Download Speciation - Seattle Central College

Speciation
What are species?
How do we identify them?
How do they form?
**Speciation**
• Speciation: A splitting event that creates
2 or more “evolutionarily independent”
groups from one ancestral group
• Requirements:
– Genetic isolation - cessation of gene flow
(reproductive isolation)
• this is all you really need
– Genetic divergence follows- selection, drift,
mutation promotes divergence
What are species?
• Any reproductively cohesive group that is
“evolutionarily independent”
(reproductively isolated & on different
evolutionary trajectory) from other such
groups
– This forms the basis of our understanding of
how life diversified
• When species appear similar, how do we
identify them as reproductively isolated?
How does reproductive
isolation develop?
• In Sympatry (“together homeland”)
– Speciation occurs in same geographical
location
• In Allopatry (“different homeland”)
– Speciation occurs in geographic (or temporal?)
isolation, when populations are separated by
some barrier
Isolation in Allopatry
• What puts
populations in
allopatry?
– Vicariant events
• Splitting event
– Dispersal events
• Founding event
Causes of vicariance
• Plate tectonics
– Plate movements; mountain building;
vulcanism
• Changes in sea level
– Land bridge removal; island formation
• Glacial cycles
– Expanding glaciers intrude
• Surround population in refugia; habitat change or
loss; ice sheets divide range
Plate
movements
• Ratite distribution
– Ostrich, emus, kiwis,
rheas, cassowaries
• Hypothesized
ancestor occurred
throughout Gondwana
Present
18,000 kya
Isolation in
Sympatry?
• How is this possible,
since gene flow stifles
incipient species
formation?
– Strong, multifarious
selection
– Chromosomal
duplications
With natural selection
• Soapbugs
– Host plant switch
– Choice of host
correlates with beak
length
– Got reduced gene
flow?
– Got diversification?
15
10
5
0
Beak length
Generation of Polyploids
• Many plant species are 4n, 6n, or
more…obvious duplications
• How do these form? Why?
– Reproductive cells have many stages of mitosis
prior to meiosis…lots of options for error
– Autopolyploid - mutation doubles chromosome
number in a single species
– Allopolyploid - parents of different species
mate; produce polyploid offspring
Mating of allopolyploids
• Homologous
chromosomes aren’t
– Early duplication can
save it
• No pairing during
metaphase I
– Unequal segregation
• Tetraploid individual
results
Speciation & hybrid formation
• Multiple interbreeding
populations (gene flow)
• Barrier to gene flow
appears between one
population
Speciation & hybrid
formation
• Population diverges
in isolation.
• Speciation is
imminent
• Barrier retreats, gene flow is reestablished in a
stable hybrid zone:
– Zone of overlap between parent species where hybrids
are common & fit, but backcrosses are not.
• Long or short-lived, narrow or wide, stationary or moving
• Fusion: Meet & mate successfully. Hybrids
and backcrosses are fit & gene flow
eradicates divergence
• Reinforcement:
– Hybrid offspring (with intermediate phenotype) have
lower fitness than either P
– Individuals tending to NOT hybridize leave more
offspring = selection for some isolating mechanism
– Selected traits reinforce diversification that began in
isolation
• Postzygotic isolation prevents multi-generation
reproduction
• Prezygotic isolation prevents mating
What if diverging populations
reconnect?
• Fusion: Meet, mate successfully, & gene
flow eradicates divergence
• Reinforcement: Hybrids are unfit, so
selection for a reproductive isolating
mechanism eliminates gene flow
• Stable Hybrid Zone
• Speciation via hybridization
Types of reproductive isolation
• Prezygotic mechanisms - prior to embryo
formation
–
–
–
–
–
Habitat
Temporal
Behavioral
Mechanical
Gametic
• Postzygotic mechanisms -after embryo
formation
– Hybrid inviability
– Hybrid sterility
Prezygotic mechanisms
• Habitat: parasite
infects new host
species
Prezygotic mechanisms
• Temporal: Flies active
at different time of
day (breed at
different
temperatures)
Prezygotic mechanisms
• Behavioral: Courtship
sequence differs
(sallys)
• Mechanical: Bits don’t
fit together
(hemipenes)
• Gametic:
Incompatible
gametes; sperm
cannot penetrate egg;
dies early in female
repro tract
I.D.-ing reproductive isolation
• Prezygotic mechanisms - prior to embryo
formation
• Postzygotic mechanisms -after embryo
formation
• Requires observation of mating behavior,
reproductive success and offspring viability
• OK for some contemporary species, but what
about:
– Fossil species?
– Hard to watch or catch species?
– Rare or endangered species?
Need Operational species
definition
• Species Concepts
– Biological: Are two groups actually or
potentially interbreeding?
– Morphological: Are two groups (fossils)
sufficiently different from each other?
– Phylogenetic: Are two groups bound by
common ancestry? Do they form monophyletic
groups?
Morphological
• Often applied to newly
discovered species,
before matings are
observed
• Applied to fossil
remains of same size,
shape and bone
arrangement
• Problem =subjective
Phylogenetic
• If genetically similar,
good chance they are
interbreeding…or
could, or recently did
• Uses monophyletic
groups: An ancestor,
ALL its descendents
& ONLY its
descendents
• Precise - no guessing
about who mated