The document discusses isolating mechanisms that prevent interbreeding between populations, leading to speciation. There are two main types: geographical isolation, which involves physical barriers, and reproductive isolation, which includes various prezygotic and postzygotic barriers. The Dobzhansky-Muller model explains how genetic incompatibilities arise between diverging populations, contributing to reproductive isolation and the formation of new species.

1. Isolating mechanisms

2. Isolation
• Any external or internal barrier, that
prevents interbreeding between
populations, is called isolation.
• As a result, the population of a species is
either separated into smaller units or the
exchange of genes (gene flow) between
them is prevented

3. • -Isolation is the separation of the population of a particular species into
smaller units which prevents interbreeding between them.
• -Some barrier that prevents gene flow or exchange of genes between
isolated populations is called isolating mechanism.
• -A number of isolating mechanisms are operated in nature and therefore
divergence and speciation may occur.
• -The isolating mechanism is of two types namely geographical isolation
and reproductive isolation.
Isolation Mechanism

4. • The reproductive characteristics that prevent interbreeding between different species
• They are important for maintaining the biological species concept, which defines
species by reproductive isolation
• There are different types of isolating mechanisms, such as
• Geographical
• Reproductive
Isolating mechanisms

7. Geographic isolation
• Physical separation of populations of organisms from one another due to geographical
barriers.

8. Geographical Isolation
• The separation of species by physical barriers like water forms, oceans, mountains,
etc.
• The organisms are ultimately separated from exchanging genetic material with
other organisms of the same species
• As there are fewer chances for variations to occur, new species are not formed in
asexually reproducing individuals undergoing geographic isolation
• This type of isolation can lead to the development of distinct genetic and physical
characteristics in different populations, and ultimately, the evolution of new
species

9. I. GEOGRAPHICAL ISOLATION
• It is also called as physical isolation.
• it occurs when an original population is divided into two or more groups
by geographical barriers such as river, ocean, mountain, glacier etc.
• -these are barriers prevent interbreeding between isolated groups.-The
separated groups are exposed to different types of environmental factors
and they Requires new traits by mutation.
• •The separated Population develop distinct gene pool and they do not
interbreed. Thus new spacies have been formed by geographical isolation
e.g Darwin's finches Original population

12. • An earthquake causes two
populations to become separate from
each other.
• Over time, each species experiences
genetic makeup specific only to their
own smaller, less diverse populations.

13. Reproductive isolation
The inability of a species to breed successfully with related species due to
• Geographical
• Behavioral
• Physiological
or
• Genetic barriers or differences

15. Reproductive isolation

17. Reproductive isolation
• is a key concept in evolutionary biology, referring to the mechanisms
that prevent different species from interbreeding or producing viable
and fertile offspring when they do attempt to mate.
• There are several types of reproductive isolation, and they can be
categorized into two main groups
• : prezygotic barriers and postzygotic barriers.

23. 1. Prezygotic barriers
1. Temporal Isolation:
2. Habitat Isolation:
3. Behavioral Isolation
4. Mechanical Isolation:
5. Gametic Isolation:
6. Ethological Isolation:

24. 1. Postzygotic barriers
1. Gametic mortality
2. Zygotic mortality
3. Hybrid inviability
4. Hybrid sterility
5. Hybrid breakdown

30. Temporal isolation
• A type of reproductive isolation mechanism among sexual organisms
in which the differences in the timing of critical reproductive events
prevent members of closely related species

34. courtship is the period wherein some couples get to know each other prior to a possible marriage or committed
romantic, de facto relationship

39. 1. Prezygotic Barriers:
• These barriers occur before the formation of a zygote (fertilized egg).
• a. Geographic Isolation:
• Populations are separated by geographic barriers such as mountains, rivers, or oceans.
Over time, this can lead to the development of distinct species. Example: The
Galápagos finches, where different species evolved on different islands due to
geographic isolation.
• b. Temporal Isolation:
• Species may have different mating seasons or times of day when they are active,
preventing them from encountering each other for mating. Example: Two species of
frogs, one that breeds in the spring and another in the summer, do not interbreed.

40. c. Behavioral Isolation: Species have different courtship rituals, behaviors, or signals that prevent
them from mating with individuals of other species. Example: Male fireflies use specific light
patterns to attract females of their own species; if the pattern is not matched, mating does not
occur.
d. Mechanical Isolation: The reproductive organs of two species are structurally incompatible,
making it physically impossible for them to mate. Example: Some species of snails have shells that
coil in different directions, preventing them from mating.
e. Gametic Isolation: Even if mating occurs, the sperm of one species may not be able to fertilize
the egg of another species due to differences in gametes (sperm and egg) or their chemical
compatibility. Example: Sea urchin sperm may not successfully fertilize the eggs of a different
species due to gametic differences.

47. Gametic Mortality
 Gametic mortality refers to the death or failure of reproductive cells (gametes) before they
can successfully fertilize and contribute to the formation of a new organism.
 Gametes are specialized cells involved in sexual reproduction: sperm in males and eggs (ova)
in females.
 Gametic mortality can occur for various reasons, including
 Genetic abnormalities
 Environmental factors, or physiological issues.
 Gametes are unable to successfully fuse during fertilization due to problems such as
 Genetic mutations,
 chromosomal abnormalities, or the inability to reach each other in the reproductive
tract
 It can result in the failure to conceive or produce viable offspring.

49. Bufo fowleri
• Mating and sperm transfer takes place but egg is not fertilized.
• In Drosophila vaginal wall swells killing spermatozoa should interspecific crosses
take place.
• If mating takes place between Bufo fowleri and Bufo valliceps, sperms cannot
penetrate the egg membrane of each other, leading to the mortality of gametes.
Bufo valliceps

54. 2. Postzygotic Barriers:
• These barriers occur after the formation of a zygote (fertilized egg).
• a. Hybrid Inviability: The zygote is formed but does not develop properly, leading to the death of the
hybrid embryo. Example: A horse and a donkey can mate to produce a mule, but mules are often
sterile and unable to reproduce.
• b. Hybrid Sterility: Hybrids may develop but are sterile, preventing gene flow between species.
Example: The hybrid between a lion and a tiger, called a liger, is often sterile.
• c. Hybrid Breakdown: The first-generation hybrids may be viable and fertile, but when they mate with
each other or with either parent species, the subsequent generations are often weak or infertile.
Example: Some cultivated rice varieties show hybrid breakdown when the second-generation hybrids
have reduced yield and quality.
Reproductive isolation mechanisms play a crucial role in the process of speciation, where new species
arise as populations become reproductively isolated from one another and accumulate genetic
differences over time

56. DOBZHANSKY-MULLER MODEL
• The Dobzhansky-Muller model, also known as the Dobzhansky-Muller incompatibilities or
speciation genes model, is a theoretical framework in evolutionary biology that helps explain
the genetic basis of reproductive isolation between two diverging populations or species.
• This model was developed independently by two prominent evolutionary biologists,
Theodosius Dobzhansky and Hermann J. Muller, in the mid-20th century.

57. • The central idea of the Dobzhansky-Muller model is that when two populations of a common
ancestral species diverge and accumulate genetic changes (mutations) independently over time,
they may eventually become reproductively isolated from each other.
• It means that individuals from these two populations can no longer interbreed or produce viable and
fertile offspring
• The model proposes that these reproductive barriers arise due to the evolution of genetic
incompatibilities between the genes of the two populations.
• Specifically, it suggests that when alleles (alternative forms of genes) from one population interact
with alleles from the other population, they can produce negative epistatic interactions.
• Negative epistasis occurs when the combined effect of two alleles together is deleterious, even if
each allele is individually functional or beneficial within its own population

58. The key elements of the Dobzhansky-Muller model are:
 Ancestral Population: Initially, there is a single, interbreeding population.
 Genetic Divergence: Over time, this population becomes geographically isolated or
undergoes genetic changes due to various factors like mutation, natural selection,
genetic drift, or geographic isolation. These changes result in the accumulation of
different alleles in each population.
 Incompatibility Alleles: Some of the alleles in each population may be functionally
adapted to the genetic background of that population. However, when individuals from
these divergent populations come into contact or attempt to mate, certain
combinations of alleles may lead to incompatibilities. These incompatibilities can result
in reduced fitness or reproductive failure in hybrid offspring.

59. Reproductive Isolation:
• The genetic incompatibilities between the populations act as a barrier to gene flow.
• This can lead to reproductive isolation, where hybrids between the populations have
reduced fitness or are inviable, preventing further interbreeding between the
populations

60. • In the ancestral population, the genotype is AABB.
• When two populations become isolated from each other, new mutations can arise.
• In one population A evolves into a, and in the other B evolves into b.
• When the two populations hybridise it is the first time a and b interact with each other.
• When these alleles are incompatible, we speak of Dobzhansky–Muller incompatibilities.

61. Significance of isolating mechanism.
 The isolating mechanism protects the gene pool of a species and prevents hybridization.
 It prevents the wastage of gametes and energy.
 A weak isolating mechanism leads to the production of new species through hybridization.
 The absence of an isolating mechanism leads to the production of new species by instant
speciation.
 Geographical isolation followed by reproductive isolation ultimately leads to the production of
new species.
 Isolating mechanisms protect the identity of a species, which all species fiercely guard