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SPECIATION
I. Speciation = the formation of new species. It is important to note
that evolution does not necessarily lead to speciation.
A. Species = groups of populations that have the potential to
interbreed in nature and produce viable offspring. Stated another way, species
are reproductively isolated groups of populations.
1. Population = a group of interbreeding individuals of the same
species sharing a common geographical area
2. Horses and donkeys are different species because their offspring,
mules, are sterile.
B. Recall the four processes of evolution:
1. mutation - changes in nucleotide sequences of DNA
2. recombination - reshuffling of the genetic material during meiosis
3. natural selection - differential reproduction
4. reproductive isolation (discussed shortly)
C. Mutation and recombination provide natural variation, the raw
material for evolution.
D. Natural selection acts upon that variation. Those individuals with more
advantageous variations have greater success passing on their genes to
succeeding generations.
II. Reproductive isolating mechanisms - If species are defined as
groups of populations that have the potential to interbreed in nature and
produce viable offspring, then reproductive isolation is necessary for
speciation to occur.
A. Reproductive isolating mechanism = any structural,
functional, or behavioral characteristic that prevents successful reproduction
from occurring
1. Premating isolating mechanisms = those that prevent
the possibility of mating:
a. Habitat isolation - two species occupy different habitats,
even within the same geographic range, so that they are less likely to
meet and to attempt to reproduce
Geographical isolation is probably the most common reproductive
isolating mechanism, rivers, lakes, mountains, oceans, deserts,
glaciers. Islands, eastern and western Hemlock trees.
Ecological isolation - specialized soils, wet vs. dry sites,
parasites develop different hosts
b. Temporal isolation - two species may live in the same
location, but each reproduces at a different time, so they do not attempt
to mate
E.g. spring and fall flowering Witch Hazel plants, flowers which open
during the night vs. those that open during the day
c. Ethological isolation - behavioral phenomena prevent
interbreeding between different species
E.g. Elaborate mating rituals in birds, mating calls of frogs and
birds, flashes of light in fireflies, cricket calls
d. Mechanical isolation - differences between two species
reproductive structures or other body parts prevent mating
E.g. male and female genitalia incompatible, different flower shapes
require different pollinators.
2. Postmating isolating mechanisms = developmental or
physiological differences between the members of two species that prevent
successful reproduction after mating has taken place:
a. Gamete isolation - physical or chemical incompatibility of
gametes of two different species so that they cannot fuse to form a
zygote; an egg may have receptors only for the sperm of its own species
b. Zygote mortality - hybrids (offspring of parents of two
different species) do not live to reproduce
c. Hybrid sterility - the hybrid offspring are sterile (e.g.,
mules)
III. Types of speciation
A. Allopatric speciation - some type of physical barrier forms,
preventing gene flow among members of a population. Once reproductively
isolated the two populations may diverge genetically
E.g. the Pleistocene glaciations caused considerable speciation in the
plants of North America, uplift of Sierras in California caused speciation
in newly created deserts, populations of pup fishes diverged in the Great
Basin
B. Sympatric speciation - speciation that takes place within the
home range of an existing species
Common in plants due to polyploidy (multiple sets of chromosomes) -
hybrids may become fertile and persist.
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species
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hybrid
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chromosomes
double
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polyploid
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AA
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BB
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---------->
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AB
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----------------------------->
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AABB
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(24)
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(30)
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(27) sterile
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(54) fertile
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E.g. wheat is the result of two wild species that hybridized, doubled
their chromosome number, and then hybridized with another species
IV. Rates of speciation
A. Gradual - speciation takes place over long periods of time by the
gradual accumulation of many small changes. This is thought to be the norm for
most speciation events.
B. Punctuated - speciation takes place very rapidly and then
there are long periods of stasis.
C. Sometimes bursts of evolution may take place in one lineage, resulting
in the formation of many new species in a wide range of habitats. This is
called adaptive radiation. This often happens on newly formed islands.
The Galapagos tortoises and finches are examples.
D. Just as new species emerge, some species die out or become extinct.
Extinctions may be quite gradual, for example certain organisms with only
slight statistical disadvantages eventually lose out in the struggle for
survival.
E. Extinctions may also be quite abrupt, when a catastrophic event
wipes out major groups of organisms. For example, it is believed that climatic
effects resulting from an asteroid impact wiped out the dinosaurs.
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