Speciation

Speciation
• Formation of species is known as speciation.
• Species: A species may be defined as an
interbreeding population, which is reproductively
isolated from other similar but morphologically
distinct populations. (or)

• Species may be envisized as an isolated pool of

genes flowing through space and time, constantly
adapting for changes in its environment as well as to
the new environments encountered by its extensions
into other geographical regions.
 Speciation
• At various intervals of time and in
different environments of this gene pool
may become isolated such isolates may
1.Become extinct
2.Reunite with the parental species
3.Differentiate during isolation to form
new species which in turn pass through
space and time.
 General characters of animal species
1. Each species is an isolated pool of genes possessing
regional (racial, populational) characteristics in gene
complexes which are inter-connected by gene flow.

2. Each species fills an ecological niche not exactly

utilized by another species.

3. Each species remains in process of continually

adjusting to its environment.
General characters of animal species
4. Each species posses a constellation isolating
mechanisms that directly or indirectly prevent
exchange of genes with related species.

5. Each species has the capacity to gives rise to new

species provided some form of geographical or
spatial isolation gives its isolates an opportunity to
develop a unique gene pool without being
swamped by gene flow from the parental species.
 Demes
• A species is a population of
individuals with similar
structural and functional
characteristics, which have a
common ancestry and in nature
breed only with each other.

• Each deme is isolated to some

extent from adjacent demes but
there is always the possibility of
genetic exchange between them.
 Demes
• Even distantly located demes may contribute
genetic material to one another over a period of
time by the gradual passage of certain genotype
of one deme to another.

• Thus, demes are open genetic systems that are

affected by gene flow from adjacent
populations, that is, they are only partially
isolated populations.
 Race
•An intermediate population unit
which exists in between species
population and local population is race
(or) geographical (or) subspecies.

•A subspecies is an aggregate of
local populations of a species
inhabiting a geographic subdivision of
the range of the species and differing
taxonomically from other populations
of the species.
 Nature of speciation
• Ever since the speciation process became a separate
topic in the 1930s (Mayr, 1931) alternative methods of
speciation have been proposed.

• The principal ones are quantum evolution proposed by

Simpson (1944), quantum speciation defined and
discussed by Grant (1963, 1971) and Carson (1973,
1975), and called saltational speciation by Ayala
(1975); parapatric speciation discussed by Murray
(1972) and Bush (1975); and sympatric speciation
described by Stebbins (1950), Mayr (1963, 1970) and
Grant (1971).
 Nature of speciation
• The true meaning of the term “origin of
species” used by Darwin was understood only
rather recently.
• The pre-Darwin evolutionists did not
understand its exact significance.
• Speciation in its restricted modern sense,
however, means the splitting of a single species
into several, that is multiplication of speciation.
 Nature of speciation
• The opposite of splitting of a single species into several
daughter species is the complete fusion of two species.

• Since the product of such a process would be an entirely

new unit, truly a new biological species, it is legitimate to
consider the fusion of two species into one as a form of
speciation, which is the reverse of a multiplication of
species.

• Since species are defined as reproductively isolated

populations, the fusion of two species is, on the whole, a
logical contradiction.
 Potential modes of speciation
•According to Mayr (1970), true speciation or multiplication of species
may occur by the following agencies:
A.Instantaneous speciation (through individuals)
1.Genetically
(a)By single mutation in ”asexual species”
(b)By macrogenesis
2. Cytologically, in partially or wholly sexual species
(a)By chromosomal mutation or aberrations (translocation, etc.)
(b)By polyploidy
B. Gradual speciation (through populations)
1.Geographical speciation
2.Sympatric speciation
 Instantaneous speciation
• The process of instantaneous speciation may
be defined as the production of a single
individual that is reproductively isolated from
the species to which the parental stock belongs
and that is reproductively and ecologically
capable of establishing a new species
population.

• It can be achieved by the following methods:

 Instantaneous speciation through
ordinary mutation
• Mutation is a genetic phenomenon of such
relatively high frequency that in a higher animal
with more than 10,000 gene loci almost every
individual will be the carrier of a new mutation.
• Such mutations merely increase the heterozygosity
of a population; they do not lead to the production
of new species.
• It is evident that ordinary mutation can not produce
new species in sexually reproducing species.
 Instantaneous speciation through
ordinary mutation
• Certain phenomena among rotifers,
cladocerans and nematodes suggest the
occasional occurrence of asexual speciation
by mutation.
• For example, in the rotifer order Bdelloidea,
there occurs no male individual in entire order
and so it is assumed that the ancestral species
of this order was parthenogenetic.
• This ancestral species evolved into over 200
species, about 20 genera and 4 families only
due to asexual speciation by mutation.
 Instantaneous speciation through
Macrogenesis
• The sudden origin of new species, new higher
taxa, or quite generally of new types by some
sort of saltation (a change by leap across a
discontinuity) has been termed macrogenesis.

• The macrogenesis theory of speciation has

been supported by from geneticists such as
Goldschimdt (1940, 1948) and Schindewolf
(1936, 1950).
 Instantaneous speciation through
Macrogenesis
• According to this theory the production of a
new type by a complete genetic reconstruction
(Macrogenesis) or by a major systemic
mutation or macromutation is the crucial event
in speciation.

• Such an event will produce a hopeful monster

which will become the ancestor of new
evolutionary lineage.
 Instantaneous speciation through
chromosomal aberrations
• Closely related species often differ more
conspicuously in their karyotype than in
their morphology.

• Among aspects of the karyotype that differ

are chromosomal number; the number of
metacentric or acrocentric chromosomes;
the presence and kind of paracentric or
pericentric inversions; or supplementary
chromosomes and just about every aspect
of chromosomal aberrations.
 Chromosomal rearrangement
without speciation
• Except deleterious chromosomal mutations,
most kind of chromosomal aberrations lead to
chromosomal polymorphism rather than to the
development of isolation mechanisms.
Example:
• Drosophila are a well known example of
such chromosomal polymorphism.
 Speciation coinciding with a
chromosomal mutation
• They have the following two advantages in
chromosomal speciation –
I. chromosomal mutations have the potential to serve as
isolating mechanisms and
II. the locking up and protection of a particularly
favorable gene complement through a chromosomal
mutation may create a new super gene.
• Both of these components of chromosomal speciation
can subsequently be improved by natural selection.
Chromosomal mutations as potential
new isolating mechanisms
• Any change in the structure of the
chromosomes is called chromosomal
mutations or chromosomal aberrations,
whether it is an inversion,
translocation, duplication, or any other
change in the linear sequence of the
genes is the mechanics of the
chromosome.

• Chromosomal mutations are estimated

to occur at a rate of 1 in 1000.
Chromosomal mutations as potential
new isolating mechanisms
• The heterozygotes containing
some kind of chromosomal
mutation that seems important in
speciation, usually encounters the
following difficulties during
meiosis:
• (i) meiotic synapsis
• (ii) malorientation of multivalent
at the first meiotic metaphase or
both. Both of these difficulties
lead to a significant reduction in
the fecundity of male hybrids.
 Chromosomal mutation and the
production of new supergenes
• Most structural rearrangements of chromosome inhibit or prevent
crossing over in heterozygotes.
• A new gene arrangement may “lock up” a coadapted gene
sequence and by protecting it from crossing over, create a new
supergene.
 Chromosomal mutation and the
production of new supergenes
• There will be a steady selection for an
improvement of the adaptation supergene and
this will tend to produce an increase in genic
heterozygote inferiority.

• This in turn, will strengthen the effectiveness

of the cyto-mechanic isolating mechanisms.
 Instantaneous specification through
polyploidy
• Polyploidy is a multiplication of the normal
chromosome number. In plants one-third of all
species of plants have arisen by polyploidy.

• Among the animals it is rare but it occurs only in

those groups which reproduce parthenogeneticaly
such as lumbricid earthworms, turbellarians.
 B. Gradual speciation
• Gradual speciation is the gradual
divergence of populations until they
have reached the levels of specific
distinctness. Two modes of gradual
speciation have been postulated.
1. One involving geographical separation
of the diverging populations
(Geographic speciation).
2. The other involving divergence without
geographic separation (Sympatric
speciation).
Geotropic or allopatric speciation
• Allopatric speciation states that in
sexually reproductively animals
and new species develops when a
population that is geographically
isolated from the other populations
of its parental species acquires
during this periods of isolated
characters that promote or
guarantee, reproductive isolation
after the external barriers break
down (Maye, 1942).
 Sympatric Speciation
• The method of origin of
reproduction isolating mechanisms
within the dispersal area of the
offspring of a single deme is called
sympatric speciation.
• The size of the dispersal area is
determined, for instance, in marine
organism by the dispersal of the
larval stage.
• In most insects it is determines in
the adult stage by the more mobile
sex.
 Sympatric Speciation
• Since there are normally a great
many ecological niches (the
constellation of environmental
factors in to which a species (or)
other taxon) fits within the dispersal
area of a deme, niche specialization
is impossible without continued
new pollutions by immigrants in
every generations.
 Reasons for postulating sympatric
speciation
• Numerous biological phenomenons suggest, at
first sight, the occurrence of sympatric
speciation.
• One is the abundance of finally adapted
sympatric species in local fauna.
• For instance, there are several hundred species
of bees of the genus Perdita in North America,
all are oligolectic, confignig their visits to the
flowers of the single species or of group of
closely allied species.
 Reasons for postulating sympatric
speciation
• Many species of Perdita may be found at a
single locality but never together because they
occur on different plants or on some cases at
different times of the year.
• A similar ecological specificity characterizes
the species of many other genera of animals.
• All such species are perfectly adapted to the
particular environment in which they occur.
 Biological and host races
• A necessary corollary of any theory of gradual
speciation is that there should exist in nature
some forms or varities or populations that are
incipient species.

• The supporters of sympatric speciation have

always cited the occurrence of biological races
as examples of such incipient species.
 Biological and host races
• Any direct population characterized by non-
morphological characters was accordingly
interpreted as biological race and incipient
species.

• Thus, kinds of animals that show no structural

differences, although clearly separately by
biological characters, are called biological
races.
 Biological and host races
Occurrence Population
Same Different
Polyploids
Sympatric Morphs Ecological races
Allopatric Clones Host races
Host “race” Sibling species
Geographical races
Semi species
Sibling species

Table: Phenomena listed as biological races

 Seasonal races
• A species with a very long breeding season
might be sympatrically split into two if the
genetic continuity between the earliest and last
breeders of year could somehow be
interrupted.

• This occurrence of seasonal races is often

cited as evidence for such a process of
speciation.
 Host races
• In many species of animals, particularly,
nematodes and insects, temporary strains called
host races may develop on specific host plants
or animals. Host races have the following
characteristics:
1. Preference for a given species of host plants
may and nearly always does have a double
basis - conditioning (including larval
conditioning) and a genetic predisposition.
 Host races
2. Nearly all species with host races concentrate in a
given distinct on one host : yet they also have the
ability to establish themselves on a variety of other
host plants, particularly under crowded condition, and
may have different preferred host plants in different
districts.

3. Local populations of insects often have considerable

genetic variability with respect to host specificity.
 Host races
3. Any monophagous (e.g. solitary bees, beetles)
or oligophagous species of insect will come in
contact, during its dispersal phase, with
numerous plant species other than its normal
host. If the species has the appropriate genetic
constitution it will establish itself on the new
host and this, according to the Luwing
theorem, leads to an expansion of the food
niche of the species.
 Host races
4. It is also found that if such a mixing of demes
or local populations is prevented artificially
and an inbred strain is selected experimentally
on a single one of several original hosts, such a
strain may become progressively less tolerant
ecologically until a stage is reached when it
may be difficult to reestablish it on any of the
other original host species.
 Host races
5. Mortality occurs whenever a strain is
established on a new host. The more usual the
host. The more usual the host, the heavier the
initial mortality.

• Among the many kinds of so called

biological races, it is the host races that have
best claim to the designation of biological races.
 Hypothesis of Sympatric Speciation

• Certain evolutionists have forwarded the

following hypothesis for sympatric speciation, but
all of these are not supported by evidences.
 Homogamy
• According to this concept, the most similar
individuals of a population tend to mate with
each other.
• Further, it postulated that monogamy would
lead to Homogamy.
• Both of these assumptions are supported by
facts (Mayr, 1947) except that in a single case
of birds where monogamy is rule but never
leads to homogamy.
 Homogamy
• In case of two kinds of geese, namely snow goose
and blue goose, monogamy leads to homogamy
and both are found to belong to a single species,
(Anser coerulescens) and are found to have
differences in colour pattern due to a single gene,
semidominant for blue.
• However, a mild form of homogamy occurs in
many animals with highly variable adult size but
that is quite insufficient for sympatric speciation.
 Conditioning
• Many workers such as Thorpe (1945) and
others have suggested that the establishment of
a new sympatric species population (e.g,
insects) in a new niche might be achieved
through conditioning.
• But Mayr (1947) contradicted it by showing
that complete isolation of the two populations
was never achieved by any of the conditioning
experiment of Thorpe with insects.
 Preadaptation and niche selection
• The concept of sympatric speciation by
preadaptation is quite typological in making the
assumption that a single gene preadapts an
individual for a new niche.
• Indeed it would require a veritable systemic
mutation to achieve the simultaneous appearance
of a genetic preference for a new niche, a special
adaptedness for this niche, and a preference for
mates with a similar niche preference.
• The known facts do not support these
assumptions.
 Sympatric Speciation by
Disruptive Selection
• The method of natural selection for phenotypic
extremes in a population until a discontinuity
is achieved is called disruptive selection or
diversifying selection.

• Muller (1940) was first to suggest that the

accumulation of different sets of specific
modifiers might lead to sympatric speciation.
 Sympatric Speciation by
Disruptive Selection
• K. Jordan suggested that
selection of different
physiological varieties within a
population can have only two
outcomes, either
polymorphism or extinction of
inferior types.

• Jordan’s conclusion is well

illustrated by the North
American butterfly Limenitis
arthemis.
 Differences between allopatric and
sympatric speciation
• The theory of allopatric
speciation lets an extrinsic
event which separates the
single gene pool into
several gene pools, with the
ecological factors playing
their major role after the
population have become
geographically separated.
 Differences between allopatric and
sympatric speciation
• According to the theory
of sympatric speciation,
the splitting of the gene
pool is itself caused by
ecological factors, and
any spatial isolation of
the populations formed
thereby is a secondary,
phenomenon
 Quantum speciation
• The budding off of a new and very different
daughter species from a semi-isolated peripheral
population of the ancestral species in a cross
fertilizing organism (Grant, 1963).
• The process of quantum speciation by which rapid
evolutionary transition is achieved in small
populations, is not fully understood, but
chromosomal rearrangement (translocations) and
change in gene regulation often play important
roles (Stanley, 1979).
 Quantum speciation
• The quantum speciation differs from the conventional
speciation in the following ways:
1. Quantum speciation is rapid, requiring only a few
generations.
2. The ancestors of new species do not include a large
proportion of the populations belonging to the pre-
existing one and may consist of only one or a few
individuals. Conventional speciation is a process of
splitting, quantum speciation is budding process.
 Quantum speciation
3. Conventional speciation may be promoted by drastic
reduction in population size, but this is not necessary as
it is for quantum speciation.
4. Conventional selection in its entirety is either guided by
or is the by-product of natural selection. Quantum
selection usually includes one or more chance events.
 Parapatric speciation
•In parapatric speciation there is no specific extrinsic barrier to gene flow.
• The population is continuous, but nonetheless, the population does not mate
randomly. Individuals are more likely to mate with their geographic neighbors
than with individuals in a different part of the population's range.
•In this mode, divergence may happen because of reduced gene flow within the
population and varying selection pressures across the population's range.
•We may be observing the first steps of parapatric speciation in the grass
species Anthoxanthum odoratum (at right).
 Parapatric speciation
•Some of these plants live near mines where the soil has become contaminated
with heavy metals.
•The plants around the mines have experienced natural selection for genotypes
that are tolerant of heavy metals.
• Meanwhile, neighboring plants that don't live in polluted soil have not
undergone selection for this trait.
•The two types of plants are close enough that tolerant and non-tolerant
individuals could potentially fertilize each other — so they seem to meet the first
requirement of parapatric speciation, that of a continuous population.
•However, the two types of plants have evolved different flowering times.
•This change could be the first step in cutting off gene flow entirely between the
two groups.