Bio152: Speciation (I)
this week: Dr. Monika Havelka
3059 SE
[email protected]
reading: Ch 25
the “tree” of life
branches are
speciation
events
macroevolution:
evolution at or
above species
level
1
how are species defined?
species = evolutionarily independent
groups of organisms
how are species identified?
tricky!
1. biological species concept
2. morphospecies concept
3. phylogenetic species concept
biological species concept
interbreeding
population
reproductively
isolated
species = gene pool
2
Peromyscus leucopus
Peromyscus gossypinus
Peromyscus maniculatus
Biological species concept:
• criterion is reproductive isolation
• reproductive isolation = no gene flow
• reproductively isolated if fail to
interbreed or do not produce viable,
fertile offspring
• problem:
what about asexual species?
what about fossils?
3
e.g. bacteria:
• genetic material passes
uni-directionally
between cells
• reproductive isolation
cannot be defined
morphospecies concept:
• species are morphologically distinct
groups
• can infer: evolutionary independence
• when there is no information about gene
flow
• advantages:
easy to do!
can apply to all organisms
4
looks can be deceiving!
5
phylogenetic species concept:
• species = smallest monophyletic group in
a phylogeny
• phylogeny: hypothesis about evolutionary
relationships among groups of organisms
• monophyletic group: ancestor + all
descendents
• monophyletic groups are nested in a
phylogeny
• smallest group =
species
phylogenies constructed using many types
of data (esp. molecular and genetic)
6
advantages of phylogenetic species
concept:
• applies to all organisms
• monophyletic groups are, by definition,
evolutionarily distinct
problems:
• phylogenies are difficult to construct
• different types of data may give
ambiguous results
7
grey wolf
coyote
red wolf
are these distinct species?
• 2 hypotheses:
1. red wolf is a separate species
2. red wolf is a recent hybrid of grey wolves
and coyotes
• recent molecular evidence:
1. common ancestor in N. Am. ~1 – 2 mya
2. ancestors of grey wolf migrated to
Europe & Asia; returned to N. Am.
~300,000 years ago
3. coyotes & red wolves began to diverge
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4. grey & red wolves hybridized; red wolves
and coyotes hybridized
5. Eastern Canadian Wolf population:
mixture of different blood lines (grey
wolves, red wolves, and hybrids)
conclusion:
1. biologists use all 3 species definitions
2. can be difficult to define!
3. has major implications for conservation
(see case study of dusky seaside
sparrow in text – pp. 541-2.)
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How do new species arise?
2 patterns of speciation:
1. anagenesis (phyletic evolution): a
population changes significantly over
time so that it becomes a new species
2. cladogenesis (branching evolution):
splitting of an ancestral species into 2
daughter species – probably most
common pattern
change over
time
splitting into 2
daughter species
10
cladogenesis + anagenesis :
speciation events:
divergence along X axis:
morphological change
Modes of speciation:
• Allopatric speciation model:
time
geographic
barrier
separate
gene
pools
isolation
vicariance = physical splitting of geographic range
11
• e.g. island archipelagos:
1) invasion of new
islands
2) over time,
divergence of pop’s
may lead to speciation
12
adaptive radiation:
a single colonizing
species arriving on
different islands may
give rise to many
species!
e.g. Hawaiian
honeycreeper
factors affecting speciation:
1. genetic drift: differences in allele
frequencies among subsets of a large
population
13
2. selection: if isolated populations
subject to different selective pressure →
differentiation & speciation
**sexual selection can be very important!
3. gene flow: “homogenizes” populations
→ reduces differentiation
• population genetics: Wright (1977)
“shifting balance”:
drift
(local pop’s)
+ selection
contribute to speciation
-
gene flow
(among pop’s)
counteracts
speciation
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How do we know speciation has
occurred?
complete speciation = complete reproductive
isolation
Mechanisms:
1. prezygotic: prevent gametes from meeting
2. postzygotic: reduce success of hybrid
zygotes
prezygotic isolating mechanisms:
1. mates don’t meet
• temporal
bishop pine
Monterey pine
15
•
habitat
spadefoot toads: occupy different soil types
2. meet, but don’t mate
16
3. copulation attempted, but no sperm
transferred
4. mate, but gametes incompatible
17
postzygotic isolation
1. egg is fertilized, but zygote or embryo
dies
6% of hybrid offspring hatch
2. hybrid offspring mature, but are sterile
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conclusion:
• most common mode of speciation is
allopatric
• drift, selection in divided pop’s cause
divergence
• gene flow reduces divergence
• over time, speciation complete if
populations are reproductively isolated
e.g. Lake Malawi cichlids:
•
•
•
lake is old (>8 my)
varied in depth (dried into separate
basins)
different habitats within lake
19
3 steps in adaptive radiation:
1. lineage split into different habitats
2. competition → specialization on different
foods
3. sexual selection promoted divergence
(assortative mating)
result: > 500 species!!
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