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
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)
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
Modern
Synthesis diagrams are "slanted" "slow
& gradual"
Post-Modern
texts 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
