Transcript Species Concept

Species Concept
Chapter 16
• Write down your best definition of a
“species.” Include any characteristics that
scientists might use to determine if two
populations of organisms that look similar
are the same species or are different.
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What is a species?
• Defining “species”
is more difficult that
many people
imagine.
• The “Barnyard
taxonomy” mentality
makes it look easy:
a cow is a cow, a
horse is a horse.
Natural species
• In nature, defining
species gets much
harder. A species may
live over a wide
geographic range and
vary slightly from one
region to another, so
that the individuals at
one end of the range
look different from
those at the other end
of the range.
Can hybridize
Can hybridize
Can hybridize
Can hybridize
Can hybridize
Do not hybridize
One species or two?
Cordillerian Flycatcher
Pacific Flycatcher
Reproductively isolated species
One species or two?
Myrtle Warbler
Audubon’s Warbler
Varieties within the same species
One species or two?
Silene douglasii var. oraria
Silene douglasii var.
douglasii
Two subspecies of the same species.
Defining “species”
• Among animals and for the most part
with plants, a species is generally
considered to be a group of
interbreeding populations whose
offspring are fertile and produce offspring
similar to the parents. (i.e. “breed true”)
• Defining “species” among the fungi and
the one-celled organisms is much more
difficult.
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• If two populations of organisms that look
similar are different species, what keeps the
two species separate? Think of behaviors,
location, other factors that keep the two
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populations from mixing.
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Reproductive isolation
• While species may be hard to define, we
do know that reproductive isolation is
how animal and plant species are
maintained.
• Two types of isolation:
• Premating isolation
• Postmating isolation
Premating Isolation
• Premating isolation prevents organisms
from interbreeding:
• Geographic isolation
• Temporal isolation
• Behavioral isolation
• Mechanical incompatibilities
Geographic isolation
• Populations of the same species which
become separated from one another
may be subject to different selective
pressures that favor slightly different
traits.
• Without interbreeding between the two
populations, genetic differences between
the two groups are maintained.
Geographic Isolation
Kaibab Squirrel
Abert Squirrel
Geographically-isolated sub-species.
Geographic Isolation
•Colonization
of islands can
lead to strong
geographic
isolation.
Islands far from
the mainland
have few
migration
events, and
strong
isolation.
Ecological Isolation
• Two populations may live in the same
geographical region, but occupy different
habitats within the region.
• Ecological isolation slows, but does not
eliminate, gene flow between
populations.
Ecological Isolation
White-crowned Sparrow:
Occupies fields, meadows,
forest edges.
White-throated sparrow:
Lives and feeds within
dense thickets.
Both are found in the same regions.
Temporal isolation
• Two species may
live in the same
habitat, but
reproduce at
different times.
• Different breeding
seasons reduce
gene flow between
populations.
Temporal isolation
• Would isolation have a bigger effect on the
genetics of small populations or large
populations over several generations?
Why?
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Behavioral isolation
• Two populations may share a habitat and
breed at similar times, but animals
especially may have different courtship
rituals.
• An animal that does not recognize
another animal’s courtship ritual will be
unlikely to breed with that animal.
Behavioral isolation
• The dark-phase and
light-phase Western
Grebes occupy the
same habitat and
breed at similar times,
yet maintain their color
forms by differing sets
of courtship behaviors
(probably learned).
Mechanical isolation
• Mechanical isolation occurs when
individuals from two populations attempt
to interbreed, but incompatible body
shapes, pollen types, etc. may foil the
attempt.
Mechanical isolation
• Salvia mellifera and
Salvia apiana are
similar species of sage
that occupy the same
desert regions.
• S. mellifera takes
pollen off of the backs
of bees, while S.
apiana takes pollen
from the sides of bees.
Mechanical isolation
• Chihuahuas and
Great Danes are
the same species,
shaped by human
selection.
• However,
mechanical
isolation prevents
them from
interbreeding.
• Suppose no one had seen Chihuauas and
Great Danes before. Suppose you are a
naturalist who comes across an island
where both Chihuahuas and Great Danes
live. Would you describe them as the same
species or different species? Why?
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Postmating Isolation
• Sometimes two species can breed. Two
mechanisms can maintain reproductive
isolation:
• Gametic incompatibility
• Hybrid inviability
Gametic Incompatibility
• Even if mating or pollination takes place,
sperm may fail to fertilize an egg.
• Chemical defenses may exist to ward off
sperm from another species.
• In plants that rely on pollinators that visit
many flowers (such as honeybees),
strong gametic incompatibility systems
often exist.
Hybrid Inviability
• Hybrid embryos may die early in
development. If born, some hybrid
animals are weak.
• Viable hybrid animals (mules, ligers) are
often infertile. Hybrid birds, even if fertile,
may have “hybrid” courtship rituals that
interfere with successful mating.
Effects of Isolation
• Geographic isolation can lead to
allopatric speciation if the separation is
maintained for many generations.
• Ecological, temporal, and behavioral
isolation can lead to sympatric speciation
over many generations.
Allopatric Speciation
• Allopatric speciation occurs when
populations are completely separated
from one another by geographical
barriers:
• Formation of a river
• Rise of a mountain range
• Accidental migration to an island
Allopatric speciation
requires strong
geographic separation
that prevents gene
flow between the main
population and the
smaller, new
population.
Sympatric Speciation
• Sympatric speciation occurs within or on
the margins of a larger population.
• Behaviorial, ecological, or temporal
isolation reduces gene flow between
populations, allowing the populations to
diverge.
The stronger the
isolating mechanism,
the more likely it is
that the two
populations will
diverge. Populations
on the margins of
the range are
subject to different
selection pressures
than the main
population.
Extinction
• If environmental conditions change
rapidly and dramatically, some
organisms will go extinct.
• The more specialized an organism is to
its habitat, the more vulnerable it is
during a rapid climate change, even
though that specialization was an
advantage before the change.
Major Extinction Events
• The earth’s history is punctuated with
several major extinction events.
• For some of these events, we have clues
about the cause, such as an asteroid
crash or a global climate change.
• We are living in the middle of a major
extinction event -- and we are the cause.
Recap
• “Species” and other taxa are humandefined categories. Not all organisms fit
into our neat taxonomic boxes!
• Divergence that leads to speciation is the
result of isolation, followed by different
selection pressures.
• Isolation may be geographic, ecological,
behavioral, or physiological.