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Chapter 4Group selection and individual selection
© 2002 by Prentice Hall, Inc.
Upper Saddle River, NJ 07458
#2Chapt. 04
Outline• Group selection vs. individual
selfishness• Altruism• Benefits and trade-offs of group
living
#3Chapt. 04
living in Groups ：
.Increasing Vigilance 警戒
. Dilution Effect
#4Chapt. 04
Altruism 利它主義：In an evolutionary sense,
enhancement of the fitness of an unrelated individual by acts that reduce the evolutionary fitness of the altruistic individual.
#5Chapt. 04
Kin selection ： 同族選擇A form of genetic selection in
which alleles life in their rate of propagation because they influence the survival of in who carry the same alleles.
#6Chapt. 04
Natural selection 自然選擇：
只有那些具有有利變異的後代可以在生存競爭中生存下來，通過以後各代有利變異得到累積，使這樣的後代漸漸與其親代不同。
#7Chapt. 04
5. Natural selection 自然選擇： (1). Stabilizing selection 穩定選擇： (2). Direction selection 定向選擇 :
(3). Disruptive selection 分裂選擇 :
#8Chapt. 04
(1). Stabilizing selection 穩定選擇：
環境條件有利於族群的表現型性狀常態分布線的平均值附近時，對於兩側的極端個體有較高的淘汰率。例如人的出生死亡率和出生重的關係。
#9Chapt. 04
(2). Direction selection 定向選擇 :
選擇對於一側極端的個體有利，從而使族群的平均值向這一側移動。例如大部分的人工選擇。
#10Chapt. 04
(3). Disruptive selection 分裂選擇 :
選擇對兩側極端的個體有利，而不利於中間的個體，從而使族群分成兩個部份。
#11Chapt. 04
自然選擇的條件：
1. 任何生物單位具有複製自身（繁殖）的能力。2. 子代的數目超過其替代的需要。3. 子代的存活決定於某些特徵（外表型或是基因型）。4. 這些特徵具有遺傳傳遞的機制。
#12Chapt. 04
Soft selection:
特定基因型的個體比族群內的其他個體，具有更強取得資源的競爭力，因此可以有較高的活存機率。
#13Chapt. 04
Hard selection:
一個個體的適應度（ Fitness: 存活率、死亡率等量化差異）和其他基因型無關，一種突發的外界環境因素可能導致高死亡率的發生。
#14Chapt. 04
Gamete selection 配子選擇：
選擇對基因頻率的影響，可以發生在配子上，例如精子的活動力差異可以受物理的或化學的狀況所影響。
#15Chapt. 04
Kin selection 親屬選擇：
相關個體間（親屬間）利他行為所產生的總適應度提高的一種選擇。例如土撥鼠發出警告叫聲的土撥鼠可以使其他親屬有較高的活存率，但是本身較易受攻擊而死亡。
#16Chapt. 04
Sexual selection 性別選擇：
最強壯或最活躍的個體具有較高的交配機率，因此這種個體的特徵在後代中會不斷的強化發展。例如孔雀的尾羽、鬥魚的鰭、雄鹿的角。
#17Chapt. 04
Frequency-dependent selection 頻度相關的選擇：
自然選擇作用在出現頻度最多的外表型個體上較高，其結果將造成其生殖程度下降，如此可以使一個群維持平衡式的多形態性。如果選擇對於某種頻度的個體最有利，則將提高這種有個體的適應度。
#18Chapt. 04
The evolution of interactions among speciesMimicry 擬態： Coevolution 共同演化 Parasitism 寄生 :Mutualism 互利共生：Competition 競爭：Predator-prey 掠食者與獵物：Herbivore-plant 草食性動物與植物：
#19Chapt. 04
Mimicry 擬態：從模仿其他物種的外表上獲得好處的現象。 .Bastesian mimicry 貝氏擬態：無毒害的物種藉由模擬有害物種而獲利的情形。 .Mullerian mimicry木氏擬態 : 兩種不同物種之間的擬態。 .Aggressive mimicry 攻擊性擬態： 有毒的種類模擬無讀得種類，以提升其偽裝效果，增加掠食成功率。
#20Chapt. 04
Coevolution 共同演化：
例如植物和昆蟲間的共同演化。
#21Chapt. 04
Group and Individual Selection
• Regulation of populations – early thoughts– Levels below which competition
becomes important– Avoid wastefulness– Development of Group Selection
•Territoriality of birds
#22Chapt. 04
Group and Individual Selection
– Development of Group Selection (cont.). • Increase in emigration correlated with
increase in numbers•Mechanisms operate in the absence of
limitations•High variation in reproductive rates•Examples of self-regulation
#23Chapt. 04
Group and Individual Selection
•Examples of self-regulation (cont.). » 1940, David Lack and Alexander Skutch» Self-regulation of song birds» Tropics vs. temperate » Clutch size based on food
– 1962, V.C. Wynne-Edwards» Animal Dispersion in Relation to Social
Behavior
#24Chapt. 04
Group and Individual Selection» Groups of individuals control their
numbers to avoid extinction» Theory known as Group Selection
– Development of Individual Selection
•Successful groups – individuals would not act selfishly•Selfish groups – overexploit their environment and die out.
#25Chapt. 04
Group and Individual Selection
– Development of Individual Selection (cont.).•1966, G.C. Williams
– Adaptations and Natural Selection– Arguments against Group Selection
» Mutation
•Cheater scenario•Clutch size based on maximizing the number of surviving chicks (Figure 4.1)
#26Chapt. 04
Group and Individual Selection– Arguments against Group Selection (cont.).
» Immigration
» Individual selection
» Resource prediction
•Selfish individuals can migrate to new areas
•Individuals die out more quickly than groups•Individual selection a more powerful evolutionary force
#27Chapt. 04
Group and Individual Selection– Arguments against Group Selection (cont.).
» Resource prediction
•Self-Regulation– Intraspecific competition– Individuals strive to command as much
resources as they can.
•Group selection needs a reliable and predictable source of food•No evidence
#28Chapt. 04
Group and Individual Selection
•Self-Regulation (cont.).– Act in self-interest.
» 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)
#29Chapt. 04
Group and Individual Selection– Act in self-interest.
» Ex. Female giant water bugs kill eggs in masses being taken care of by males (Figure 4.3)
#30Chapt. 04
Altruism
• Apparent cooperation– Grooming– Hunting– Warning signals
#31Chapt. 04
Altruism
• Caring for copies of one’s genes– Genes in offspring– Coefficient of relatedness = r– Probability of sharing a copy of a
particular gene
#32Chapt. 04
Altruism•Probability of sharing a copy of a
particular gene (cont.).– 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
#33Chapt. 04
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)
#34Chapt. 04
Altruism– Coefficient of relatedness = r (cont.).
• Implications of relatedness to altruism– 1964, W.D. Hamilton– Importance of passing on one’s genes through
offspring as well as related individuals.
#35Chapt. 04
Altruism– Coefficient of relatedness = r
• Implications of relatedness to altruism (cont.).
– Inclusive fitness» Total copies of genes passed on to all
relatives– Kin selection
» Lowers individual chance of reproduction
#36Chapt. 04
Altruism– Kin selection
» 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
#37Chapt. 04
Altruism– Kin selection (cont.).
» Ex. Caterpillars
•Aposematic – contain colors to warn predators of bad taste or poison•Datana caterpillars (Figure 4.5)
#38Chapt. 04
Num
ber
of
cate
rpill
ar
speci
es
0
10
20
30
40
50
Aposematic Cryptic
Large family groups
Solitary
#39Chapt. 04
Altruism» Ex. Caterpillars (cont.).
• Predator must kill one to learn• Advantage of animals to congregate in groups (Figure 4.6)
#40Chapt. 04
Altruism» Alarms from ‘sentries’
• 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
#41Chapt. 04
Altruism
• Altruism between unrelated individuals– “You scratch my back, I’ll scratch
yours”– Reciprocal altruism
#42Chapt. 04
Altruism
• Altruism between unrelated individuals (cont.).– Evidence
•Brooding success correlated to availability of helpers
•Social hunting– Benefit: Bigger prey
#43Chapt. 04
Altruism•Social hunting (cont.).
– Cost: Sharing meat
• Altruism in social insects– Extreme example of altruism – sterile
castes in social insects– Female workers
•Rarely reproduce
#44Chapt. 04
Altruism– Female workers (cont.).
•Assist queen with her offspring (eusociality)
– Soldier castes (Figure 4.7)
#45Chapt. 04
Altruism– Social insect reproduction (Table
4.1)
#46Chapt. 04
#47Chapt. 04
Altruism– Relatedness
•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
#48Chapt. 04
Altruism•Sister relatedness
– 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.
– Genetic system termed haplodiploidy– Relatedness and the Queen
» Sons and daughters; r = 0.5
#49Chapt. 04
Altruism•Sister relatedness (cont.).
» Maximize reproductive potential. 50:50 sex ratio
» Average relatedness for sterile workers would be 0.5
– Relatedness and the Queen» Better for female workers to have more
sisters
#50Chapt. 04
Altruism– Relatedness and the Queen
» Colonies usually have more females than males
•Non-haplodiploid colonies– Termites– Mole rat from South Africa (Figure 4.8)
#51Chapt. 04
Snake predators may venture into surface burrows
5 cmBlocked off burrow
Larger “non-workers”act in defense
20cm
40-50 cm
Mean burrow length= 545 feet
Mean number of animals= 60
#52Chapt. 04
Altruism– Lifestyles that promote eusociality in
mammals• Individuals are confined to burrows or
nests•Food is abundant enough to support
high concentrations of individuals•Adults exhibit parental care
#53Chapt. 04
Altruism
• Lifestyles that promote eusociality in mammals
•Mothers can manipulate other individuals
– Lifestyles that promote eusociality (cont.).•Heroism is possible
#54Chapt. 04
Group Living
• Dense living• Promote intense competition• Significant advantages to
compensate
#55Chapt. 04
Group Living
• predators (Figure 4.9)
#56Chapt. 04
Sch
ool co
hesi
on
7
6
5
Few 1 2 3 4 5 6 Many
Predator abundance (streams in rank order)
#57Chapt. 04
Group Living
• “Many-eyes hypothesis”– Success of predator attacks
•Prey alerted to attack (Figure 4.10)
#58Chapt. 04
Group Living – Success of predator attacks (cont.).
• 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)
#59Chapt. 04
1 2-10 11-50 50
Number of pigeons in flock
0
20
40
60
80
100
Att
ack
succ
ess
(%
)
#60Chapt. 04
Group Living – Success of predator attacks (cont.).
•Cheating vs. the advantages of not cheating
• Selfish-herd theory– Predators usually only take one prey
per attack.
#61Chapt. 04
Group Living
• Selfish-herd theory (cont.).– 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
#62Chapt. 04
Group Living
• Selfish-herd theory (cont.). – 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
#63Chapt. 04
Group Living – Geometry of the selfish herd(cont.).
•Peripheral prey easier to visually isolate•More difficult for predator to reach the
center of herd– Large herds are better able to defend
themselves
#64Chapt. 04
#65Chapt. 04
Group Living – Conflicting variables
•Competition for food•Presence of predator •Figure 4.12
#66Chapt. 04
Perc
enta
ge o
f ti
me
Perc
enta
ge o
f ti
me
Perc
enta
ge o
f ti
me
Optimal flock size
Extra scanning in presence of hawk
Optimal flock size Optimal flock size
Feeding
Scanning
Fighting
Increase inaggression bydominantsat higherfood levels
(c)
(a) (b)
#67Chapt. 04
Group Living – Conflicting variables (cont.).
•Figure 4.13
#68Chapt. 04
Perc
ent
of
tim
e s
pent
in e
ach
act
ivit
y
0
20
40
60
80
1 3-4 6-7
Flock size
Scanning
Fighting
Feeding
#69Chapt. 04
Applied Ecology
• Tragedy of the Commons– 1968, Garrett Hardin– “Tragedy of the Commons”– Humans and cattle grazing– Carrying capacity of land
#70Chapt. 04
Applied Ecology•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.
#71Chapt. 04
Applied Ecology– Tragedy (cont.).
» What would happen if all ranchers did this?
» Overgrazing» Not sustainable
– Benefits of the environment often accrue to the individual
#72Chapt. 04
Applied Ecology– Cost of using the environment is
usually borne by the entire population
#73Chapt. 04
Summary
• Group Selection– Past theory– Population maintained at equilibrium
based on group selection•Self-regulation of individuals•Prevent overexploitation of resources
#74Chapt. 04
Summary
• Group Selection (cont.).– Several flaws – mutation,
immigration, and resource prediction.– Individual Selection
•More likely
#75Chapt. 04
Summary•Explanations for altruism
– Kin selection– Caste systems of social insects– Haplodiploid mating systems
•Occurrence of eusociality and cooperation
– Haplodiploid organisms
– Non-Haplodiploid organisms» Confinement to burrows
#76Chapt. 04
Summary•Occurrence of eusociality and
cooperation» High food concentrations» Parental care of offspring» Mothers can manipulate other individuals
– Non-Haplodiploid organisms Cont.).» Opportunity for heroism
#77Chapt. 04
Summary•Group Size - Trade offs• Competition for food•Protection from predators
#78Chapt. 04
Discussion Question #1
• If kin selection occurs in nature, how do you think animals recognize their kin?
#79Chapt. 04
Discussion Question #2
• If it is equally valuable for a female to raise her own young or help raise sisters (both having an r = 0.5), why do we see most females preferentially raising their young?
#80Chapt. 04
Discussion Question #3
• Unrelated vampire bats that roost communally often rest next to the same neighbors every night and sometimes regurgitate meals of blood to their hungry neighbors. How can you explain this apparent act of altruism?
