Speciation and hybridization
• Where do species come from and where are
they going?
• Darwin’s “Origin of Species” said little about
how new species arise
– We now know that physical isolation is an
important factor (allopatric speciation)
– Can new species arise in sympatry? If so, how?
Phenotypic Diversity
• Consequence of genes and environment
• Heritable differences constitute a reservoir
for adaptative change in the face of
environmental change
Geographic Variation in Song Sparrows:
Incipient Speciation?
For a conservationist or evolutionary biologist, what is the most important thing to
know about the source of this variation? Why care what the origin is?
• Where does genetic diversity come from?
• How is genetic diversity maintained in a
population or populations?
• Why don’t we call these populations
species?
Species Diversity
• A species is a special type of population. How is it
special?
• What are factors that can lead to speciation?
– Isolation (why isolation—what happens there?)
• What are some types of isolation?
– Can speciation happen without physical isolation?
• Species concepts
– Biological Species Concept
– Morphological Species Concept
– Evolutionarily Significant Units
• Terminology
– The species “diverged”
• The frequency of alleles in that population changed over time
• Some alleles were lost, new ones were gained, until members of
that population can no longer mate with the original population from
which the species diverged.
Speciation in Darwin’s Finches
• Stages of Speciation noted by Grant and
Grant
– Establishment of new population (isolation)
– Divergence of population in isolation
– Reproductive incompatibility in sympatry with
orignal population
• How do they infer this without seeing any
single population go through all the
stages?
Darwin’s finches cont’d:
• Newly established populations of similar species
on the same island don’t interbreed
– Why not?
– What sorts of reproductive barriers exist?
– Can competition for resources lead to further
divergence?
• Populations of the same species on different
islands look different
– Why?
– Is this incipient speciation?
• How have species changed with environmental
changes over the long-term?
– Is evolution directional?
– How likely is the kind of persistent divergence that
leads to speciation?
How isolated are the Darwin’s
Finch species from each other?
• Divergence in mate preference has
happened before the evolution of genetic
incompatibility
• Different populations can function as
biological species before becoming true
species (in the sense of the biological
species concept)
• Pre-zygotic isolating mechanisms lead to
functional isolation, even if post-zygotic
isolation is not yet complete.
Adaptive Radiation
• What is it?
– Many descendent species in a short amount of time,
from one ancestor.
– Involves adaptive diversification into new niche space
– Niche space must last long enough for directional
selection to lead to reproductive isolation
• Can speciation happen without adaptive change
leading the way? For example, can a random
change in mating signals lead to reproductive
isolation? Or do mating signals diverge to
reinforce adaptive divergence?
Hybrid zones: Windows into the speciation process
What happens when two recently diverged geographically
isolated populations come into secondary contact?
Scenario 1
Scenario 2
X
time
If hybrids are selected
against—suggests that
ecological specialization
has begun.
reinforcement of
mating signals
(Prezygotic
isolation)
Full reproductive
isolation
Scenario 1 is probably more common than 2
(Postzygotic
isolation)
If hybrids are less fit than parentals, then you can get selection for
reinforcement.
Example: African Tinkerbirds—one species occupies savannah
and the other forest. At the forest/savannah interface there is
selection against hybrids and hence greater ability for females to
distinguish males of each population.
Frequency (Hz)
•Divergence in sexual and social signals
leads to prezygotic isolation (in animals)
Allopatry
Sympatry
Sympatry
Allopatry
reinforcment
Time (sec)
Kirschel et al. PNAS 2009
Reinforcement
• The enhancement of prezygotic isolating
mechanisms in sympatry by natural
selection
• Applies to populations that interbreed, but
in which hybrids are less fit than parentals
Pre-zygotic vs. post-zygotic
isolation?
• Pre generally comes before post, but can post evolve
without pre?
– Probably yes, but usually in allopatry
– Prezygotic isolating mechanisms play a bigger role in sympatric
populations where there is a cost to hybridization
• Must there be some existing genetic incompatibility for
pre to arise? Or can pre- lead to genetic incompatibility?
– Ongoing debate among bird researchers
– Birds learn songs. Local dialects may lead to isolation of
populations, followed by full genetic (post zygotic) isolation. (But
song can be learned, so not a complete barrier to geneflow. A
juvenile may disperse into another dialect zone and learn that
dialect, importing outside genes).
– Generally thought that some adaptive (and genetic divergence)
must precede or accompany divergence in prezygotic isolating
mechanisms for speciation to occur. E.g. divergence in song
alone, not enough to start speciation process.
White crowned sparrow
song dialect zones on west
slope of Cascades
Seattle Dialect Zone
Dialect 1
Dialect 2
2 dialects from Oregon (4 individuals each)
Baptista 1976
What drives speciation in this genus of plants? What is the
ancestral pollinator? Is this pre- or post zygotic isolation?
Most of these species can hybridize if forced crosses are made.
Most of these species are pollinated by different species. At least
2 are pollinated by the same species and live in the same place,
yet do not hybridize? Why not?
Kay and Schemske
Same pollinator, but no hybrids, why?
What is a scenario where genetic
isolation exists without pre-zygotic isolation?
Should there be selection for different pollinators now, or not?
With these two species, allopatry may have been a bigger factor in their
evolution. Post-zygotic isolation likely arose due to a long period of geographic
isolation. Now, even though they share the same pollinator and are still
closely related, hybridization is not possible because the pollen tube morphology
is incompatible. Interspecific pollen tubes germinate (graph 1), but do not grow
long enough to reach ovaries (2 and 3). So pre zygotic isolation is unlikely to
evolve now. The only thing wasted by cross species pollination is time and pollen.
Limnetic
Benthic
Benthic
Assignment
probability
Lake Apoyo,
new and deep!
Limnetic species
and deep water
species
Speciation in Sympatry: Barluenga 2006
• Analagous to stickleback system in N. America (featured
in C. 16)
• How does divergence begin when no geographic
barrier?
– How does disruptive selection happen?
– Intermediate phenotypes have lower fitness
– limited dispersal (and hence geneflow) among habitats—habitats
are separated in space even though no physical barrier
• How is speciation completed?
– Selection for stronger pre-zygotic isolation
• Females selected to mate assortatively with their own kind because
mating with the wrong phenotype leads to less fit intermediate
offspring
• Evolution of divergent signals to facilitate assortative mating
– Ultimately leads to post-zygotic isolation (genetic incompatibility)
Species
collapse in
secondary
contact.
These two species likely
separated during the last
ice age. Two populations
migrated to separate
summer habitat in the
Rockies and along
the coast. Note that the
habitat in the Rockies was
smaller. The population
there evolved to be more
aggressive because
territories were in short
supply. Now the more
aggressive species has
expanded its range and
Is starting to overtake the
range of the other species
hybridizing as it goes.
Currently, the hybrid zone is
in western Washington.
Hermit Warbler
Townsend’s Warbler
Rohwer et al. 2001
Species
collapse in
secondary
contact.
These two species likely
separated during the last
ice age. Two populations
migrated to separate
summer habitat in the
Rockies and along
the coast. Note that the
habitat in the Rockies was
smaller. The population
there evolved to be more
aggressive because
territories were in short
supply. Now the more
aggressive species has
expanded its range and
Is starting to overtake the
range of the other species
hybridizing as it goes.
Currently, the hybrid zone is
in western Washington.
Hermit Warbler
Townsend’s Warbler
Rohwer et al. 2001
How do we know Townsend’s moved this way?
Hybrid
zone
Townsend’s males are more
aggressive than Hermit
males. Kick out the Hermit males
and Hermit females then mate with
Townsend’s males.
What should songs look like in the
hybrid zone?
Note that Northwards along the coast,
all the birds bear mixed mitochondrial DNA
Even though they look like Townsend’s warblers.
This is testament to the fact that male Townsend’s
Warblers have been moving south along the
coast
hybridizing with female hermit warblers as they go.
The only pure hermit warblers left are in Oregon
and California.
Hermit warblers may be completely extinct in another
1000 years or less (but their mtDNA may live on).
Similar scenarios could happen in other warblers
“species” pairs too.
Pies represent the
proportions of hermit
mtDNA haplotypes to
Townsend’s
haplotypes.
Adaptive radiation
Grant and Grant 2003
• Back to Darwin’s finches:
– Divergence in bill characters in isolation on
separate islands.
– Further divergence through competition for
resources (when they fly to an island already
inhabited by slightly different population)
– Song differences related to bill differences
(males learn song from father, and bill
size/shape may determine how fast trills can
be moved up and down)
– Song may reduce geneflow enough to maintain
morphological differences (females prefer bill
shape and song type of father)
– But, no penalty to hybridization…it does occur.
Hybrids survive as well as parentals in some
cases.
– So how “good” are these species? Will they
collapse back into 1? Depends on whether in
long run birds with divergent characters survive
and reproduce better than hybrids with
intermediate phenotypes.
– If resources specialized upon are separated in
space or time on an island this can help along
the process of divergence by semi-isolation.
Review Questions to think about:
• Should prezygotic isolation happen before or after post- zygotic?
• What influences the evolution of prezygotic isolating mechanisms?
• What influences fitness of hybrids?
• When do we expect divergence and when do we expect
convergence? (answer: Hybrids less fit than parentals, and hybrids
equally fit to parentals, respectively)
• What if hybrids are more fit than parentals?
– Can hybridization itself lead to speciation?
• What are some examples of prezygotic isolating mechanisms in
plants and animals?
• Examples of postzygotic isolating mechanisms in plants and
animals?
Natural History of Birds
What is natural history to you?
Focused, observational, descriptive study of organisms in a natural (their present
environment) or historical context, including description of anatomical form and
function. Does a modern definition include description of cells and genomes?
Who does natural history?
Georg Wilhelm Steller Alexander Wilson
1709-46
1766-1813
John James Audubon Thomas Mayo
Brewer 1814-1880
1785-1851
We are part of the tradition…
Bird habitats of Washington
• Washington has low alpha diversity (any
one particular habitat typically has
relatively low diversity)
• High gamma diversity—many unique
habitats, partially due to the moisture
gradient across the state from rainforest to
rainshadow (desert).
• Elevational gradient too, including coastal
habitat and year-round snow fields.
Note forest fragmentation
3.5 million world population
Declining, down from 6.5 million
in 1973
Grays Harbor Estuary
What is an estuary?
Marbled murrelet
Brady Loop
Road
Managed flooding of farm fields for migratory
birds. Picture actually from Skagit delta.
Note Urbanization and forest fragmentation
Most intact
forest is in
mountains
Union Bay
>100 in. rain / year
40 inches of rain / year
Exit 38
Union Bay and Seattle
East
I90
Shrub Steppe ecosystem
Quilomene Wildlife Area
<10 in. rain / year
Potholes Reservoir
I 90
looking east
Crab Creek Rd.
Saddle Mtns.
Vantage
Russian Olive
Irrigation is negatively impacting wildlife habitat
Snoqualmie Pass
I 90
Taneum Creek
Eastern Foothills
20-30 in. rain/year
Veery
Pondersoa Pine
(Fire adapted)
Quaking Aspen
Shrub-steppe-pine forest transition
Field Trips by the numbers
• Total Species Observed by BIOL 444:
– ~153 species in 35 families
• Coast (~280 miles driven)
– 68 species (34 unique)
– Species per mile = .24
• Eastern WA (~400 miles driven)
– 85 species (50 unique)
– Species per mile = .21
• Union Bay (~1 mile walked)
– 63 species (12 unique)
– Species per mile = 63
Note: ~434 species of birds have been recorded in WA in 48 families
~260 species breed in WA.
Eastern Washington
What do you think of when you
think of birds?