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ISOLATING MECHANISMS
ISOLATING MECHANISMS
• Factors that prevent gene exchange amoung
populations are called isolating mechanisms.
• Dobzhansky introduced the term ‘isolating
mechanism’ to define any agents that hinders
interbreeding of group of individuals.
• Mayr restricted this term to sympatric populations &
defined it as ‘biological properties of individuals
which prevent interbreeding of populations that are
actually or potentially sympatric’(This definition
excludes geographic barriers)
ISOLATING MECHANISMS
PREZYGOTIC
ISOLATING
MECHANISMS
HABITAT
(ECOLOGICAL)
SEASONAL
(TEMPORAL)
ETHOLOGICAL
(BEHAVIOURAL)
POSTZYGOTIC
ISOLATING
MECHANISMS
GAMETIC
MORTALITY
ZYGOTE MORTALITY &
HYBRID INVIABILITY
HYBRID
STERILITY
MECHANICAL
HYBRID
BREAKDOWN
PREZYGOTIC ISOLATING
MECHANISMS
Isolating mechanisms which operate
before fertilization.
HABITAT(ECOLOGICAL) ISOLATION
Potential mates donot meet
because they flourish in different
habitats
Eg-5 species of Fly catchers of
North America
Least flycatcher(Empidonex minimus)
• Seen in open woods,farms and orchards
Acadian fly catcher (Empidonex virescens)
•Found in deciduous forests particularly in beech
trees and swampy woods
Alder fly catcher (Empidonex alnorum)
• seen in wet thickets of alder trees.
Yellow bellied flycatcher(E.flaviventris)
• nests in conifer woods
Willow fly catcher (E.traillii)
• seen in bushy pastures and willow thickets.
SEASONAL (TEMPORAL) ISOLATION
Differences in breeding season prevent mating of
potential individuals of different species.
Eg-Wood frog & Leopard frog
Seasonal isolation
Wood frog (Rana
sylvatica)
It usually mates in late
March or early April when
water temperature is
about 7.2°C (45°F)
Leopard frog (Rana
pipiens)
It usually mates in mid-
April when water
temperature is
12.8°C(55°F)
ETHOLOGICAL(BEHAVIOURAL)
ISOLATION
•Differences in courtship behaviour
prevent mating between individuals
of different species
•Eg-chiff chaff,wood warbler &
willow warbler
Wood
warbler
Willow
warble r
ETHOLOGICAL
(BEHAVIOURAL)
ISOLATION
Chiff chaff,wood
warbler & willow
warbler are 3
different species which looks almost exactly same,but they
donot interbreed due to difference in mating songs
MECHANICAL ISOLATION
•Body parts of a species may not
physically match with those of other
species that could otherwise be mates or
pollinators
•Eg-black sage & white sage
Salvia mellifera
(Black sage)
Salvia apiana
(White sage)
Black sage &White sage grow in the same areas, but hybrids
rarely form because flowers of 2 species have become
specialized for distinct pollinators.Black sage flowers are
pollinated by small bees &white sage flowers by large bees
Dufour’s hypothesis
Genital armatures act like lock & key and prevent hybridization
between individuals of different species.Eg-Drosophila
Karl Jordan’s contradiction
Out of 698 species of sphingidae family of insecta,48 were not
different in genitalia from other species of family,while about
50% of species with geographic variation in colour,there was
geographic variation in structure of genitalia.This indicate
slight importance of genitalia as isolating mechanism.
POSTZYGOTIC ISOLATING
MECHANISMS
Isolating mechanisms which operate
after fertilization
GAMETIC MORTALITY
•In this mechanism interspecific cross destroys
either sperm or egg.
•Sperm encounter antigenic reaction in genital
tract of female and will be immobilized and
killed before it has a chance to reach egg.
•Eg- insemination reaction in
Drosophila(Patterson)
ZYGOTE MORTALITY &
HYBRID INVIABILITY
The egg is fertilized but zygote either doesn’t
develop or develop into an organism with
reduced viability
HYBRID STERILITY
•Hybrid has normal viability but is sterile.
•Hybrid sterility is common if 2 parent
species have different chromosome
number.Eg-Mule which is a cross between
Horse and donkey (Jackass)
Female horse
(2n=64)
Male donkey
(2n=62)
Mule (2n=63)
Sterile,because synapsis and segregation
cannot occur properly
HYBRID BREAKDOWN
•Even though viable and fertile hybrids between 2
species are obtained,some barriers still exist if hybrids
of second generation or from backcross of F1 hybrid to
parental species are inviable.
•Eg-Drosophila pseudoobscura &
Drosophila persimilis
D.pseudoobscura D.persimilis
F1
Fertile female Parent Sterile male
F2
Hybrid weaker than
parent and F1 hybrids
•Isolating mechanisms always have a
partially genetic basis.
•For vast majority of animals it is still not
known which particular mechanism prevent
closely related species from interbreeding
•Coaction-If one isolating mechanism fails
another must overcome
Isolation mechanisum

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Isolation mechanisum