Download Natural selection

大學部 生態學與保育生物學學程 (必選)
2010 年 秋冬
天擇 (Natural selection)
─ 動物行為學(Ethology)
鄭先祐(Ayo)
國立 臺南大學 環境與生態學院
生態科學與技術學系 教授
Ayo NUTN Web: http://myweb.nutn.edu.tw/~hycheng/
Part 1. 動物行為的研究途徑 (個體行為)
 歷史背景 (History of the Study of Animal




Behavior ).
基因分析 (Genetic Analysis of Behavior ).
天擇 (Natural Selection and Behavior ).
學習與認知 (Learning and Cognition.)
生理分析 (Physiological Analysis)
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
(一) 神經細胞 (Nerve Cells and Behavior ).
(二) 內分泌系統 (The Endocrine System).
 發育(The Development of Behavior ).
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04 天擇 (Natural Selection)
 Natural selection
 Common misunderstandings
 Genetic variation
 Response to Natural selection
 The maintenance of variation
 Test hypotheses about natural selection and adaptation
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Natural selection and behavior
Species behaviors are well suited to their environments
 Kittiwake gulls treat strange chicks as
offspring


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They nest on steep cliffs, where the
chances of the wrong chick ending up
in the nest are minimal
The deeper nests are less likely to
allow eggs to roll off cliffs
Predators can’t reach the nests
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Natural selection and behavior
 Herring gulls recognize their
chicks

They won’t care for
neighboring chicks that
wander into their nests
 Predators move freely through
a herring gull colony


Nests must be less obvious
Parents remove eggshells and
droppings
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Closely related species are very different
 “Nothing in biology makes sense except in the light
of evolution”
 Evolution: a change in the frequencies of different
alleles in a population of organisms over generations
 Population: an interbreeding group of organisms of
the same species
 Natural selection is the most important type of
evolutionary force

It is the reason why species are well suited for their
environments
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Natural selection
 Darwin contemplated many esoteric topics, including
pigeons

Over generations, extraordinarily bizarre pigeons could
be bred through artificial selection
 Darwin noticed a parallel between the process that
was happening in pigeon lofts and what might be
going on in nature
 He published On the Origin of Species by Natural
Selection or The Preservation of Favored Races in the
Struggle for Life
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Core concepts of natural selection
 Observation 1: Individuals in a population vary
They differ in appearance, behavior, physiology, etc.
 Observation 2: Some variable traits are genetically based
 Traits inherited from parents can be passed to offspring
 Observation 3: Some inherited traits improve an individual’s
chances of leaving more offspring
 Conclusion: Because offspring are likely to inherit their
parents’ beneficial traits, these traits become more common in
the population relative
 This is evolution by natural selection

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Natural selection selects traits
 In natural selection, nature “selects’’ those traits that
enhance reproductive success

i.e. male bighorn sheep that win head butting contests
leave more offspring
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Evolution must occur
 Because of natural selection, the population’s genetic
composition changes in future generations

More individuals have alleles that code for “winning”
traits
 Adaptations: traits that evolved because they allow
individuals to survive and reproduce better


Have a genetic basis
Excludes learned behaviors, but the capacity to learn
may be an adaptation
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Nonscientists misunderstand natural selection
 Reinforced by poorly written articles in the popular
press
 The terminology of evolution uses words that have
other meanings


In artificial selection, the selective force is imposed
by humans that have particular goals in mind (有目
的)
Natural selection is not capable of long-range or even
short-range planning (沒有目的)

It works only generation by generation
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A troublesome phrase: “survival of the fittest”
 “Survival” is only one of many traits acted on by natural
selection
 An animal must survive, compete, find a home and mates,
and produce offspring
 These abilities are improved through natural selection
 “The fittest” suggests that the most physically fit, strongest,
and aggressive individuals dominate all others
 In an evolutionary sense, fitness describes the reproductive
success of a gene or an individual
 Other traits can be more important to evolutionary success
than being the biggest or strongest
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Different measures of fitness
 Direct fitness: the number of surviving offspring an
individual produces
 Relative fitness: the average fitness of a gene or
individual compared with the rest of its population

Dictates how a population will change over generations
 Indirect fitness: fitness gained by helping relatives
 Inclusive fitness: direct and indirect fitness together
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Avoid the phrase “for the good of the species”
 Traits do not evolve to help a species survive
 Natural selection does not act with the species’ future in
mind
 Traits that increase an animal’s fitness increase in the
population



Even if it means trouble for the species in the long term
i.e. through natural selection, the frequency of a gene
increases that allows a female to have more offspring
The population can outstrip its available food resources and
crash
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Macroevolution vs. microevolution
 Macroevolution: large-scale changes over geological time
Such as birds evolving from reptilian ancestors
 The concept of evolution also encompasses microevolution:
small changes that happen over only a few generations
 An evolutionary change within species
 For example, some Colorado potato beetles have alleles that
allow them to survive pesticides
 Finally, populations, not individuals, evolve
 Evolutionary change only happens to populations from one
generation to the next

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Variation in individuals is the rule
 Variation is the rule, rather than the exception
 Not even the offspring of the same parents are identical
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Some differences are obvious
 Some differences between
individuals are obvious (i.e. size
or color pattern)

Others are harder to detect (i.e.
metabolic rate)
 Variants can be distinct or
continuous, changing gradually
from one extreme to the other

Most individuals fall midway
between the extremes
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常態分布
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Phenotypic variation has two sources
 Genetic variation: the key component of evolution
If all individuals are genetically identical for a particular
trait, that trait cannot evolve by natural selection
 The environment
 Evolution does not act directly on the genotype (the
genetic makeup), but upon the phenotype (the observable
traits)
 Selection cannot act on genetic differences if they have
no effect on the phenotype

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Mutations: raw material of genetic
variation
 Mutation: a change in the DNA sequence of an organism
They can be passed on to offspring
 Some mutations affect only a small part of the genotype
 Dramatically affect the function of structural and
regulatory genes
 Other mutations are larger: genes may be duplicated or
deleted
 Entire pieces of chromosomes can move or be reversed
 Variation produced by mutation is likely to be
disadvantageous
 A random change is unlikely to be an improvement

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Recombination: raw material of genetic variation
 Occurs during meiosis: cell division that results in the
formation of gametes (eggs or sperm)

Crossing over: pieces of chromosomes containing
alleles for the same gene are swapped
 The combination of alleles is scrambled during gamete
production
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Stabilizing selection
 Genetic variation provided by mutation and
recombination provides raw material on which
natural selection can work

Populations change when they undergo natural
selection
 Stabilizing selection: under stable environmental
conditions, animals with traits at the center of the
distribution do best


Are most successful in the current environment
Those at the extreme ends of the distribution are less
well-suited to current conditions
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Directional selection
 Directional selection: the
environment changes and the
optimum phenotype shifts over
time



Those at one extreme become
favored
The curve that represents the
population’s phenotype shifts in
that direction
The degree of change can be
weak or strong
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The existence of nonadaptive traits
 Why haven’t individuals with alternate traits been
eliminated?

Natural selection is not the only force that changes allele
frequencies
 Gene flow: genes from populations mix
 It makes populations more similar
 It can slow or halt the effects of local adaptation
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案例：Gene flow in the funnelweb-building spider
 This species occupies a wide variety of habitats from
northern Wyoming to southern Mexico
 Some spiders live in lush riparian vegetation along
rivers and lakes of Arizona


Abundant insect prey
Common predators of spiders
 Other spiders live in desert grassland
 Insect prey are few
 It’s too hot to forage through the day
 There are fewer good places to build a web
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Funnel web spiders
 Funnel web spiders (Agelenidae)
fit into the web building spider
group. Their webs function as
their primary hunting tool.
 The above picture shows its
unique web design. It looks like
a mass of silk with a hole, or
funnel in the middle. Normally
the spider sits at the end of the
funnel waiting to pounce on a
visiting insect.
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Spiders have very different behaviors
 Grassland spiders are much more
aggressive
 Don’t allow other spiders near their
webs
 Intense territorial disputes
 Fighting results in injury or death
 Very aggressive toward prey
 Riparian spiders have other problems
 Abundant web sites
 Abundant prey
 However, birds and other predators
make riparian spiders very cautious
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Behavioral differences in spiders are genetic
 Genetic differences exist between desert and riparian
populations of funnel-web-building spiders

Laboratory-raised spiders behaved like those that lived in
the wild
 The behaviors responsible for territory size are
genetically, rather than environmentally, determined

Even under lush conditions, grassland spiders still
maintain a large web
 Spiders collected from one habitat died when they were
transferred to the other habitat
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One spider population does not fit
the behavioral pattern
 One riparian population shows more variability in
behavior

Including the highly aggressive territorial behavior of desert
populations
 This population is not isolated from desert populations,
but has a constant influx of immigrants
 When individuals were prevented from moving from one
population to another


The population evolved to become less aggressive and more
cautious
And more adapted to its local environment
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Genetic drift: an evolutionary process
 Genetic drift: the change in allele frequencies in a
population due purely to chance events
 Allele frequencies in populations drift over generations
 An allele might even drift to fixation - it is carried by
every member of the population
 Genetic drift is more important as population size gets
smaller
 Populations go through bottlenecks – a sharp reduction in
population size
 Because of natural events
 Especially likely in rarer animals of conservation
concern
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Correlated traits
 Traits may be correlated with one another for several
reasons

Pleiotropy: one gene, such as a regulatory gene, may
affect several traits
 Genes can be tightly linked when they are physically
close together on the same chromosome

Until recombination and selection break the link
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Tightly correlated traits can be negative
 Two traits may share an underlying morphological and
physiological basis that

It is difficult to uncouple them
 When traits are tightly correlated, even negative traits
might be maintained in the population if the net effect
on the genotype is positive
 Behaving optimally in every situation is impossible
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案例：A correlated trait: finch beaks
 Many finch species in the Galapagos Islands look similar
But have very different beaks
 Darwin suggested that the species shared a common
ancestor, but over time diverged and specialized on
different food resources
 i.e. beaks for seeds vs. beaks for poking into flowers
 But finches use their beaks for more than just feeding: beak
shape influences how males sing
 Females choose mates on based on their song
 Selective forces act on feeding, singing and mating behavior

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Darwin Finches
 Galapagos Finches
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Changing environmental conditions
 A population may seem to be poorly adapted to current
conditions

Today’s traits reflect past evolutionary pressures
 Humans change the environment
 An opossum’s naked tail and ears make it vulnerable to
cold
 Opossums survive winter by living near humans
 Over time, there is selection for less fearful opossums
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Opossums (負鼠)
 North American opossums can benefit from proximity
to human habitation.
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Human-caused changes pressure species
 Environmental changes are occurring so rapidly
Many populations cannot evolve fast enough to keep up
In Britain, newts(蝾螈) have responded to warming
temperatures by entering ponds earlier than they used to
 But frogs still reproduce at the same time
 Frog eggs and tadpoles are exposed to more newt predators
Habitat is lost to development
Pollutants and fertilizers change water chemistry
Traditional migratory stopovers disappear
Light pollution interferes with animal navigation

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The environment includes biotic factors
 Birds on New Zealand had no mammalian predators in
their evolutionary past

So they do not have antipredator skills
 Birds will land near dangerous animals
 Some, such as the kakapo (a parrot)(鸚鵡) , have lost
the ability to fly
 Cats, rats and other predators became established
 Many bird populations are in dramatic decline
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 New Zealand parrot (鸚鵡)
 New Zealand robin (知更鳥)
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Aversive conditioning can save species
 Some researchers have tried to instill fear of predators into
naive birds by using aversive conditioning techniques
 For example, New Zealand robin chicks were presented with
dead stuffed cats and ferrets while hearing robin alarm and
distress calls
 Robins learned to associate cats and ferrets with danger
 And reduced their tendency to approach them
 Techniques such as these are time-intensive
 But may be useful as a last-ditch measure to save severely
threatened populations
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Adaptations in a species affect others
 Adaptations that evolve in one species may change the selection
pressures on other species
 Which may change the selection pressures on the first
 For example, insectivorous bats locate flying moths by sonar
 In response, some moth species have evolved the ability to
detect the ultrasonic signals emitted by the bat
 And undertake evasive action with a fast erratic flight
 Bats are under even greater pressure to detect and follow
moths
 This coevolution is known as an evolutionary arms race
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Frequency-dependent selection
 Variation can be maintained in a population because different
genotypes are favored at different times
 Frequency-dependent selection: an allele has a greater
selective advantage when it is rare than when it is common
 As a result, the frequency of any given allele fluctuates
 It increases until it is common and then decreases once the
alternative allele is favored
 Two examples of frequency-dependent selection are
 Frequency-dependent predation
 Frequency-dependent reproduction
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Frequency-dependent
predation maintains variation
 Although predators have a varied diet, they often attack one
prey type more often than expected by chance
 A predator might concentrate on the most common prey
 The more common individuals are attacked until their
numbers, and their alleles, decline in frequency
 The rarer form survives and reproduces and its relative
frequency increases
 Then the predator switches to the new most common prey
 Which eventually decreases in number, and the cycle
begins again
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Predators choose the most common prey
 Frequency-dependent predation can maintain variation in
prey appearance
 Especially if the prey density is low
 For example, blue jays “preyed on” virtual moths presented
on computer screens by pecking on the screen
 The blue jays preyed on the most common form of moth
 And switched to alternative forms when that form became
less common
 In nature, the maintenance of prey polymorphism (“many
forms”) would also maintain the genetic variation underlying
it
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Frequency-dependent reproduction
 Frequency-dependent reproduction (the “rare male”
effect) can maintain a variety of male phenotypes in the
population
 A male with a rare phenotype mates more expected
 The alleles of the rare phenotype increase in the
population until they become common and are no longer
favored
 The allele frequencies of different phenotypes seesaws
 Male guppies have extremely variable coloration
 Females choose males with novel color patterns—rare
males—over males with a familiar color pattern
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Both types of frequency dependence in crickets
 Male Texas field crickets rub their wings to attract females
Males vary in the time they devote to calling every night
 Some rarely or never call, and others call for hours
 Why would there be this much variation?
 Calling also attracts parasitoid flies, which lay their eggs on
the crickets, eventually killing them
 When flies are common, the calling males are soon
parasitized, and the males that call less end up with more
mates over their (longer) lives
 When flies are rare, the calling males have the advantage

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 Male Texas field crickets
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Evolutionarily stable strategies
 A individual’s success may depend on what others are doing
A “strategy:” the set of behaviors available to an animal
 “Winning:” the individual’s fitness increases more than its
competitor’s does (i.e., it leaves more offspring)
 Evolutionarily stable strategy (ESS): the optimal strategy
for an individual to follow when the rewards (payoffs) depend
on what others are doing
 When adopted by most members of a population, this
strategy cannot be beaten by a different strategy: no other
strategy confers more fitness benefits

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An ESS: herring gulls (銀鷗)
 A herring gull (銀鷗) will not care for a neighboring
chick that wanders into its nest
 This is an ESS: there is no alternative behavior that will
yield greater reproductive success


The alternative strategy: caring for other birds’ chicks
But herring gull parents would waste time and energy
caring for offspring that are not their own
 An ESS is unbeatable and uncheatable in the long run
 A pure ESS: a single strategy
 A mixed ESS: several strategies in a stable equilibrium
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The favored strategy maximizes benefit
 A hypothetical population of fish-catching birds has two
strategies for getting dinner

Catch your own fish or steal one from another bird
 Thievery is favored first: it minimizes its costs and gets
full benefits from the efforts of others



As the number of bandits increases, so does the chance of
encountering another robber or a bird that had its fish stolen
Then, honesty becomes the best policy
When hard-working birds become common, thievery once
again becomes profitable
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Nesting strategies of digger wasps
 The nesting behavior of female digger wasps is a
mixed ESS, with two strategies
digger wasps (掘鑿蜂)
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Female digger wasps must make a choice
 A female can dig her own nest
 Digging is expensive in time and energy
 Another female may take the nest
 Ants or centipedes could invade the nest
 She can enter an existing burrow, reaping benefits
without costs

This is the favored strategy if the burrow is
abandoned
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A female digger wasp’s alternative nesting
strategies and their outcomes
 But there is no way to determine whether the nest is
abandoned or whether the resident is just out hunting

Eventually the two females will meet and fight
sometimes to the death, and winner takes all
 The two available strategies: to “dig” and to “enter”
 Three possible outcomes of a decision to dig
 Two possible outcomes of a decision to enter an
existing burrow
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A female digger wasp’s alternative
nesting strategies and their outcomes
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Female digger wasps: to dig or not to dig
 Depends on what other members of the population are
doing
 Entering an existing burrow is the successful strategy when
it’s rare



As entering becomes more common, there are fewer diggers
Which increases the chance of entering an occupied nest,
along with costly fights
Eventually digging becomes a better strategy
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 The digger wasp’s strategies to “dig” and
to “enter” are a mixed ESS.
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Mixed ESS strategies cycle
 Mixed ESS strategies cycle between generations
 For example, fitness changes among male side-blotched
lizards with alternative reproductive strategies
 Male lizards come in three genetically determined
throat colors: Orange, Yellow, Blue
 Each color morph displays a different reproductive
strategy

A population of only one morph is not evolutionarily
stable
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Uta stansburiana
 Side-blotched lizards are some of the most abundant and commonly
observed lizards in the deserts of western North America.
 Males often have bright throat colors. Orange-throated males establish
large territories and accommodate multiple females. Yellow stripe
throated males (sneakers) stay on the fringe of orange-throated lizard
territories and mate with their females while the orange-throat is
absent as the territory to defend is large. Blue-throated males defend a
small territory large enough for one female. They can fend off the
yellow stripe throated males but they can't withstand attacks by
orange-throated males.
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Different reproductive strategies of lizards
 Orange throats: very aggressive, defend large territories
with several females

Can’t defend every female all the time
 Yellow throats: don’t defend territories
 “Sneak” matings from females in orange territories
 Blue-throated males: defend territories holding a single
female

Successfully defends her against yellow-throated males
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Side-blotched lizards: a mixed ESS
 A population of only orange-throated males is not




evolutionarily stable; yellow males can win over orange
males
Only yellow-throated males is not evolutionarily stable,
because they can be invaded by males with blue throats
Orange-throated males can invade a population of bluethroated males and have higher reproductive success
When yellow-throated sneaker males are rare, it pays to
defend large territories with several females
In this mixed ESS, yellow beats orange, blue beats yellow,
and orange beats blue
 But, different color morphs predominate in different years
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Negative-assortative mating: opposites attract
 Negative-assortative mating also preserves genetic variation
in a population, but is uncommon
 Females choose mates with a different phenotype from
theirs
 It is not rare-male advantage (where females of all
phenotypes prefer unusual males)
 Females of different phenotypes have different preferences
 If the difference has a genetic basis, variability is enhanced
 Can prevent inbreeding
 Mice can determine whether others share certain alleles by
the smell of urine
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案例：Negative-assortative mating of
sparrows
 It maintains the tan-striped and white-striped morphs of
white-throated sparrows in equal numbers in a population
 Both female morphs prefer tan-striped males
 They are better parents because they spend more time
feeding chicks
 White-striped females outcompete tan-striped females for
access to the tan-striped males
 So, tan-striped females pair with the white-striped males
 As a result, 93% to 98% of the population mates with an
individual of the opposite morph
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Tan-striped form
white-striped form
 white-throated sparrows
 house sparrows (麻雀)
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A trait does not need to be optimal to exist
 Natural selection acts on the total phenotype of the
individual
 Which consists of good and bad traits, so perfection is
elusive
 Natural selection can act only on the available alternatives
 Which depend on the population’s evolutionary history
and
 Each individual’s present conditions—ecological,
anatomical, and physiological
 Natural selection works in a given environment
 Conditions vary from place to place or change over time
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A nonadaptive behavior is actually adaptive
 A seemingly nonadaptive behavior of black-headed gulls is
that Parents do not immediately remove broken eggshells
 Eggshells in the nest can attract predators
 This trait is adaptive
 Newly hatched, wet chicks are eaten by neighboring gulls


They are easier to swallow than dry chicks
Delaying egg shell removal until the chicks were dry
decreased the likelihood of the chicks’ being cannibalized

While the parents were away removing the shells
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 black-headed gulls
 Kittiwakes
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The experimental approach tests
multiple hypotheses
 Kittiwakes, with low predation rates, leave eggshell
pieces in the nest

Ground-nesting gulls, with high predation rates, remove
eggshells
 The survival value of shell removal: to reduce predation
on the young

White eggshells attract predators
 Other hypotheses to egg removal
 Sharp shells may hurt chicks
 The shell might suffocate a chick
 It might attract parasites
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Testing multiple competing hypotheses
 California ground squirrels and rock squirrels chew shed skins




of rattlesnakes, a major predators, and lick their fur
 It depends on who applies the scent (氣味)
Hypothesis 1: it defends against parasites (e.g., fleas, ticks)
 Juveniles: they have more parasites\
Hypothesis 2: it distracts conspecifics during aggressive
interactions
 Males: they fight each other
Hypothesis 3: it deters(嚇住) predators
 Juveniles and adult females are most vulnerable
The experimental approach supports hypothesis 3
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The comparative approach
 Related species inherited common genes because they
have a common ancestor
 If related species live in different situations, they
experience different selection pressures
 Unrelated species in the same environment and
experiencing the same selection pressures may display
similar behaviors
 The comparative approach is seen in the herring and
kittiwake gull example
 They descended from a recent, common groundnesting ancestor but now live in very different
ecological situations
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Correlation may be coincidental
 A correlation between behavior and the environmental
might be just a coincidence

A large sample size is needed to rule out random chance
 This method works best when a taxonomic group has
been well studied


The phylogeny (historical relationships) among a group
of organisms is known
The phylogeny show the order in which behavioral and
morphological traits evolved
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The comparative method: swordtail fish
(劍尾魚)
 Males of some species in the swordtail fish have long
tailfin extensions called swords

Males in other species do not
 Females prefer males with swords
 Even females in species where the males have no swords
 A phylogenetic reconstruction suggests that the female’s
preference for swords evolved before the sword itself

Females were predisposed to be attracted to swords even
before males evolved them
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 swordtail fish (劍尾魚)
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Another use of phylogenetic information
 It can examine the relationship between behavior and
various ecological variables (i.e. predation risk)
 It’s best to have multiple species from different
environments


i.e. the Galápagos swallow-tailed gull chooses nest sites
with characteristics between kittiwakes and herring gulls
It also shows intermediate behavioral patterns
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swallow-tailed gull
kittiwakes
herring gulls
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Phylogenetic trees
A hypothetical example illustrating the effect of wise
choice of species on our ability to test hypotheses about
the influence of the environment on traits. (有20個物種)
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 (b) Sleeping in groups evolved eight times.
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Limitations of the comparative approach
 The comparative method must be applied carefully
 Alternate hypotheses must be considered, tested, and
ruled out
 Confirmation of predictions lends more weight to some
hypotheses
 Correlations between traits and ecological variables do
not prove there is a common cause

For example, is diet a cause or an effect of sociality in
weaver birds (織巢鳥) ?
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 weaver birds (織巢鳥)
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案例：Diet and sociality in weaver birds
 兩種類型：


Species in the forest eat insects and forage alone
Species in the savannah eat seeds and feed in flocks
 Does diet cause flocking?
 Groups are likely to find a patch of seeds that can feed them
all
 Does flocking protect birds?
 Seeds may be the only food source that could supply enough
food to a flock
 The correlation does not answer the question
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The lack of a pattern can be important
 The lack of a pattern between traits and ecological
variables can also answer questions
 One hypothesis for why birds roost in groups at night
is because of thermoregulation

Huddling together conserves body warmth
 Hypothesis: species that spend time in cold areas and
species that have lower body masses need this
thermoregulatory boost

A phylogenetically-based study of distantly related
groups of birds did not find this pattern
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Be careful of confounding variables
(困擾的變項)
 A confounding variable: any variable other than the factor
of principal concern that may contribute to or cause the
correlation

i.e. age and body size
 These factors must be controlled to prevent incorrect
conclusions
 For example, antler size is correlated with reproductive
success among male red deer.
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案例：紅鹿的 antler size
 antler size is correlated with reproductive
success among red deer stags ?



Antler size is also correlated with age
and body size
When one controls for age no
association is found between antler
length and reproductive success
Antler size is not the primary factor in
determining reproductive success
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Monitoring selection in the field
 It’s very hard to measure evolution in
the field
 需要非常長時期的研究。
 In an intensive 30-year-long field
study, Rosemary and Peter Grant and
their colleagues documented changes
in beak size in medium ground
finches (Geospiza fortis) in
response to the environment.
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案例： Ground finches and beak size
 beak size and shape in Darwin’s finches change in
response to the environment
 During drought, medium ground finches with deeper
beaks were able to eat hard seeds and produced more
offspring

The offspring also had deeper beaks
 During rainy years, more small seeds were available -
birds with smaller bills had the advantage
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Modeling the costs and benefits of traits
 The best choice: the strategy in which the
advantages outweigh the disadvantages by the
greatest amount
 The decision is difficult because it requires
integration of concerns along different dimensions


For example: animals must decide to stay in a safe
place where there is not much to eat
Or go out to forage in a place with abundant food but
predators
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Optimality modeling
 Optimality modeling weighs the pros and cons (the costs
and benefits) of each available strategy
 A model: a mathematical expression of the costs and
benefits of each strategy
 All costs and benefits are translated into common units
that represent a measure of fitness

Currency: the measure of fitness which allows different
strategies to be compared
 The optimal strategy: the behavioral alternative that
maximizes the difference between the costs and benefits

In terms of evolution, this choice maximizes fitness
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Models gives insight into behavioral
rules
 The term “decision rules” does not imply that animals make
conscious decisions to find the optimal course of action
 Natural selection has shaped behavior over generations
 The animal responds appropriately to a set of
circumstances
 Complex behavior may occur by following a simple
strategy
 Models provide insight into how simple behavioral rules can
generate complex behavior
 Given just a few rules to follow, computer-generated
animals exhibit behavior that resembles territoriality
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Summary
 Animals match their environments through natural selection
 Natural selection occurs when there is phenotypic variation




in a population
 Changing allele frequencies in a population causes
evolution
In small populations, genetic drift becomes very important
Selection pressures change over time, and evolution lags
behind
Selection pressure on a genotype may depend on its
frequency in the population
An evolutionarily stable strategy cannot be beaten by
another
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問題與討論
[email protected]
 Ayo 台南 NUTN 站
http://myweb.nutn.edu.tw/~hycheng/
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