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
Ramapithecusargued
to be a hominid at 15 MYBP
=> human / ape split very old
Now recoginzed as extinct pongid: human split ~4 MYBP
Australopithecushas
intermediate
size, stance, skull
"Lucy" is oldest human ancestor (3.5 MYBP)
[but see Kenyanthropus,
"Nature," 22 Mar 2001]
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,
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
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 natural selection
continued steadily 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
HYPOTHESIS:
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 Ne < 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.
:
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
Classical diagrams depict the Cone
of Diversity
Modern Synthesis diagrams are "slanted" 
"slow
& gradual"
Modern texts (>1980) show "rectangular"
phylogeny diagrams "punctuated"
[See also Campbell (1999)]
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
