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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
#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.
#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。
#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。
#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。
#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。
#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)
#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)
#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
#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)
#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
#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)
#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
#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?
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