by the
number of eggs lain on one host before
– May lay smaller clutches than predicted by
Lack’s hypothesis because it may be more
optimal to begin the search for a new host
How Many Offspring to Have?
• Lack’s hypothesis is a good starting
point for evolutionary analysis of clutch
• Violations represent presence of
• Current parental effort may be
negatively correlated with parent or
offspring future reproductive success
or survival
How Big Should Each
Offspring Be?
• We have been assuming that size of
offspring was fixed
• Principle of Allocation states that if
organisms use energy for one function
the amount of energy available for
other functions is reduced
– Leads to trade-offs between functions
such as number and size of offspring
– Can either have many small young or few
large young
How Big Should Each
Offspring Be?
• Smith and Fretwell’s analysis
– Two assumptions
– Tradeoff between size and number of
– Individual offspring survival correlated to
– Created a mathematical plot of expected
parental fitness versus offspring size
How Big Should Each
Offspring Be?
• Smith and Fretwell’s analysis
– Optimal offspring size for parents is often
small than optimal offspring size for
• Offspring always want to be bigger to
survive better
• Parents want to ration out resources to all
offspring, current and future
– Can only test Smith and Fretwell’s analysis
if there is high polymorphism in offspring
size in a population
How Big Should Each
Offspring Be?
• Offspring Size Selection in Uta
– Side-blotched lizards live in western US
– Show heritable variation in egg size
– Sinervo surgically manipulated egg size
variation further
• Got lizards to lay unusually small eggs by
removing some yolk from each
• Got lizards to lay unusually large eggs by
destroying all but two follicles so that yolk
would be transferred to remaining eggs
How Big Should Each
Offspring Be?
• Offspring Size Selection in Uta
– Found tradeoff in number and size of eggs
– Probability of juvenile survival was a
function of egg mass
– In 1989, selection favored larger eggs
– In 1990, selection favored intermediate
– Maternal fitness always favored medium
– Conflict of interest between mother and
offspring for egg size
How Big Should Each
Offspring Be?
• Phenotypic Plasticity in Beetle Egg Size
– Seed beetle lays eggs on various seeds
– Larvae burrow inside, feed, and pupate
– Fox studied seed beetle grown on acacia
and palo verde seeds
– Acacia is a good host
• Most larvae survive
– Palo verde is a poor host
• Less than half survive
– Females lay larger eggs on palo verde than
How Big Should Each
Offspring Be?
• Phenotypic Plasticity in Beetle Egg Size
– Phenotypic plasticity selected for in this
generalist species
– Researchers switched the host seed midway through oviposition and the females
switched egg sizes
– 0.3% of larvae from small eggs on palo
verde survived
– 24% of larvae from large eggs on acacia
– Poor environment forces females to expend
more energy on large egg size