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Evolution and Natural Selection. Chapters 1.4-1.6, Bush. Introduction to Natural Selection. History of Evolutionary Thought Theory of Natural Selection Examples of Natural Selection. Introduction to Natural Selection. History of Evolutionary Thought Theory of Natural Selection

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Evolution and natural selection

Evolution and Natural Selection

Chapters 1.4-1.6, Bush


Introduction to natural selection
Introduction to Natural Selection

  • History of Evolutionary Thought

  • Theory of Natural Selection

  • Examples of Natural Selection


Introduction to natural selection1
Introduction to Natural Selection

  • History of Evolutionary Thought

  • Theory of Natural Selection

  • Examples of natural selection


Paving the way for darwin
Paving the way for Darwin

  • Charles Darwin’s theory relied upon the findings of other scientists

    • Casting doubt on Divine Creation

      • Cuvier, Georges

      • Lyell, Charles

      • Darwin, Erasmus

  • Contributing to the theory itself

    • Lamarck, Jean-Baptiste

    • Malthus, Thomas Robert

    • Wallace, Alfred Russell

      http://goldberg.history.ohio-state.edu/naturalselection/


Georges cuvier 1769 1832
Georges Cuvier (1769-1832)

  • leading palaeontologist of his time

  • found that many species have gone extinct


Charles lyell 1797 1875
Charles Lyell (1797-1875)

  • geologist

  • Earth was way older than the 5000 years or so allowed according to Biblical chronology


Erasmus darwin 1731 1802
Erasmus Darwin (1731-1802)

  • Charles Darwin’s grandfather

  • proponent of the theory that species change over time


Jean baptiste lamarck 1744 1829
Jean-Baptiste Lamarck (1744-1829)

  • Believed scientists like Erasmus Darwin that life forms could change over time

  • Lamarckism: acquired traits can be inherited

    • e.g. a giraffe with a short neck stretches to get at vegetation high up a tree and manages to make its neck longer. This giraffe passes its long neck to its offspring

  • got Darwin thinking about inheritance


  • Thomas malthus 1766 1834
    Thomas Malthus (1766-1834)

    • found that all species have the potential to create far more offspring than there are resources to support


    Alfred russell wallace 1823 1913
    Alfred Russell Wallace (1823-1913)

    • came up with the theory of natural selection independently of Darwin

    • spurred Darwin to publish his own work on the subject


    Charles darwin 1802 1882
    Charles Darwin (1802-1882)

    “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.”

    (The Origin of Species)

    http://www.interaktv.com/Darwin/Darwin.html



    Introduction to natural selection2
    Introduction to Natural Selection

    • History of Evolutionary Thought

    • Theory of Natural Selection

    • Examples of natural selection


    The theory of natural selection
    The “Theory” of Natural Selection

    Natural Selection is a “Theory” in the same way that we consider gravity or Einstein’s relativity to be a theory


    Theory of natural selection
    Theory of Natural Selection

    • Three conditions for Natural Selection:

      1) Variation in traits

      2) Heritability

      3) Survivorship/Competition

    • Natural selection  “Survival of the fittest”


    Variation and heritability
    Variation and Heritability

    • Observations from Lamarck and Erasmus Darwin that offspring are not exactly like parents (change can occur in a single generation)

    • Observed the commonly known facts that:

      • all individuals are not alike (i.e., there are different phenotypes)

      • Offspring inherit the majority of their traits from their parents.


    Variation within a species
    Variation within a species

    • Variation can be:

      • CONTINUOUS: having a multitude of variants (e.g., colour bands in the snail)

      • DISCRETE: limited # of types (such as blood types)


    Heritability in diploids
    Heritability in Diploids

    • Two copies of each gene (diploid)

      • Humans have 23 chromosomes, 2 copies of each, for a total of 46 chromosomes)

    • Each egg or sperm has only one copy of each chromosome


    Passing on genes is like tossing coins
    Passing on genes is like tossing coins

    • Two copies exist for each gene

    • Whether you pass on a certain copy of a gene is an independent event for each child

    • If you have two children, sometimes you will pass on the same copy to both children (leaving the second copy passed on to neither child)



    Competition
    Competition

    • From Malthus: more offspring are produced than there are resources to support

    • Creates a “struggle for existence”

    • Some offspring will be better at surviving and reproducing than others (i.e., have higher fitness)


    Fitness
    Fitness

    FITNESS:

    • the number of offspring an individual produces that survive to reproduce themselves

  • Fitness = 1.0 means that individuals of this phenotype are successfully passing on 100% of their genes, on average


  • How is fitness calculated
    How is fitness calculated

    • Fitness = the number of genes passed on to the next generation

    • Because diploid organisms (i.e., most organisms) only pass on half of their genes to each child, they must have two offspring living to reproductive age to have Fitness = 1

    • Fitness = 1 does not exactly mean that you have passed on 100% of your genes to the next generation (Remember: sometimes you send two copies of the same gene and zero copies of the other)


    Outcome
    Outcome

    • Some phenotypes will be better represented in the next generation than they are in the present generation

    • Could be extended: some entire lineages may be more successful than others as well resulting in some lineages going extinct (as Cuvier had found)


    Natural selection will not take place if
    Natural selection will not take place if:

    • there is no variation

      • E.g., No humans have gills, so we cannot select for them, regardless of how beneficial they might be

  • If the gene is not heritable

    • E.g., Working out and getting a strong heart might make you live longer and have more children but selection can not act upon it if is not a genetic trait

  • If there is no difference in survivorship or reproductive ability between variants

    • E.g., Having attached or free earlobes doesn’t really matter


  • Survival of the fittest
    “Survival of the fittest”

    • This saying is a bit misleading and doesn’t quite capture the essence of what is natural selection

    • You can be as “fit” an individual as can be but it is the ability to reproduce that is the key feature for an increase in representation in the next generation



    Gregor mendel father of genetics
    Gregor Mendel – father of genetics

    • conducted experiments on pea plants

    • discovered that most organisms have two copies of their genes, one from each parent.



    Introduction to natural selection3
    Introduction to Natural Selection

    • History of Evolutionary Thought

    • Theory of Natural Selection

    • Examples of natural selection


    Ground finch geospiza fortis
    Ground Finch (Geospiza fortis)

    • beak size has a lot to do with how well a finch feeds on certain seeds

    • seeds of Tribulus have the toughest seed coat that requires a large beak to break


    Natural selection in finches
    Natural selection in finches

    Drought causes collapse of food supply, survival plummets

    High mortality in smaller individuals, strong selection for large birds that can crack large, tough seeds


    Human induced selection
    Human-induced selection

    • Natural pop’n with variation for insecticide resistance

    • Insecticide appl’n kills all but those with resistance

    • Surviving insects breed new generation of insecticide resistance population




    Summary
    Summary

    • Darwin put together a number of ideas from different disciplines to come up with the Theory of Natural Selection

    • Natural selection states that heritable phenotypes that are well-suited to their environment will have more offspring and so will be better represented in the next generation.

    • Natural selection can operate so quickly that we can observe it in a single generation



    Natural selection continued
    Natural Selection – continued

    • Characteristics of natural selection

    • Types of natural selection

    • Natural selection  Evolution


    Natural selection continued1
    Natural Selection – continued

    • Characteristics of natural selection

    • Types of natural selection

    • Natural selection  Evolution


    Characteristics of natural selection
    Characteristics of Natural Selection

    • Natural Selection:

      • dependent on the variation present in the population

      • Short-sighted – acts only present selection pressures


    Sources of variation
    Sources of variation

    • Gene flow: immigration

    • recombination

    • ultimately, from mutation


    Immigration leads to new variation
    Immigration leads to new variation

    • Immigration provides new genetic material for selection to act upon



    Mutation at the phenotype level
    Mutation at the Phenotype Level

    • Mutations can be:

      • beneficial

      • detrimental

      • neutral


    Mutation at the dna level
    Mutation at the DNA Level

    • A mutation is caused when the chromosomal machinery makes a mistake


    Mutagens
    Mutagens

    • Many things may increase the mutation rate:

      • radiation

      • certain chemicals (e.g. carcinogens)


    Variation is random
    Variation is random

    • When a new recombinant or mutant genotype arises, there is no tendency for it to arise in the direction of improved adaptation

    • Natural selection imposes direction on evolution, using undirected variation


    Natural selection continued2
    Natural Selection – continued

    • Characteristics of natural selection

    • Types of natural selection

    • Natural selection  Evolution


    Types of natural selection
    Types of Natural Selection

    • Three kinds of natural selection:

      • Directional selection

      • Stabilizing selection

      • Disruptive selection


    Directional selection
    Directional Selection

    Larger individuals may have higher fitness

    (i.e., produce more offspring) than smaller

    individuals.


    Directional selection1
    Directional Selection

    Fishing industry produces selection that favours smaller cod and can produce a decrease in average body size.


    Stabilizing selection
    Stabilizing selection

    The average members of the population may

    have higher fitness than the extremes.


    Stabilizing selection1
    Stabilizing Selection

    Babies of intermediate

    birth weight have higher

    survivorship than very

    small and very large babies


    Disruptive selection
    Disruptive selection

    Natural selection could favour both extremes

    over the intermediate types


    Disruptive selection1
    Disruptive Selection

    In the finch, Pyrenestes ostrinus

    both very large and very small bills are beneficial for eating large and small

    seeds, respectively


    Natural selection continued3
    Natural Selection – continued

    • Characteristics of natural selection

    • Types of natural selection

    • Natural selection  Evolution



    Other factors in evolution
    Other factors in evolution

    • If there is no relation between fitness and the character in question, then natural selection is not acting on it

    • Chance events can still make these traits show change over time = RANDOM DRIFT



    Summary1
    Summary

    • Natural Selection acts on whatever variation is present at the time. This variation is generated randomly with respect to selection pressures

    • Selection can be directional, stabilizing or disruptive

    • Random factors can also play a part in evolution


    "nothing inbiology makes sense except in the light of evolution"

    -Theodosius Dobzhansky (1900-1975)


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