Chapter 16 – 17 Short Version
Species and Similarity:
On Being the Same
Yet Different
Figure CO: Booby
© rebvt/ShutterStock, Inc.
The Species Problem
• In the first three decades of the 20th
century, species were envisioned as
arising from saltations or macromutations, not from the gradual
accumulation of small hereditary changes
by natural selection
• One important voice for natural selection
during its period of eclipse was Sir
Edward Bagnall Poulton, (1856 - 1943), a
British evolutionary biologist who was a
life-long advocate of natural selection
• Ernst Mayr gave credit to Poulton as an
early originator of the Biological Species
concept
Sir Edward Bagnall Poulton
The Species Problem
Ernst Mayr
• The many biologists whose work generated
“The Modern Synthesis” conceived of
species in terms of populations, demes and
gene pools
• Ernst Mayr, revered as the greatest
evolutionary biologist of the 20th Century,
proposed the Biological Species Concept in
1942, and revised it from time to time over
his six decade profession career
• His last version, from 2001 states: "Species
are groups of interbreeding natural
populations that are reproductively isolated
from other such groups."
Limitations of the Biological
Species Concept
• Reproductive isolation cannot be determined
in fossil species
• Reproductive isolation cannot be determined
in asexual species
• Populations separated geographically for long
periods, often exhibiting vacariant
distributions, may be shown not to have
reproductive isolation in the laboratory, and
yet have been distinct for thousands, even
millions of years
Limitations of the Biological
Species Concept
“Catawba trees”
Catalpa ovata (left) from China and
Catalpa bignoides (right) from the
eastern US produce viable fertile
offspring when crossed in the
arboretum, but have been isolated
on different continents for millions of
years and are best considered as
separate species; another species is
found on Caribbean islands
The Species Problem Again
• Genetic species
– A set of organisms exhibiting similarity of DNA
• Agamospecies
– A species in which sexual reproduction does not occur,
represented typically as a collection of clones
– The absence of sexual reproduction means that the
biological species concept cannot be applied, and instead
taxonomists must rely on identifying certain diagnostic traits
to distinguish between closely related asexual lineages
– Consequently, the boundaries of agamospecies are often
hard to define
– Taraxacum: the genus of dandelions, an agamic complex; it
includes 26 (2n) species, [all of which can be crossed], and ~
200 “microspecies” are recognized!
The Recognition Species Concept
• According to Patterson (1993), species have a
specific mate recognition system (SMRS)
• Species can be defined as a set of organisms
with a common method of recognizing mates
• Advantages:
– Specific mate recognition systems (SMRS) are
easier to observe than interbreeding in nature
– The Recognition Species Concept may more
accurately represent what happens when a new
species originates
The Species Problem
• Ecological Species: Populations that are adapted to
certain ecological niches, and because of their
adaptations, will form discrete morphological clusters
• Advantages: Acknowledges the role the environment
plays in controlling morphological development
• Disadvantages:
– Cryptic species can be missed
– Ecological niches tend to be assumed and are difficult to
define and describe completely
– Many taxa exploit overlapping resources, or can suddenly
switch if a resource becomes scarce
– This is not a very robust species concept
Leigh Van Valen
(1935 – 2010)
Ring Species
• Ring species are species with a geographic
distribution that forms a ring and overlaps at
the ends.
• Although song sparrow (Passarella melodia) at
the northern and southern extremes of the
geographic range do not interbreed directly,
the unity of the species is maintained by
extensive gene flow between neighboring
populations all along the Pacific coast
Shown on the map are representatives of 3 of
the 34 subspecific populations of the song
sparrow. Lines indicate the approximate
breeding ranges of the illustrated subspecies,
while those of 14 other distinct populations are
shown by medium shading.
Chronospecies
• A chronospecies is a species which changes in
morphology, genetics, and/or ecology over
time on an evolutionary scale such that the
originating species and the species it becomes
could not be classified as the same species
had they existed at the same point in time
• This implies directional selection
• But does not rule out disruptive selection
Chronospecies
• A single lineage a evolves
over time into a
morphologically distinct
form b (left)
• If the connection is
recognized, both forms will
be assigned to a single
evolutionary species
• Especially if the fossil record
of the lineage is incomplete
(right), forms a & b may be
regarded and named as
separate biological species
organism a undergoes a
"pseudoextinction" when it
evolves into organism b
and
the fossil data can be interpreted as an
example of punctuated equilibrium
The Species Problem
• Evolutionary Species
– George Gaylord Simpson: “An evolutionary
species is a lineage (an ancestor-descendant
sequence of populations) evolving separately
from others and with its own unitary
evolutionary role and tendencies”
– One of the central “problems” of The Species
Problem is that there are many options for
species classification because there are many
options for the process of speciation itself
The Species Problem
• Phylogenetic (Cladistic) Species
Willi Hennig
1913 - 1976
– A group of organisms that shares an ancestor; a
lineage that maintains its integrity with respect to
other lineages through both time and space
– At some point in the progress of such a group,
members may diverge from one another: when
such a divergence becomes sufficiently clear, the
two populations are regarded as separate species
– The smallest diagnosable monophyletic group
sharing a set of unique synapomorphies
Phylogenetic (Cladistic) /
Evolutionary Species
• To be called a species under the PSC definition,
populations must have been evolutionarily
independent for a long enough time for diagnostic
shared derived traits (synapomorphies) to emerge
• Actually, time for evolutionary independence is a
necessity for most species definition concepts
A LIST OF 26 SPECIES CONCEPTS
Grades Versus Clades
• It has always been useful to classify
higher taxa based on common traits,
though this classification often implies a
ladder of life
• When groups are united by common
characteristics inherited from their
common ancestor, the taxon is a grade
• Unfortunately, despite their descriptive
value, identifying groups as grades can
cloud their evolutionary relationships
Similarity: Knowing When Characters
Are the Same or Different
• When similarity means shared ancestry:
Homology
• When similarity means parallel evolution:
Parallelism
• When similarity means similar solutions in
independent lineages:
Convergence
Homology and Analogy
• Darwin’s contemporary, the
comparative anatomist and
paleontologist, Richard
Owen (1804-1892), coined
the terms homology and
analogy (and Dinosauria)
• To Owen, homologies
indicated that organisms
were created following a
common (Divine/Vitalist)
plan or archetype
Homology and Analogy
• Homology: Any similarity between
phenotypic characters that is due to their
shared ancestry. Homologous structures may
retain the function they served in the
common ancestor or they may evolve to fulfill
different functions.
• Analogy: Any phenotypic characters that
perform the same or similar function by a
similar mechanism but evolved separately.
Analogies
• Analogy: Any phenotypic characters present
in different taxa which perform the same or a
similar function by a similar mechanism but
which evolved separately
– The structural carbohydrate chitin in fungal cell wall and in
arthropod exoskeletons
Analogies
• Thylacosmilus is a marsupial sabretooth cat
• Smilodon is a placental saber-tooth
cat
Traits
their
common
ancestor
lacked
Analogy: Oxygen-Binding Proteins
• Hemoglobin combines iron heme groups and globin proteins, red pigment found in
vertebrate erythrocytes
• Myoglobin combines iron heme groups and globin proteins, red pigment found in
vertebrate muscle tissues
• Hemocyanin combines copper prosthetic groups and globin proteins, blue pigment
found in arthropod and mollusc blood plasma
• Hemerythrin combines iron-containing non-heme globin proteins, pink/violet vs
colorless pigment found in some marine invertebrate blood plasma
• Chlorocruorin combines iron heme groups and globin proteins, red vs green
pigment found in many annelids
• Vanabins (aka vanadium chromagens) combine the rare metal vanadium in
prosthetic groups and globin proteins, colorless molecules found in sea squirts
blood plasma
• Erythrocruorin is a giant free-floating blood protein containing many dozens —
possibly hundreds — of iron- and heme-bearing protein subunits bound together
into a single protein complex with a molecular mass greater than 3.5 million
daltons Found in many annelids, including earthworms
• Pinnaglobin is a brown manganese-based porphyrin protein only found in the
mollusc Pinna squamosa
• Leghemoglobin combines iron heme groups and globin proteins, colorless
compound found in leguminous plants, such as alfalfa or soybeans
Homology
• A Brief History
– Richard Owen (1804–1892):
regarded organs as the same
(homologs), though they served
different functions
– After Darwin, scientists realized
that homologies are found in
organisms with a shared
evolutionary history, in
particular, those with a shared
common ancestor
Developmental Homology
• Developmental
Homology: A related
concept meaning that
structures arose from the
same tissue in embryonic
development
• the ovaries and clitoris of
female humans and the
testes and penis of male
humans are homologous
in this sense
Classic Structural Homologies
Determining
that
Phenotypic
Traits are
Homologous
Allows
Scientists to
Build
Phylogenies
Figure 07: Skeletal structures of the forelimbs of
representative terrestrial vertebrates
Classic Structural Homologies
The vertebrate forelimbs have the same basic bone design: one long bone attached to
two other long bones attached to a forefoot with many carpals attached to five or
fewer digits. The vertebrate hind limb offers similar evidence of homology
Classic Structural Homologies
early fossil amphibians
6
8
6
5
• The number of digits was variable in the earliest fossil
amphibians but pentadactyly became the standard for
all later vertebrates, though there have been many
reductions in number and a much fewer increases in
certain finned marine reptiles
Digit Increases From the Standard
Vertebrate Pentadactyl Limb
mosasaur standard;
plesiosaur not
ichthyosaur, like the plesiosaur, exhibits hyperphalangy
Note that a small change (mutation) in a homeotic gene
might produce an initial dramatic skeletal change quite easily
Homologies
• Homologous structures may be adapted for different roles in
which case their structure may vary considerably
• Consider mammalian incisors and canines
A Deep Homology: The Genetic Code
a few microbes have
a few differences
This is one of the most powerful of all
homologies, as it links all living organisms
on Earth to a common ancestor!
Protein Homologies
• The gene that codes for the protein hormone insulin
is homologous in humans and other mammals; the
similar DNA sequences descended from a common
ancestral gene sequence
• Human insulin (right) and pig insulin (left), showing
their one amino acid difference
Protein Homologies
• Fig. 1. Phylogeny of the insulin superfamily. A typical phylogenetic
tree of the animal kingdom is drawn and highlighted balloons indicate
the animal classes/phyla in which insulin/insulin-related peptides
were unambiguously identified in 1922, 1975 and 1998.
Physiological Homologies
•
•
•
•
•
Many primates, a few bats, and the elephant shrew
Phylogeny showing the distribution of
menstruation in placental mammals and
the inferred states of ancestral lineages
Menstruating species/lineages are
colored in pink, non-menstruating
species/lineages in black
Species in which the character state is
not known are not colored, and lineages
of equivocal state are represented with
black lines
Monodelphis (short-tailed opossums)
represents the outgroup
Note that there is strong evidence for
three independent originations of
menstruation among placental mammals
Analogy / Homology /Homoplasy
• homology is a common element in structure
between parts of different organisms due to
evolutionary differentiation from a
corresponding part in a common ancestor
• analogy is a trait or an organ that appears
similar in two distantly related organisms
– Biological analogies are often the result of convergent evolution
• homoplasy is when the organs of two
different species have similar characteristics,
functions and features as a result of similarity
of environment rather than common heredity
– something of a special case of analogy, or
– a more modern term for analogy
– and an example of convergent evolution
Homoplasy
• Homoplasy is the logical opposite of homology
• Homoplasy is the structural similarity between
two traits in two species without phyletic
continuity
• Even though the traits are similar, the common
ancestor of species A and B did not present the
trait
– The last common ancestor may not be so far away in
time or phylogenetic history
• There are three different types of homoplasy:
convergence, parallelism, and reversal
Homoplasy
• Convergence is the evolution of
similar traits in response to
similar adaptive pressures, but
not built upon similar genes or
developmental processes
• An example of convergence is
found in the electroreception of
mormyrids (African weak electric
fish) and gymnotoids (South
American electric eels): while
the organs responsible for this
perceptual capacity are similar,
they are not derived from the
same organ in their last common
ancestor
Convergent Electric Organs in Fish
• A fish is considered to be an electric
fish if it can generate electric fields
• Some electric fish live in the ocean and
some live in freshwater rivers of South
America and Africa
• The figure shows the location of the
electric organ in each fish, and a
sample of the waveform of the electric
organ discharge
• Most electric fish use electroreception
to swim in murky waters; some use
electric shock as a defensive weapon
If a fish can generate electric fields it is said to be electrogenic. If a fish
has the ability to detect electric fields, it is said to be electroreceptive.
Most fish that are electrogenic are also electroreceptive.
Ecological Equivalents
Without Convergence
Pileated
Woodpecker ►
And
Woodpecker
Finch ▼
Madagascar Aye-Aye (lemur)
Mimicry/Analogy/Convergence
• Pollination in the Bee orchid (Ophrys apifera)
and Spider orchid (O. fuciflora) is enhanced by
reproductive mimicry and the pheremones
that attract specific male bumblebees
Homoplasy:
Parallelism
• Parallelism occurs in closely related taxa,
and is defined as the independent
development of a descendant character
trait that is not present on a common
ancestor
• The evolution of similar features in closely
related lineages that do not share a last
common ancestor who exhibited the trait
An extinct South American Litoptern
camel
Homoplasy
• Parallelism occurs when two taxa
develop the same character after
evolutionary divergence; since the
trait is absent in a common ancestor,
but present in both descendant
species, it is probable that the
developmental genetic program that
produces the structures in the
different taxa is the same, which
means the developmental genetic
program was inherited from the
common ancestor
• Thus, there is homology between the
developmental and genetic materials,
but not on the final structure
• (Also ecological equivalents)
litoptern
camel 
Convergent vs. Parallel Evolution?
• Convergent evolution takes place • Parallel evolution involves the
when distantly related organisms
independent origin of similar features
give rise to species that resemble
in more closely related organisms
one another because they adapt in
comparable ways
Parallel
Evolution
Cichlid species from large lakes migrated to
smaller lakes and in both the smaller lakes, a
more elongated form evolved independently
Recknagel, H., Kusche, H., Elmer, K.R. and Meyer, A., Two new endemic species in the Midas cichlid
species complex from Nicaraguan crater lakes: Amphilophus tolteca and Amphilophus viridis
(Perciformes, Cichlidae), aqua, International Journal of Ichthyology, vol. 19 no. 4 - 25 October 2013.
Homoplasy: Reversals
• hair and heterodont teeth
brown bear
pelycosaur
cynodont
orca
The Panda’s Thumb?
2nd elongated
carpal
• The panda has five digits
like most mammals;
however, opposing these
is another digit, a “thumb,
“ which is actually not a
thumb at all but an
enlarged wrist bone
analogy
Human (Intelligent) Design
• Machine functions vary and machine designs vary
accordingly, using unrelated parts created from scratch
• Unlike biological organisms, human designed and built
machines show no correspondence of parts from
planes (flight), to mining machines (burrowing), or to
boats (swimming)
From Homology to Phylogeny
• Homologous traits can be grouped into series
• Homologous traits can include adaptive traits,
maladaptive traits, vestigial features and atavisms
• All homologous traits can be potentially useful in
deriving phylogenetic trees
• However, care must be taken to avoid using
analogous traits by mistake
• Finally, homologous traits are excellent evidence
that evolution has, in fact, occurred!
Chapter 16
End
Chapter 17
Origin of Species
Figure CO: Mimulus Flower
© LubaShi/ShutterStock, Inc.
What Interests Us About Speciation?
• Speciation provides evidence that evolution
occurs
• Speciation provides important insights into
the mechanisms of evolution
• Patterns of speciation provide insight into the
distribution patterns of extinct and living
organisms
• Speciation explains patterns in the ecology
and reproductive biology of organisms
What Interests Us About Speciation?
• What are the causes of speciation?
• What are the rates of speciation and do they
differ among different taxa?
Species Definitions
• Morphological / Typological Species: a set of organisms
sharing structural similarities between members and
discontinuities in structure between different species
• Mayr’s Biological Species: groups of interbreeding natural
populations that are reproductively isolated from other such
groups
• Ecological Species: a set of organisms adapted to a particular
set of resources, called a niche, in the environment
• Genetic species: A set of organisms exhibiting similarity of DNA
Species Definitions
• Agamospecies: a set of organisms in which sexual reproduction
does not occur, represented typically as a collection of clones
• Chronospecies / Paleospecies*: a set of extinct organisms which
changes in morphology, genetics, and/or ecology over time on an
evolutionary scale such that the originating species and the
species it becomes could not be classified as the same species
had they existed at the same point in time
– [*Note: experts establish fine distinctions between chronospecies and paleospecies]
• Phylogenetic (Cladistic) / Evolutionary Species: a set of
organisms that shares a common ancestor and maintains its
integrity with respect to other lineages through both time and
space
• Ring Species: a set of generally hybridizing species with a
geographic distribution that forms a ring and overlaps without
hybridization at the ends
An Important Reminder
• Regardless of species definitions, a species, to be a
biological entity, must exist in an ecological niche
• Can a population of organisms which no longer has a
niche still be a biological entity?
• Only 400-500 Siberian tigers still exist within their range
Problems Defining Species Through Time
a) Morphospecies:
Viewed today, at
one moment in
time, species A, C,
and E are clearly
distinct species,
demarcated by
current natural
discontinuities
between them
Problems Defining Species Through Time
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
morphological
discontinuities
between them [still
an oversimplification]
Species Can Change Without
Speciation Being Initiated
• Local adaptations may occur among the
populations and demes of a species
• Even when a species consists of many
subpopulations, gene flow between
subpopulations may slow or even inhibit
local specializations
Speciation
• Branching speciation and
adaptive radiations can
be initiated when genetic
exchange within or
among populations is
impeded
• Directional selection
produces anagenesis
• Disruptive selection
produces cladogenesis
Speciation
• Four different processes have been defined to
describe different modes of speciation
(cladogenesis) in sexually reproducing
species, most of which satisfy the definition
of Mayr’s Biological Species Concept
• These same four processes may also explain
speciation in many asexual organisms, but
isolating mechanisms will be different
• The processes differ in how the evolving
demes are distributed geographically
Spatial Aspects of Speciation
• Allopatric speciation – a physical barrier
divides a continuous population
• Peripatric speciation – a small founding
population enters a new or isolated niche
• Parapatric speciation – a new niche
found adjacent to the original niche
• Sympatric speciation - speciation occurs
without physical separation inside a
continuous population
Spatial Aspects of Speciation
As you can see in
this diagram,
peripatric and
parapatric are
very similar
Peripatric does
involve crossing
some sort of
barrier to find
favorable habitat
Speciation Initiated
by Geographical Isolation
• three steps
– geographical isolation
– local adaptation
– reproductive isolation
• three forms of geographical isolation
sometimes called a porous
or permeable barrier
Figure 01C:
Parapatric
Figure 01A: Allopatric
Figure 01B: Peripatric
Allopatric Speciation
• Mechanisms that prevent interbreeding may be:
• ecological:
• seasonal breeding, migration, etc.
• habitat preference
• differing abiotic factors, etc.
• behavioral
• activity times
• food acquisition, etc.
• physiological
• reproductive biology
• fertilization
• embryonic development
[More on this later in the chapter.]
Allopatric Speciation
Four steps lead to speciation:
1.
2.
3.
4.
A single species is an
interbreeding reproductive
community
A barrier develops, or a
dispersal event occurs, dividing
the species
Separated into different
habitats, the divided
populations diverge through
the accumulation of gene and
trait differences
The separate populations
become so different that, if
and when the barrier
disappears and they overlap
again, interbreeding does not
occur
mountains and deserts are classic examples of new barriers
Allopatric Speciation
• The populations of Tamarin
monkeys (family Callitrichidae)
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
Incomplete or Peripheral Isolation
• Ernst Mayr and Theodosius Dobzhansky,
speaking for the Modern Synthesis,
emphasized that most speciation was
allopatric, i.e., geographical isolation required
• Two modifications or variants have been
proposed:
– Peripatric speciation – a small population enters a
new or isolated niche
• [originally proposed by Mayr, and related to the
founder effect and genetic drift altering the isolate’s
gene pool]
– Parapatric speciation – a new niche found
adjacent to the original niche
Incomplete or Peripheral Isolation
• Peripatric speciation: when a population is
divided because of the budding off of a small
completely isolated founder colony from a
larger population so that gene flow is minimal
• Parapatric speciation: when a population at
the periphery of a species adapts to a
different environment but remains contiguous
with its parent so that gene flow is possible
between them
Peripatric vs. Parapatric speciation
• Peripatric speciation is caused
by being at the edge of the
range and almost isolated
geographically ↔ geographic
isolation leads to genetic
isolation
• Parapatric speciation is by
becoming genetically isolated
which leads the population to
become geographically
isolated ↔ genetic isolation
leads to geographic isolation
Both are less common and more difficult to demonstrate since
small niche and habitat differences rarely have fossil records
Peripatric
Speciation
Two male hammer-headed picture-wing
Drosophila battle for mating territory.
Figure B01A: “Picture-winged" group of Drosophila
Figure B01B: “Picture-winged" group of Drosophila
Figure B01C: “Picture-winged" group of Drosophila
Modified from Edwards K.A, Doescher L.T., Kaneshiro K.Y., Yamamoto
D., (2007) A Database of Wing Diversity in the Hawaiian Drosophila.
PLoS ONE 2(5): e487. doi:10.1371/journal.pone.0000487. Courtesy of
Kevin Edwards , Illinois State University.
Figure B02: “Picture-winged" group of Drosophila
Adapted from Carson, H.L. Drosophila Inversion Polymorpism. CRC Press, 1992.
Parapatric Speciation
dispersal
• Potential parapatric speciation in sweet vernal
grass/buffalo grass, Anthoxanthum odoratum,
triggered by adaptation to heavy metal contaminated
soils in many locations globally
• Divergence in flowering times (reproductive
isolation) between the two populations suggests that
incipient speciation is under way
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
(dotted lines). (Simplified originally from
Mayr, 1963)
Ring species are often considered examples
of parapatric speciation
Speciation Without Geographical Isolation
• Recall that the other modes of speciation require three
steps:
1. geographical isolation
2. local adaptation
3. reproductive isolation
• Is it possible to skip the first step and have speciation
without any geographical isolation within the
continuous ancestral population?
• The biometricians thought so . . .
Sympatric Speciation
• Sympatric speciation is the
process through which new
species evolve from a single
ancestral species while
inhabiting the same geographic
region
• there is no geographic constraint
to interbreeding
• the term was invented by the
British entomologist Sir Edward
Bagnall Poulton (1856-1943) in
1904
Sympatric Speciation
• The first model was put forward by John
(1920 – 2004)
Maynard Smith in 1966
• Maynard Smith suggested that homozygotes
(AA and aa) might, under particular
environmental conditions, have a greater
fitness than heterozygotes (Aa) for a certain
trait
• Because of disruptive selection, therefore,
homozygotes would be favored over
heterozygotes, eventually leading to speciation
Sympatric Speciation
• Ernst Mayr completely rejected sympatry
• Debate continues on how important and widespread
sympatric evolution may be
• But well documented empirical evidence for sympatric
evolution exists, and sophisticated theories incorporating
multilocus genetics have been developed
• Australian biologist Michael J.D. White (1910-1983) wrote
one of the first books to document the evidence: Modes of
Speciation (1978)
Sympatric Speciation
• Sympatric divergence could also result from
sexual selection
• Reproductive isolation may develop through
changes in behavior, microhabitat, seasonality
of breeding, or chromosomal mutation or
ploidy events
• Sympatric speciation is common in plants; less
common in animals
• It is often difficult to confirm sympatric origin
Sympatric Speciation
• Sympatric African Indigobirds are host specific nest
parasites
• Their hosts rear their young but their young do not destroy
the host’s young, as cuckoos often do
Sympatric Speciation
Heliconius cydno alithea
• Polymorphic mimicry in Heliconius
cydno alithea in western Ecuador,
where the white form (middle
left) mimics the white species
Heliconius sapho (top left) and the
yellow form (middle right) mimics
the yellow species Heliconius
eleuchia (bottom right).
• Sympatric Neotropical Heliconius butterfly species are Mullerian
mimics
• Their common toxicity is cyanide derived from cyanoglucosides in
various Passiflora , passion flower vines, host plants eaten by the
larvae
• The two H. cydno alithea color morphs exhibit assortative mating
Speciation Types: Summary
Recall that non-sympatric
speciation requires three steps:
geographical isolation
local adaptation
reproductive isolation
Notice that sympatric speciation
may end up producing the same
species distribution patterns as
allopatric speciation
Figure 04a-e: Two modes of speciation
Adapted from Strickberger, M. W. Genetics, Third edition. Macmillan, 1985.
This is one reason it has been
difficult to demonstrate sympatric
speciation
Speciation Types: Summary
All four modes of geographical speciation assume that
local adaptation and reproductive isolation will occur
Individual cases may show local adaptation to precede
reproductive isolation; others may show reproductive
isolation to precede local adaptation (assortative
mating, changes in ploidy, etc.); and still others may
show local adaptation and reproductive isolation to
occur simultaneously
Another accomplishment of the Modern Synthesis was to
characterize the mechanisms of reproductive isolation
Reproductive Isolating
Mechanisms (RIMs)
• Different types of mechanisms that can prevent reproduction
between individuals of different species
• RIMs are also referred to as prezygotic or premating, versus
fertilization or mating, versus postzygotic or postmating
mechanisms
• prezygotic or premating mechanisms
– geographic, ecological, behavioral, and temporal isolation
• (mating mechanisms) still prezygotic
– mechanical mechanisms and gamete incompatibility
• postzygotic or postmating mechanisms
– zygotic mortality, hybrid inviability, hybrid sterility, and
hybrid breakdown
Geographical (Reproductive) Isolation
Iguana iguana
Conolophus subcristatus
Amblyrhynchus cristatus
• The two Galapagos iguana genera are, themselves, ecologically
isolated from each other
Ecological (Reproductive) Isolation
Agkistrodon piscivorus
Agkistrodon contortrix
• Water or cotton-mouth
moccasin is semiaquatic, feeds on
aquatic vertebrates, and
is aggressive
• Copperhead is
terrestrial, feeds on
terrestrial vertebrates,
and is less aggressive
Behavioral (Reproductive) Isolation
Anolis garmani
Anolis opalinus
Anolis trinitatis
• Members of the genus Anolis
on Jamaica chose different
perches and use different
patterns of head bobbing to
attract female anoles
• They also have separate
ecological niches
Temporal (Reproductive) Isolation
13 year cicada
17 year cicada
• Members of the genus
Magicicada, exist in
temporaly separated
populations, three species
of 17 year cicadas, and
four species of 13 year
cicadas
• There is also some
geographical isolation
within the 17 year cicadas
in the northeastern US and
the 13 year cicadas in the
southeastern US
Mechanical (Reproductive) Isolation
• Members of the genus
Parafontaria, Japanese
millipedes, differ in body size
and in the size and shape of
their reproductive gonopodia
Mechanisms Facilitating Reproductive
Isolation
• Prezygotic mechanisms may be somewhat more
advantageous to the species when they develop,
because they prevent individuals from mating with
members of the other species – this saves energy
and prevents the waste of gametes
• Postzygotic mechanisms may be less advantageous
from that perspective, but, on the other hand, may
allow some alleles to pass from one species to the
other, a form of horizontal gene transfer often
called introgression
Introgression
• A 2010 study by Svante Pääbo and his colleagues confirms that
genetic “introgression” occurred in the human lineage, and
that between 1% and 4% of the DNA (SNPs) of Europeans,
Asians, and Papua New Guineans is attributable to
hybridization between “modern” humans and Neanderthals
Reproductive Isolating
Mechanisms (RIMs)
• Prezygotic mechanisms: Factors which prevent individuals from
mating
– Geographical isolation already discussed
– Temporal isolation; Ecological isolation; Behavioral isolation; Mechanical
isolation ―
– Gametic incompatibility: Sperm transfer takes place, but the egg is not
fertilized
• Postzygotic isolating mechanisms: Genomic incompatibility,
hybrid inviability or sterility
–
–
–
–
Zygotic mortality: The egg is fertilized, but the zygote does not develop
Hybrid inviability: Hybrid embryo forms, but is not viable
Hybrid sterility: Hybrid is viable, but the resulting adult is sterile
Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but
further hybrid generations (F2 and backcrosses) are inviable or sterile
Zygotic Mortality
In sheep and goat crosses fertilization takes place but the hybrid
embryos die in the early developmental stages (Dobzhansky, Ayala, Stebbins
and Valentine, 1977. Evolution. W. H. Freeman)
Hybrid Inviability
Drosophila simulans
Drosophila melanogaster
Drosophila melanogaster and D. simulans have
incompatible nuclear pore protein alleles
Hybrid
Inviability
• The four groups of leopard frogs (Rana sp.) resemble one another closely
in their external appearance
• But early tests of interbreeding produced defective embryos (hybrid
inviability) 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 behavioral isolation separates and
reproductively isolates members of each group, producing four species:
(1) Rana pipiens; (2) Rana blairi; (3) Rana utricularia; (4) Rana berlandieri
Hybrid Sterility
mule
hinny
liger
tigon
These hybrids have reduced, if not absent, fertility, though they are often otherwise healthy
Hybrid Breakdown
• In the parasitoid wasp genus Nasonia,
F2 hybrid males of Nasonia vitripennis
and Nasonia giraulti experience an
increased larval mortality rate relative
to the parental species. Previous
studies indicated that this increase of
mortality is a consequence of
incompatibilities between multiple
nuclear loci and cytoplasmic factors of
the parental species, but could only
explain ∼40% of the mortality rate in
hybrids with N. giraulti cytoplasm.
Hybrid larvae that carry the
incompatible allele on chromosome 5
halt growth early in their
development and ∼98% die before
they reach adulthood.
Nasonia giraulti
Reproductive Isolating
Mechanisms (RIMs)
• Over time, as a pair of sibling
species diverge, reproductive
isolation may increase by the
development of multiple
forms of both premating and
postmating isolating
mechanisms, as illustrated
here with two hypothetical
species of salamander
Biotic Clines
• A biotic cline, in reference to population biology, is a
gradual change of phenotype (trait, character or feature)
and underlying gene pool allele frequencies in a species
over a geographical area, often as a result of
environmental heterogeneity
• This meaning of "cline" was introduced by Sir Julian
Huxley
Many speciation events seem to track or
parallel these environmental discontinuities
Clinal Variation
• In the leopard frog (Rana pipiens), tadpoles exhibit a range
of temperature tolerances, generally enduring colder
temperatures in higher (northern) latitudes and warm
temperatures at lower (southern) latitudes
Reproductive Success
• In a study by J. Moore in 1949 of the
leopard frog (Rana pipiens), eggs from
females in the north were fertilized 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)
• This study and others prompted biologists
to divide leopard frogs into several
different species
egg
mass
Speciation for Sexual Species
• If species reproduce asexually, reproductive
isolation is inherent in their formation;
offspring form asexual clones, but may acquire
new alleles through horizontal gene transfer
• If species reproduce sexually, the degree to
which species may hybridize varies greatly
• The ability to hybridize does not necessarily
contradict the reality of species distinction
• Some sister species never have the
opportunity to reproduce across populations
or form hybrids in nature
Genes and Speciation
• General statements about the evolution of
new species:
• Adaptive and Sexual traits first undergo
disruptive selection for differences between
populations during speciation
• Adaptive and Sexual traits later undergo
stabilizing selection for uniformity within
sister species after attaining speciation
• Hybridization is limited by Reproductive
Isolating mechanisms
Darwin’s Particular Genius
Over his career, Darwin wrote about 3 kinds of selection:
(1) Natural selection
- makes a species better adapted to its environment
- increases survival
(2) Sexual selection
- makes one sex more appealing to the other
- increases reproduction
(3) Artificial selection
- humans choose the desired traits and amplify them through
selective breeding of domestic organisms
In the 150 years since, scientists have added group selection and kin
selection and species selection as additional kinds of speciation
Darwin’s Particular Genius
Darwin defined 3 kinds of selection. However, you will hear
biochemists use the term Directed Evolution: “A laboratory process
used on isolated molecules or microbes to cause mutations and
identify subsequent adaptations to novel environments.”
A typical directed evolution experiment involves three steps:
Diversification: The gene encoding the protein of interest is mutated and/or recombined at random to create a large
library of gene variants. Techniques commonly used in this step are error-prone PCR and DNA shuffling.
Selection: The library is tested for the presence of mutants (variants) possessing the desired property using a screen or
selection. Screens enable the researcher to identify and isolate high-performing mutants by hand, while selections
automatically eliminate all nonfunctional mutants.
Amplification: The variants identified in the selection or screen are replicated manyfold, enabling researchers to
sequence their DNA in order to understand what mutations have occurred.
Together, these three steps are termed a "round" of directed evolution. Most experiments will perform more than one
round. In these experiments, the "winners" of the previous round are diversified in the next round to create a new
library. At the end of the experiment, all evolved protein or RNA mutants are characterized using biochemical methods.
Biology Students: This is nothing but Darwin’s Artificial Selection using molecular
tools. The biochemists are just showing their ignorance of the history of science.
3 Modes of Natural Selection
• Stabilizing
• Directional
• Disruptive
Figure 02: Selection and their effects on the mean (dashed lines)
and variation of a normally distributed quantitative character
Summary of
Geographic Modes of Speciation
• Allopatric speciation – a physical barrier
divides a continuous population
• Peripatric speciation – a small founding
population enters a new or isolated niche
• Parapatric speciation – a new niche found
adjacent to the original niche
• Sympatric speciation - speciation occurs
without physical separation inside a
continuous population
Summary of Reproductive
Isolating Mechanisms (RIMs)
• Prezygotic mechanisms: Factors which prevent individuals from
mating
– Temporal isolation; Ecological isolation; Behavioral isolation; Mechanical
isolation
– Gametic incompatibility: Sperm transfer takes place, but the egg is not
fertilized
• Postzygotic isolating mechanisms: Genomic incompatibility,
hybrid inviability or sterility
–
–
–
–
Zygotic mortality: The egg is fertilized, but the zygote does not develop
Hybrid inviability: Hybrid embryo forms, but is not viable
Hybrid sterility: Hybrid is viable, but the resulting adult is sterile
Hybrid breakdown: First generation (F1) hybrids are viable and fertile, but
further hybrid generations (F2 and backcrosses) are inviable or sterile
Chapter 17
End