Classification
 Organisms
are classified into a
hierarchical classification that groups
closely related organisms and
progressively includes more and more
organisms.
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.
Evolutionary Species Concept
 George
Gaylord Simpson proposed the
Evolutionary Species Concept in the
1940’s to add an evolutionary time
dimension to the Biological Species
Concept.
Evolutionary Species Concept
 Evolutionary
species concept “A single
lineage of ancestor-descendant
populations that maintains its identity from
other such lineages and that has its own
evolutionary tendencies and historical
fate.”
Evolutionary Species Concept
 Definition
applies to both sexually and
asexually reproducing species and
emphasizes common descent. As long as
diagnostic features are maintained a
lineage will be recognized as a single
species.
Phylogenetic species concept
 A third
species concept is the phylogenetic
species concept.
 “an irreducible (basal) grouping of
organisms diagnosably distinct from other
such groupings and within which there is a
parental pattern of ancestry and descent.”
Phylogenetic species concept
 The
phylogenetic species concept also
emphasizes common descent and covers
both sexually and asexually reproducing
organisms.
 Under
the PSC any population that has
become separated and has undergone
character evolution will be recognized as a
species.
Phylogenetic species concept

Criterion of irrreducibility requires that no more
than one diagnosibly distinct population can be
included in a single species. Thus, the
emphasis is placed on monophyly: lineages that
contain all the descendents of a single common
ancestor.

Main difference in practice between ESC and
PSC is that PSC recognizes as species the
smallest groupings of organisms that have
undergone independent evolutionary change.
Phylogenetic species concept

The ESC would group into one species a series
of geographically disjunct populations that show
some genetic divergence, but the PSC would
treat them as discrete species.

Thus, subspecies under the ESC would be
species under the PSC and in general more
species would be recognized under the PSC
than either the BSC or ESC.
Typological Species concept

For historical interest this is the pre-Darwinian idea
that species are defined by fixed and unchanging
features and do not change over time (i.e., evolve).

Biologists discarded the idea after Darwin’s theory
of evolution by natural selection became
established.

Creationist’s still cling to the typological species
concept and you’ll often see “types” referred to in
creationist writings.
Applications of species concepts
 Diversification
 Copepods
in marine copepods.
are small abundant
crustaceans. Numerous populations of
Eurytemora affinis have been described
from estuaries in the northern hemisphere
and traditionally grouped into one species
on the basis of similarity of appearance.
Diversification in marine copepods

A study by Lee (2000) in which she compared
gene sequences of populations and also carried
out breeding trials showed that at least 8
phylogenetic species exist, which are
reproductively isolated.

Clearly, assuming species identity on the basis
of morphology alone will underestimate species
diversity.
16.3
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 phylogenetic 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:



Isolation of populations.
Divergence in traits of separated populations
(e.g. mating system or habitat use).
Reproductive isolation of populations that
maintains isolation when populations come
into contact again (secondary contact).
How species form
 Recent
research shows that steps one and
two may take place simultaneously in the
same place and often the third step does
not occur.
Genetic Isolation: physical
isolation

Physical separation reduces or stops gene flow
between populations and as a result there may
be a balance between gene flow and natural
selection (recall the Lake Erie water snake
example from chapter 6).

On the islands selection favors elimination of
alleles for banding, but migration constantly
introduces them. If the islands were to be
completely separated so no snakes migrated
natural selection would result in the island
populations becoming different from the
mainland ones.
Allopatric speciation
 This
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.
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 te
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.
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.
 An
estimated 70% of flowering plants
appear to have had polyploid events in
their evolutionary history as have 95% of
fern species.
Mechansims of divergence
 Dispersal,
vicariance and polyploidization
create opportunity for speciation to take
place.
 For
speciation to occur populations must
diverge genetically from each other.
Genetic drift
 Genetic
drift is a sampling phenomenon in
which only some alleles occur in a
population as a result of its small size
because of founder effect and
bottlenecking.
 If
a population remains small for a period
of time many alleles may be lost from the
gene pool.
Genetic drift

The length of time the population is bottlenecked
has a strong influence on how great allele
frequency changes will be. Theoretical studies
show that if populations remain very small for
only a short time then only rare alleles are likely
to be lost and little effect on speciation is likely.

Thus, scientists are increasingly focusing on
natural selection as a more important force
driving speciation than drift.
Natural selection and speciation in
apple and hawthorn 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 speciation in
apple and hawthorn 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 speciation in
apple and hawthorn 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 speciation in
apple and hawthorn 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 speciation in
apple and hawthorn 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 speciation in
apple and hawthorn 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 speciation in
apple and hawthorn 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
Hybridization

In many cases hybrid offspring have reduced
fitness and this maintains two distinct gene pools
and incipient species.

However, in some instances, hybridization
appears to promote speciation, especially in
plants, as some hybrids may obtain
combinations of genes from parental species
that enable them to occupy habitat that neither
parental strain can.
Hybridization
 For
example, Helianthus anomalous a
southwestern species of sunflower,
possesses a unique combination of genes
from H. annuus and H. petiolaris and is
clearly the result of a hybridization event.
Genetics of speciation
 How
much genetic differentiation is
needed to separate populations enough
that two new species are produced?
 Historically,
it was considered that large
differences would be necessary, but more
recent thinking is that large differences are
not necessary
Genetics of speciation: pea aphids

Pea aphids are small sap-sucking insects.

Via et al. have studied two populations one that
lives on red clover and the other on alfalfa.

They have shown that members of each
population actively chooses its preferred plant
and each does poorly if reared on the other
plant.
Genetics of speciation: pea aphids
 Crosses
between the two populations
produce F1 hybrids that do poorer than
either parental population on both plants.
 Via
et al. have identified alleles at several
locations in the genome that increase
fecundity on one plant, but decrease it on
the other.
Genetics of speciation: pea aphids

These data suggest that there is a genetic tradeoff and that alleles that lead to high fitness on
one plant lead to low fitness on the other.

In addition, alleles for plant preference and
success on that particular plant appear closely
related, which suggests the same allele may
have multiple effects or that alleles for success
and preference are closely linked.
Genetics of speciation: pea aphids

If it is common for the same genes or closely
linked sets of genes to simultaneously alter
preference and increase success on host plants
then mutations of these genes should lead to
speciation on the basis of host plant use.

Because there are millions of plant-feeding
insects, this may be an important mechanism of
speciation.