EVOLUTIONARY ECOLOGY SOME USEFUL DEFINITIONS Lecture #2

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EVOLUTIONARY ECOLOGY EVOLUTIONARY ECOLOGY SOME USEFUL DEFINITIONS SOME USEFUL DEFINITIONS Lecture #2 Lecture #2

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EVOLUTIONARY ECOLOGY SOME USEFUL DEFINITIONS Lecture #2. 1. “Nothing in biology makes sense except in the light of evolution” Theodosius Dobzhansky 1973 2. The book is one written by G.E. Hutchinson 1965 “ The ecological theater and the evolutionary play”. - PowerPoint PPT Presentation

Transcript of EVOLUTIONARY ECOLOGY SOME USEFUL DEFINITIONS Lecture #2

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EVOLUTIONARY ECOLOGYEVOLUTIONARY ECOLOGY

SOME USEFUL DEFINITIONSSOME USEFUL DEFINITIONS

Lecture #2Lecture #2

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1. “Nothing in biology makes sense except in the light of evolution” Theodosius Dobzhansky 1973

2. The book is one written by G.E. Hutchinson 1965 “The ecological theater and the evolutionary play”

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The quote is self-explanatory The quote is self-explanatory and the intent of the books and the intent of the books title is to state that the process title is to state that the process of evolution takes place in an of evolution takes place in an ecological contextecological context

- evolutionary events occur because of the relationships between an organismic unit and their environment

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To develop the tie between To develop the tie between ecology and evolution we will ecology and evolution we will begin with a discussion of begin with a discussion of Natural SelectionNatural Selection

1. The theory of natural selection is a truly fundamental unifying theory of life

2. A thorough appreciation of it is is essential background for understanding evolutionary ecology

3. Natural Selection comes as close to being a “fact” as anything in biology

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Many think that Natural Many think that Natural Selection is synonomous Selection is synonomous with Evolution; however, with Evolution; however, it’s not!it’s not!- Evolution refers to temporal

changes, whereas natural selection specifies one particular way in which these changes are brought about

- Evolution more specifically refers to a change in gene frequencies or descent with modification

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Darwins’ Definition of Darwins’ Definition of Natural SelectionNatural Selection1. In any population, more offspring

tend to be produced than can survive to reproductive age.

2. Members of the population vary in form and behavior. Much of the variation is heritable.

3. Some varieties of heritable traits are more adaptive than others

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Darwins’ definition Darwins’ definition 4. Because bearers of adaptive traits have a

greater chance of reproducing, their offspring tend to make up an increasingly greater proportion of the reproductive base for each new generation. This tendency is called differential reproduction.

5. “Differential reproduction” is natural selection. Adaptive forms of traits show up (are selected for) with increased frequency in a population, because their bearers contribute proportionately more offspring to succeeding generations

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Endler’s (1986) definition Endler’s (1986) definition of Natural Selectionof Natural Selection

a. variation in some attribute or traitb. a consistent relationship between that trait

and mating ability, fertilizing ability, fecundity and/or survivorship: fitness differences

c. a consistent relationship, for that trait, between parents and their offspring, which is at least partially independent of common environmental effects: i.e., there is a heritable component

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Then:Then:

1. There will be a within-generation effect; individuals of a given age will differ predictably from the individuals which do not survive to that age

2. There will also be a between-generation effect; the offspring generation will differ predictably from their parental generation

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The modern statement of The modern statement of the theory of natural the theory of natural selection is in terms of selection is in terms of genes; i.e., what changes genes; i.e., what changes during evolution is the during evolution is the relative frequency of relative frequency of genesgenes

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Modern restatement in Modern restatement in genetic termsgenetic terms

1. All organisms have genes which code for protein synthesis. These proteins regulate the development of the nervous system, muscles and structure of the individual.

2. Within a population many genes are present in two or more alternative forms, or alleles, which code for slightly different forms of the same protein. These will cause differences in development and so there will be variation within a population.

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Modern restatementModern restatement

3. Any allele that can make more surviving copies of itself than its alternative will eventually replace the alternative form in the population.

Therefore, natural selection is the differential survival of alternative alleles

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Modern restatementModern restatement

- genes will be selected through a complex interaction with their environment, including their genetic environment (other genes) and ecological environment (climate, competitors, herbivores, etc.)

- the individual can be regarded as a temporary vehicle by which genes survive and are replicated

- the most successful genes will be those which promote an individuals survival and reproductive success

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SelectionSelection

The three most common include– Stabilizing– Directional– Disruptive

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Stabilizing Selection - Stabilizing Selection - reducing the variance of reducing the variance of extremesextremes

Fitness

FreqBefore

FreqAfter

Phenotype

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Disruptive Selection - can Disruptive Selection - can increase the variance in a increase the variance in a traittrait

Fitness

FreqBefore

FreqAfter

Phenotype

or

Phenotype

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Directional Selection - Directional Selection - affects the mean value of affects the mean value of the trait; the variance may the trait; the variance may also changealso change

Fitness

FreqBefore

FreqAfter

Phenotype

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Other types of selection Other types of selection encountered:encountered:

- Frequency-dependent- Density-dependent and density-independent- Hard and soft- Artificial- Sexual- Mortality- Phenotypic- Correlational

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Frequency Dependent Frequency Dependent SelectionSelection- occurs when the fitness of a

phenotype depends upon the frequency of that phenotype in the population. Selection of this sort could favor either rare or common phenotypes

- leads to oscillations in the mean value of a trait

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Density-dependent & Density-dependent & Density-independent Density-independent SelectionSelection- density-dependent selection, selection

may only emerge when competition between individuals becomes pronounced, i.e., when density is high

- by contrast, the effects of extreme weather may be density-independent, so the survival or death of an organism occurs regardless of density

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Hard & Soft SelectionHard & Soft Selection

Parallels density-dependent & density-independent

- Soft selection occurs when individuals are in competition - only a certain proportion will be able to survive

- Hard selection - the survival or death of one individual is not conditional on the suvival or death of another individual

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Artificial SelectionArtificial Selection

Selective breeding; fitness differences imposed by humans - goal oriented

By contrast, natural selection is blind and operates without a goal; humans can certainly impose natural selection

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Sexual SelectionSexual Selection

Traits that seem more a hinderance than a help

Selection that assisted individuals in gaining mates– competition for mates– preference for particular traits in the

other sex

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Mortality SelectionMortality Selection

Consistent phenotypic-specific mortality

A single component of fitness variation and therefore an incomplete definition

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Phenotypic SelectionPhenotypic Selection

Phenotypic selection requires conditions a and b of Endler’s definition - trait variation & fitness differences

Does not include genetic change

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Correlational SelectionCorrelational Selection

- certain combinations of traits or alleles are favored

- this will result in patterns of gametic phase (“linkage”) disequilibrium for suites of traits

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Example of Correlational Example of Correlational SelectionSelection

Brodie, E.D. 1989. Genetic corelations between

morphology and antipredator behavior in

natural populations of the garter snake Thamnophis

ordinoides. Nature.

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ExamplesExamples

Gibbs and Grant. 1987. Oscillating selection on Darwin’s finches. Nature

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Directional Selection - Directional Selection - Darwin’s FinchesDarwin’s Finches

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Hori, M. 1993. Frequency-dependent Hori, M. 1993. Frequency-dependent natural selection in the handedness of natural selection in the handedness of scale-eating cichlid fish. Sciencescale-eating cichlid fish. Science

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Frequency-Dependent Frequency-Dependent Selection in Cichlid FishSelection in Cichlid Fish

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Disruptive Selection in Disruptive Selection in SalmonSalmon

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Fitness (Dawkins 1982)Fitness (Dawkins 1982)

1. Historical2. Genotypic3. Individual (Classic)4. Inclusive5. Personal

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Historical FitnessHistorical Fitness

Definition used by Wallace, Spencer and Darwin

- The fittest individuals would be those with the keenest eyes, the strongest muscles, the sharpest ears, the swiftest reflexes - a definition close to everyday usage, physical fitness

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Genotypic FitnessGenotypic Fitness

Used by population geneticists- May be regarded as a measure of

the number of offspring that a typical individual of genotype Aa is expected to bring up to reproductive age

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Individual Fitness (Classic Individual Fitness (Classic Fitness)Fitness)

Individual reproductive success- Fitness is a property of an

individual organism, often expressed as the product of survival and fecundity

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Inclusive FitnessInclusive Fitness

Calculated from an individuals own reproductive success plus his effects on the reproductive success of his relatives, each one weighed by the appropriate coefficient of relatedness

- IF = Wind + (Wrel ) X (r)i

Inclusive Fitness = Direct Component + Indirect Component

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Personal FitnessPersonal Fitness

Backwards way of looking at inclusive fitness

- Personal fitness focuses on the effects that the individual’s relatives have on his or her fitness

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Natural Selection as a Natural Selection as a TautologyTautologyNatural selection is defined as

“survival of the fittest” therefore the fittest survive is circular or an inherently true statement

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There are at least two There are at least two reasons why this is reasons why this is misleadingmisleading1. Fitness can be directly related to biological

properties and the design of organisms.2. It is heritable variation in the biological

properties of organisms which gives rise to fitness differences and the process of natural selection - From heritable variation in some attribute or trait and a correlation between that trait and fitness we can deduce the effects - evolutionary change

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Therefore, natural selection Therefore, natural selection is a syllogism (2 premises is a syllogism (2 premises leading to a conclusion) leading to a conclusion) rather than a tautology (or rather than a tautology (or circular reasoning)circular reasoning)

New insights can be gained New insights can be gained from an understanding of from an understanding of the processthe process

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How did we get into this How did we get into this tautology mess?tautology mess?

Herbert Spencer & Alfred Russel Wallace were

responsible for the phrase“Survival of the fittest”