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Species
 The
species is the basic biological unit
around which classifications are based.
 However,
what constitutes a species can
be difficult to define and there are multiple
definitions of species in use today.
What is a species?
 The
species is a basic biological unit and
humans seem to intuitively recognize
species.
 However, why do species exist?
 Why
don’t we see a smooth continuous
blending of one species into another?
Why do we see discrete species?
 Because
intermediate forms between
closely related organisms are usually
selected against.
 If
they were not selected against, then the
two forms would merge into one as their
gene pools mixed.
Why do we see discrete species?
 Organisms
are very well adapted to their
environments having evolved over millions
of years.
 Each organism has specialized
characteristics such as camouflage,
feeding structures, behavior, and genitalia
that equip it to survive well in its
environment.
Why do we see discrete species?

An offspring that results from a cross between members
of two different species or between members of different
populations that have been evolving in isolation from
each other, will probably have traits intermediate
between its parents.

As a result, it likely will be less well adapted to its
environment than either parental form and be selected
against.

Thus, we see distinctively different species.
What is a species?
 John
Ray (1627-1705) gave first general
definition of a species.
A
species consists of all individuals that
can breed together and produce fertile
offspring.
A female donkey mated to a male horse
produces what?
A mule (which is sterile)
Hence, donkeys and horses
are separate species.
Biological Species Concept

Ray’s idea was updated into the Biological Species
Concept. Two definitions of the BSC are given below:

“Species are groups of actually or potentially
interbreeding natural populations, which are
reproductively isolated from other such groups.” Ernst
Mayr.

“A species is a reproductive community of populations
(reproductively isolated from others) that occupies a
specific niche in nature.” Ernst Mayr.
Biological Species Concept

The biological species concept emphasizes that
a species is an interbreeding population of
individuals sharing common descent and that
members of that community because they share
a niche constitute an ecological entity in nature.

Members of a species we expect to be similar to
each other but different from other organisms,
Criticisms of the Biological Species
Concept

The BSC has been criticized for several
reasons:
 1. It applies only to sexually reproducing
species.
 2. Distinguishing between species on the basis
of reproductive separation is problematic
because it can be difficult to determine how
much reproductive separation is needed to
distinguish between species.
 3. The definition refers only to current
populations and ignores the species status of
ancestral populations.
How many species of African
elephants are there?

Traditionally one species of elephant Loxodonta
africana has been recognized in Africa (a
second species Elephas maximus occurs in
Asia).

However, recent morphological studies have
pointed out that forest dwelling elephants in
West Africa appear to differ from elephants
found in Savannah habitats elsewhere on the
continent.
How many species of African
elephants are there?

A comparison of DNA from 21 populations
suggests that two species exist and it has been
suggested by Roca et al. (2001) that forest
elephants be named Loxodonta cyclotis.

Whether the two populations are capable of
interbreeding is unclear, but the clear genetic
differences between populations suggest that
conservation biologists should be attempting to
conserve members of both populations.
How species form
 Classically,
speciation has been viewed as
a three stage process:
1. Isolation of populations – populations
become separated from each other.
2. Divergence in traits of separated
populations (e.g. mating system or habitat
use). Isolated populations adapt to their local
environemnts and become different from the
ancestral population
How species form

3. Reproductive isolation of populations
occurs. The populations become so different
that they can no longer mate successfully.

These differences result in mating isolation
between populations being maintained if and
when populations come into contact again
(secondary contact).
Allopatric speciation
 The
three-step process described is the
essence of Ernst Mayr’s allopatric model
of speciation.
A
physical barrier isolates a population or
populations from the rest of the species
and selection favors genetic divergence of
that population.
Allopatric speciation
 Separation
of populations can occur by
two major means:


Dispersal of some individuals across a
barrier creating new isoloated populations.
Development of a new barrier that separates
populations [Vicariance] (the vicariance event
could be e.g. change in flow of a river, lava
flow, development of a mountain range,
habitat destruction)
Geographic isolation through
dispersal

We have already encountered example sof
speciation after individuals crossed a barrier.

The ancestors of Darwin’s finches colonized the
Galapagos Islands after dispersing from South
America and speciated into the current range of
species.

Similarly, the Hawaiian Islands were colonized
by ancestral Drosophila fruit flies that appear to
have speciated to produce more than 500
endemic species of Drosophila on the islands.
Evidence for founder hypothesis of
speciation in Hawaiian Islands

The main hypothesis for how the Hawaiian
Islands became populated with a diverse variety
of endemic species most of which occur on only
a single island is the founder hypothesis.

According to the founder hypothesis new
species are formed when a small population of
individuals disperses to a new island and after
being separated diverges from the ancestral
form.
Evidence for founder hypothesis of
speciation in Hawaiian Islands

The Hawaiian Islands were formed by a
stationary geological “hot spot” over which the
continental plate drifts northwest.

Periodically, the hot spot produces magma
flows, which form islands that are then carried
away on the plate and ultimately erode away.
Thus the newest islands are close to the hot
spot and the oldest further northwest.
Evidence for founder hypothesis of
speciation in Hawaiian Islands
 Based
on the geological information the
founder hypothesis makes two predictions
about the pattern of speciation that should
be observed.


Closely related species should be found on
adjacent islands and
Some speciation sequences should match the
sequence in which islands formed.
Evidence for founder hypothesis of
speciation in Hawaiian Islands
A
study of mitochondrial DNA of four
species of closely related Drosophila by
DeSalle and Giddings (1986) found the
predicted patterns.
 The
most recent species occur on the
youngest islands and several of the
branching events match the order of island
formation.
15.7
Geographic isolation through
vicariance events

There are many ways in which a species
distribution may be split into two by a physical
event. Some such as mountain formation are
slow, others such as a lava flow are rapid.

The Isthmus of Panama closed about 3 million
years ago separating marine populations on
either side. Did these populations speciate?
Geographic isolation through
vicariance events

A DNA sequence study by Knowlton et al. 1993
of snapping shrimp populations from both sides
of the isthmus suggests they did.

Seven pairs of morphologically closely related
species pairs occur, one of each pair on each
side of the isthmus and the DNA sequence
results confirm that these are each others
closest relatives, which is consistent with the
vicariance hypothesis.
Phylogenetic tree of numbered species of
snapping shrimp. P and C refer to
Pacific and Caribbean species
respectively.
Geographic isolation through
vicariance events
 Mating
experiments with the snapping
shrimp fund that males and females with
the greatest genetic divergence were least
interested in each other and almost none
the pairs produced clutches that yielded
fertile young.
Speciation without physical
isolation
 Not
all speciation requires physical
isolation of populations.
 Such
speciation without physical isolation
is called sympatric speciation.
Polyploidization as a mechanism of
speciation
 Polyploidy
(production of multiple sets of
chromosomes) appears to have played a
major role in the speciation patterns of
plants.
 In
such an event a new species can be
formed in a single or just a few
generations and no physical separation is
required.
Polyploidization
 An
estimated 70% of flowering plants
appear to have had polyploid events in
their evolutionary history as have 95% of
fern species.
Natural selection and sympatric
speciation in apple maggot flies
 The
apple maggot fly (Rhagolestis
pomonella) is a major pest of apples that
occurs throughout the northeastern U.S. It
also parasitizes hawthorn trees a close
relative of apples.
 Maggot
flies recognize trees on the basis
of visual, tactile and olfactory cues and
mate on or near the fruit.
Natural selection and sympatric
speciation in apple maggot flies
 Eggs
are laid on fruits and larvae develop
in them. When the fruit falls the larvae
burrow into the ground and pupate
emerging as adults the next year.
 Apple
trees are a novel food source for
these native flies, which exploited apples
after they were introduced about 300
years ago.
Natural selection and sympatric
speciation in apple maggot flies
 The
question is does the new food source
represent an island and are the
populations that breed on apples
genetically distinct form those that breed
on hawthorn trees?
 Do
apple and hawthorn populations
interbreed or not and are they diverging?
Natural selection and sympatric
speciation in apple maggot flies
 Hawthorn
and apple trees are often in very
close proximity so it would seem hard for
the populations to diverge.
 However,
a protein electrophoresis study
by Feder et al. (1988,1990) showed that
the populations are genetically distinct.
Natural selection and sympatric
speciation in apple maggot flies

Each population shows a strong preference for
its own fruit type, which because mating takes
place on fruit results in strong nonrandom
mating.

There is gene flow between populations
because about 6% of matings are crosspopulation matings, but despite this gene flow
natural selection appears to driving the
populations apart.
Natural selection and sympatric
speciation in apple maggot flies

Natural selection favors divergence because
hawthorn fruits ripen 3-4 weeks after apples. As
a result hawthorn fly larvae experience cool
temperatures before pupating whereas apple fly
larvae experience warmer temperatures.

Hawthorn flies and apple flies thus depend on
different temperature signals to time their
pupation and emergence the next spring and
have different developmental timetables.
Natural selection and sympatric
speciation in apple maggot flies

Experimental tests show that these
developmental schedules have a genetic basis
and individuals need the correct alleles to
develop under each temperature regime.

Individuals that are the result of crosses
between apple and hawthorn flies are thus
selected against and the populations have
diverged and continue to do so.
Secondary contact

Theodosius Dobzhansky (1937) the famous
geneticist reasoned that for populations that had
diverged and come back into contact hybrid
offspring between them would have reduced
fitness.

As a result there should be strong selection
favoring assortative mating (individuals mating
within their own population) and as a result a
variety of isolating mechanisms should evolve to
reduce the likelihood of interbreeding.
Isolating mechanisms
 Isolating
mechanisms fall into two
categories:


prezygotic (those that reduce chances of
mating and fertilization taking place) and
postzygotic (those that reduce the viability or
hybrid offspring).
Isolating mechanisms
 Examples
of prezygotic isolating
mechanisms:



Different habitat choice
Activity at different times of day
Differences in sexual advertisements: calls,
displays, pheromones.
Isolating mechanisms
 Examples
of postzygotic isolating
mechanisms:



Failure of zygote to develop
Reduced viability of zygote
Sterility