Transcript The evolution of sociality

Interactions, variability, altruism and sociality
Primitive animals behave largely in the same way. Individualistic behaviour is very limited.
Higher animals evolved individualism. The highest birds and mammals, but even higher
insects, evolved individualistic characters (moods), motions and fears.
Classical population genetic does not predict individualism because it focuses on
optimisation and equilibrium states that are the same for all members of a population.
Evolutionary theory has to explain:
•
Altruism (the help of others despite of own costs)
•
Cooperation of related and unrelated individuals
•
The evolution of cheating
•
Sexual selection (the existence of differentiated sexual behaviour and mating rituals)
•
Biased sex ratios (the prevalence of either males or females in a population)
•
The existence of highly altruistic insect societies (eusociality)
•
The existence of infanticide in many mammals and birds
•
The existence of homosexuality in many mammals and birds
•
The existence of large intellectual differences in mammals, birds, and even higher insects.
•
The appearance of common beliefs and religion in man
The unit of selection and evolution
Unicellular organisms
Multicellular organisms
Higher taxonomic level
Species
Classical population genetics (Fisher,
Haldane, Sewall Wright)
Population
Higher taxonomic level
Group
Species
Family
Population
Individual
Cell
Organelle
Genome
Gene
Nucleotid
Wynne Edwards (1962)
to explain cooperation
A more liberal view sees any trait
inducing carrier of information as a
potential unit of evolution. These
include genes, individuals, and
even groups but not species.
The basic unit is the gene as
the smallest essential carrier
of information
C. Richard
Dawkins (1941-
The game theory approach
The classical hawk and dove game
Assume two players:
• a hawk that will always fight until
injured or until the opponent retreats
John F. Nash
(1928-
• a dove that will always retreat.
John Maynard
Smith (1920-2004)
Contests are associated will potential
benefits (B) and potential costs (C).
Hawk v. Hawk: Each
contest has a 50%
chance to win. The net
gain is the difference
between benefits and
costs of the contest
Dove v. Hawk: The
dove will always loose
The pay-off matrix
Hawk
Dove
Hawk
½(B-C)
B
Dove
0
½B
Hawk v. Dove: The
hawk will always win
Dove v. Dove: Each
contest has a 50%
chance to win. There are
no costs
The pay-off matrix
Hawk
Dove
Hawk
½(B-C)
B
Dove
0
½B
Is Hawk an EES?
𝑝
𝐵−𝐶
𝐵
+ 1 − 𝑝 𝐵 > (1 − 𝑝)
2
2
𝐵 > 𝑝𝐶
The idea behind game theory is now to define equilibrium
conditions that define which game (strategy =
behavioural phenotype) will have the highest payoff in
the long run.
Maynard Smith defined such equilibria that cannot be
beaten by other strategies as evolutionary stable
strategies (ESS).
Populations of individuals playing an ESS cannot be
invaded by immigrating individuals or by mutants playing
other strategies.
If the benefits are higher than the costs hawk is an EES, otherwsie dove is the EES.
The Retaliator game
(fight when meeting a hawk and retreat when
meeting a dove)
Hawk
Dove
Retaliator
Hawk
½(B-C)
B
½(B-C)
Dove
0
½B
½B-e
Retaliator
½(B-C)
½B+d ½B-¼C+g
Retaliator and a mixed strategy are the two
ESS of this game. Realization depends on
the initial frequencies of players.
Even simple games favour mixed
strategies.
This is the start of individualistic
behaviour.
The evolution of cheating or the Prisoner’s dilemma
Assume two prisoners have the alternative either of defect the other or to
cooperate. Defection means shorter imprisoning.
B>C
The pay-off matrix
Defect Cooperate
0
Defect
0
Cooperate
0
B(A)
B(B)
0
C(B)
C(A)
If both prisoners defect they do worse than if
both cooperate. However cheating the other
is superior irrespective of what the other
makes.
Hence pure cooperation can never evolve.
Now assume an iterative game where the players
play many times. What would be the best strategy?
In the long run there are several possible strategies
One EES is Tit for Tat (defect if prior being defected
and cooperate if the other prior also cooperated).
Defect CooperateTit for Tat
Defect
0
e
0
Cooperate
-d
g
g
Tit for Tat
0
g
g
The prisoners dilemma cannot fully be
resolved analytically.
The first software solution was provided
by Rapoport in 1980.
The program played
Tit for Tat or reciprocal altruism.
The other EES of this game is always
defect.
Small
individual
Occassional
mates
Uta stansburiana
Large
individual
An intermediate behaviour
is not a stable strategy
Gains
Costs
Trade offs
Trade offs in morphological
and behavioural traits allow for
the existence of multiple stable
traits that of contrast
Permanent
mates
The lizard males have three mating strategies
Orange strategy: They are very aggressive occupying a large territory,
mating with numerous females
Yellow strategy: They are unaggressive mimicking the females lizards and
sneakily slipping into the Orange territory to mate with some females
Blue strategy: They mate with and carefully guard ONE female, prohibiting
sneakers to succeed and therefore to overtake their place in a population.
Blues always loose in competition with orange males.
The majority of plant and (probably) animal communities
are structured by competitive loops (A>B>C>A)
according to the rock-paper-scissors game.
In rock-paperscissors games
Such loops increase biological diversity
multiple strategies
and diversity of behavioural traits
might co-occur
Bumble bee intelligence
16
100
16
120
10
120
In bumble bees the picture is similar:
there are smart and there are dump bees
Colonies having a majority of smart bees
were shown to collect about 40% more
nectar that colonies where dump bees rule.
Bombus terrestris
There should be a strong pressure for
becoming smarter.
In great tits smart individuals on average
lay more eggs and are more efficient
foragers
For unknown reasons, smart birds are
also more likely to abandon their nests,
negating any reproductive advantage.
Parus major
If intelligence would be
generally adaptive we
expect a constant trend to
higher IQ.
This would also imply a
decrease in standard
deviation.
Dump bees make more
errors in flower recognition
and accidentally find new
and nectar rich flowers.
That means smart bees
are not able to play both
strategies.
Energetic costs
Parental Investment
Brain power
Net reproduction rate
Trade offs in intelligence
The trade off between parental investment
and energetic costs of larger brains might
favour two strategies: cheap reproduction
and expensive reproduction.
One reproductive strategy invests in
parental care (larger brains) of fewer
offspring, the other in a large number of
offspring.
Brain power
Females
Males
Retherford, Sewell 1986
This study does not consider inclusive
fitness!!
Data from 9000 Wisconsin
inhabitants graduated in 1957 point
to
1) sex differences in reproductive
output and to
2) a decline in female reproductive
output with increasing intelligence
and to
3) peaks in reproductive output in
males at low and high intelligence.
All studies in intelligence – fertility relationships
are highly controversial!
Experimental evolutionary research
Yeast (Saccharomyces cerevisiae) multicellularity
Radcliffe et al. 2012 (PNAS) selected single celled yeast for
settle down behaviour.
After 60 selection steps (one week of experiment) all replicates
were dominated by star like clusters of aggregated cells.
These clusters were uniclonal, with division of labour,
apoptosis, and reproduction by propagules.
Aptoptosis was in line with the inclusive fitness model.
Females
In guppies (Poecilia reticulala)
artificial selection for larger and
smaller brains resulted in lower
gut mass and reduced
reproductive output in large
brained species (Kotrschal et al.
2013, Current Biology).
Males
Experimental evolutionary research
Rotifer sex evolution
Brachyonus calyciflorus
Becks et al. (2010) reared predominately sexually reproducing rotifers in homogeneous and
heterogeneous environments.
After 20 weeks the homogeneous environment favoured asexual reproduction.
Bell (2012) cultured green Chlamydomonas algae
for 12 month in the dark.
A few strains survived evolving alternative
metabolic pathways to use acetate. They evolved
significant morphological and genetic diversity.
Some lost the ability for photosynthesis and
became obligatory heterotrophs.
William D. Hamilton
(1936-2000)
Local mate competition
In 1967 W. D. Hamilton proposed that in the long run organisms
should preferentially invested in the cheaper sex.
The cheaper sex is the one that promises more offspring at
equal costs.
pM rM CM  pF rFCF
p: probability to produce a son; r: expected
reproductive success, C: cost of reproduction
Which sex to produce?
The probability that
a son reproduces
is high
The probability
that a daughter
reproduces is low
For a proper choice a female
• needs knowledge about the actual sex ratio and
• must have the ability to control which sex she produces
Many Hymenoptera and some
other insects have these abilities
Mammals and birds perform
selective infanticide
Two examples of sex ratio allocation
Figs and fig wasps
Parasitic wasps
Agaonidae are closely
connected to figs.
Depositing eggs into the
ovaries they pollinate figs.
Sex ratio
ro
0.6
0.5
0.4
0.3
0.2
17 species of fig
wasp species
(Agaonidae)
0.1
0
0
0.2
0.4
0.6
0.8
z
Sex ratio second female
Males are wingless and
mate only with the local
clutch
1
Proportion
of of
fruits
Proportion
fruitparasitized
parsitized
Sex ratio is defined as the proportion of males
Secondary
parasitism of the
parasitoid wasp
Nasonia vitripennis
parasitoid of blow
and flesh flies
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
y = 0.27x-0.43
0
1
2
3
4
Offspring second female /
Offspring first female
5
Selective infanticide in man
The sex ratio is the proportion of males: SR = males / (males + females)
The normal cross cultural sex ratio at birth is 105 males to 100 females = 0.512
(range 101 to 107: 100)
Some reported sex ratios in childhood of preindustrial societies:
Inuit Eskimos: 0.67
Yanomamö Indians: 0.56
Cashinahua, Peru: 0.60
Rajput caste, India: > 0.9
Upper class medieval Florence: 0.57
Selective infanticide is found in nearly all cultures.
Often it serves to
• stabilize population size
• to adjust sex ratios to marriage probabilities in cases of highly unequal reproductive success
• to adjust to a culturally preferred gender (frequently the male gender)
Reciprocal altruism
Reciprocal altruism beween non-related
individuals needs:
Blood sharing in the vampire bat
• Long term association of group members.
• Donorship can be predicted from past helping.
• Roles of donors and recipients reverse.
z
• Benefits of the recipients outweigh donor costs.
Percentage of prefeeding weight
• Donors can detect cheaters.
130
Exponential vampire bat
weight loss function due
to starvation
120
• Primary social groups contain 8 to
12 adults with depending young.
110
100
• 30% of the blood sharing events
involve adults feeding young other
than their own.
Weight Donor
lost
90
Recipient
• Blood sharing intensity depends on
the degree of relatedness.
Weight gained
80
Time lost
Time gained
70
0
20
40
60
Hours
Benefits outweigh costs
80
• Blood sharing is often reciprocal.
• Cheaters have not been observed.
Cooperative breeding and helpers at the nest
In the pied kingfisher Ceryle rudis
primary and secondary helpers at the
nest occur.
Helpers occur in many higher bird
species and help adults to raise the
offspring.
Male
helpers
S
Primary helpers are older sons that are yet
unable to breed.
Additional young
fledged per helper
P
They increase their fitness via their younger
sisters and due to additional experience.
>5
Number
of adults
providing
care
4
Secondary helping males are unrelated to the
pair they help.
3
2
0
1
2
3
Young fledged
4
5
Secondary helpers increase their fitness due to
the chance to become the widow’s mate if the
breeding male dies.
Kin selection and the evolution of sociality
Members cooperate
Part of the members loose
but retain reproductive
own reproductivity in favour
ability
of other group members
Individualistic life
→
Sociality
→
Eusociality
(superorganisms)
Joined parental care
→
Cooperative breeding
and defence
Most bacteria and
True multicellular
→ organisms (Metazoa,
Colonies
→
single cell
eucaryotes
Fungi, Plantae
Most ‘primitive’
animals and
plants
→
Social spiders,
isopods,
many insects,
many fishes
Higher birds and
mammals
Often intensive common
parental care, aunt behaviour,
playing groups, and group
defence
→
Isoptera (autapomorphy)
Some Aphidae and Thripidae
At least 14 independent
lineages of Hymenoptera
Eucalyptus ambrosia beetles
(Australoplatypus
incompertus)
Sponge shrimp (Synalpheus
regalis)
Naked mole rats
(Heterocephalus glaber and
Cryptomys damarensis)
All termites (Isoptera).
They have male and
female workers and
different casts.
Some Aphidae and Thripidae
(Homoptera) have sterile
soldiers. Sometimes
rudimentary parental care.
All ants (Hymenoptera). At least 14 groups of eusocial Apidae
They have female workers and Vespidae (Hymenoptera). They
only and highly
have female workers only. Some
differentiated cast systems. bumble bees may be either solitary
or eusocial depending on
environmental conditions.
Two species of mole rats have
Some flukes of the genus
non-reproducing workers and a Himasthla (Trematoda) have a
queen. Colonies have up to
reproductive and a soldier caste
300 members.
larval form (redia).
Inclusive fitness
In the Hawk - Dove game the EES for C > B was
B<pC → pB>C
p was the probability of a trait to occur. This is formally identical with the probability of a gene to
occur via descent, it is identical to the coefficient of inbreeding.
Inclusive fitness = individual fitness + proportional fitness of all relatives
𝟏 < 𝟏 − 𝑪 + 𝒑𝑩
Hamilton’s rule of inclusive fitness
𝑪 < 𝒑𝑩
A simple example
Assume a new gene A that promotes parental care.
The probability of transmitting A from mother to daughter
is 0.5.
Even if the mother would die due to parental care (cost =
1) two additional raised offspring (B = 2) satisfy
Hamilton’s rule.
0.5 = 1 / 2
Parental care should therefore be widespread in animals.
In cockroaches (Phoraspis
and Thorax) the young bite
wholes in the mothers
thorax to feed from their
haemolymph.
What favours Hymenoptera to become eusocial?
Hymenoptera are haplo-diploid organisms
The haplo-diploid system
Queen King Daughter Son Brother
Fertilized eggs become females
Unfertilized eggs become males
Queen
A,B
King
C
Daughter
0.5
0.5
0.75
0.25 0.25
King
0
1.0
1.0
0
0.5
Queen
1.0
0
0.5
0.5
0.25
The diploid-diploid system
Son
A
Daughter
A,C
Son
B
Daughter
B,C
Hamilton’s rule of inclusive fitness
𝑪 < 𝒑𝑩
Queen King Daughter Son Brother
Daughter
0.5
0.5
0.5
0.5
0.5
King
0
1.0
0.5
0.5
0.5
Queen
1.0
0
0.5
0.5
0.5
Queen - daughter
0.5 
C
B
Queen - sister
C
0.75 
B
Given that costs and benefits of reproducing are similar it pays for a hymenopteran female more
to invest in her sisters than in her own brood.
This explains why eusocial Hymenoptera all have sterile female workers and never sterile males.
For instance a hymenopteran female helps her sister at the cost of no reproduction.
At equlilibrum the number of surviving offspring should be 2. Hence C = 2
The sister raises one additional offspring
0.75 
2
 0.67
2 1
0.5 
2
 0.67
2 1
Even for one additional offspring of the sister it pays to resign of own offspring
But be careful
Most of the haplo-diploid Hymenoptera are solitary.
The theory requires that queens a priori invest more in daughters than in sons.
Interestingly, many Hymenoptera are able to decide whether to lay male or female eggs.
They are able to control sex ratios
Termites are diplo-diploid
Today’s reading
The game theory site:
http://www.holycross.edu/departments/biology/kprestwi/behavior/ESS/ESS_index_frms
et.html
Selfish gene theory: http://en.wikipedia.org/wiki/Gene-centered_view_of_evolution
The evolution of eusociality:
http://www.thornelab.umd.edu/Termite_PDFS/EvolutionEusocialityTermites.pdf
Biology and sexual orientation:
http://en.wikipedia.org/wiki/Biology_and_sexual_orientation
http://www.newscientist.com/article/mg20427370.800-homosexual-selection-thepower-of-samesex-liaisons.html
Biased sex ratios in man: http://huli.group.shef.ac.uk/lummaaproceedins1998.pdf
and http://www.jstor.org/cgibin/jstor/printpage/00664162/di975349/97p0109i/0.pdf?backcontext=page&dowhat=Ac
robat&config=jstor&[email protected]/01cce4405a00501c7b1f1&0.pdf
and http://en.wikipedia.org/wiki/Gender_imbalance
Figs and fig wasps: http://www.figweb.org/Interaction/index.htm