An example story…

• Questions: – What is the role of selection in diversification?– Can spatial heterogeneity promote the emergence of

multiple, coexisting ecotypes?

(Nature 394:69-72)

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

• 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 only arises in still environment

• Complex dynamics seen repeatedly in still medium

• Strong diversifying selection

Still Shaken

Maintenance of high diversity requires continued heterogeneity

• Diversity declined, but did not completely disappear

shaken still

shaken

still

still still

Fitness of each is ‘frequency-dependent’

• Rate of invasion when rare (10-2)– Used pantothenate auxotroph

(panB)– Competition assay

• All but one pair have a fitness advantage when rare

• Tradeoffs underlie coexistence – competition caused radiation

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…

• 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

• 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 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 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

• 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• 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…

• Futuyma outlines 20 main points:

1. Phenotype due to genotype and environment2. Environmental effects not inherited*3. Heredity due to discrete genes4. Genes mutate to new alleles5. Environment does not bias mutations6. Evolution is a population process (change in genotype or

allele frequencies)7. Changes in frequencies can be due to drift or selection8. Small selective differences matter9. Selection can alter populations beyond original variation due

to recombination between alleles10. Natural populations are genetically variable

Modern synthesis

Modern synthesis• Futuyma outlines 20 main points:

11. Geographic differences can have a genetic basis12. Phenotypic differences often due to sum of many small

genetic changes13. Natural selection continues to act today14. Geographic differences are often adaptive15. Phenotypic differentiation can occur within a species; species

represent distinct gene pools16. Degree of differentiation is a continuum17. Speciation is often linked with geographic isolation*18. Differences between higher taxa sum of small differences19. Gaps in fossil record due to incompleteness, but also show

intermediate forms20. Paleontology is concordant with evolutionary theory