Paleobiology and Macroevolution

1. Paleobiology and Macroevolution Chapter 22

2. 22.1 The Fossil Record • Fossils form when organisms are buried by sediments or preserved in oxygen-poor environments • Fossil record provides an incomplete portrait of life in the past

3. 22.1 (cont.) • Scientists assign relative and absolute dates to geological strata and the fossils they contain • Fossils provide abundant information about life in the past

4. Fossilization • Occurs when organismal features are preserved long after death • Tissue to stone; trace fossils; lack of oxygen • Fossil record incomplete • Soft-bodies preserved less than hard • Rare species • Lack of sedimentation

5. Geological Dating • Geological strata arranged in order formed • Oldest deepest • Geological processes may move strata after formation • Fossils in particular strata represent relative age • Radiometric dating from isotope half-life provides absolute age

6. Fossil Information • Only direct record of past • Chronicles evolutionary patterns to infer processes • Provide indirect data • Ecology and behavior from location and structure • Document changes in Earth’s climate

7. 22.2 Earth History, Biogeography, and Convergent Evolution • Geological processes have often changed Earth’s physical environment • Historical biogeography explains the broad geographical distributions of organisms • Convergent evolution produces similar adaptations in distantly related organisms

8. Geological Processes • Plate tectonics • Theory that Earth’s crustal plates float on semisolid mantle • Continental drift occurs with moving plates • Earth climate highly variable over long time periods • Continental drift changes climate • Volcanic eruptions • Asteroid impacts

9. Historical Biogeography • Continuous distribution • Species occur in habitats throughout geographical areas • Disjunct distribution • Species occur in widely separated locations • Dispersal creates new populations far from original • Vicariance fragments populations by external factors

10. Biogeographical Realms • Biota • All organisms living in a region • Biogeographical realms • Defined by biota • Nearctic, Neotropical, Ethiopian, Palearctic, Oriental, Australian • Endemic species occur nowhere else on earth • Endemism increases with geographic isolation

11. Convergent Evolution • Similar adaptations in distantly related organisms • Similar selective pressures produce similar adaptations

12. 22.3 Interpreting Evolutionary Lineages • Modern horses are living representatives of a once-diverse lineage • Evolutionary biologists debate the mode and tempo of macroevolution

13. Horse Evolution • Fossils test hypotheses of evolutionary history • Evolutionary lineages inherently disorderly • Originally hypothesized as sequence of stages with linear progression • Fossils from 100 extinct horse species reveal highly branched evolutionary tree • Only one branch survived to modern times, others “pruned” by extinction

14. Modes of Macroevolution • Anagenesis • Accumulation of changes in lineage from adaptation to environment • No increase in species number • Cladogenesis • Evolution of two or more descendant species from common ancestor • Increase in species number

15. Tempo of Evolution (1) • Gradualist hypothesis • Large changes result from slow, continuous accumulation of small changes • Perfect evidence (complete series of transitional fossils) rarely found

16. Tempo of Evolution (2) • Punctuated equilibrium hypothesis • Periods of no change punctuated by rapid change • Mechanism: Isolated populations with different natural selection forces • Transitional forms would be rare

17. 22.4 Macroevolutionary Trends in Morphology • Body size of organisms has generally increased over time • Morphological complexity has also generally increased over time • Several phenomena trigger the evolution of morphological novelties

18. Body Size Evolution • Many lineages show increases in body size • Larger organisms often more successful from regulation to reproduction • Major exception: Insects (external skeleton)

19. Species Selection • Evolutionary success of species that produce many new species • Natural selection favors species with traits that lead to cladogenesis • Not yet well tested

20. Morphological Complexity • Many lineages have increased morphological complexity • Larger organisms have more cells for specialization • Many exceptions to increasing complexity • Snakes, horses

21. Morphological Novelties (1) • Preadaptations • Traits advantageous under new selective environment (always serendipitous) • Traits never evolve in anticipation of future needs • Each step in evolutionary lineage was advantageous in particular environment

22. Morphological Novelties (2) • Allometric growth • Differential growth of body parts over time • Few genes may separate species due to allometric growth

23. Morphological Novelties (3) • Heterochrony • Changes in timing of developmental events • Causes differential morphology of related species • Paedomorphosis • Reproductive capability with juvenile characteristics • Often simple mechanism to separate species

24. 22.5 Macroevolutionary Trends in Biodiversity • Adaptive radiations are clusters of related species with diverse ecological adaptations • Extinctions are common in the history of life • Biodiversity has increased repeatedly over evolutionary history

25. Adaptive Radiation • Biodiversity • Number of species in given area • Adaptive radiation • Group of closely related species occupying different habitats • Ancestral species move into unfilled adaptive zone (unoccupied or open from extinction)

26. Extinctions • Background extinction rate • Low rate, from environmental change and poor adaptations • Over global time scales, most species go extinct • Mass extinctions • High rate over short time • Climate changes from geological activity and asteroid impacts

27. Biodiversity • Mass extinctions create evolutionary opportunities • Adaptive radiations fill vacated adaptive zones • New adaptations allow new groups of species to replace other groups • Ecological interactions allow biodiversity • Adaptive radiations in one group allow adaptive radiation of ecologically dependent groups

28. 22.6 Evolutionary Developmental Biology • Most animals share the same genetic tool kit that regulates their development • Evolutionary changes in developmental switches may account for much evolutionary change

29. Evo-Devo • Evolutionary developmental biology (evo-devo) • Evolution in genes of embryonic development • Genes of development also regulate morphology • Homeoticgenes • Control transcription of development genes • Small changes in homeotic genes can produce large changes in morphology

30. Homeotic Genes • Many organisms share common genetic tool-kit for development • Common animal genes for 500 million years • Common genes in animals, plants, fungi and prokaryotes from earliest life

31. Hox Genes • Control animal body plan • Homeobox • 180-nucleotide sequence • Codes for homeodomain (part of a transcription factor)

32. Developmental Switches • Regulatory sites of transcription factors switch on or off downstream genes • Small changes in regulation produce large changes in morphology • Natural selection still determines success of regulatory genes