Bio152: Speciation (II)
from last lecture:
1. species definition
2. patterns of speciation
- anagenesis & cladogenesis
3. allopatric speciation
4. reproductive isolating mechanisms
today:
1. sympatric speciation
2. genetics of rapid speciation
3. reinforcement and hybrid zones
N.B.: review session Monday June 27th
1 – 3 pm, Rm 3007
1
recall: allopatric speciation
• population is divided into geographically
isolated subpopulations
• initial allele frequencies differ among subpop’s
• gene flow is prevented
• biotic & abiotic factors exert different
selective pressures
e.g. allopatric speciation of antelope
squirrels in Grand Canyon:
Ammospermophilus harrisii
south rim of canyon
Ammospermophilus leucurus
north rim of canyon
2
to confirm allopatric speciation: determine if
2 “species” can no longer interbreed
(reproductive isolation)
Expt:
Drosophila pseudoobscura
some raised on
starch
some raised on
maltose
Q: does adaptation to different media
result in reproductive isolation?
1. mix “starch” and “maltose” pop’s; observe
matings
2. control: mix pop’s raised on same media
results:
22
9
8
20
females
same pop
diff pop
males
same diff
males
maltose starch
females
starch
maltose
18
15
12
15
3
conclusion:
• flies preferred to mate with flies adapted
to same medium
• did not prefer flies from same or diff pop if
raised on same medium
• isolation not yet complete
selection for different traits in isolated
pop’s leads to reproductive isolation
sympatric speciation
can speciation occur if populations are not
subdivided?
•
1.
2.
3.
mechanisms:
chromosomal changes
habitat differentiation (host shifts)
assortative mating
4
changes in ploidy
•
•
•
•
ploidy = # of chromosome sets
polyploidy: extra sets of chromosomes
results from mutational error
common in plants
1. autopolyploidy
2. allopolyploidy
autopolyploidy
> 2 sets of chromosomes derived from
same species
5
1. tetraploid can interbreed with diploids
(original stock) → triploid (3n)
• 3n cannot interbreed with
parental type (abnormal meiosis)
• 3n is sterile
2. tetraploid (4n) can self-fertilize or
interbreed with other 4n
reproductive isolation in 1 generation!
allopolyploidy
> 2 sets of chromosomes derived from
different species (through hybridization)
• hybrid undergoes mutation that increases
ploidy; becomes fertile new species
6
hybridized with
European species
Spartina townsendii
sterile hybrid
Spartina alternifolia
native to N. Am.
increased ploidy; fertile
new species
Spartina anglica
allopolyploidy in Spartina:
7
• bread wheat (Triticum aestivum):
allohexaploid
6 sets of chromosomes;
2 from each of 3 species
8
sympatric speciation: host shift
• a subset of population exploits a new
resource:
• e.g. apple maggot fly (Rhagoletis):
• native host is hawthorn – larvae are laid
on fruit
• ~ 200 years ago, European settlers
introduced apple trees
• some Rhagoletis laid eggs on apples
• apples mature fast; selection for fast
development
• apples & hawthorns fruit at different times
(temporal isolation)
9
• soapberry bugs:
• introduction of exotic plants → host shift
• selection for different beak lengths
• host specificity is leading to speciation
sympatric speciation: assortative
mating
• assortative mating: non-random with
respect to phenotype
• positive: prefer same
• negative: prefer different
• can lead to sympatric speciation
10
e.g. Lake Victoria cichlids:
• young (~12,000 years old)
• rapid speciation partly driven by sexual
selection:
Pundamilia pundamilia
Pundamilia nyererei
• mate choice exp’ts:
• normal light: females prefer males of own
species
• orange light: males appear identical;
females mate indiscriminantly
sexual selection maintains
reproductive isolation
11
parapatric speciation:
• selection for different alleles in adjacent
populations → cline in allele frequency
• strong selection: populations can
differentiate
Agrostis tenuis
• hybrid zones may arise from parapatric
speciation rather than from secondary
contact
12
secondary contact
•
what happens when isolated pop’s come
into contact?
1. complete reproductive isolation: no gene
flow; separate species
2. incomplete reproductive isolation??
fitness costs of “hybridization”
• costs lead to reinforcement → selection
to maintain differences occurring during
isolation
13
hybrid zones:
• areas of overlap between species where
interbreeding occurs
1. characteristics of hybrid zone depends
on fitness of hybrids; extent of
interbreeding
2. hybrid zone may be transient: e.g.
Townsend’s & hermit warblers
14
• Townsend’s males aggressive; mate with
hermit warblers to form hybrids
• hybrid zone is moving south; hermits may
go extinct!
• same thing in black duck & mallard:
hybrids can become new species
• if intermediate phenotype advantageous in
some environments, hybrids may
eventually form a separate species
Larus occidentalis
Larus glaucescens
15
• 2 gulls hybridize in Western U.S.
• hybrids do better than either parental type
in hybrid zone:
• nest in vegetation (like glaucescens) but
eat mostly fish (like occidentalis)
• could lead to new species!
conclusion:
1. allopatric speciation most common
2. sympatric speciation occurs with changes
in ploidy, host shifts and assortative
mating
3. with secondary contact, hybrid zones
may form if isolation incomplete
4. selection to reinforce differences if cost
to hybridization
16
5. if intermediate phenotype favoured,
hybrids may become new species
6. some hybrid zones may be result of
parapatric speciation
17