Download History of Evolutionary Thought in Biology Lecture 1 Slide 2

History of Evolutionary Thought in Biology
Lecture 1
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Evolution
Evolution: a continuous development distinguished by each stage growing from the one before. 2 types
considered in class.
Biological- a change in gene frequencies in a population
Cultural-the continuous development from simple to complex forms of culture
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Charles Darwin
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Pre-Darwinian Theories of Evolution
PreDarwinian Theories were either
• saltational- sudden changes due to catastrophes, spontaneous generations (Catastrophism) or
transformational gradual change within a lineage (Lamarkianism)both believed in the theory of inheritance of acquired characteristics
i.e., giraffe’s neck
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Inheritance of Acquired Characteristics
The theory that species evolve in response to environmental stimuli on phenotype. These traits the passed
on to succeeding generations.
Also called Lamarckian Evolution.
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Adaptation
Adaptation: the process by which an organism adjusts to its environment,
Change in organism resulted from exposure to certain environmental conditions that make it react more
effectively to those conditions i.e. is concerned w/ function
Best Suited for Ecological Thought- Problem in Evolutionary theory is that it is tautological (circular).
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Pre-Darwinian Theories of Evolution
Biologists were biased by a perception that species were fundamentally different kinds of things that could
not inter-grade.
scala naturae- great chain of being
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Development of a Modern Theory of Evolution
Charles Darwin worked on selectionist theory for 20 years.
Independent Discovery by Alfred Russell Wallace.
Darwin Publishes On the Origin of Species in 1859.
The Descent of Man and Selection in Relations to Sex published in 1871.
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Inspirations of Darwin’s New Theory
Voyage to the Galapagos Islands, observed geographical variation of species in different islands- different
from temporal perspective emphasized in past (variation in space)
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Homology
Sharing of Traits Because of Ancestry
Traits may share similar structures because of common evolutionary history.
However, traits may have divergent adaptive functions.
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Analogy
Sharing of Traits Because of Common Function or Adaptation
Traits may share similar design because structures evolved to meet similar functions.
However, traits may have divergent structures and evolutionary history.
Convergent Evolution
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Darwin’s New Theory
These gave Darwin three new insights on which he built his theory
1) Common descent- all species descended from a common ancestor- therefore species could grade into one
another (this had not been appreciated before
2) Natural Selection
3) Sexual Selection
Inspirations of Darwin’s New Theory
Artificial breeding of domesticated species (particularly pigeons) artificial selection by breeders served as a
model for natural selection (variation in time)
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Inspirations of Darwin’s New Theory
Social theory- particularly the work of Thomas Malthus (18th century economist)
Populations intrinsically grow faster than means of subsistence.
Famine and Disease keep population in check
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Inspirations of Darwin’s New Theory
Geological theory- particularly the work of Charles Lyell
Principle of Uniformitarianism
Past Phenomena Caused by Natural Processes Observed in the Modern World.
Operating over immense spans of time.
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Natural Selection
Principal process of evolutionary change originally identified by Darwin.
Theory states that those individual within a population having beneficial characteristics will contribute
more offspring to the succeeding generation than those having less advantageous traits.
If those traits are inheritable the composition of the total population will changes
In other words, natural selection is selective and emphasizes individuals
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Principles of Natural Selection
Variation
Hereditability
Competition
Reproductive Success
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Sexual Selection
Sexual Selection- the increased reproductive success of a small percentage of individuals owing to their
possession of characters that enhance either their ability to compete with members of the same sex or
attractiveness to the opposite sex
I.e., peacock’s tails
Darwin believed that sexual selection accounted for racial differences among Homo sapiens.
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Differences from Pre-Darwinian Theories
Common descent
“Selectionist”
Emphasis on individuals rather than groups
Gradual Change
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After Darwin 1880-1900
Darwin’s work revolutionized biology, but contrary to popular belief his selective theories were not
accepted in biology at first.
From Darwin, biologists accepted
that evolution had occurred
that all species descended from a common ancestors (species could change)
that man was included
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However most biologists rejected Natural Selection
The problems were
NS implies gradual change and transformations from one species to another, but the fossil record still
seemed to imply long periods of no change and sudden replacements.
Darwin could not account for origin of variation and inheritance of characters (genetics not understood)
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Rediscovery of Mendel
Austrian monk.
Contemporary of Darwin, but work unrecognized.
Performed crossbreeding experiments with peas.
Discovered that inheritance is particulate, not blending.
Refute inheritance of acquired characteristics and provided a mechanism (mutation) by which new
variation would enter the population.
Led to development of genetics.
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1900-1940
But natural selection still not accepted- geneticist emphasized mutation as an evolutionary force- more
consistent with saltational theories
Mendel’s theories were seen as fundamental challenge to Darwinism
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Mutation
Sudden random changes in chromosomal DNA. Most mutations are deleterious, or neutral. However, some
may convey an adaptive advantage, that is subsequently selected.
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The Evolutionary Synthesis of the 1940s
Genetics, natural selection, and the fossil record reconciled into a single theory in 1940s
Ronald Fischer - statistician
JBS Haldane- biometrist/ biochemist
Sewell Wright -geneticist
Chetverikov Dobzhansky- geneticist
Ernst Mayr- ornithologist
G. G. Simpson - paleontologist
G. Ledyard Stebbins Jr. - plant geneticist
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Geneticist documented that almost all mutations were small-scale- large mutations were likely harmful.
Also identified Genetic Drift as a force.
Paleontologists suggested that characters of fossil record was likely due to geographic speciation and
taphonomic process.
Agreed on a restatement of the principles of natural selection according to genetic theory.
This synthesis has been the party line of most biology ( a unified theory of evolution) for the last 50 years
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Restatement of natural selection in terms of modern genetics
All organisms have genes that regulate the development of the individual and influence behavior.
Within a population, many genes will have two or more variants (alleles) that cause variation in the
population.
There will be competition between the alleles of a gene to reproduce on the chromosome.
Any allele that can make more surviving copies of itself than its alternative will eventually replace the
alternative form in the population
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The Evolution of the Peppered Moth
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Genetic Drift
Chance shifts occurring in gene frequencies over time. Most likely to occur in small, isolated populations
(Founder and Bottleneck Effects)
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Biological Evolution
Cumulative change in the characteristics of a population occurring over the course of successive
generations related by descent (reflecting changing gene frequencies). Evolution occurs by a combination
of several natural processes:
natural selection,
mutation,
drift,
sexual selection
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Ecosystems Theory and the Return of Group Selection
V.C. Wynne Edwards (1962) Animal Dispersion in Relation to Social Behavior
observed that species frequently fail to reproduce up to their physiological capabilities
populations of many species seemed below the carrying capacity of the environment
saw this as an adaptation to the environment (population levels remain in equilibrium w/ the ecosystem)
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Ecology
Study of the interrelations of various plants and animals (including humans) to their surrounding organic
and inorganic environment.
Ecological study in biology and anthropology hinges on the concept of adaptation
Ecosystem- An ecological community together with its environment, functioning as a unit
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Elements of WE’s Ecosystems Theory
Group Selection-natural selection operates on groups, populations that overextend their food supply lose
out to groups that do not.
Explained altruistic behavior as group selection
Envisage natural selection as a process that ensures that individuals behave for the good of the species or
ecosystems.
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Altruism
Behavior that benefits another organism at a cost to the actor.
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Selfish Genes and Sociobiology
George Williams- Adaptation and Natural Selection
Pointed out the problem of evolution of altruism.
Group level selection should be rare in nature because individual selection (or gene selection) should
overrule.
Much altruistic behavior could be explained in terms of fitness.
Advocated a more Darwinian approach that saw behavior as selected to maximize the reproductive fitness
of individuals (or genes).
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Led to perspective that reemphasized selection on the gene.
Dawkins- Selfish Gene, Wilson - Sociobiology
Explained behavior as adaptation that serves to maximize reproductive fitness individuals
Adaptation – a trait brought about during evolution by natural selection which functions (ultimate) to
increase the reproductive success (fitness) of individuals bearing the trait.
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Punctuated Equilibria and Macroevolution
Paleontologists Steven J Gould and Niles Eldridge
Return to paleontological evidence that species don’t change gradually.
Natural selection alone cannot explain how species evolve.
Advance theory of punctuated equilibrium to explain periods of speciation.
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Punctuated Equilibrium
Theory that most evolutionarily important events took place during short bouts of speciation and that once
species are formed they are relatively stable for long periods
speciation must be understood at a macroevolution and ecosystems level
species reach equilibrium in ecological relationships with other species, prevent expansion into new niches,
and dampens evolutionary change, genetic drift becomes major force
Until historical events disrupt the equilibrium
New mutation, environmental change, cataclysm
Initiate rapid environmental change as species readjust
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Gould and Lewonton- Spandrels of San Marcos
Challenged sociobiology notion of natural selection as an optimizing agent
Dr Pangolss “our noses were made for carrying glasses so we have glasses”-Best of all possible worlds
Everything exits because it is adapted and is adapted because it is
Watch out for just-so stories- tautology
Cannot assume that every trait has been optimally honed by natural selection some traits are neutral
One cannot assume that that natural selection shaped a structure or behavior to perform the particular
function now observed
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The Tautological Problem of Adaptation
Any trait can be interpreted in ecology as an adaptation.
Leads to circular reasoning- everything adaptive has evolved, everything evolved is adaptive.
Involves necessity the mother of invention.
However, a trait’s observed adaptive function may not explain its evolution.
Some adaptive traits too complex (flight in birds)
Traits may change adaptive function
Not all evolved traits are adaptive
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An Example of an Adaptive Tautology
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Non-tautological Adaptation
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Preadaptation
Cannot assume that every trait has been optimally honed by natural selection some traits are neutral
One cannot assume that that natural selection shaped a structure or behavior to perform the particular
function now observed
Exaptation or preadaptation- any characteristic of a population that suits it for a niche that it does not yet
occupy and hence facilitates the populations evolving to occupy that niche
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Principles of Natural Selection
Individuals w/n a species differ in morphology, physiology, and behavior (variation).
Some variation is heritable- offspring tend to resemble parents more than other individuals in the
population.
Organisms produce far more offspring than can survive to reproduce. Therefore there is competition
between individuals for resources
As a result of competition some variants will leave more offspring than others. These will inherit the traits
of their parents leasing to their successful reproduction.
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Post-Synthesis Evolutionary Theories in Biology
Ecosystems Theory
Group selectionist explanations for altruistic traits
Sociobiology
Individual or gene level explanations for altruistic traits
Macroevolution
Punctuated equilibria
Preadaptations- Neutral Traits
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Evolutionary Ecology in Biology
Lecture 2
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Evolutionary Ecology
aka Behavioral Ecology
The study of evolution and adaptive design in ecological context
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Evolutionary Ecology deals with Ultimate Causation
Why has animal behavior evolved?
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Tinbergen’s Four Why Questions
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Why do Birds Fly South for the Winter?
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Why do birds fly south for the winter? Because
Foraging in warmer temperatures allows more offspring to survive (ultimate causation).
Because decreasing day length and lower temperatures triggers a hormonal response (proximate causation).
Because they leaned to do so from their parents (ontological causation).
Because Pleistocene climates forced their ancestors to breed and forage in climatically different areas
(phylogenic causation).
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Tinbergen’s Four Questions
Ultimate Causation - Why is a trait maintained in the population (what is its advantage to the fitness of
individuals)?
Proximate Causation - How does a trait work?
Ontological causation - How is a trait passed on?
Phylogenic causation- Why did a trait evolve in a population?
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Evolutionary Ecology
The study of evolution and adaptive design in ecological context.
When features under examination include behavior the field is termed “behavioral ecology”
Characteristics
Reliance on Natural selection theory
Hypothetical deductive method based on models
Focus on individuals as unit of selection
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Basic Tenets
Natural Selection requires three conditions to operate
There must be phenotypic variation between individuals (includes behavior)
Some of this variation must be inheritable
Variants must differ in their ability to survive to reproduce.
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Note that the link between phenotype and genetics does not need to be established to assume that Natural
Selection operates as long as three conditions are met!
Darwin never knew about genes!
Assume that selection will favor advantageous traits irrespective of particulars of inheritance.
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The Phenotypic Gambit of Behavioral Ecology
(also called Charnov’s bet with God)
Analyze traits as if the very simplest genetic system controlled it.
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Behavior is an Aspect of Phenotype
Behaviors are responses of organism to stimuli in environment.
Some aspects instinctive, some learned.
Both instinctive and learned behavior have a genetic basis.
If behavioral mechanisms evolved by natural selection, then behaviors must function in ways that improve
reproductive success.
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Fitness
In evolutionary biology: any characteristic of an organism that improves its chances of leaving descendents,
based on the number of offspring potentially left to the succeeding generation as a result of the trait
Fitness refer to reproductive success
Note this is different from the concept of adaptation
I.e., Peacocks tail
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Main Principles of Behavioral Ecology
Natural Selection favors individuals who behave in ways that maximize their genetic contribution to future
generations.
Which behaviors are selected depends on ecological conditions.
Selection will design animals to behave in efficient ways.
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Why do male lions kill other lions clubs while females cooperate in raising young?
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Note: evolutionary ecology rarely uses group selection explanations
Ultimate causation explained in terms of individual benefit.
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Evolutionary Ecology Developed in response to Wynne Edwards
Behaviors among social animals are selected if they encourage reproductive self-restraint, because they
keep populations below carrying capacity (i.e., “good for the species)
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Level of Selection
Individual Selection- form described by Darwin, selects between individuals who differ in heritable
characteristics and favors traits that leave largest number of offspring
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Level of Selection
Group Selection- acts on heritable variation between groups of individuals and favors characteristics that
result in increased survival of groups.
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Tragedy of the Commons
Economic example showing the problems of methodological collectivism, when individuals act in own
self-interest.
Pastoralists share common grazing lands.
Group benefit achieved when herd size and grazing times limited to sustain grazing.
However, individuals will benefit when they cheat rules.
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In developing a hypothesis to explain some behavioral trait, it will always be more profitable to use an
approach based on individual selection rather than a species-benefiting approach.
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Tragedy of the Commons
Economic example showing the problems of methodological collectivism, when individuals act in own
self-interest.
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How can altruism evolve from an individual perspective?
Parasitism (you looser)
Kin Selection (Keep it in the family))
Reciprocal Altruism (Scratch my back and I will scratch yours)
Tolerated Theft (go ahead and take it, I don’t care)
Handicap Principle (you show-off)
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Parasitism
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W.D. Hamilton- Kin Selection
Kin Selection- selection for shared components of the genotype in individual related by common descent.
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W.D. Hamilton- Kin Selection
Altruism will be favored by selection when
C < rB, where
R = the coefficient of relationship(or probability that two related individuals share same gene).
B= benefit received from the altruist.
C= Cost incurred by the altruist
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W.D. Hamilton- Kin Selection
Simply stated: kin selection will be favored when the cost to the altruist is less than the benefit to his kin
time the degree of relatedness
“I would give my life for three brothers or nine cousins” - J.B.S. Haldene
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Inclusive Fitness
Inclusive Fitness- the sum of an individuals reproductive success and their influence on the reproductive
success of related individuals (other than direct descendants) weighted by the degree of relatedness.
Leads to idea that organisms are designed by natural selection to contribute to the replication of their genes.
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Social Insects, the exception that proves the rule about group selection
Social insects (bees, ants, wasps) live in large societies.
Marked by cooperation in defense, foraging, care of young.
Labor conducted by sterile workers while a single queen lays all eggs.
How does this case of reproductive altruism arise?
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Kin Selection In Social Insects
Eusociality associated with haplodiploid reproductive systems (eleven cases in order Hymenoptera, only
one in normal diploid insects).
Females are diploid, males are haploid.
Females are more closely related to sisters (r=.75) than daughters (r=.5).
Allows extreme kin altruism to be selected for.
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Tolerated Theft
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Zahavi’s Handicap Principle- Showing Off
Usually associated with sexual selection
Females choose traits that are honest indicators of male fitness (peacock’s tail)
The cost of the signal ensures that it is honest (too expensive to fake)
The signal may either take the form of “wasteful” displays or an altruistic act
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Reciprocal Altruism
Scratch my back and I will scratch yours
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Reciprocal Altruism
“ I will gladly pay you Tuesday for a hamburger today”
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Prisoners' Dilemma
Two prisoners separated and given reward to fink (defect) on partner
If ego defects on partner and partner cooperates, ego goes free (0 years) and partner gets heavy sentence
(10 years )
If both prisoners keep quiet (cooperate) then both get light sentences (1 year apiece)
If both prisoners defect, both get light sentence (5 years )
If partner defects and ego cooperates, partner goes free (0 years) and ego gets heavy sentence (10 years)
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Payoff Matrix
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Implications of PD
As long as payoffs are set this way, ESS is always to defect (i.e., altruism could never invade).
However, if game is played multiple times an altruist could invade.
Cooperate first and then follow what everyone else does.
Tit for Tat Reciprocity
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MAD
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Why Should Anthropologists Care About the Evolution of Altruism?
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Methods of Evolutionary Ecology
Test hypotheses against observations
Comparative Approach- observes and compares different populations or individuals to see if differences in
behavior correlate with predicted ecological differences.
Experimental - controlled experiments to determine if the behavior of individuals varies in different
ecological conditions in the manner predicted.
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Methods of Evolutionary Ecology
Develop hypotheses (predictions) from models about what traits should natural selection favor
2 kinds of models
Optimality
Game theory
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Optimality Models
Useful for predicting responses to external environment
Optimization- assumption that natural selection favors behaviors that maximize reproductive fitnessIf constraints are correctly identified assumes that natural selection will move behaviors toward optimal
solution
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Optimality Models
Focuses
on behavior of individual making decisions
about available set of behavioral options
using some currency (calories, offspring) that permits the cost/benefits of each option to be evaluated
within a set of constraints that determines options and benefits
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Diet Breadth Model
aka Prey Choice Model
Ranks resources by ratio of calories to handling time.
Adds Search time (abundance) as the variable.
Predicts whether a forager will take or ignore and encountered resource.
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Diet Breadth Model
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Diet Breadth Model
Makes Three General Predictions
Foragers always take high ranked resources when encountered.
Whether low ranked resource taken depends on the abundance of higher ranked resources.
Changing abundance of high ranked resources can cause diet breadth to expand or contract
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Bluegill Sunfish Prey Choice
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Game Theory
Useful in predicting responses to social environment
More than one strategy may simultaneously be selected for if frequency dependent
Involve Evolutionary Stable Strategies
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Hawk -Dove Game
Two individuals compete over resource (100 fitness points)
Hawks fight aggressively
Always win against doves (100 points)
Lose half time against hawks (-100 points)
Doves display and retreat
Always lose against hawks (0 points)
Win half time against doves (50 points)
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Payoff Matrix
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ESS
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Evolutionary Stable Strategy (ESS)
A strategy that when common in a population cannot be displaced via natural selection by specified
alternative strategies
ESS is not determined by group good
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Why is Sex Maintained?
Costs
Meiosis
Recombination
Mating
Reduced Genetic
Contribution per offspring
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Why are there two sexes instead of one or three?
Basic difference between males and females concern strategy for producing gametes
Females produce few gametes and invest highly in each (competition for resources)
Males produce many gametes with little investment in each (competition for mates)
An intermediate strategy would not be able to produce sufficiently well provisioned gametes to compete
with females or sufficiently numerous gametes to compete with males.
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Why are sex ratios @50:50 in most species?
Since males can produce enough sperm to fertilize many females, a higher ratio of females to males would
make more sense from a group perspective.
Answer is that in a species rich in one sex, parents who produce the rare sex are more fit.
A 50:50 ratio of males and females is an ESS in most species.
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Differences in Male and Female Reproductive Behavior Have Large-scale Implications for Mating and
Parental Behavior
Females have greater investment per offspring than males.
Females usually have certainty of parentage, males often do not.
Females that invest in parental care more likely to be selected for than females who do not (competition for
resources).
Males that invest in mating opportunities more likely to be selected for than females who do not
(competition for mates).
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Implications of Sexual Reproduction from a Evolutionary Ecology Perspective.
Sex has benefits that outweigh costs from a gene perspective
Two sexes at a 50:50 ratio is an ESS (disruptive selection)
Reproductive strategies involve high investment in a limited # of gametes vs. high investment in large
number of gametes
Females compete for resources and choose mates- Males compete for mates
ALWAYS EXCEPTIONS
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Male Competition and Female Choice Interact with Ecology
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When Females Congregate in Groups
Males Compete with Males
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When Females Compete with each other for Resources
Males control territory
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Polygyny Threshold Model
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When Males Cannot Restrict Access to Female Groups or Control Territories
Female Choice= Sexual Selection
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Characteristics of Darwinian Evolution
Selective, not transformational (i.e., Lamarckism)
Individual is unit of selection, not species or group
Driven by fitness and adaptation in the social and natural environment
Non-Directional, i.e., no inherent direction or goal to process (non-teleological)
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Non-Teleological View of Evolution
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Teleological (Goal or Purpose Driven) Evolution