Understanding Macroevolution: Insights from the Cambrian Explosion and Molecular Clocks

1. Ch. 17 Macroevolution

2. Prior to the Cambrian 543 Ma • Ctemophora • Cnidaria • Ctenophora • After the Cambrian • Suddenly everything else

3. molecular clocks • calibrate – (e.g., hemoglobin) measure rate of evolution among vertebrates with known fossil ages • compare sequences of vertebrate and invertebrates to infer divergence time • Independent studies • Runnegar ’82 • Levinton and Shapiro ’96

4. What was the Cambrian explosion? • Independent molecular clocks date divergences (Ba) • 1.2 proto/deuterostomes • 1 • 500 million year discrepancy between fossil record and independent molecular clocks

5. Cambrian ecological explosion? • Due to: • Rising [O2] – larger, more energetic organisms • Mass extinction – opened niches • Neoproterozoic fauna were poor fozzilizers

6. Rates of evolution • Darwin predicted gradual change • The actual pattern in the fossil record often looks like (a) • morphological change is associated with speciation • circularity: morphology defines species in fossil record

7. Avoiding circularity: ancestral and derived species co-occur (e.g. bryozoans) • O and P co-occur over time therefore speciation can be inferred • R and S don’t co-occur over time: is S a new species or did R rapidly evolve into S (resolution 150k years) • Conclusions: • stasis very high • change often associated with bifurcation (speciation) • speciation or anagenesis?

8. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Punctuated equilibrium (Eldredge and Gould ’72): • epistatic genetic relationships prevent substantial evolution • coadaptation disrupted by founder event (genetic revolution)

9. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Punctuated equilibrium • epistatic genetic relationships prevent substantial evolution • coadaptation disrupted by founder event (genetic revolution) • Implications • evolution occurs by species selection not by genotype selection within populations

10. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Punctuated equilibrium • epistatic genetic relationships prevent substantial evolution • coadaptation disrupted by founder event (genetic revolution) • Implications • evolution occurs by species selection not by genotype selection within populations • Why this is frass • Tremendous number of examples of populations evolving substantially without speciating • Stasis is overstated: fluctuation occurs about a mean, implying stabilizing selection

11. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Punctuated equilibrium • epistatic genetic relationships prevent substantial evolution • coadaptation disrupted by founder event (genetic revolution) • Implications • evolution occurs by species selection not by genotype selection within populations • Why this is frass • Tremendous number of examples of populations evolving substantially without speciation • Stasis is overstated: fluctuation occurs about a mean, implying stabilizing selection • habitat selection in animals can contribute to stasis • Resolution of fossil record is rarely < 100,000 years; substantial opportunity for transitions • Individual selection should be much faster than species selection b/c of greater opportunity • Genetic mechanisms undocumented

12. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Speciation preserves change (Futuyma, 1987) • 1. different populations are regularly diverging to local conditions time pop A pop B habitat A habitat B morphological trait value

13. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Speciation preserves change (Futuyma, 1987) • 1. different populations are regularly diverging to local conditions time pop A pop B habitat A habitat B morphological trait value

14. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Speciation preserves change (Futuyma, 1987) • different populations are regularly diverging to local conditions • these populations are usually brought back together before reproductive isolation • recombination (interbreeding) erases divergence pop A pop B time pop A pop B habitat A habitat B recombination morphological trait value

15. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Speciation preserves change (Futuyma, 1987) • different populations are regularly diverging to local conditions • these populations are usually brought back together before reproductive isolation • recombination (interbreeding) erases divergence pop A pop B time pop A pop B habitat A habitat B recombination morphological trait value

16. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Speciation preserves change (Futuyma, 1987) • 1. different populations are regularly diverging to local conditions • 2. these populations are usually brought back together before reproductive isolation • 3. recombination (interbreeding) erases divergence • 4. rarely, reproductive isolation evolves before (2) in which case morphological changes are not erased by recombination and possibly preserved in the fossil record reproductive isolation time pop A pop B habitat A habitat B No recombination morphological trait value

17. Conclusions: stasis very high; change often associated with bifurcation (speciation) • Why? • Speciation preserves change (Futuyma, 1987) • different populations are regularly diverging to local conditions • these populations are usually brought back together before reproductive isolation • recombination (interbreeding) erases divergence • rarely, reproductive isolation evolves before (2) in which case morphological changes are not erased by recombination and possibly preserved in the fossil record • Consistent with fossil record, extant observations, and population genetic theory