Qurra-tul-Aien
Role of isolation in
evolution
Contents
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Evolution
Speciation leading to evolution
Reproductive isolating mechanisms
Types of reproductive isolation
Role of isolation
Extinction
Role of isolating mechanisms
 Evolution:
Changes in the genetic composition of a population with
the passage of each generation lead to evolution.
o Evolution is the gradual change of living things from one
form into another over the course of time, the origin of
species and lineages by descent of living forms from
ancestral forms, and the generation of diversity.
 Isolation:
It is the quality or condition of being isolated. It consists of
a separation of cases (persons or animals)
Types of evolution
 1) Microevolution:
• Small genetic changes in a population such as
the spread of a mutation or the change in the
frequency of a single allele due to selection
(changes to gene pool) that is not possible
without genetic variability in a population.
 2) Macroevolution:
• Long term large scale evolutionary changes
through which new species are formed and
others are lost through extinction.
 Evolution involves speciation.
That may occur by isolation of
one species from other or also
by isolation of one group from
other within species.
 Speciation is the process by
which daughter species evolve
from a parent species.
Populations of a species have a
shared genetic history.
Speciation
1) Morphological Species
Concept:
• Based on appearance alone
2) Biological Species Concept:
• A species is one or more
populations of individuals that
are interbreeding under
natural conditions and
producing fertile offspring, and
are reproductively isolated
from other such populations
Speciation leading to evolution
1) Allopatric speciation:
– Speciation that occurs when 2
or more populations of a
species are geographically
isolated from one another
– The allele frequencies in these
populations change
– Members become so different
that can no longer interbreed
2) Sympatric speciation
– Populations evolve
with overlapping
ranges
– Behavioral barrier or
hybridization or
polyploidy
Reproductive isolating mechanisms
• Any heritable feature of body,
form, functioning, or
behavior that prevents
breeding between one or
more genetically divergent
populations is called
reproductive isolation.
Pre-zygotic isolation
• Mating or zygote formation is blocked by
– Ecological Isolation
– Temporal Isolation
– Behavioral Isolation
– Mechanical Isolation
– Gamete Mortality
Post-Zygotic Isolation
• Hybrids don’t work
– Zygotic mortality - Egg is fertilized but zygote or
embryo dies
– Hybrid infertility - Hybrid is fully or partially sterile
– Hybrid inviability - First generation hybrid forms
but shows low fitness
I) Ecological isolation
• Species occupy different habitats. Two species
may live in the same area but in different
habitats.
• Living in these different habitats (in water, living
on land, or living in tree tops) effectively
segregates these organisms from each other.
• Since there is little if any contact the possibility
of successfully mating is drastically reduced.
• That is habitat or ecological isolation. It involves
physical separation.
• The lion and tiger overlapped in
India until 150 years ago, but the
lion lived in open grassland and the
tiger in forest.
• Consequently, the two species did
not hybridize in nature (although
they sometimes do in zoos).
• So these two species living in same
area but do not hybridize causing
the chances of evolving new species.
Hence reduce the rate of evolution.
II) Temporal isolation
• Species breed at different times leads to temporal
isolation.
• Two species that breed at different times of the
day, season, or year cannot mix their gametes.
Since the breeding times are different there is no
chance of reproductive contact.
• Temporal isolation is genetic isolation achieved
due to temporal differences in breeding.
• For example, one population
would be temporally isolated
from another if its breeding
season was in the fall while the
other's was in the spring.
• For example, In North America,
five frog species of the genus
Rana differ in the time of their
peak breeding activity.
III) Behavioral isolation
• Species engage in distinct courtship and
mating rituals.
• Species-specific signals and elaborate
behavioral patterns are used by closely related
species to insure contact with the proper
mate.
• Birds, mammals, and insects have pre-mating
rituals that attract the proper mate.
• These signals can be chemical or physical in
nature.
• Other organisms pay little or no attention to
these behaviors or scents and remain isolate.
IV) Mechanical isolation
• When interbreeding is prevented by structural or
molecular blockage of the formation of the
zygote, mechanical isolation done.
• Mechanisms include the inability of the sperm to
bind to the egg in animals, or the female
reproductive organ of a plant preventing the
wrong pollinator from landing.
• Anatomical incompatibility may prevent sperm
transfer between two closely related species.
Example
• The absence of certain
appendages or their modification
may inhibit a male from grasping
and successfully fertilizing the
female.
• Difference in floral structure may
prevent pollen from reaching the
stigma of the intended flower.
V) Gametic Isolation
• If for some reason foreign sperm is introduced
into a female there are several preventative
measures to insure that there is no union
between the sperm and egg, gametic isolation
occurred.
• Internal environmental conditions may cause
the sperm to die.
• Gamete recognition sites on the sperm do
not fit with the intended egg. If the two
species differ in the type of fertilization
(external and internal) there is no chance of
the sperm ever contacting the egg.
• All of the above types of isolation prevent the
formation of hybrid zygotes.
2) Post-zygotic isolating mechanisms
I) Zygotic mortality
II) Hybrid sterility
III) Hybrid inviability
I) Zygotic mortality
• Development of the zygote proceeds
abnormally and the hybrid is aborted causing
zygotic mortility.
• For instance, the hybrid egg formed from the
mating of a sheep and a goat will die early in
development. Hence causes barrier to
evolving new organisms and prevent chances
to overlapping of gene frequencies for the
development of new species.
II) Hybrid sterility
• Some times the hybrid is healthy but sterile called
hybrid sterility.
• For example, The mule, the hybrid offspring of a
donkey and a mare, is sterile; it is unable to
produce viable gametes because the
chromosomes inherited from its parents do not
pair and cross over correctly during meiosis
• This hybrid sterility reduces rate of evolution by
arising new species.
III) Hybrid inviability
• First generation hybrid forms but shows low
fitness or infertility appears in later generation
called as hybrid inviability.
• As witnessed in laboratory crosses of fruit
flies, where the offspring of secondgeneration hybrids are weak and usually
cannot produce viable offspring.
• That further may go to extinction of later
descendants.
Role of isolation
• When an animal or plant is isolated they evolve
by their surroundings.
• For example, say tigers live on mainland but a few
get separated by some act of nature. Millions of
years down the line they will have evolved
differently to adapt better to their little isolated
island then their cousins on the main land.
• To develop new species by making the isolated
groups change with their environment separated
from others of their species.
Role of isolation
• Eventually the isolated group will
change so much they won’t be able
to reproduce with the others,
making a new species.
• All these types of isolation favor
speciation leading to evolution.
• With the process of speciation,
isolation may affect evolution in
negative way also by the extinction
and less biodiversity.
Extinction
• If the environment changes rapidly and the
species living in these environments do not
already possess genes which enable survival in
the face of such change and random
mutations do not accumulate quickly enough
then all members of the unlucky species may
die.
Role of isolating mechanisms
• Is to increase the
efficiency of mating.
Where other closely
related species do not
occur, courtship signals
can “afford” to be
general, non specific and
variable.
Role of isolating mechanisms
• Where other related species
coexist, however, non
specificity of signals may lead
to wasteful courtship and
delays, even where no
heterospecific hybridization
occurs under these
circumstances there will be a
selective premium on precision
and distinctiveness of signals.
Role of isolating mechanisms
• As each species is a delicately integrated genetic
system that has been selected through many
generations to fit into a definite niche in its
environment.
• Hybridization usually leads to breakdown of this
system and results in the production of
disharmonious types.
• It is the function of isolating mechanism to
prevent such a breakdown and to protect the
integrity of genetic system of species.
Role of isolating mechanisms
• Any attribute of the species that would favor
the production of inferior hybrids is selected
against, since it results in wastage of gametes.
Such selection maintains the efficiency of the
isolating mechanisms and indeed helps to
perfect them.
• Isolating mechanisms are among the most
important biological properties of species
leading to evolution.