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Chapter 24
The Origin of Species
“Observe always that everything is the result of a
change, and get used to thinking that there is
nothing Nature loves so well as to change existing
forms and to make new ones like them.”
--Emperor Marcus Aurelus Antoninus
Essential Idea
Gene pools change over time.
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TOK

Punctuated equilibrium was long
considered an alternative theory of
evolution and challenge to the long
established paradigm of darwinian
gradualism. How do paradigm shifts
proceed in science and what factors are
involved in their success?
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Gene Pools
Gene pools consist of all the genes and
their different alleles present in an
interbreeding population.
 For instance, the genes for making type
A, B, and O blood.
 The different alleles for doing so: IA, IB,
i.

Evolution

Evolution requires that allele
frequencies change with time in
populations.
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Speciation

The origin of a new species is the focal
point of evolutionary theory. The
appearance of a new species is the
main point of diversity.


Microevolution describes the adaptations that
arise within a gene pool.
Macroevolution occurs when evolutionary change
occurs above the species level. These are big
changes.
Speciation

Speciation due to divergence of isolated
populations can be gradual as we’ll see
with a variety of examples.
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Speciation

Speciation can also occur abruptly as
we’ll see later on with Gould and
Eldridge’s Punctuated Equilibrium.
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Ernst Mayr and the Biological Species
Concept

This concept defines a species as a
population or group of populations
whose members have the ability to
interbreed in nature and produce viable,
fertile offspring, and are unable to
produce viable, fertile offspring with
other populations.
Reproductive Isolation

Reproductive isolation is a mechanism
by which many species are isolated
from one another due to the existence
of many different types of biological
barriers.
 1.
Prezygotic barriers impede mating or
hinder fertilization.
 2. Postzygotic barriers prevent viable,
fertile adults from forming.
Prezygotic Barriers to Mating

Impede mating/hinder fertilization:
 1.
 2.
 3.
 4.
 5.
Habitat isolation
Temporal isolation.
Behavioral isolation.
Mechanical/Morphological isolation.
Gametic isolation.
1. Habitat Isolation

Occurs as 2 species live in the same
area, but encounter each other rarely, if
ever.
 Example
2 species of garter snakes. One
lives in the water, the other lives on land.
2. Temporal Isolation

This occurs when there are differences
in breeding times, seasons, or years
which prevents gamete mixing.

Example: Eastern and Western spotted skunk.
Their geographic ranges overlap, but the Eastern
skunk mates in the late winter, and the Western
skunk in the late summer.
3. Behavioral Isolation

Courtship rituals which occur between
closely related, but different individuals
produce effective reproductive barriers.

Example: Blue footed boobies of the Galapagos.
The courtship ritual for these organisms males do
a dance where he shows off his blue feet to a
female in high-step form.
4. Morphological Isolation

Occurs when differences in appearance
and/or anatomy cause different species
to be unable to mate.
 Example:
Plants of different color attract
different pollinators preventing cross
pollination. Also, differences in flower
shape and design prevents cross
pollination.
5. Gametic Isolation

This happens when the sperm of one species
is unable to fertilize eggs of another species.

Reasons for why this does not occur: The sperm
may not survive the reproductive tract of a female
of a different species; the sperm may not be able
to penetrate the egg once it gets there.
 Example: Sea urchins are a variety of closely
related aquatic animals. They reproduce in a
similar way, but they are distinct enough that
their gametes do not fuse to form zygotes.
Post Zygotic Barriers to Mating:

Prevent production of viable/fertile
adults.
 1.
Reduced hybrid viability.
 2. Reduced hybrid fertility.
 3. Hybrid breakdown.
1. Reduced Hybrid Viability

Occurs when genes of different parent
species interact and impair normal
growth and development of the
organism.
 Example:
A specific subspecies of
salamander live in areas where they
occasionally meet and breed. Often times
the offspring do not develop fully and those
that do are not very fit.
2. Reduced Hybrid Fertility

Sometimes 2 species can mate and
reproduce a viable species that is
sterile. The sterility is often a result of
the two parents having a different
number or structure of chromosomes.
Thus, meiosis fails to produce normal
gametes.
 Example:
A donkey and a horse mate and
a mule is formed.
• Donkey
+
Horse
=
Mule
3. Hybrid Breakdown

Occurs when 2 parents meet and
produce offspring. When these
offspring mate with each other or the
parents, the resulting offspring is very
weak and sterile.
 Example:
different strains of rice that each
carry a number of recessive alleles. When
the offspring mate, the recessives
accumulate in the F1. Thus, the resulting
F2 offspring are very weak and sterile.
Limitations of the BSC



These barriers to mating are not easily
seen because we can’t observe the
matings of fossilized remains.
We also can’t evaluate the reproductive
isolation of prokaryotes and other
organisms.
Additionally, there are a lot of animals we
don’t know much about making it difficult
to apply this concept.
2 Main Ways of Speciation
1. Allopatric (other country) speciation.
 2. Sympatric (same country)
speciation.

1. Allopatric Speciation

Occurs when gene flow in a population is
interrupted by a geographic barrier.


Example: Lakes may rise and fall separating
groups of individuals. Rivers may split land that
was formerly as one.
Once the barrier has been set up and
populations begin to diverge, mutations and
natural selection take over and allele
frequencies change as genetic drift alters the
gene pool.
1. Allopatric Speciation
Allopatric speciation is likely to occur in
small populations as they are more
affected by genetic drift than larger
populations.
 To confirm allopatric speciation,
scientists bring together 2 species in a
laboratory and see if they can
successfully breed and produce viable,
fertile offspring.

1. Allopatric Speciation

The Galapagos ground finch Geospiza
dificilis:
 Females
respond to songs from males
from the same island and ignore songs
from males of the same species from
different islands (allopatric populations)
 This demonstrates that prezygotic barriers
have developed in these allopatric
populations and that they are on their way
to becoming separate species.
2. Sympatric Speciation

This occurs in geographically
overlapping populations. Even though
direct contact remains between
members of the same species,
mechanisms such as chromosomal
changes and nonrandom mating alter
gene flow.
2. Sympatric Speciation

A general example:


Polyploidy occurs when accidents happen during cell
division, and cells end up with extra sets of chromosomes.
If a diploid cell becomes a tetraploid (4n) as a result of an
error, and the organism is able to self-fertilize, it can become
reproductively isolated in just one generation.
Polyploidy
Polyploidy occurs a lot in the genus
Allium (onions).
 This is a mechanism that adds a lot of
genetic diversity to the species and
allows for speciation to occur.

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Polyploidy
Allium is a genus of monocot
angiosperms.
 It includes:

 Onions
 Chives
 Scallions
 Shallots
 Garlic
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Polyploidy

In many of these species of Allium,
polyploidy has created multiple
phenotypes resulting in a large number
of reproductively isolated populations
with similar characteristics.
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2. Sympatric Speciation

Example:
 The
North American apple maggot-fly
usually colonizes Hawthorn trees--and
eats haws.
2. Sympatric Speciation

Some flies have switched from haws to
apples and lay their eggs on the apples while
the fruit is still on the tree.
 They often compete with each other for
territory on apple trees, rather than with other
flies on the hawthorn trees.
 These flies are becoming more and more
entwined with the cycle of the apple trees.
2. Sympatric Speciation

Apples ripen and fall about a month earlier
than the haws.
 One month is a lifetime for these flies, so the
switch from haws to apples by these flies is
pushing the two gene pools apart.
 One fly is reliant upon the life-cycle of the
hawthorn tree, the other is reliant on the lifecycle of the apple tree and they are shifting
their breeding cycles to account for their new
habitat.
Adaptive Radiation

This occurs when a few organisms make
their way into a new environment and give
rise to diversely adapted populations of new
organisms that have descended from a
common ancestor.
 This is often seen following mass extinctions
when numerous niches open up.
The Tempo of Speciation


Speciation can occur abruptly.
Niles Eldridge and Stephen Jay Gould coined
the term ‘punctuated equilibrium’ to describe
the tempo of speciation.
 Punctuated equilibrium is marked by periods
of apparent stasis in the evolution of an
organism and then is followed by points
where speciation occurs rapidly.
The Tempo of Speciation

Sometimes we see fossils in the strata that
never change and then disappear.
 This doesn’t mean that the organism didn’t
change or evolve. It just means that the
events that produced the new species may
have occurred too rapidly to have been
preserved in the fossil record.
The Tempo of Speciation

For example:
 Say
a species survived for 5 million years,
and many of its major morphological
changes occurred in the first 50,000 years
(1%). Often times this is too quick to be
preserved in the fossil record. Then, it
would seemingly appear suddenly, linger
with no little/no change, and then become
extinct.
The Tempo of Speciation

Stasis can also be explained this way. Often
times, changes go undetected by the fossil
record. Consider changes in biochemistry-they can’t be detected by paleontologists.
The Tempo of Speciation

Speciation due to divergence of isolated
populations can be gradual as we’ll see
with the following example in
Stickleback fish.
Macroevolution

Macroevolution results as species diverge
and speciate again and again resulting in
differences that accumulate and become
more pronounced. Speciation is the
beginning of macroevolutionary change.
 These cumulative changes occur as a result
of thousands of small speciation events.
 Thus, if you accept microevolution, you get
macroevolution for free.
The Arguments

Many arguments against evolution fail to
recognize the fact that many complex
structures evolve in small increments from
simple ancestral structures that perform the
same basic function.
 It’s hard to imagine millions of years when
you can’t comprehend what a million actually
is.
The Arguments

“I believe that modern opposition, both overt
and cryptic, to natural selection, still derives
from the same resources that led to the now
discredited theories of the nineteenth century.
The opposition arises, as Darwin himself
observed, not from what reason dictates but
from the limits of what the imagination can
accept.”
 George
Williams, evolutionary biologist.
The Arguments

For example, consider the human eye. It is a
very complex structure that works to form an
image and transmit the information to the
brain for processing.
 How could such a complex structure evolve
in gradual increments?
 How could a partial eye be of any use to our
ancestors?
The Arguments

The flaw in the argument is the assumption
that partial eyes have no use. Simple light
sensors are useful, even though they can’t
focus an image.
 Many different eyes evolved on different
organisms as they diverged from a common
ancestor with light-sensing photoreceptors.
Remember These Things:

Evolution is not goal oriented, it is the result
of the interactions of organisms with their
current environments. The most fit
organisms survive. As the environments
change over time, so too do the organisms.
 These small changes (microevolution)
accumulate and slowly give rise to big
changes (macroevolution).
Remember Also,

“Speciation is a process, NOT an
event.”