Download Rate of Evolution

4/17/2009
Rate of Evolution
Juliana Senawi
Rate of Evolution
Measurement of the change in an Measurement
of the change in an
evolutionary lineage overtime
Radiometric and paleomagnetic dating p
provide an effective basis for determining g
the age of fossil and hence the rates of evolution of the species to which they belong 1
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Rates of Evolution
G. G. Simpson:
G
G Simpson:
“Tempo & Mode in Evolution” (1944)
• applied
applied principles of principles of
modern synthesis (e.g. population genetics) to fossil record
How to measure evolution
1) Phylogenetic Rate
‐ Morphological Rate
rate of change of character or group of characters in a lineage
‐ Quantitative measures
Q
i i
Rate = Change/ Unit Time
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Rates of Evolution of Single Characters
• J.B.S. Haldane (1949):
J B S Haldane (1949):
Evolutionary rate (r) = (ln x2‐ ln x1) /change in t
Measured in ‘darwins’
darwin = change in e / my
Bruce MacFadden
• The measuring the rate of evolution – illustrated by MacFadden on horse teeth: classic materials in the study of evolution.
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Evolution of Equine Lineage
Comparing Rates
• Determined by selection?
– Can’t
Can t prove selection is responsible for changes in prove selection is responsible for changes in
fossil record
– But can find out the results in two area are consistent.
e.g. Fossil evolution rate are higher than rates observed in artificial selection ‐ suggest selection not the only cause of evolution
But other way round‐ mechanism of population genetics, natural selection and random drift can accommodate the fossil observations.
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Fluctuations in Rate
• Inverse relation between rate of evolution and time interval over which it was measured:
rapid evolution have tended to be for shorter interval than slower evolution
• the rate of evolution measured over a short h
f
l i
d
h
interval is inevitably higher than the rate measured over a longer time interval because the short‐term changes cancel out.
width
Fluctuations in Rate
time
e.g. Galapagos finches’
beaks
-Larger
Larger in time of food
shortage and smaller in
times of abundance
- overtime these changes
in size cancel each other
out
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Character Types
• characters
characters evolve at different rates evolve at different rates
(mosaic evolution)
• rate of change is not constant
• conservative characters: canalized; general adaptations
general adaptations
• derived characters: specialized, rapid evolution
2) Taxonomic Rate
• replacement of forms
• origination & extinction
• Quantified: ((# taxa originate ‐
g
# taxa extinct) / unit )/
time
• Or the inverse of the average duration of a species 6
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Cladogenesis & Anagenesis
• Speciation at t1 & t2
• a & c contemporary
• b goes extinct
Chronospecies
• Problem: Fossil record: taxonomy based on morphology characters. Hard to separate anagenesis from cladogenesis.
g
Identification of many chronospecies
(descendent recognized as separate spp.) Taxonomic
Pseudoextinction
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↑Phylogenetic Rate = ↑ Taxonomic Rate
morrphology
• rapid rate of morphological change leads to high rate of taxonomic replacement
to high rate of taxonomic replacement
time
↑ Taxonomic Rate ≠ ↑ Phylogenetic Rate
morrphology
• high rate of turnover; little morphological change
time
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Relationship between phylogenetic rate and taxonomic rate depends on characters used to determine taxa C
Comparison of taxonomic rates :
i
ft
i
t
balance of origination & extinction
Recent Taxa
• rapid evolution rapid evolution
• poor fossil record
• typical of Adaptive Radiations
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Problem of stasis:
Fossil Deposits: 50 ‐ 100 my apart
• short term changes are lost
h
h
l
However, observe:
1) long periods without change
2) rapid appearance of new forms
3) no transitional forms
Real or Artifact?
Quantum Evolution
Problem: new taxa without fossil intermediaries
Simpson 1953:
• rapid, substantial evolutionary change with
shift into new adaptive zones
• once a threshold passed in acquisition of new adaptation, strong directional selection shapes feature into new forms
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Quantum Evolution • Within the taeniodonts, a group of extinct placental mammals, two lineages evolved. • One was the original group of taeniodonts, the beaver‐sized conoryctines that survived into the late Paleocene; conoryctines
Quantum Evolution • The other taeniodont lineage was stylinodonts ‐ evolved rapidly (quantum evolution) idl (
t
l ti )
across a transition to a new adaptive zone (lifestyle). • The bear‐sized stylinodonts evolved specialized dentition especially suited to rough and highly abrasive foods, well‐developed claws, and strong muscles suggesting a digging foraging style
stylinodonts
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Hypotheses
1) Phyletic Gradualism
• constant anagenetic change
constant anagenetic change
• speciation gradual
• transitional forms lost in fossil record
2) Punctuated Equilibrium
2)
Punctuated Equilibrium
• stasis is real
• evolution occurs during speciation
• long‐term trends in morphology due to spp. selection
Hypotheses
• Phyletic evolution (anagenesis) envisions gradual divergence of a lineage as the bell‐shaped mean of successive populations changes, until a new species is formed. 12
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Hypotheses
• Punctuated equilibrium (cladogenesis) envisions long periods of more or less unchanging species persistence, suddenly interrupted by speciation, producing a new species.
3) Rate of Molecular Evolution
• Total amount of genetic change between two generations or within a succession of
generations or within a succession of generations – between fossil and living taxa)
• Techniques (retrieve and sequence DNA) from fossil – demonstrate base‐pair differences in small segments of particular g
p
genes in many extinct organism
– but rarity in fossil DNA makes this impractical
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• DNA and RNA sequence
– Provide a means for establishing relationships measuring relative rates of evolution in all taxonomic group
• This important
– Fossil
Fossil record is incomplete record is incomplete
– Study of the sequence of divergence of populations and subspecies
Rate of Molecular Evolution vary
• Changes in DNA not occur the same rates in all taxa
all taxa
– In common with changes in morphology
• Mitochondrial DNA changes much more rapidly than nuclear DNA
– Different portions of the same gene evolve more rapidly than others
• Since almost infinite amount of DNA
– Details of phylogenies will differ depending on what genes are used
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Molecular Clock
• Molecular evidence used ‐ establishing the nature of interrelationships of different
nature of interrelationships of different lineages and the relative time of their divergence
– But cannot establish the actual time when lineages originated by itself
•M
Molecular clock must be ‘set’ on basis of l l
l k
t b ‘ t’
b i f
some other evidence such fossil record or a geological event
Example
• Separation of South America
South America and Africa (continental drift)
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• Give minimum time for the separation of populations that inhabited those
populations that inhabited those continental
– The time at which their genome was identical must have been prior to that split
• Estimated rate of molecular evolution =
Total amount of change / elapsed time • If the rate constant
– It can be used to establish the sequence of divergence of individual lineages within the groups that subsequently evolved separately in
groups that subsequently evolved separately in South America and Africa • This provides information as to the rate of divergence and the relative longevity of the living members of these lineage
– If several different genes in all taxa is constant –
supports assumption the rate of genetic change is constant
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Summary
• Genes used in taxonomic studies ‐ not provides direct evidence of the evolution of
provides direct evidence of the evolution of morphological structures.
• Although phylogenies based on molecular evidence reflect the relative time of divergence ‐
g
cannot be used to establish how or when specific characters evolved.
• Taxonomic groups are recognized by presence of unique morphological traits by which all known members can be identified. • Divergence of a group may be accompanied by emergence new character ‐ it is very unlikely that more than a single character will arise in synchrony with the divergence. • No matter how accurate the phylogenetic resolution ‐ the fossil record remains the primary basis for establishment of the
primary basis for establishment of the patterns and rates of morphological change.
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Thank You!
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