Species and
Speciation
Species are different “kinds” of organisms.
Species are the product of divergence of genetic lineages.
When one genetic lineage becomes two each lineage
can have an independent evolutionary future.
The difference between species can be slight or dramatic.
Biologists differ in their concept of species. The difference in
concepts used can depend on the group of organisms studied or on
the goal of the researcher who studies them.
• Biological species concept – a population or groups of
populations that are actually or potentially interbreeding and
reproductively isolated from other populations.
• Evolutionary species concept – a single lineage of populations
that maintains an identity separate from other such lineages and
which has its own evolutionary tendencies and historical fate.
• Phylogenetic species concept – the smallest monophyletic group
distinguished by a synapomorphy.
synapomorphy
• Recognition species concept – the most inclusive population of
individual organisms that share a common fertilization system
Ultimately, the idea of a species is a human construct that allows us
to communicate about different forms of life.
1
Deciding what populations constitute a species can be difficult
because reproductive isolation or other species criteria can’t
always be easily assessed.
In practice, species are often identified by consistent differences
in one or more characteristics. This is the phenetic species
concept. Asexual organisms can only be designated phenetically.
It is assumed that
consistent differences
are associated with
p
reproductive
isolation.
If this is disproven,
then the species
designation can be
revised.
Even though the biological species concept can be problematic in
practice, and it only applies to sexually reproducing organisms, it is
the species concept that is most often used by evolutionary biologists
because it seems to correspond to what occurs in nature. Forms that
are reproductively isolated maintain separate identities.
Reproductive isolation – the
restriction of genetic exchange
between groups even when those
groups are sympatric (live in the
same region). The restriction is
often not absolute but sufficiently
restrictive
t i ti so that
th t eachh group
maintains a separate genetic
identity.
A single species can have a range of forms but so long as the forms
are not reproductively isolated, there is the potential for them to
exchange genes and share the same evolutionary fate.
2
Because the divergence
of lineages to the point
d ti isolation
i l ti
off reproductive
can be a long process,
many lineages may be
only partially isolated,
or isolated in some areas
but not in others.
The divergence of lineages
can be gradual and their
genetic separation may not
become complete. This can
result in hybrid zones. If
those zones are narrow, and
stable, they may be called
different species
3
Members of one species
often vary geographically.
Intermediate forms often
i t andd show
h evidence
id
exist
off
genetic exchange.
Sometimes geographic
variants of a species are
called subspecies.
Some species, are difficult to distinguish using external and easily
observable features, but are reproductively isolated.
Such cases are called “sibling species.”
Sibling species show that appearance is not
the critical criterion for different species.
4
Some plants, classified as different species broadly hybridize.
Populations, subspecies, and reproductively isolated species have
gradations of genetic differences.
5
The biological species concept can only be applied to organisms
that are sexual and outcrossing. Asexual forms may be called
different species, but a different criterion must be used. For
asexuals two forms are called different species if they differ in
some consistent way
way.
In spite of difficulties, the biological species concept is broadly
applicable. Many closely related species are reproductively
isolated. There are many barriers to gene flow that result in
reproductive isolation. Barriers are classified as premating or
postmating.
Reproductive isolation is due to barriers to gene exchange
I. Premating barriers – keep species from mating
II. Postmating barriers – mating occurs but gene flow between
species does not occur because of
A. Prezygotic barriers – mating occurs but zygotes are not
f
formed
d
B. Postzygotic barriers – zygotes are formed but have
reduced fitness
6
Premating barriers
1. Ecological differences result in
potential mates not meeting
a. Temporal (timing) differences
b Habitat
bi differences
diff
b.
2. Potential mates meet but do not mate
a. Behavioral differences
b. Pollinator differences
Premating barriers
1. Ecological differences result in
potential mates not meeting
a. Temporal (timing) differences
b Habitat differences
b.
2. Potential mates meet but do not mate
a. Behavioral differences
b. Pollinator differences
7
Postmating, prezygotic barriers
A. Mechanical barriers – poor fit of mating structures results
in ineffective gamete transfer
B. Copulatory behavioral barriers – mating occurs but there
is no fertilization because of behavioral differences or
lack of proper stimulation
C. Gametic isolation – mating occurs but gametes are
incompatible (lack proper enzymes, attractants, etc.)
8
Postmating, postzygotic barriers
A. Ecological inviability – the hybrid is not fit in either parents’
niche
B. Behavioral sterility – the hybrid is unable to attract either
parental species as a mate
C. Hybrid inviability – the hybrid has reduced survival due to
developmental problems
D. Hybrid sterility – the hybrid has reduced ability to form
viable gametes
Hybrid sterility is most often seen in the heterogametic sex –
males in mammals and insects – females in birds and butterflies.
This is called “Haldane’s Rule.”
Hybrid problems may not develop in the F1 offspring, but crosses
among the F1 and backcrosses to the parental species may produce
inviable or sterile offspring . This is called “F2 breakdown.”
9
Coyne and Orr used genetic distance to estimate time of divergence
of many species pairs of Drosophila and compared their degree of
prezygotic isolation and postzygotic isolation with their divergence
time.
The strength of isolation
increases with time – for both
types of barriers
Full reproductive isolation
evolves with variable amounts
of divergence (0.3 to 0.5) y
~1.5 to 3 million years
Among recently diverged forms the strength of prezygotic isolation
is greater than the strength of postzygotic isolation.
For one species to become two, separate populations of the
same species must become reproductively isolated.
For reproductive isolation to evolve
evolve, some change must occur
in one or both lineages in ecology, behavior, physiology,
biochemistry, or genetic system that makes them
reproductively incompatible. How one lineage can become
incompatible with its closest relative lineage is the key
question of how new species are formed.
10
How can an allele that makes an individual reproductively
incompatible with its relatives increase in frequency in a
population?
Dobzhansky-Muller Incompatibility
Allele A1 increases
in one population
due to fitness
advantages or due
to genetic drift.
Allele B1 increases
in one population
due to fitness
advantages or due
to genetic drift.
Alleles A1 and B1 are incompatible with each other and
hybrids (A1A2B1B2) are either not formed or have low
fitness when the populations come into contact.
Speciation can involve the gradual development of
reproductive isolation, or in the case of some types of
chromosomal change, be nearly instantaneous
Gradual speciation can be defined through the geography of
the populations involved.
11
Allopatric speciation is the evolution of
reproductive barriers between populations that
are geographically separated.
When allopatric populations expand their
ranges and come into contact they might
• interbreed and blend to become a single
continuous species
• interbreed in the region of contact and form
a stable
t bl hybrid
h b id zone
• not interbreed due to some barrier to
reproduction that evolved while they were
allopatric
The evidence for
allopatric differentiation
of geographically
separated populations is
clear
12
Peripheral isolation (peripatric speciation) - the development of
reproductive isolation in small marginal populations of a species.
There are many examples of new species that
arise from single populations of a widespread
i
species.
Moths of
the genus
Greya
This may not be different from simple
allopatric speciation or it may involve some
component of genetic drift.
13
Mayr hypothesized that founder populations, because they are
small, may have reduced genetic variation and low fitness due to
genetic drift. Drift may increase the frequency of alleles that
were rare in the ancestral population. In such a situation,
selection for new combinations of alleles that are compatible with
h newly
l fixed
fi d alleles
ll l may occur andd allow
ll increased
i
d fitness
fi
i
the
in
the new conditions. A possible result is a reorganization of the
genome that makes it incompatible with the ancestral population.
Mayr envisioned a fitness topography
where the founder population went
through a low fitness valley due to
drift and after selection and
reorganization, the population
evolved to a new fitness peak that is
incompatible with the ancestral
population.
In theory, natural selection can result in the evolution of barriers
to reproduction while the populations are allopatric.
Alternatively, selection can increase the degree of prezygotic
isolation among populations that have partial postzygotic
i l ti
isolation.
If the hybrid of two forms has
lower fitness than nonhybrid
offspring, any variation in a
prezygotic barrier in the two
forms mayy result in selection
that increases the frequency
of the alleles that are the basis
for the barrier.
The preference may be for any prezygotic barrier - ecological or
behavioral.
14
If the fitness of the hybrid is not reduced (there is no postzygotic
isolation), then there will be no selection to reinforce the
prezygotic barrier and the populations will likely blend.
Reproductive character displacement - species are more
similar in allopatry than in sympatry.
It can be produced by any selection for prezygotic differences.
It can also the product of competition for resources in the zone
of sympatry.
15
In tree frogs, where partial
postzygotic isolation is known,
song characteristics are most
similar between allopatric
and most different
ppopulations
p
between sympatric populations.
Females of each species show
greater preference for males of
their own species when they
come from populations that are
sympatric with the other species.
species
The strength of preference
decreases with the volume of the
call of the other species.
Prezygotic
than
P
i isolation
i l i is
i stronger among sympatric
i forms
f
h
among allopatric forms.
There is some evidence that postzygotic isolation can select for
prezygotic differences between species.
16
Parapatric speciation - the origin of new species over the former
range of the ancestral species.
The populations can only diverge if there is
relativelyy strongg selection across the ggeographic
g p
range of the species. Often due to an ecological
cline.
A stable hybrid zone may result if there is
moderate selection against the hybrids.
Complete divergence can occur if there is strong
selection against the hybrids - as in reinforcement
of reproductive isolation in formerly allopatric
populations.
Ring species are a special
case of parapatric
speciation over
geographic range.
The ends of the range are
geographically close to
each other but genetic
exchange can only occur
through a great distance.
The ends of the range are
more different from each
other genetically than
any of the intervening
populations.
17
The pattern produced by parapatric speciation and the
reestablishment of contact of formerly allopatric populations is
difficult to distinguish.
The best case for parapatric speciation
is in populations of plants on
contaminated soils.
Adaptation to contaminated soils results
in hybrids that are unfit in either
environment.
Selection against hybrids has resulted in
divergence
in
di
i flowering
fl
i time
ti in
i adjacent
dj
t
populations and selection for self
pollination in the population on the
contaminated soil.
Anthoxanthum odoratum - a grass
Sympatric speciation - the development of reproductive isolation
between forms of a species that live entirely in the same
geographic region.
Strong disruptive selection for habitat differences or differences in
reproductive timing may result in divergent phenotypes that
produce hybrids that are unfit for the same environment for which
the parental types are well suited.
Potential scenario: Two homozygous genotypes A1A1 and A2A2 are
well suited to different host plants and their hybrid A1A2 has low
fitness on both host plants. If another gene is introduced that
produces
d
a difference
diff
in
i mating
i behavior
b h i that
h is
i correlated
l d with
i h the
h
host plant it will reinforce mating among like genotypes and
potentially lead to complete divergence.
18
Apple maggot flies may be a case of the
beginning stages of sympatric speciation.
The apple race emerges
early and parasitizes apples.
Th Haw
H race emerges late
l t
The
and parasitizes haws. Any
mating between a late apple
fly and an early haw fly will
produce hybrids with an
intermediate emergence time
with fewer opportunities to
parasitize apples.
Those that avoid mating with the other race will produce offspring
with emergence times appropriate for apples or haws.
The two races already show some preferences in mating for
members of their own race.
Speciation by polyploidy and recombination
Hybrid organisms receive two different sets of chromosomes, one
from each parent species. They are usually sterile because
differences in gene arrangements among chromosomes results in
gametes
improper synapsis and aneuploid gametes.
Duplication of
whole sets of
chromosomes
(allopolypoidy)
may result in
gametes that
can produce
balanced sets of
chromosomes.
19
Allopolyploids with a diploid number of sets of chromosomes
from each parent (2NA + 2NB) produce gametes that are euploid
with one set of chromosomes from each parent (NA + NB).
Such organisms are potentially interfertile or self-fertile but they
can t produce fertile offspring in backcrosses with either parent
can’t
species.
Gamete (NA + NB) combined with gamete (NA) produces an
allotriploid (2NA + NB) that produces unbalanced sets of genes in
gametes.
Thus, allopolyploids are reproductively isolated from each of
their parent species. They can only reproduce with other
allopolyploids or through self-fertilization. They are new species
as soon as they are formed.
Many species of plants and some animals are polyploid. At least
50% of all flowering plants are polyploid.
Read: Evolutionary History of Humans
20
Classically there have
been two hypotheses for
the evolution of humans,
the multiregional
hypothesis
- modern
yp
Homo sapiens evolved
simultaneously
throughout the old world
from archaic Homo
sapiens with exchange of
genetic information by
gene flow
and the out-of-Africa
hypothesis - modern
humans evolved in
Africa and moved out
replacing previously
widely dispersed
archaic humans
21
Mitochondrial DNA analysis of modern
humans suggests that Asian, European,
Australian and Indonesian populations
all share a common ancestor that
dispersed from Africa about 80,000
years ago
ago. Multiple dispersals out of
central Asia appear to account for
European populations.
22
Read: Did Humans and Neanderthals Interbreed?
23