Download biological species concept

biological species concept
Bio153: lecture 8
Rates of Evolution
microevolution = evolutionary changes
within species
natural selection
sexual selection
genetic drift
can lead to speciation
interbreeding
population
reproductively
isolated
species = gene pool
Modes of speciation:
• e.g. island archipelagos:
• Allopatric speciation model:
time
geographic
barrier
separate
gene
pools
1) invasion of new
islands
isolation
2) over time,
divergence of pop’s
3) “re-invasion”
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• population genetics: Wright (1977)
“shifting balance”:
drift
gene flow
(local pop’s)
(among pop’s)
anagenesis and cladogenesis:
anagenesis
+ selection
evolution
may lead to large, adaptive changes in
a widespread population
- directional change in
characters in a lineage
cladogenesis
- branching by speciation
evolutionary changes
cladogenesis without anagenesis
sibling species: similar species that do not
interbreed
gray flycatcher
anagenesis without cladogenesis:
e.g. mimetic butterflies
-much geographic variation
without speciation
dusky flycatcher
Empidonax spp.
cladogenesis + anagenesis :
speciation events:
interpreting the history of evolution:
• in fossil record, taxonomy defined by
morphological characters
• difficult to separate anagenesis from
cladogenesis
divergence along X axis:
morphological change
how do you define a species??
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1) rate of ∆ of character in a lineage
(anagenesis)
2) replacement of “species”
origination and extinction
(cladogenesis)
morphology
rapid anagenesis
slow anagenesis
time
enough change
to look like
new species
morphological change without
speciation
• chronospecies: anagenesis makes
descendents look like separate species →
taxonomic pseudoextinction
no
speciation
looks like
speciation!
morphology
rates of evolution :
time
many speciation events; little
morphological
change
problem of stasis:
fossil deposits: 50 - 100 my apart; short
term ∆’s are lost
however:
1) long periods without change (stasis)
2) rapid appearance of new forms
3) no transitional forms
Changes in the shape of molar teeth of the
early Eocene mammal Hypsodus
real or artifact?
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two hypotheses:
1. Phyletic Gradualism
- constant anagenetic change
- speciation gradual
- transitional forms lost in fossil record
basic tenets of PE:
1) stasis; rapid change is real
2) speciation by peripheral isolates
3) adaptive ∆ mostly during speciation
2. Punctuated Equilibrium
- stasis is real
- evolution occurs during speciation
- long-term trends in morphology due to
species selection
• Peripheral isolate model:
-
species selection (species sorting)
environment homogeneous
population small
founded by few individuals
low genetic diversity
genetic drift
must be no gene flow
• related species overlap in niche space
competition → displacement; extinction
• 2 main processes leading to anagenetic
change:
differential speciation
differential extinction
differential speciation
differential extinction
t2
time
time
t2
t1
body size
t1
body size
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• examples: comparisons of speciation rates
African antelopes: impalas: slow
wildebeest : fast
marine snails : planktonic larva: slow
non-planktonic : fast
next lecture:
from 2 kingdoms to 3 domains
why? genetic isolation
same in:
burrowing rodents: coruros: slow
tuco-tuco : fast
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