Macroevolutionary patterns in the fossil record

Paleontology catalogs appearance & disappearance of 'types' in geological timescale
    Origins of major taxa correspond to evolution of distinct types of organisms
       REMEMBER that new taxa are recognized retrospectively
       Lineages may not go extinct, even though forms change
What sorts of evolutionary phenomena are apparent in the fossil record?

Evolutionary Trends: patterns of change within taxa that persist over time
        Trends may be observed across many taxa => 'rules' (cf. ecogeographic rules)
       Cope's Rule: descendant species are larger than ancestral species
                        Ex.: >10X increase from Eocene Hyracotherium to Recent Equus
       Williston's Rule: serial homologues become specialized & reduced in number
                        Head, thorax, & abdomen of Insecta from segmented ancestors
       Trends can be quantified

Adaptive Radiations: proliferation of taxa after morphological innovation  or entry into new niche
       Ex.: Placodermi were first gnathostomes (opposable jaw elements)
         Six orders evolve in Devonian: first vertebrate predators
         Eutheria [placental mammals] at K / T boundary (Cretaceous / Tertiary, 65 MYBP)
         Mammalia are small noctural insectivores for first 120 Myr [see Eomaia at 125 MYBP]
               Extinction of dinosaurs opens aquatic & aerial niches
                From late Cretaceous  Paleocene  early Eocene ( = 20 Myr)
                                 >20 new orders: including bats & whales

Living Fossils: persistance of taxa unchanged with low diversity over long periods
       Mosaic evolution: different features evolve at different rates
                'Fossils' are not necessarily primitive
                'Living Fossils' do not necessarily represent ancestral type
       Tadpole shrimp (Triops: Notostraca)
                Fossils from 180 MYBP assigned to extant species
       Coelacanth (Latimeria: Crossopterygia) survives from Carboniferous (250 MYBP)
                Re-discovered in 1938 in Indian Ocean
                Fin structure resembles immediate ancestors of terrestrial vertebrates,
                Extinct crossopterygia are freshwater: marine habit is derived
       Tuatara (Sphenodon: Rhynchocephalia) confined to islands off New Zealand
                Parietal 'third eye', diapsid temporal openings in skull are ancestral
                    [Poor taxonomy may have led to extinction of other species]

Taxon cycles: Taxa originate, proliferate, 'rusticate'
       Lungfish (Dipnoi: Sarcopterygia) are first terrestrial vertebrates
                High diversity in Paleozoic (20 genera),
                Low diversity in Cenozoic (3 species in 3 Gondwanaland continents)
               [But: fleshy-limbed, air-breathing relatives attend university]
       Horses (Equidae: Perrisodactyla) evolved in the New World
                Originate in Paleocene as digitigrade 3-toed browsers (65 MYBP)
                Proliferate in Miocene grasslands as cursorial 3-toed grazers (20 MYBP)
                Replaced in Plio-Pleistocene by Artiodactyla (5 MYBP)
                Artiodactyla have more efficient digestion & locomotion
                    Extant Equus survives only in Old World as cursorial 1-toed grazer

Missing Links: expected fossils that show transitions between groups are absent
        Historically: applied to fossil intermediates between apes & humans
            Neanderthals were first hominid fossils (1856):
                        now Homo sapiens neanderthalensis
           "Piltdown Man" had cranium of modern human, jaw of ape
                       A hoax: two modern, artificially aged skulls combined
           "Nebraska Man" identified from fossil molar teeth:
                        Tentatively identified as New World hominid (unlikely)
                        Subsequently shown to be an extinct pig
            Ramapithecus argued to be a hominid at 15 MYBP
                        human / ape split very old
                        Now recoginzed as extinct pongid: human split ~4 MYBP
            Australopithecus has intermediate size, stance, skull
                    "Lucy" is oldest human ancestor (3.5 MYBP)
                            [but see Kenyanthropus, "Nature," 22 Mar 2001]

            Homo floresiensis : the original "hobbit" ?
                      1-m tall hominin with small encephalization quotient
                       A H. erectus descendant contemporary with H. sapiens at 18KYBP ?

       Major adaptive types appear 'suddenly'
               First bird (Archaeopteryx) flies, has fully-formed feathers
               First bat (Icaronycteris) is fully-formed Microchiropteran (w/ long tail)
                        moth scales in gut region echolocation
               First whales (Archaeocetes) are fully aquatic
                Origins & relationships of invertebrate taxa are notoriously poor

Mass Extinctions: 'Catastrophism' revisited
        End of Paleozoic & Mesozoic Eras characterized by major extinctions

        "Great Permian Die-off" (250 MYBP): Extinction of marine invertebrates
                > 50% of families, > 95% of species
                marine vertebrates, terrestrial life unaffected
        "Cretaceous (K/T) Extinction": Extinction of Dinosauria
                All vertebrates > 25 kg gone; plankton & benthic invertebrates gone
                Marine fish & terrestrial plants unaffected
               "Asteroid Holocaust" Theory
                        "Iridium Anomaly" at K / T Boundary (Alvarez 1980)
                        "Nuclear Winter" scenario predicts mass extinction
                        Impact seems certain: biological effects uncertain
                 26 MY extinction cycle may be related to extraterrestrial impacts
        "Pleistocene Extinction" (10,000 BP)
                Disapperance of New World megafauna (large mammals & birds)
                     ground sloths, mammoths, sabre cats, Diatryma, etc.
                Coincides with arrival of first humans: the "Overkill" Hypothesis

        "After Man" (Dougal Dixon)
                What will Life be like 50 MYr after the extinction of Homo?
                Premise: "ecological disaster" wipes out all large mammals
                                survivors are small, r-selected, nocturnal
            Rabbucks are deer-like rabbits
            Falanx are predatory, cursorial rats
            Night Stalkers are flightless bats

Gradualist versus Punctuationalist Interpretations of the fossil record

Modern Synthesis: Evolution is 'slow and gradual'
        Microevolution is the result of  Natural Selection:
                (differential survival & reproduction of individuals)
        New variation arises from two main sources:
         Mutation produces new alleles
              Genetic recombination produces new allelic combinations

Macroevolutionary patterns result from microevolutionary processes,
        continued over vast periods of time: "Gradualism"

        'Evolutionary Trends' are due to directional selection
                acting persistently & consistently over millions of years
             Ex.: Hypsodont molars of horses evolve at 40mD s = 10-6
                        1 death / 106 individuals / generation for 16 MYR
                Ex.: Cranial capacity of Homo 850cc  1330cc in 1 Myr
                        increase of  ~ 0.01 cc / generation

        Natural Selection with selection coefficients so small as to be immeasureable
                can produce major evolutionary change in long term

        'Adaptive radiations' are due to diversifying selection:
                Evolutionary innovation allows entry into new niche

        'Living fossils' are due to stabilizing selection:
                Successful lineages become specialized

The Origin of Species
        Speciation occurs mainly by adaptive divergence
                Differences among species accumulate by microevolution.
                This results in phyletic evolution:
                    species gradually change into new species
       Ex.: Evolution of proloculus size in Lepidolina (Foraminifera) during Permian

        Speciation is not adaptive per se:
          i.e., creation of new species does not create selective advantage
       RIMs arise by chance while population are isolated
       SRMsevolve within populations by adaptation during divergence
                anagenesis produces cladogenesis

       The fossil record is very incomplete
       Very few organisms leaves fossils
             Ex.: Oceanic & tropical species unlikely to fossilize
                'species gaps' are only apparent, not real
                        A more complete record would show infinite gradation
            Ex.: Evolution of molars in Pelycodus (Primata: Mammalia) during Eocene
                     Notharctus arises from Pelycodus gradually
                'missing links' are rare & temporary not seen

 Microevolution explains macroevolution: end of story (& the course)

The Punctuated Equilibrium Model (Eldredge & Gould 1972)
    A  challenge to conventional interpretations

    The fossil record is complete
       'species gaps' are real:
             Intermediates don't exist
         Rapid 'founder effect' speciation is the rule
             Speciation is geologically instantaneous [< 50K years] => no fossils

       Cladogenesis produces anagenesis
                Species' genotypes are too stable: phyletic evolution is rare
       A (tricky) population genetic argument (Lande 1983):
                  Recall: 40mD horse molar evolution implies s = 10-6
                        Because genetic drift > selection if Ne ~ 1/s
                            A "gradual" trend requires Ne > 1,000,000 over 16 MYr
       But: observed horse N <  10 ~ 50,000
                       genetic drift would swamp such a weak selection coefficient
                  Thus, if horse evolution is the result of natural selection
             s must have been quite high (s > 10-4 ~ 5)
                       and evolution would have occurred quite "quickly": 10s ~ 100s K yrs

                 Horse evolution can be explained by brief periods of rapid change
                        interspersed among longer period of little or no change.

       Important: a "geologically instantaneous"  interval of 50,000 yr
                                  comprises many, many generations

Evolution proceeds as a punctuated equilibrium:
        New species are produced by 'founder effect' speciation:
             'Genetic revolutions' & selection for SRMs leads to a rapid 'adaptive shift'

        Extant lineages are replaced by their descendants:
            Geological picture is an extant lineage "suddenly" replaced by new lineage
                [i.e., an anagenetic 'equilibrium', 'punctuated' by cladogenetic speciation]

       Ex.: Lake Turkana snails show punctuated events correlated with changes in water level
       Ex.: Re-interpretation of Pelycodus fossil record shows
                        episodes of rapid change & reversal (statistical artefacts?)
                        origins of new taxa might arguably be "instantaneous"

So which is correct? Balance of supporting evidence is not clear
       Paleontological "connect the dots" on same data can support
            "gradual", "punctuated", or "mixed" models (See Fig. 6-12)

       Iconography of evolution is changing in textbooks
            "gradual" diagrams are being replaced by "punctuated" diagrams
            Ex.: Horse evolution is one of the best studied examples
                Historically, evolution is shown as a linear progression
                Modern Synthesis diagrams are "slanted  "slow & gradual"
                Post-Modern texts show "rectangular" phylogeny diagrams  "punctuated"
                    [See also Campbell (1999)]

Can Macroevolution & Microevolution be 'decoupled'?

       Microevolution by natural selection works through
            differential survival & reproduction of individuals:

       Evolutionary trends may be due to species selection =
          differential 'survival & reproduction' of evolutionary lineages
                'survival' = persistence (avoidance of extinction)
               'reproduction' = production of new lineages (speciation)
        Might different biological properties be selected at individual & species levels?

       Differential extinction: The evolution of sex
       Asexual propagation may be favored in short term
                        Uniform clone may be favored by stabilizing selection
             But: any single clone is subject to extinction
       Sexual reproduction may be favored in long term
                        Recombinant variants are eliminated by selection
             But: multiple new adaptive types may survive extinction
                Ex.: Daphnia populations (research of D. Innes)
                        asexual reproduction favored in stable environemtns
                        sexual reproduction favored in variable environments
     An ecological disaster might destroy a plentiful, uniform asexual clone,
                   while an uncommon, diverse sexual clade would survive

       Differential speciation: The evolution of dispersal strategies
                'Stay at home' may be safer strategy in short term
                        chances of survival better in known environment
             But: survival is poor during high density
                'Dispersal' may be favored in long term
                        most colonies fail (recall 'founder' model)
             But: some succeed as new species
                Ex.: Rodents undergo population cycles
                        Lemmings (Microtus) are territorial at low density,
                             become 'migratory' at high density
                        Migratory individuals may found successful new colonies

                Speciation per se is an adaptive strategy:
             Lineages that disperse more persist longer
                             because dispersion leads to speciation
                        Rodents are extremely speciose (about 1/4 of mammalian diversity)

Macroevolution may require a new set of evolutionary explanations
        a complete theory of evolution will consider extinction & speciation patterns

Text material © 2010 by Steven M. Carr