Evolution

• Definition?– Most broadly: “change” (Latin evolvere “to unfold or

unroll”– Still broad: “descent with modification”– Biological evolution: “change in the properties of

populations of organisms, or groups of such populations, over the course of generations” (Futuyma, p.4)

• Fact & Theory…

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 dicovering 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

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)

Darwin & natural selection

• Darwin vs. Lamarck:

Darwin & natural selection• Through time…

– (only figure in The Origin of Species):

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

Variation

• Sources of phenotypic variation?– Genotype– Environment– Epigenetics (maternal effects)

• How separate these causes?– Control for environment with “common garden” or

randomize potential effects– For microbes – grow in same environment, and move

the alleles…

Hardy-Weinberg (in one slide)• Concerns alleles present in diploids• Concept: The proportion of alleles (p,q) can

predict the proportion of genotypes (A1A1 = D, A1A2 = H, A2A2 = R), and vise versa

Blood types in Sicilian village

Hardy-Weinberg (in one slide)• Concerns alleles present in diploids• Concept: The proportion of alleles (p,q) can

predict the proportion of genotypes (A1A1 = D, A1A2 = H, A2A2 = R), and vise versa

• Critical assumptions:– Mating is random– Population infinitely large (no drift)– No gene flow/migration– No mutations– No fitness differences

• Utility as a null model…

Genetic variation in natural populations

• Is there a “wild-type”?

• Indirect measures:– Phenotype (if cause known)– Crosses to extract

chromosomes

• Direct measures– Polymorphic proteins

(Lewontin & Hubby, 1966)

– Restriction fragment length polymorphism (RFLP)

– Sequence (Kreitman, 1983)

protein electrophoresis

sequence data

Mutation types• Point mutation (transition, transversion)

• Insertion/deletion (sometimes frameshift)

• Recombination with homologous sequence

• Transposable elements (Insertions, duplications, deletions, inversions)

Mutation rates

• How estimate?– Classically: phenotypic

changes…– Accumulated changes

between lineages (but need a ‘clock’)

Mutation rates

• Do such low rates matter? – For our Methylobacterium populations:

(~0.0035 mutations replication-1 genome-1) x (2 x 108 genome replications ml-1 transfer-1) x 10 ml

= 7 x 106 mutations transfer-1