Chap. 4 Group selection and Individual selection

Chap. 4 Group selection and Individual selection

鄭先祐生態主張者 Ayo

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2003 Chap. 4 Group and Individual生態學 2

Chap. 4 Group selection and Individual selection

1. Group selection vs. individual selfishness

2. Altruism ( 利他主義 )

3. Benefits and trade-offs of group living

Road Map


4.1 Group and Individual Selection

• Regulation of populations – early thoughts– Levels below which competition becomes important– Nature is neat, tidy and harmonious, avoid wastefulness1. Development of Group Selection

1. Territoriality ( 領域行為 ) of birds2. Increase in emigration correlated with increase in numbers3. High variation in reproductive rates

– Examples of self-regulation ( 自我調控 ) or external regulation– Tropics vs. temperate (Self-regulation of song birds)– 1940s, David Lack vs. Alexander Skutch


High variation in reproductive rates• 現象：

– Songbirds typically lay a clutch of four to six eggs in temperate regions of North America and Europe

– Only two or three in the tropics.

• 解釋– Lack, birds in the tropics couldn’t gather enough reso

urces to fledge more than two to three young, so the availability ot resources provided a limitation on reproduction.

– Skutch, tropical populations were self-regulated to ensure that no resources were wasted.


Self-regulation viewpoint• In 1962, the self-regulation viewpoint was

championed by Wynne-Edwards, who articulated the full concept of self-regulation in a book called Animal dispersion in relation to social behavior.– Groups of individuals control their numbers to

avoid extinction– Theory known as Group Selection

• In the late1960s, the idea came under severe attack.


Individual selection• Williams (1966), Adaptation and Natural

selection , argued against group selection

1. Mutation– Cheater scenario– Clutch size based on maximizing the number of

surviving chicks (Figure 4.1)

2. Immigration– Selfish individuals can migrate to new areas

3. Individual selection

4. Resource prediction


Individual selection

3. Individual selection– Individuals die out more quickly than groups– Individual selection a more powerful

evolutionary force

4. Resource prediction– Group selection needs a reliable and

predictable source of food– No evidence that they can.


Fig. 4.1 Great tits, parus major. There are four surviving nestlings.

• Group selection implies that individuals should not over utilize their resources for the good of the group.

• Individual selection entails an”every one for themselves more likely than group selection in nature.


Self-Regulation?

• Come from Intraspecific competition– Individuals strive to command as much resourc

es as they can.• Ex. Male lions that that kill existing cubs when

they take over pride. Increase their own offspring

• Ex. Male langur monkeys kill infants (Figure 4.2)


Self-Regulation?

• Come from Intraspecific competition• Ex. Female giant water bugs kill eggs in masses

being taken care of by males (Figure 4.3)


4.2 Altruism

• Apparent cooperation– Grooming– Hunting– Warning signals

• Caring for copies of one’s genes– Genes in offspring– Coefficient of relatedness = r– Probability of sharing a copy of a particular

gene


Caring for copies of one’s genes

– Probability of sharing a copy of a particular gene

• Parents to its offspring; r = 0.5• Brothers and sisters; r = 0.5• Grandparents to grandchildren; r = 0.25• Cousins to each other; r = 0.125• Figure 4.4


0.25 0.25

grandparents

father

0.5 0.5

mother

0.250.25

0.25

0.25

0.25

0.25

0.5

0.5

1 0.125

grandparents

mate self

daughteror son

granddaughter or grandson

half sib

aunt/uncle

niece ornephew

cousinbrother/sister(full sib)

Fig. 4.4 Degree of genetic relatedness to oneself in a diploid organism. Open circles represent completely unrelated individuals.


Implications of relatedness to altruism• 1964, W.D. Hamilton

– Importance of passing on one’s genes through offspring as well as related individuals.

• Inclusive fitness– Total copies of genes passed on to all relatives

• Kin selection – Lowers individual chance of reproduction– Raises chances of relatives’ reproduction


Quantifying kin selection

• rB – C > 0

• r = coefficient of relatedness

• C = number of offspring sacrificed by donor

• B = number of offspring gained by recipient


Kin selection• Aposematic – contain colors to warn pred

ators of bad taste or poison

• Datana caterpillars (Figure 4.5)– Predator must kill one to learn

All the larvae in the group are likely to be the progeny of one egg mass from one adult female moth.


Num

ber o

f cat

erpi

llar s

pecie

s

0

10

20

30

40

50

Aposematic Cryptic

Large family groups

Solitary

Fig. 4.6 Brightly colored species of caterpillars of British butterflies are more likely to be aggregated than are cryptic species.

• Advantage of animals to congregate in groups (Figure 4.6)


Alarms from ‘sentries’ ( 哨兵 )

– Increased risk of being attacked– Animals living near‘sentry’most likely relatives– Favors kin selection

• Alternative to kin selection– ‘Sentries’ that are forced to live at the fringe– Alert for their own safety– If ‘sentry’ is successful, predator may seek new

area– Sentry’ increases chances of own survival


Unrelated individuals• Altruism between unrelated individuals

– “You scratch my back, I’ll scratch yours”– Reciprocal altruism

• Evidence– Brooding success correlated to availability of

helpers ( 台灣藍鵲 )– Social hunting

• Benefit: Bigger prey• Cost: Sharing meat


Altruism in social insects

• Extreme example of altruism – sterile castes in social insects

• Female workers– Rarely reproduce

– Assist queen with her offspring (eusociality)

• Soldier castes ( 士兵身份 ) (Figure 4.7)

• Social insect reproduction (Table 4.1)


Fig. 4.7 A soldier Amazonian termite

• Altruism in social insects may arise from the unique genetics of their reproduction.



Relatedness (haplo-diploid organisms)

• Females are diploid

• Males are haploid– Formed without meiosis

– Each sperm is identical

• Sister relatedness– Each daughter receives an identical set of genes from

her father

– Half of a female’s genes come from her diploid mother

– Total relatedness of sisters: 0.5 from father + 0.25 from mother = 0.75.


Relatedness (haplo-diploid organisms)

• Sister relatedness– Sons and daughters; r = 0.5

– Average relatedness for sterile workers would be 0.5

• Queen, Maximize reproductive potential = 50:50 sex ratio

• However from the workers; viewpoint, it is far better to have more sisters.

• Colonies usually have more females than males


Non-haplodiploid colonies

• Termites

• Mole rat from South Africa (Figure 4.8)– There is only one breeding female, the queen.

– The other castes perform different types of work.

– Frequent workers, infrequent workers, nonworkers


Snake predators may venture into surface burrows

5 cm Blocked off burrow

Larger “non-workers”act in defense

20cm

40-50 cm

Mean burrow length= 545 feetMean number of animals= 60

Fig. 4.8 Cross section of naked mole rat colony


Lifestyles that promote eusociality in mammals1. Individuals are confined to burrows or nests

2. Food is abundant enough to support high concentrations of individuals

3. Adults exhibit parental care

4. Mothers can manipulate other individuals

5. When “heroism” is possible, whereby individuals give up their lives and, by so doing, can save the queen.


4.3 Group Living

• Dense living, Promote intense competition

• Significant advantages to compensate – Ex. predators (Figure 4.9)


Scho

o l c

ohe s

ion

7

6

5

Few 1 2 3 4 5 6 Many

Predator abundance (streams in rank order)

Fig. 4.9 variation in group size may be related to defense against predators.

Guppies (Poecilia reticulata)


Group living• “Many-eyes hypothesis”

– Success of predator attacks• Prey alerted to attack (Figure 4.10)

– Ex. Goshawks less successful attacking large flocks of pigeons (Columba palumbus)

• The bigger the flock (more eyes) the more likely the prey will be alerted to the presence of a predator (Figure 4.11)

– Cheating vs. the advantages of not cheating• 值班觀察到掠食者，本身逃走的機會較大，這可以 discourage “cheating”.


Fig. 4.10 For these snow geese large flocks may be better able to detect predators, such as the bald eagle shown here just skyward of the flock.


1 2-10 11-50 50

Number of pigeons in flock

0

20

40

60

80

100

Atta

ck su

cces

s (%

)

Fig. 4.11 The larger the flock of pigeons, decreasing the goshawk’s rate of success in attacking.


Group living• Selfish-herd theory

– The bigger the herd, the lower the probability of an individual prey being taken

– Larger herds are attacked more, but probability of being taken would still favor individual

– Geometry of the selfish herd• 1971, W.D. Hamilton• Prey prefer middle of herd to avoid predator• Predator difficulty in tracking large numbers of prey• Peripheral prey easier to visually isolate• More difficult for predator to reach the center of herd

– Large herds are better able to defend themselves


A model of optimal flock size

• Conflicting variables– Competition for food

– Presence of predator

• Figure 4.12


Perc

enta

ge o

f tim

e

Perc

enta

ge o

f tim

e

Perc

enta

ge o

f tim

e

Optimal flock size

Extra scanning in presence of hawk

Optimal flock size Optimal flock size

Feeding

Scanning

Fighting

Increase in aggression by Dominants at higher food levels

(c)

(a) (b)


Perc

ent o

f tim

e sp

ent i

n ea

ch a

ctiv

ity

0

20

40

60

80

1 3-4 6-7Flock size

Scanning

Fighting

Feeding

Fig. 4.13 The increase in fighting and decrease in scanning of yellow-eyed juncos with increasing flock size yields the highest rate of feeding at intermediate flock size.


The tragedy of the commons• Garrett Hardin (1968) “Tragedy of the Commons

” ( 公共財的悲劇 )

• Ex. Carrying capacity on a piece of land - 1000 cattle – 10 ranchers share land, each with a 100 cattle

– One individual wants to add one cattle more than his/her share

– Maximizes his/her profits at expense of others

– All of the cattle suffer very little.

Applied Ecology


The tragedy of the commons

– What would happen if all ranchers did this?• Overgrazing• Not sustainable

• Benefits of the environment often accrue to single individuals, but the Cost of using the environment is usually borne by the entire population.

Applied Ecology


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Ayo 台南站： http://mail.nutn.edu.tw/~hycheng/

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Chap. 4 Group selection and Individual selection