Download Speciation and Macroevolution

Speciation
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Speciation: splitting of one species into 2 different species.
What is a species? Based on ability to reproduce.
“Biological species concept”: a species is a group of organisms that
interbreed under natural conditions and that are reproductively isolated from
each other.
– Reproductively isolated: don’t produce fertile hybrids.
– Natural conditions: artificial breeding doesn’t count. For example, artificial
insemination, keeping 2 species locked up together.
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In contrast, the older “morphological species concept”: members of the
same species look similar to each other. Many examples of organisms that
look similar but can’t produce fertile offspring.
Problems with biological species concept:
Doesn’t work with fossils or extinct species.
Doesn’t work with asexual species , such as most bacteria.
How to deal with what is “natural”.
Reproductive Isolation
• How do populations become reproductively isolated?
• Result of gene mutations and altered allele frequencies due to
selection and genetic drift.
• To maintain as a single species, there must be gene flow between
populations: matings between members of separated populations
that allow mixing of alleles.
• In the absence of gene flow, mutations in different populations will
be independent, and allele frequencies will change independently of
each other.
• In most cases, migration is the key to gene flow.
• Once gene flow stops: genetic divergence occurs. The two
populations gradually become genetically different.
• Speciation sometimes occurs very quickly, other times more slowly.
In most cases it is not an instantaneous event.
Genetic Divergence
Reproductive Isolation Mechanisms
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Pre-zygotic (before mating) vs. postzygotic (after mating).
Pre-zygotic: different breeding
seasons (either yearly or time of day),
different signals: sight, sound, smell,
behavior, mechanical problems—
genitalia don’t fit together. Also
gametic isolation: sperm don’t detect
the egg or can’t fertilize it.
Post-zygotic: hybrid inviability: hybrids
don’t survive to birth or adulthood;
hybrid sterility: as with the mule, a
sterile hybrid.
Even having hybrids that are
significantly less fit (able to survive and
reproduce) than purebreds is an
isolating mechanism, a way to prevent
the two populations from fusing into
one population.
Allopatric Speciation
• The simplest and
most common
mechanism of
speciation is allopatric
speciation: 2 groups
of one species are
isolated
geographically, and
diverge into separate
species.
More Allopatric Speciation
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Geographical barriers: mountains,
oceans, rivers. A few members of
a species manage to cross by a
rare chance event.
This is the mechanism by which
Darwin’s finches evolved into
separate species in the
Galapagos islands. Only very
rarely can birds cross the ocean to
get to other islands.
Or, the barrier develops slowly as
conditions change: the gradual
formation of the Grand Canyon
split a population into 2 isolated
groups, that have diverged into
separate species, the Kaibab and
Albert squirrels.
Sympatric Speciation
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Geographical isolation is the easiest
way for species to form, but there are
other possible mechanisms.
“Sympatric speciation” means
speciation that occurs within the same
geographical location.
An example: cichlid fish in Lake
Barmobi Mbo in Cameroon, Africa—an
isolated volcanic lake. Nine species,
all more closely related to each other
(by DNA evidence) than to similar fish
in other lakes. Lake has no distinct
geographical zones, and the fish can
easily swim anywhere in it. They feed
in different locations, but all breed in
the same location, close to the bottom.
An example of sympatric speciation,
but the mechanism is not clear.
Sympatric Speciation by Polyploidy
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About half of all flowering plants are
polyploid: more than 2 copies of each gene.
Polyploids are the result of failure of cell
division (mitosis or meiosis) to separate the
chromosomes into 2 cells.
New polyploids are usually sterile, or their
offspring are sterile: extra chromosomes
with no homologue to pair with in meiosis
leads to unbalanced (aneuploid) gametes,
which will produce dead offspring.
Triploids (3 copies of each gene) are
sterile—the source of seedless fruit.
In animals, sexual reproduction is essential
every generation, so most polyploids fail to
reproduce: need a similarly polyploid mate
to produce fertile offspring.
In plants, vegetative propagation is possible
for many generations without sex, and in
many species one plant contains both
sexes. So, reproduction with a polyploid
partner is easier in plants than in animals.
Hybrid Zones
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When two populations of a species are
separated by a geographical barrier, they
diverge genetically. Sometimes the barrier is
removed and the two groups come into
contact with one another. The region of
contact is a “hybrid zone”.
Several possibilities exist:
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If the two groups have only diverged a bit, fertile
offspring will result, and the two groups will
merge back into a single species. Geographical
differences may exist within the species:
different subspecies or varieties, but all can
interbreed freely.
If the two groups have diverged to the point that
no fertile offspring result from their matings,
sexual selection will occur to deter further
matings. New pre-zygotic reproductive isolation
mechanisms, especially behavioral differences,
arise to reinforce the division between the two
species.
This is called “parapatric speciation”: 2
species forming while in contact with each
other in a restricted region.
Parapatric Example
•Shrimp in Panama. Up until 3
million years ago, North and South
America were separated by the
ocean. As dry land rose between
the continents, shrimp who once
freely interbred were separated by
a barrier. When put together in the
lab (or by going through the
Panama Canal) now, they attack
each other rather than mating.
Patterns of Speciation
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What happens after 2 species separate from
each other?
In some cases, the species exists for
millions of years, gradually changing in
response to external conditions but always
maintaining as a single distinct species.
In other cases, many new species will form
from a single species in a very short time:
this is “adaptive radiation”. This often
happens on isolated islands, where a new
species is blown in by a storm, and finds
many different ecological niches to fill.
Darwin’s finches are an example of this.
They are thought to have originated with a
small group of finches that blew over about
1 million years ago, to islands with no
dangerous predators and very few other
land birds.
Also, sometimes a “key innovation” will
arise, and new niches will suddenly be open
to the possessors of the innovation. Or, a
disaster will allow a sudden expansion of a
few surviving species. Mammals grew
rapidly in number of species following the
extinction of the dinosaurs.
Extinction
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Extinction can happen: none left
of the species.
Various events can cause
extinction: being outcompeted for
a critical resource, having the
climate change too rapidly to
adapt.
“Mass extinctions” are caused by
catastrophic events. The Earth
has had several mass extinction
events, where the vast majority of
species die out over a short period
of time. This is what is seen when
one moves between various
geological ages. Asteroids hitting
the Earth are responsible for at
least some of these, but probably
not all.