Evolution and natural selection
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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


    Summary of the history of evolutionary thought

    Summary of the history of evolutionary thought


    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)


    Heritability of simple traits

    Heritability of simple traits


    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


    Aside darwin s nemesis was genetics

    Aside: Darwin’s nemesis was genetics!


    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.


    Darwin never read mendel s paper

    Darwin never read Mendel’s Paper


    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


    Natural selection can occur rapidly

    Natural selection can occur rapidly


    Rock plants

    Rock plants


    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 reviewed

    Natural Selection reviewed


    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


    Recombination creates variation in offspring

    Recombination creates variation in offspring


    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


    Selection pressures may conflict

    Selection pressures may conflict


    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


    Chance events influence evolution

    Chance events influence evolution


    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


    Evolution and natural selection

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

    -Theodosius Dobzhansky (1900-1975)


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