Download Speciation

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Natural Selection & Speciation
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Speciation
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First, what is a species?
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Second: Cladistics, cladograms, clades
Speciation is the process of one species diverging
(evolving) into two (or more) species.
At the most basic level, the process is dictated by two
processes:
1) Gene flow
2) Selection (Drift)
1) Gene flow is the movement of genetic
material within, and among, populations
of a given species
•
In plants, gene flow is accomplished through
pollen movement, and through seed (or clone)
dispersal.
Populations of Populus nigra.
Gene flow binds together the populations that make up a species.
Speciation is the process of one species diverging
(evolving) into two (or more) species.
At the most basic level, the process is dictated by two
processes:
1) Gene flow
2) Selection
2) (Natural) Selection is the differential success of
individuals within a population such that traits are
eliminated or emphasized over (evolutionary) time.
- variation among individuals in a heritable trait.
- variation results in difference in reproductive
success/survival.
Think about the
range of conditions
this species
experiences given
its distribution.
Flowering dogwood (Cornus florida)
http://www.plantbio.ohiou.edu/trees/Cornus%20florida.htm
Role of Selection?
(eg. habitats different)
Gene flow is holding the populations together.
Selection is driving the populations apart.
what if gene flow is prevented?
Differentiation, the development of ecotypes... maybe,
later, speciation
Ok…so how do you get from here, to speciation??
Four (broadly accepted) modes of speciation:
1) Allopatric speciation (classical model).
2) Founder effect speciation.
3) Peripheral isolates speciation.
4) Genetic hiccups.
1) Allopatric speciation (classical model).
- A species range is divide geographically by some
event, dividing the species into sub-groups and
blocking gene flow.
- Selection pressures within the sub-groups drive
them toward dissimilarity, differentiation occurs,
then eventually the two become reproductively
isolated.
Populus nigra.
- Gene flow is blocked
- Gene flow is blocked
Adaptation, differentiation, ecotypes, reproductive
isolation…speciation
2) Founder effect model of speciation.
- Some event leads to a very small population- or
with plants a single individual, arriving in some new
location, beyond the range of gene flow.
- Usually intense selection pressure (and inbreeding)
commence- the founder population diverges quickly
and becomes a new species.
Populus nigra.
Populus nigra.
- Gene flow is blocked (& inbreeding & intense selection
pressure)
Adaptation, differentiation, ecotypes, reproductive
isolation…speciation
3) Peripheral isolates speciation.
- Perhaps most common (or constant)
- Individuals at the edge of a species range are
isolated from the main body of the range.
- Gene flow is restricted- Selection intense.
- Ecotypes (varieties) develop, then reproductive
isolation, then speciation.
Puma concolor
Puma concolor
Puma concolor
Puma concolor
Gene flow from the south
to the north of this range is
highly restricted
Puma concolor
Populations on the periphery of
this very large range are
differentiating into varieties,
moving (perhaps) toward
speciation
Argentine puma
Costa Rican Cougar
Eastern South American cougar
North American Cougar (Puma concolor couguar) includes the
previous subspecies and synonyms arundivaga, aztecus, browni,
californica, coryi, floridana, hippolestes, improcera, kaibabensis,
mayensis, missoulensis, olympus, oregonensis, schorgeri,
stanleyana, vancouverensis and youngi;
Northern South American cougar
Southern South American puma
Puma concolor
Some populations are
already completely
isolated (e.g., Florida)
4) Genetic hiccup.
- Something goes wrong during the reproductive process.
- Some mutation occurs, or new portion of the genome is
turned on, that cause a wildly different morphology, or
other reproductive isolating mechanism.
- In plants, another “hiccup” involves a change in the
number of chromosomes. E.g., reproduction involving two
diploid organisms produces a triploid offspring. (which
then cannot interbreed with the parent population.
- Less well understood, the subject of much current
research.
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Speciation