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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. Microbe: Pseudomonas fluorescens SBW25

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An example story…

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  1. 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?

  2. 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

  3. 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’

  4. 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

  5. Maintenance of high diversity requires continued heterogeneity • Diversity declined, but did not completely disappear still still still shaken still shaken

  6. 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

  7. 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?

  8. 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?

  9. 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)

  10. 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?

  11. 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)

  12. 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)

  13. 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…

  14. 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

  15. 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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