Ch. 7 Evolution and the fossil record Evolution (popular definition) = descent with modification Evolution (technical definition) = change in gene frequencies or gene combinations in a series of populations, brought about by natural selection Earth History, Ch. 7 1
Evolution and the fossil record The basic tenets of evolution In natural populations, more offspring are produced than can be sustained in the environment Mutations and genetic recombinations are the sources of variability among individuals Natural selection results in the differential survival of variants Variation is heritable: therefore, the more successful variants preferentially will pass on their genes to following generations, and through time the overall composition of the gene pool will shift Earth History, Ch. 7 2
Extinction Extinction may result from one or more of these causes, if taken to extremes Predation Disease Competition from other species Change in physical environment or ecosystem Rates of extinction vary by type of organism, for example lifespan of mammal species is 1 to 2 million years (extinction rate of 50 100% every million years) lifespan of bivalve species is ~10 million years (extinction rate of 10% every million years) Earth History, Ch. 7 3
Extinction Mass extinctions = intervals during which unusually large numbers of taxa suddenly become extinct (e.g., 40% or more genera) Causes of mass extinctions may be extraterrestrial or a combination of earthly factors Five major mass extinctions in Phanerozoic history Earth History, Ch. 7 4
Earth History, Ch. 7 5
Origination Evolutionary radiation = episode of rapid evolutionary expansion (production of large numbers of new taxa) Radiations usually occur shortly after the origin of a new major taxon Radiations may follow extinctions as ecologic replacements Radiations may follow an adaptive breakthrough The appearance of some key feature that allows ecologic and morphologic diversification Earth History, Ch. 7 6
Origination Example: radiation of hexacorals in Triassic Period Radiation quickly followed the extinction of Paleozoic rugose and tabulate corals (vacant niche) Radiation was facilitated by adaptive breakthrough: the ability to quickly secrete large skeletons using relatively little CaCO 3 Earth History, Ch. 7 7
Triassic hexacorals Earth History, Ch. 7 8
Adaptive radiation of hexacorals in middle Triassic time (in the aftermath of the end-permian mass extinction) Earth History, Ch. 7 9
Extinction & origination Organisms that have high rates of origination may also have high rates of extinction, making them well suited as guide fossils ammonoid cephalopods Earth History, Ch. 7 10
Patterns of Evolution Convergence and iterative evolution Marsupial/mammal and foram examples Evolutionary trends Cope s rule Evolution of the whales Evolution of the horses Phyletic gradualism vs. punctuated equilibria Earth History, Ch. 7 11
Evolutionary convergence Convergence = the evolution of similar form in two or more distinct biologic groups Example: similarity between marsupials and placental mammals Unrelated or distantly related groups typically converge on particular forms that have high adaptive value Earth History, Ch. 7 12
Convergence Although marsupials and placentals mammals have a common ancestor, there has been no genetic communication between the two groups since the breakup of Pangaea, during the Mesozoic Era. Earth History, Ch. 7 13
Earth History, Ch. 7 14
Earth History, Ch. 7 15
Iterative evolution Iterative evolution = the repeated evolution of a particular form from the same ancestor, but at different times Ancestral form is usually a stable (morphologically conservative) and long-ranging taxon Similar descendant forms arise periodically by chance? [Or, genetic regulation may allow variation in only one direction?] Earth History, Ch. 7 16
C Iterative evolution time B A second descendant, C, which may be nearly identical to B, originates from A long after the extinction of B. Stable ancestral taxon A gives rise to slightly more complex descendant B. A morphology Earth History, Ch. 7 17
Iterative Cenozoic planktonic foram radiations Quaternary globigerinoid globigerinoid hastigerine hastigerine orbuline orbuline pulleniatine globigerine turborotalid turborotalid globorotalid globorotalid Pliocene Miocene Oligocene Eocene Paleocene Paleogene Neogene conical conical Earth History, Ch. 7 18
Evolutionary trends Cope s rule = the general tendency for body size to increase during the evolution of a particular group of organisms. Examples: Whales probably originated from even-toed ungulates ~50 mybp Progressive modification of appengages Progressive adaptation to marine environment Progressive increase in size Earth History, Ch. 7 19
Evolutionary trends (cont.) time Earth History, Ch. 7 20
Whale ancestor (note hoof-like nails and position of nostrils) Earth History, Ch. 7 21
Whale B A C ankle bones A = newly discovered bone B = ankle of extinct, carnivorous ungulate C = ankle of even-toed ungulate (like a hippo) Earth History, Ch. 7 22
Evolutionary trends (cont.) Horses Originated ~ 55 mybp Earliest forms were dog-size, with four toes on front feet and three toes on rear feet, and small molars Modern horse is large, with a single hoofed toe on each foot, and complex molars for grinding grasses Earth History, Ch. 7 23
Earth History, Ch. 7 24
Modes of evolution Phyletic transition Gradual, stepwise evolution Punctuated equilibria Long periods of stability punctuated by short bursts of evolutionary change Earth History, Ch. 7 25
Types of evolutionary change time modification of a species through time splitting of one species into two gene pool Gradual, or phyletic transition in response to directional environmental change gene pool Speciation, as a result of geographic isolation and then reproductive isolation Earth History, Ch. 7 26
Phyletic transition Gradual evolution of the Jurassic oyster, Gryphaea. Note increase in size as well as progressive flattening of the lower valve. Earth History, Ch. 7 27
Punctuated equilibrium Evolution of bowfin fishes. Note long period of stability (Cretaceous late Eocene), then rapid speciation followed by another long period of stability (late Eocene present). Earth History, Ch. 7 28