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How do new species originate?
How do we account for
the great diversity of life
in the biosphere?
www.cdnn.info/news
http://bio1903.nicerweb.com
www.botany.wisc.edu
www.condorjourneys-adventures.com
What is a species?
We say that all dogs are in the
same species, but that Norway
maples and sugar maples are
in different species. How do
we know that? What do we
mean?
What we mean by species is a
group of individuals with the
potential to interbreed under
natural conditions and produce
fertile offspring. By this
definition each species has its
own gene pool that does not
mix with the gene pools of
other species.
www.thestonemill.com
http://oregonstate.edu
Shortcomings of this definition
As discussed earlier, this species
definition does not work for
species that reproduce only
asexually.
Moreover, it is not always feasible
to determine whether mating is
occurring between two groups in
the same habitat.
And sometimes members of two
species look so much alike that
we have a hard time telling them
apart.
We sometimes make mistakes in
concluding that two groups are in
different species, when in fact
they do share genes through
mating.
Sturnella
neglecta
Sturnella
magna
www.sacsplash.org
www.stephenbodio.com
The eastern and western
meadowlarks (above, S. magna
and S. neglecta, respectively) look
similar, but their songs and other
behaviors are sufficiently different
that they would not interbreed, if
they met in nature.
And mimicry in nature can confuse
biologists even more
• As an adult, the
butterfly on the
left, Monarch
danaus, is similar
to the viceroy
butterfly on the
right, Limenitis
archippus.
• But compare their
appearance as
larvae!
www.mongabay.com
www.cirrusimage.com/
www.richard-seaman.com
How do gene pools of similar species
remain separate from each other?
• A catastrophe (earthquake, fire,
new highway, and so on) may
North rim
separate one population in two
Kaibab
smaller populations.
squirrel*
• A famous example is the two

populations of Kaibab squirrels,
isolated geographically from each
other by the Grand Canyon
(perhaps 6 to 7 million years ago).
South rim
• We do not know if these two
populations have been isolated for Abert
enough generations that mutations squirrel **
would have accumulated that have

made the two populations
reproductively isolated from each
* Sciurus aberti kaibabensis and
other.
**Sciurus aberti
Geographical Isolation
Habitat Isolation
Two other ways to ask this
question are as follows:
What keeps different
species from breeding with
each other? How to they
maintain their reproductive
isolation?
An example is two garter
snake species in the genus
Thamnophis. One species
spends most of its time in
water, while the other is
primarily terrestrial.
Such barriers may occur
before fertilization has a
chance to occur, or after
fertilization.
A first step toward
reproductive isolation is
often habitat isolation
within the same area.
http://sparkleberrysprings.com
www.wildherps.com
An example of ecological
isolation by different habitats
• There exist 750 wasp species (!) each of whose
members lay their eggs in the flower of a different fig
species. These fig species share the same habitat but
bloom at different times. This keeps the different wasp
species reproductively separate from each other.
Note the long
ovipositor on the
fig wasp. Why is
this adaptive?
Fig wasp laying her eggs
amongst the ovules of a
developing fig fruit
(www3.imperial.ac.uk/)
Emerging new generation of
fig wasps (image from
www.morning-earth.org)
Temporal isolation is another prezygotic reproductive barrier
 Two species may
breed at different
times of the day, or in
different seasons, or
in different years.
Spilogale gracilis
Image from http://fwp.mt.gov/
 For example, even
though the territories
of the western and
eastern spotted
skunks, Spilogale
gracilis, and S.
putorius, overlap, S.
gracilis mates in the
late summer while S.
putorius reproduces
in the late winter.
Spirogale putorius
Image from www.cedarcreek.umn.edu
Different courtship behaviors may
isolate two species
Below, four similar looking beetles with
very different courtship behaviors–
Posterior courtship
behavior in Linsleya
beetles 
 Mounting behavior in
Meloe beetles
Linear courtship
behavior in subfamily
Meloinae beetles 
 Anterior courtship
behavior in
Tegrodera beetles
Images from www.ipmworld.umn.edu
Sexual selection by coloration
can act as a reproductive barrier
Biologists at the University of Leiden in the Netherlands raised two
cichlid species (Pundamilia pundamilia and P. nyererei) together in
two different aquaria. One included natural light and the other
monochromatic orange light.
Under natural light the two species looked very different from each
other, but under orange light they were indistinguishable from each
other.
Campbell’s Biology, 7th ed.
Cichlid experiment, concluded
RESULTS:
With natural light, in which the
two species looked different from
each other, the females mated
only with males of their species.
However, under orange lighting
the females mated with males of
either species indiscriminately
and produced fertile offspring.
INTERPRETATION AND
CONCLUSION:
The species are very similar to
each other, differing only in their
coloration.
This color difference acts as a
prezygotic reproductive barrier.
The two species very likely
separated from each other only
recently in their evolutionary past.
P. pundamilia
Cichlid images from www.fishecology.ch
P. nyererei
Mechanical isolation may keep two
species reproductively separate
• In such a situation, even if mating is
attempted, structural differences in their
reproductive organs prevent copulation.
Both shells must
coil in the same
direction
A boa’s
hemipene
www.proexotics.com
And be sure to check out the amazing video of
the four-headed penis of the spiny anteater 
at http://scienceblogs.com/neurophilosophy/
2007/10/experimenting_with_a_fourheaded_penis.php
(scroll halfway down)
Four-headed
penis of the
spiny anteater
Coiling direction matters to a snail!
Shells of three
closely related snail
species (image a)
Two snails whose
coiling directions are
the same are able to
mate (image b)
The genitalia of two
snails with opposite
coiling direction do
not match up (see
image c).
Gametic isolation
Sperm of one species might
not be able to survive in the
reproductive tract of another
species.
Or biochemical differences
might prevent the sperm of
one species from getting
through the protective barrier
of another species’ ova (in
both plants and animals).
For example, sea urchins
of more than one species
might reproduce in the
same place at the same
time, but the sperm of
species A might be
incompatible with the ova
of species B and vice
versa.
Thus, even though members
of two such species
copulated, fertilization would
not occur.
www.dailygalaxy.com
Postzygotic barriers may exist
between two “subspecies”
For example, if cross
fertilization does occur,
the resulting hybrid
offspring may not
complete embryonic
development or may
be frail.
This is true of some of
the salamander
subspecies of the
genus Ensatina in
California.
We describe this
situation as hybrid
inviability or reduced
hybrid vigor.
http://evolution.berkeley.edu/

Reduced hybrid fertility
• Even if hybrids between two
species or subspecies (two
emerging species) are survive
and are healthy, they might
not be fertile.
• Chromosome incompatibilities
might prevent pairing at
Prophase I of meiosis and
thus limit gamete production.
• This keeps the two gene pools
effectively separate from each
other.
www.utc.edu
• The well known example of reduced
hybrid fertility is the mostly sterile
mule (see image b, below), produced
by a mating between a male donkey
(a) and a female horse (c)*:
(a)
(b)
(c)
www.talkreason.org/
• Another example comes from the
work of a friend of mine, Sandra
Anagnostakis. She crosses different
chestnut species in an attempt to
produce a tall, straight, blightresistant tree with chestnut fruits. Her
world-famous chestnut plantation is at
the east end of the Sleeping Giant.
* But see the following:
www.ncbi.nlm.nih.gov/pmc/articles/PMC1289946/pdf/j
rsocmed00204-0031.pdf !
Hybrids might have
low fertility due to
chromosomal
incompatibilities
 For example, in a hybrid
between a parental stock
and a stock with
translocations, the
chromosomes will not
match up well at
Prophase I of meiosis.
 Only some of the resulting
gametes will have full sets
of genetic information.
https://qspace.library.queensu.ca
Hybrid breakdown: A final
reproductive isolating mechanism
Some hybrids are robust and fertile and mate either
with each other or with a parent (!), but the offspring of
such matings may be either weak or infertile.
That minimizes the gene flow between the two
species.
For example, some rice species follow this pattern.
Healthy hybrids have been produced experimentally, in
attempts by farmers to increase crop yield by combining
genetic traits of both parents.
However, attempts to cross two different such hybrids
often fail; the two rice strains that produced these
hybrids are well on their way to becoming different
species.
Explanation for hybrid breakdown in
the case of rice crosses
Various strains of
cultivated rice have been
produced by artificial
selection.
*
*
Over time they have
diverged genetically from
their common ancestor
and have each
accumulated different
recessive mutations.
Crosses between such
strains often bring
together harmful mutant
alleles, often resulting in
weak or sterile hybrid
offspring.
* “N” means normal, and “S” means sterile.
** The Rf allele restores fertility, but the rf allele does not.
Summary of the different mechanisms for
reproductive isolation of species
*
(or have different niches)
*
* The terms “prezygotic” and “postzygotic” are sometimes used instead of
“premating” and “postmating”, respectively.
How do new species form?
• In addition to distilling Darwin’s theory into a few succinct
observations and inferences, Ernst Mayr has given us
the two major conditions under which speciation can
occur:
(1) Interbreeding between the populations of two emerging
species must be blocked, by their becoming isolated from each
other.
(2) Enough time must pass during this time of isolation for
mutations to accumulate and for the two populations to
become genetically distinct from each other. They must
become sufficiently different from each other that if the two
populations were brought together again, they would not be
able to share their gene pools.
• These differences might be caused by genetic drift or
accumulation of different mutations (as we have
discussed), or through different environmental pressures
that result in natural selection within each of these now
separated populations.
Biologists believe that most new species
arise from allopatric* speciation
We will consider the following examples of
allopatric speciation:
• Adaptive radiation, resulting from small
groups leaving a larger population and
colonizing new environments
• Geological and climactic changes
separating members of a population
• Human activities that split a population into
separate subgroups
* literally, “other country”
“Adaptive radiation” is well known
to occur in island populations
By this means, small subsets
of a common ancestral
population* leave at different
times, each traveling in its
own direction.
* Often on the mainland
Upon arrival in a suitable
habitat**, each such “founder”
group will become
established, responding to the
unique environmental
conditions of its new location.
** often an island
The Hawaiian islands provide many such
examples of rapidly evolving populations
• The five silversword species below are
all very closely related, based on
molecular studies, yet look quite
different from each other.
• Most Hawaiian species are endemic;
they can be found nowhere else.
http://ops.fhwa.dot.gov/freight/
freight_analysis/state_info/hawaii
•
•
The Hawaiian islands are
located ~3,500 km (about 2200
miles) from the nearest
continent.
Very young, these islands arose
volcanically; the youngest (“Big
Island”) appeared only about a
million years ago and still
demonstrates active eruptions.
Campbell’s Biology, 7th ed.
Dams built by beavers or humans create
ponds and lakes and separate populations
http://oakridgevisitor.com
www.mvhs65.com
 Studies of striped bass (Morone
saxatilis) were carried out by Irwin
Beitch 10 years after the completion
of a large dam in southeastern
Virginia.
 Kidney morphologies of populations
above (freshwater) and below (near
the Chesapeake Bay) the dam had
become significantly different from
each other.
www.highlandlakesflyfishing.com
Land masses that were connected
during the Triassic Period later
separated, taking life forms with them.
“Gondwana”
http://earthscience.files.wordpress.com
www.palaeos.com/
Check out the very cool animation at
http://www.divediscover.whoi.edu/images/gondwana2.gif!
Ratite birds– an example of early
evolution after Gondwana broke apart
Cassowary
(www.unfamiliar-image.co.uk )
www.animals.national
geographic.com
Rhea
www.uwm.edu/
Emu
The above map shows the distribution of living and recently extinct*
ratite birds. Key: C = Cassowary; E = Emu; e = Elephant Bird*; K =
Kiwi; m = Moa*; O = Ostrich; R = Rhea (see www.grisda.org)
Kiwi
Alternative models of speciation
* In such a situation,
however, there might
exist subtle microhabitat differences
within the same
space.
*
Ecological isolation within the same
space ( sympatric speciation)
One of many examples of
how microhabitat
differences might provide
environmental pressures
for genetically different
members of a species to
specialize in different
ways: palm trees in a
habitat that contains both
volcanic soil (high in ash)
and calcareous soil (of
coral reef origin)
Another example of microhabitat differences
leading to sympatric speciation
 The maggot fly Rhagoletis pomonella has a stable
parasitic relationship with the American hawthorn
tree, in which the fly lays its eggs.
 In the seventeenth century apple trees were
introduced to North America by the early colonists.
 Over the years scientists began to observe the fly
infecting apple trees, as well as hawthorns.
 The two fruit fly populations tend not to intermingle,
even if in the same habitat, and thus are unlikely to
interbreed. When offspring become adults, they
tend to lay their eggs in the same kind of fruit from
which they had hatched (perhaps based on a type
of olfactory imprinting).
 Over time these two populations have become
genetically different from each other and may be
on their way to becoming separate species.
♂
Apple maggot flies
(www.sciencecases.org/maggot_fly) 
♀
Sudden speciation by polyploidy
(happens mostly in higher plants)
• Sometimes DNA replication occurs normally (during the S) phase in
(especially in plants), but problems arise during meiosis.
• If chromosomes failed to separate from their homologs, then diploid
(2n) reproductive cells could result.
• Self fertilization of such diploid gametes would then produce
tetraploid (4n) offspring.
• This occurs surprisingly often in plants; it is estimated that 25 to
50% of all plant species evolved through polyploidy.
www.departments.oxy.edu
Speciation by polyploidy
http://trc.ucdavis.edu/biosci10v/bis10v/media/ch13/wheat_speciation.html
Polyploidy can occur during
mitotic cell division
Other common examples of
polyploid plant species
* Triploid crops: banana, apple, ginger
* Tetraploid crops: durum wheat, macaroni wheat,
maize, cotton, potato, cabbage, leek, tobacco,
peanut
* Hexaploid crops: chrysanthemum, bread wheat,
oat
* Octaploid crops: strawberry, dahlia, pansies,
sugar cane
CHALLENGE: Why are polyploids with odd
numbers of sets of chromosomes (e.g., triploid
species) sterile? And bananas are sterile, so
how do we get new banana plants?
http://ask.metafilter.com
Why triploid species are sterile