Speciation
Species Definition
• Biological species: a reproductively isolated group in which all
individuals potentially or actually interbreed amongst themselves but
are genetically isolated from other such groups.
• Morphological species: groups of individuals showing similar
phenotypes.
• Paleospecies: a chronological series of similar forms.
• Agamospecies: groups based on the amount of variation or gaps
in the variation of phenotypic features.
FIGURE 9.1 Defining Species
(a) Morphospecies. Viewed today, at one moment in time, species A, C, and E are clearly
distinct, demarcated by current natural discontinuities between them. (b) Paleospecies
(chronospecies). Viewed historically, through time, discovered fossil intermediates (B and
D) fill in the missing gaps above, giving us a more or less continuous series with no
obvious discontinuities between them.
The Process of Species Formation
Four Steps:
• Single population or reproductive community.
• Development of a reproductive barrier (formation of
allopatric populations).
• Differentiation of the separated populations.
• New species can no longer interbreed if barrier
disappears and they become sympatric.
Four steps leading to
speciation.
First, a single species is an
interbreeding reproductive
community.
Second, a barrier develops,
dividing the species.
Third, separated into
different habitats, the
divided populations become
differentiated through the
accumulation of differences.
Fourth, so different have the
separate populations
become, that is when the
barrier disappears and they
overlap again. Interbreeding
does not occur.
Allopatric speciation
The populations of
Tamarin monkeys are
separated on the
sides of the Amazon
River. Where the river
tributary is wide and
individuals on
opposite banks do not
interbreed, the
populations are
diverging toward
separate species.
Where the river
tributary is narrow, the
individuals still
interbreed.
Allopatric speciation
involves the geographic
separation of the parent
population into two
subpopulations. It can
occur if a a physical
barrier separates the
range of a population.
Allopatric speciation can also
occur following a founder event.
Such an event
may have led to
the development
of the thirteen
species of
Darwin’s finches.
Geographic isolation is usually a
prerequisite to speciation. However, there
are situations in which speciation can take
place between populations that are
sympatric. Perhaps the most common
example of this is the result of polyploidy
in plants.
Parapatric speciation
occurs between
contiguous populations.
The parent population
does not completely split.
Two species of crows,
Corvus corone and
Corvus cornix, meet
along a line in central
Europe. Along that line
they produce hybrids.
Speciation is
incomplete.
The critical evidence in the case of parapatric speciation involves hybrid
zones. Hybrid zones are defined as an area of contact between two
different forms at which hybridization takes place.
Hybrid zones are an example of a stepped cline.
How do such clines relate to speciation?
If hybrids are disadvantageous, natural selection will work to to reinforce
the reproductive isolation of the two forms. So, the process of
parapatric speciation calls for:
1. The evolution of a cline.
2. Hybrids selected against in the hybrid zone.
3. Two different forms selected to mate only with others of their own
type.
If hybrids are disadvantageous, natural selection will
work to to reinforce the reproductive isolation of the
two forms. So, the process of parapatric speciation
calls for:
1.
2.
3.
The evolution of a cline.
Hybrids selected against in the hybrid zone.
Two different forms selected to mate only with
others of their own type.
Sympatric speciation occurs
without any separation of the
ancestral geographic range. Some
evolutionary biologists doubt this
ever happens. In all likelihood, it
can only occur after the
establishment of a stable
polymorphism.
The green lacewings, Chrysoperla carnea and
Chrysoperla downesi may illustrate an example of
sympatric speciation.
C. Carnea is light green in spring and
early summer, changing to brown in the
fall. C. downesi is a darker green year
round. Their color is adapted to their
habitat. C. carnea lives in fields and
meadows in the summer and moves to
deciduous trees in the fall. C. downesi
lives mainly on conifers. Their ranges
are sympatric. The species are
separated by breeding season as well
as habitat. C. carnea breeds in winter
and again in summer. C. downesi
breeds only in the spring.
It is thought that speciation in the lacewings
began with a multiple niche polymorphism.
One morph was adapted to live on conifers and
the other in a deciduous habitat. Once the
polymorphism was established, selected would
favor assortative mating, because crosses
between the two morphs would not be well
adapted to either habitat. Divergence in
breeding season could have been the
mechanism by which assortative mating was
brought about.
Reproductive Isolation
Different mechanisms
prevent reproduction
between individuals of
different species. These may
occur premating or
postmating, as illustrated
here with two species of
salamander.
How does reproductive isolation evolve?
Premating or prezygotic mechanisms:
a) Ecological or habitat isolation
b) Seasonal or temporal isolation
c) Sexual or ethological isolation
d) Mechanical isolation
e) Isolation by different pollinators
f) Gametic isolation
Postmating or zygotic mechanisms:
a) Hybrid inviability
b) Hybrid sterility
c) Hybrid breakdown
Patterns of Speciation
Yarrow
Cline – a gradient or gradual change in a character
within a species over a geographic area.
Represent an early step in the evolutionary process.
Tadpoles of the
leopard frog show
clinal variation in
their ability to
survive in waters of
varying
temperatures.
This reflects
adaptation to
different climates in
populations found
at different
latitudes.
In 1949, Moore fertilized
leopard frog eggs from females
in the north with sperm from
males progressively farther to
the south.
The degree of embryo or
tadpole abnormalities was
scored, from A (normal young)
through progressively more
abnormalities to F (high death
rate).
Today, this study and others
prompt biologists to actually
divide leopard frogs into
subspecies or even different
species.
The four groups of leopard frogs resemble one another closely in their external
appearance. But early tests of interbreeding produced defective embryos in some
combinations, leading biologists to suspect that these might be different
subspecies or even different species.
Research on males’ mating calls indicates that the various groups differ
substantially, and that such prezygotic behavior separates and reproductively
isolates members of each group, producing four species: (1) Rana pipiens; (2)
Rana blairi; (3) Rana utricularia; (4) Rana berlandieri.
Ecotype
A local population
genetically adapted to a
local environment.
Within a species,
ecotypes do not
typically overlap
geographically.
An ecotype that may be
visually detected may
be considered a
subspecies.
A cline is a geographic
series of ecotypes.
Yarrow – Achillea sp.
A transect, cross section of the yarrow’s distribution from sea shore to
high mountains is shown across California. Note the clinal variation in
height.
Three garden plots were selected at
three different locations along the
transect- sea level (Stanford), 4,600
feet (Mather), and at 10,000 feet
(Timberline).
Yarrow seeds collected from five
locations along this transect were
planted, grown into young plants, and
then divided into equivalent tufts,
clones, planted at the three garden
sites-Stanford, Mather, Timberline.
Note especially that sea-level clones
(from San Gregorio) at high elevations
did poorly (died), and high-elevation
clones (from Big Horn Lake) at low
elevations still did not grow to large
heights.
Clearly, ecotypes have developed
within yarrow.
Ring Species - Herring Gulls
As glaciers retreated, herring gulls
(Larus argentatus) were released
out of a north Pacific refugia
spreading one way across North
America and into western Europe;
and spreading in the other direction
across Alaska into Siberia.
From Siberia, as the herring gull now
extended its range further across
Asia, it tended to differentiate,
producing a subspecies (or species
by some ornithologists) such as the
vega gull (Larus vegae) and farther
west the lesser blackbacked gull
(Larus fuscus).
Eventually its current circumpolar
distribution became established
(dashed lines). Adjacent subspecies
interbreed (solid arrows), but where
the ends of the circular range of the
herring gull meet and overlap in
Europe, there is very little
interbreeding.
Ring species—salamanders
The ensatina salamander
(Ensatina eschscholtzii)
occurs from Canada to
Southern California with
interbreeding between
adjacent populations
through this range.
The Central Valley—a dry,
hot lowland area—is
divided into a coastal arm
and inland arm. However,
where these two arms of
the species meet again in
Southern California,
interbreeding does not
occur.
Parallelism and Convergence
Just as different habitats can produce different selective
pressures and result in ecotypes, similar habitats can
produce similar selective pressures and lead to organisms
with similar characters.
Parallelism – the situation in which closely related
organisms, in similar habitats, take on similar traits.
Convergence – the situation in which distantly related
organisms take on similar traits as a result of living in
similar habitats.
Australian marsupials
resemble placental
mammals in the rest of
the world.
Within the relative
isolation of Australia, the
marsupials entered
similar habitats as
counterparts among the
placentals elsewhere.
Under similar selective
pressures, similar
features and ecological
lifestyles evolved, but
upon a marsupial theme.
Convergent Evolution-Cacti
Different families of desert plants have evolved similar adaptations to the
desert’s dry, hot conditions - namely, succulent shoots with spines. Two
such plant species are shown from Africa. The third, from the New World,
is the endemic member of the true cactus family (Cactaceae).
Latitudinal Gradients
Within taxonomic groups,
species are not distributed
evenly across the planet.
There are distinct gradients
for many groups, with the
number of species
decreasing as one moves
away from the equator.
A number of explanations
have been suggested.
Latitudinal Gradient for Mammal Species
Suggested Explanations for Latitudinal Gradients
Evolutionary Time
Climate Stability
Productivity
Ecological Effects
An interactive exercise
“Every population of a species
differs from all others. The degree
of difference between populations of
a species ranges from almost
complete identity to distinctness
almost of species level”
Ernst Mayr