Speciation Definition

Speciation is the process of formation of a new genetically independent group of

organisms, called species, through the course of evolution.

• The process of splitting of genetically homogenous population into two or more

populations that undergo genetic differentiation and eventual reproductive
isolation is called speciation.

• The entire course of evolution depends upon the origin of new populations (species)
that have greater adaptive efficiency than their ancestors.

Speciation occurs in two ways.

1. Transformation of old species into new species over time.

2. Splitting of a single species into several, that is the multiplication of species.

Speciation Causes

Speciation occurs as a result of several factors which are:

1. Natural selection

• As explained by Charles Darwin, different individuals in a species might develop

specific distinct characteristics which are advantageous and affect the genetic makeup
of the individual.

• Under such conditions, these characteristics will be conserved, and over time, new
species might be formed.

• However, in this case, the essential aspect of this factor is that speciation occurs only
when a single species splits into several species resulting in the multiplication of
species.

2. Genetic drift

• Genetic drift is the change in the allele frequencies in a population as a result of

“sampling error” while selecting the alleles for the next generation from the gene pool
of the current population.

• It has been, however, argued that genetic drift doesn’t result in speciation and just
results in evolution, that is, change from one species to another, which cannot be
considered speciation.

3. Migration

 • When a certain number of species from a population migrate from one geographical
region to another, the species might accumulate characteristics which are different
from that of the original population.

• Migration usually results in geographical isolation and ultimately leads to speciation.

4. Chromosomal Mutations

• Chromosomal mutations have the potential to serve as (or contribute to) isolating
mechanisms, and the locking up and protection of a particularly favorable gene
complement through a chromosomal mutation.

• These mutations, when preserved from one generation to another, might result in the
formation of new species.

5. Natural causes

• Sometimes, natural events imposed by the environment like a river or a mountain

range might cause the separation of what once a continuous population is divided
into two or smaller populations.

• These events result in geographical isolation of the incipient species followed by

reproductive isolation leading to speciation.

6. Reduction of gene flow

• Speciation might also occur in the absence of some extrinsic physical barriers.

• There might be a reduced gene flow over a broad geographical range where
individuals in the far east would have zero chance of mating with individuals in the
far western end of the range.

• In addition, if there are some selective mechanisms like genetic drift at the opposite
ends of the range, the gene frequencies would be altered, and speciation would be
ensured.

Speciation process (how does speciation occur?)

Classically, speciation has been observed as a three-stage process:

1. Isolation of populations.

2. Divergence in traits of separated populations (e.g. mating system or habitat use).

3. Reproductive isolation of populations that maintains isolation when populations come

into contact again (secondary contact).

 • Recent research shows that steps one and two may take place simultaneously in the
same place, and often the third step does not occur.

• The process of speciation begins with the isolation of subpopulation of a species

which could either occur through physical isolation (allotropic speciation) or genetic
isolation (sympatric speciation).

• Once the population is separated, a gradual accumulation of small genetic changes

results in a subpopulation of a species that eventually accumulate so many changes
that the subpopulations become different species.

• Over time, the subpopulation now becomes genetically independent and will continue
to diverge by mutation, selection, and genetic drift.

• The genetic differentiation might cause a slight change in the mating dance or even a
small change in the shape of the male genitalia or some changes in the habitat or
feeding habits of the subpopulation, which results in reproductive isolation.

• Eventually, the genetic differentiation between the subpopulation becomes so high

that the formation of hybrids between them would be physiologically,
developmentally, or behaviorally impossible even if the modes of the separation were
abolished.

Types of speciation/Modes of speciation

• The classification of the modes or types of speciation is based on how much the
geographical separation of the original population contributes to the reduced gene
flow and ultimately, the formation of new species.

 The modes of speciation are:

Allopatric Speciation

• Allopatric speciation is the mode of speciation in which the original population is

divided into two by a barrier resulting in reproductive isolation.

• The model for allopatric speciation was presented by Mayr.

• It is based on the concept that new species arise when some physical geographic
barrier divides the large population of a species into two or more small populations.

• The individuals of these isolated populations cannot interbreed because of their

physical isolation.

• Physical isolation might occur either due to physical barriers like vast expanses of
ocean, high mountains, glaciers, deep river valleys, wide rivers or deserts, or a
considerable distance due to a larger geographical range.

• Each isolated population starts to adapt to their separated environments while

accumulating differences and evolving independently into new species.

• Allopatric speciation can occur even in cases in which the barrier allows some
individuals to cross the barrier to mate with the members of the other groups.

• For speciation even to be considered “allopatric,” gene flow between the soon-to-be
species must be significantly reduced—but it doesn’t have to be entirely reduced to
zero.

Examples of Allopatric speciation

• The classic example of allopatric speciation is that of Darwin’s finches. The divergent
populations of finches inhabiting the Galapagos Islands were observed to have
differences in features such as body size, color, and beak length or shape. The
differences resulted because of the different types of food available in various Islands.

• Another example is of Grand Canyon Squirrels which were separated during the
formation of the Grand Canyon and resulted in two different species of squirrels.

Peripatric Speciation

• Peripatric speciation is a special condition of allopatric speciation which occurs when

the size of the isolated subpopulation is small.

• In this case, in addition to geographic separation, genetic drift also plays an

important as genetic drift acts more quickly in small populations.

 • The small isolated subpopulation might carry some rare genes which upon
reaching the new geographical region become fixed over the course of a few
generations as a result of genetic drift.

• As a result, the entire population of the new region ends up having these rare genes.

• Over time, new genetic characters, as well as natural selection, cause the survival of
individuals which are better suited to the climate and food of the new region.

• Finally, under the influence of all these factors, new species are formed.

• However, it is very difficult to explain what role genetic drift played in the divergence
of the two populations, which makes gathering evidence to support or refute this
mode very challenging.

Examples of Peripatric speciation

• The Australian bird Petroica multicolour and London Underground mosquito, a variant

of the mosquito Culex pipiens, which entered in the London Underground in the
19th century are the examples of Petripatric speciation.

Parapatric Speciation

• Parapatric speciation is a mode of speciation in which there is no extrinsic barrier

between the population but, the large geographic range of the population causes
the individuals to mate with the neighboring individuals than with the
individuals in a different part of the geographical range.

• In this case, the population is continuous, but the population doesn’t mate randomly.

• Here, the genetic variation occurs as a result of reduced gene flow within the
population and varying selection pressures across the population’s range.

• This occurs in population which is distributed over a large geographical range. Thus,
the individuals in the far west region cannot mate with the individuals in the far east
region.

• Through a few generations, new species might be formed within the existing
population.

Examples of Parapatric speciation

• The grass species Anthoxanthum odoratum where some species living near the mine
have become tolerant to heavy metals; however, other plants that don’t live around
the mines are not tolerant.

 • But because the plants are close together, they could fertilize each other and result in
a new species.

Sympatric Speciation

• Sympatric speciation is the process of the formation of new species from an original
population that are not geographically isolated.

• It is based on the establishment of new populations of a species in different ecological

niches and the reproductive isolation of founders of the new population from the
individuals of the source population.

• Gene flow between daughter and parental population during sympatric speciation is
postulated to be inhibited by intrinsic factors, such as chromosomal changes
and non-random mating.

• Exploiting a new niche might automatically reduce gene flow with individuals
exploiting a different niche.

• This mode of speciation is common in herbivore insects when they begin feeding
and mating on a new plant or when a new plant is introduced within the
geographical range of the species.

• The gene flow is then reduced between the species that specialize in a particular
plant which might ultimately lead to the formation of new species.

• The selection resulting in specialization needs to be really strong for the

population to diverge.

• Thus, sympatric speciation is a sporadic event in multicellular organisms or randomly

mating populations.

Examples of Sympatric speciation

• Sympatric speciation is observed in apple maggot flies which 200 years ago laid eggs
and bred only on hawthorns but now lays eggs on both hawthorns and domestic
apples.

• As a result, gene flow between parts of the population that mate on different types of
fruit is reduced, and in fewer than 200 years, some genetic differences between these
two groups of flies have evolved.

Reproductive Isolating Mechanisms

• A biological species is defined as a group of similar organisms able to interbreed (under

natural conditions) to produce fertile, viable offspring.
• Biological species are reproductively isolated from one another. Evolution of
reproductive isolating mechanisms prevents nascent species from interbreeding. Isolating
mechanisms can operate at two basic levels.
Prezygotic Mechanisms prevent formation of viable zygotes.
Postzygotic Mechanisms prevent hybrids from passing on their genes.

A. Pre-Zygotic Reproductive Isolating Mechanisms

•
I. Ecological Isolation
Closely related species may inhabit different ecosystems within a region.
• Different habitat preferences lower their probability of mating.

Example: Anopheles maculipennis group

• Once believed to comprise one species, Anopheles actually contains six species.
• Each occupies a different estuarine niche (freshwater, brackish water, marine).
• Their ecological preferences make matings among them rare.

II. Temporal Isolation:

• Two related species occupying the same geographical range may have different periods
of sexual activity or breeding seasons.
•
• Example : Closely related Fruit Flies in Hawaii
▪
• Drosophila persimilis breeds in early morning.
• Closely related Drosophila pseudoobscura breeds in the afternoon.
• Never the twain shall mate.

III. Behavioral Isolation

• Animals with complex courtship behaviors usually perform species-specific "call and
response" signals between male and female before actual mating takes place.
• These rituals prevent wasted mating effort that would halt gene transmission due to
sterile or inviable hybrids.

IV. Mechanical Isolation:




 • Morphological differences between species prevent hybridization.

•
◦ Example 1: Shell Coiling in Euhadra
• In snails, the direction of shell coiling is controlled by a single (maternal effect) gene.
• Left-coiling snails cannot mate with right-coiling snails.
• Gene flow between them will cease.

• Example 2: The Bucket Orchid and the Orchid Bee

• Male Orchid Bees obtain a wax from the orchid that they use to make a substance to
attract female bees.
• The anatomy of the Bucket Orchid allows pollination only by this species of bee.
• This partnership is so precise that if either species became scarce or extinct, the other
would follow

V. Gametic Isolation

• If the sperm and ova of two species have incompatible plasma membrane receptors, they
cannot form a zygote.

• Example: Sympatric Sea Urchin Species

• Sea urchins synchronously broadcast gametes into the ocean.
• Sperm and eggs from the same species fuse to form zygotes.
• Planktonic larvae eventually settle to metamorphose into adults.
• The Giant Red Urchin (Strongylocentrotus franciscanus) and the Purple Urchin
(Strongylocentrotus purpuratus) cohabit the rocky intertidal along the western U.S., but
do not interbreed.
• Their gametes do not recognize one another, maintaining species integrity.

B. Postzygotic Isolating Mechanisms

I. Hybrid Inviability: Drosophila

• Sperm and egg from the two species may combine, but the genetic information
• is insufficient to carry the organism through normal development. The embryo dies after
a few cleavages, or some time before birth/hatching.

Example 1: Drosophila spp.

◦ Despite their superficially similar appearance, D. melanogaster and D. simulans
have incompatible alleles for nuclear pore proteins.



 ◦ Dysfunction of this vital gene results in inviable hybrids.

•
II. Hybrid Sterility
Some species are closely related enough to produce viable hybrids, but the hybrids are
sterile.
• Example 1: Tigers (Panthera tigris) and Lions (Panthera leo)
◦ Tigers and Lions have been separate for millions of years.
◦ Their hybrid offspring are viable and robust, but sterile.
▪ male tiger x lioness --> tigon
▪ male lion x tigress --> liger
◦ Non-homologous chromosomes lead to meiotic dysfunction.
•
• Example 2: Horse (Equus caballus) and Donkey (Equus asinus)
◦ Horses and donkeys have been separate species for millions of years.
◦ Their hybrid offspring are viable and robust, but sterile.
◦ male horse x female donkey --> mule
◦ male donkey x female horse --> hinny

III. Hybrid Breakdown

• Two related species can hybridize, and their F1 offspring are fertile.
• But successive generations (F2 and beyond) suffer lower viability or fecundity.
• Thus, they cannot become an established population.
Example: Rice cultivars
◦ Cultivars of domestic rice have been artificially selected for centuries.
◦ Some are closely related enough to hybridize.
◦ F1 hybrids are fertile and viable.
◦ F2 generation is stunted and sterile.