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An example story…. (Nature 394:69-72). Questions: What is the role of selection in diversification? Can spatial heterogeneity promote the emergence of multiple, coexisting ecotypes?. Experimental design. Microbe: Pseudomonas fluorescens SBW25

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An example story

An example story…

(Nature 394:69-72)

  • Questions:

    • What is the role of selection in diversification?

    • Can spatial heterogeneity promote the emergence of multiple, coexisting ecotypes?


Experimental design

Experimental design

  • Microbe: Pseudomonas fluorescens SBW25

  • Inoculated ancestral strain into replicate flasks that were either shaken (homogeneous) or static (heterogeneous)

  • Flasks destructively sampled by mixing and plating dilutions onto solid medium


Diversity observed in still medium after 7 days

Diversity observed in still medium after 7 days

  • Repeatedly saw multiple colony types:

    • ‘Smooth’ (SM)

    • ‘Wrinkly-spreader’ (WS)

    • ‘Fuzzy-spreader’ (FS)

  • Alone, each occupies a different niche ‘preference’


Diversity cell counts over time in each environment

Diversity & cell counts over time in each environment

  • Diversity only arises in still environment

  • Complex dynamics seen repeatedly in still medium

  • Strong diversifying selection

Still

Shaken


Maintenance of high diversity requires continued heterogeneity

Maintenance of high diversity requires continued heterogeneity

  • Diversity declined, but did not completely disappear

still

still

still

shaken

still

shaken


Fitness of each is frequency dependent

Fitness of each is ‘frequency-dependent’

  • Rate of invasion when rare (10-2)

    • Used pantothenate auxotroph (DpanB)

    • Competition assay

  • All but one pair have a fitness advantage when rare

  • Tradeoffs underlie coexistence – competition caused radiation


Caveat

Caveat…

  • Biggest issue: What is the true relationship between colony phenotypes and genetic (or other phenotypic) diversity?

  • Could there be at least some ‘cryptic’ diversity within shaken flasks?


And a world was created in 7 days

And a world was created in 7 days…

  • Things to be thinking about looking forward

    • How much is fitness context dependent?

    • How rapid can adaptation occur?

    • Role of population size & mutation rate?

    • How repeatable is evolution?

    • Are there tradeoffs in adapting to each niche? If so, is this specialization irreversible?

    • Genetic basis?

    • Quantitative predictions based on physiological models?

    • Phenotypic diversity/epigenetics?

    • Are ecotypes from adaptive radiations stable? Equal to species?

    • Co-evolve specificity?

    • Are the WS cooperating? Are there cheaters?


Evolution before darwin

Evolution before Darwin

  • Aristotle: Scala Naturae

    • “Great chain of being”

    • Ordered gradation from inanimate - barely animate - plants/invertebrates - higher forms

  • Linnaeus: Systema Naturae (1735)

    • “Undertaken in hope of discovering the pattern of God’s creation” (Futuyma, p.17)

  • Species are constant though time, and no extinction (imperfection)


Evolution before darwin1

Evolution before Darwin

  • Lamarck: Philosophie Zoologique (1809)

    • Species change through time (no extinction)

    • Inheritance of acquired characteristics

  • But also drew evolution as a tree…

    • Extant organisms as ancestors?


Darwin wallace

Darwin & Wallace

  • Darwin spent 5 years on HMS Beagle (1831-1836)

  • On September 28th, 1838 (29 years old), made connection between Malthus and organisms

  • In June 1858 received manuscript from Wallace “On the Tendency of Varieties to Depart Indefinitely form the Original Type”

  • Rushed to present his work at the same time, then wrote a 490 page abstract… (1859)


Darwin natural selection

Darwin & natural selection

  • Two major tenets:

    • “[A]ll species, living and extinct have descended without interruption from one (or a few) original forms of life”(Futuyma, p.21)

    • “[I]f variations useful to any organic being ever occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance, these will tend to produce offspring similarly characterised. This principle of preservation, or the survival of the fittest, I have called natural selection.”(Darwin, 1859)


After darwin

After Darwin

  • Big problem: no mechanism for generation of heritable variation

  • Solution: Mendel’s peas (1863, but not discovered until 1900)

  • In the 30s and 40s, the genetics was fully brought into the fold in the “Modern Synthesis”

    • Fisher, Haldane, Wright, Mayr, Huxley, Stebbins, Simpson, Dobzhansky…


Modern synthesis

Modern synthesis

  • Futuyma outlines 20 main points:

    • Phenotype due to genotype and environment

    • Environmental effects not inherited*

    • Heredity due to discrete genes

    • Genes mutate to new alleles

    • Environment does not bias mutations

    • Evolution is a population process (change in genotype or allele frequencies)

    • Changes in frequencies can be due to drift or selection

    • Small selective differences matter

    • Selection can alter populations beyond original variation due to recombination between alleles

    • Natural populations are genetically variable


Modern synthesis1

Modern synthesis

  • Futuyma outlines 20 main points:

    • Geographic differences can have a genetic basis

    • Phenotypic differences often due to sum of many small genetic changes

    • Natural selection continues to act today

    • Geographic differences are often adaptive

    • Phenotypic differentiation can occur within a species; species represent distinct gene pools

    • Degree of differentiation is a continuum

    • Speciation is often linked with geographic isolation*

    • Differences between higher taxa sum of small differences

    • Gaps in fossil record due to incompleteness, but also show intermediate forms

    • Paleontology is concordant with evolutionary theory


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