Evolutionary genetics - Theories,

GNT 601: DEVELOPMENTAL AND EVOLUTIONARY GENETICS Page 1 of 17
UNIT II | A |Evolutionary Genetics
UNIT II (A): EVOLUTIONARY GENETICS
INTRODUCTION:
 In simpler terms, Evolutionary Genetics is the study to understand how genetic
variation leads to evolutionary change.
 Evolutionary Genetics attempts to account for evolution in terms of changes in gene
and genotype frequencies within populations and the processes that convert the
variation with populations into more or less permanent variation between species.
 The central challenge of Evolutionary Genetics is to describe how the evolutionary
forces shape the patterns of biodiversity.
 Evolutionary Genetics majorly deals with;
a. Evolution of genome structure
b. The genetic basis of speciation and adaptation
c. Genetic change in response to selection within populations
There are several theories describing the possible causes for evolution. FOUR theories to
be understood are as follows;
1. Lamarckism
2. Darwinism
3. Mutation theory
4. Neo Darwinism/Synthetic Theory
A1. LAMARCKISM
Jean Baptiste
de Lamarck
 The first general theory of evolution was outlined in
1802 and reported in 1809 by a Frenchman, Jean
Baptiste de Lamarck (1744-1829).
 Lamarck proposed the theory of „inheritance of
acquired characters‟ or also popularly known as
`Lamarckism„.
 The important point in Lamarckism is that, the
environment affects the shape and organization of
animals.
THEORY OF INHERITANCE OF ACQUIRED CHARACTERISTICS
1) Internal Vital Force: Living organisms and their component parts tend to increase
continually in size.
2) Use and Disuse of Organs: If an organ is used constantly, it tends to become
enlarged, whereas lack of use results in degeneration.
3) Effect of Environment and New Needs: Production of a new organ results from a
new need and from the new movements which this need starts and maintains.
4) Inheritance of Acquired Characters: Modification produced by the above principles
during the lifetime of an individual will be inherited by its offspring, with the result
that changes are cumulative over a period of time.
Examples:
a. Long neck of giraffe
b. Disappearance of limbs in snakes
c. Webbed feet of ducks
d. Cave Dwellers

Long neck of giraffe as an example of Lamarckism
EVIDENCES JUSTIFYING LAMARCKISM
1. Effect of Change of Environment: Radish is a two-year crop in cold countries but
completes its growth in one year in tropical areas.
2. Effect of Chemicals: Change in the secretion of hormones results in the change of
different parts of the body.
3. Regeneration of organ reported in animals.
CRITICISM OF LAMARCKISM
1. There is no vital force extracted in organisms, which increases their body parts
2. The environment can affect the animal, but it is doubtful that new needs develop
new structures or organs.
3. The use and disuse of the organs is correct up to some extent only.
4. Mendel‟s Laws of Inheritance and Weismann‟s Theory of Continuity of Germplasm
(1892)
The inheritance of acquired characters is disputed. Therefore, Lamarck‟s concept - The
inheritance of acquired characters was discarded.
KEY POINTS OF LAMARCKISM
 Frequent or continuous use develops and enlarges any organ.
 While by permanent disuse it weakens until it finally disappears all acquisitions or
losses wrought through influence of the environment.
 Hence, through use and disuse, are preserved by reproduction.

A2. DARWINISM
CHARLES DARWIN
 Charles Darwin was a British naturalist, proposed
the theory of biological evolution by natural
selection.
 The theory of evolution by natural selection, first
formulated in Darwin's book "On the Origin of
Species" in 1859.
 Darwin defined evolution as "descent with
modification," the idea that species change over
time, give rise to new species and share a common
ancestor.
 The ship HMS Beagle (1831 -1836) - theory of
Darwin’s finches.
 Darwin clearly saw the importance of selection on a prime evolutionary force in the
natural world. He cited many examples of adaptation through selection.
 Darwin found that competition listed among all living things envisioned that struggle
for existence might be means by which the well adapted individuals survive and ill
adjusted are eliminated.
The Main Points of Darwin’s Theory of Evolution
1. OVER PRODUCTION:
 Every species in the absence of environmental checks tend to increase.
 Most species produce far more offspring than are needed to maintain the
population.
 Species populations remain more or less constant (“stable”) because a small
fraction of offspring live long enough to reproduce.
 In a given geographical area, if a population of a particular species doubles in one
year and if there is no check on its increasing population. It will quadruple in the
next year and so on.
 A great reproductive potential of various species could be observed in nature.
 In case, if all the offsprings of any species remained alive and reproduced. They will
soon crowd all the other species on this planet.

2. COMPETITION ( Struggle for existence)
 The increase in birth rate of offsprings leads to struggle for survival, Competition
for food, space and mates.
 Living space and food are limited, so offspring from each generation must compete
among themselves in order to live.
 This competition could continue as act of kill or to be killed.
 Only a small fraction can possibly survive long enough to reproduce.
3. VARIATION
 Variation is the characteristic of animals and plants. There are many ways in
which an organism can differ.
 Characteristics in individuals in any species are not exactly alike.
 Ex: Differences for Homo sapiens (humans) can be exact size or shape of body,
strength in running, or resistance to disease.
 These differences are considered to be the variations within a species.
NOTE: Darwin and Wallace did not understand the cause of variation. They assumed it
was innate property of living things. Now, we are aware that inherited variations are
resulted from mutation or recombination.
4. ADAPTATION (Survival of Fittest)
 An adaptation is an inherited trait that increases an organisms‟ chance of
survival and reproduction in a given environment.
 The consequent elimination of unfit makes it easier for survival of rest of the
population.
 The idea of Survival of Fittest is the core of the theory of Natural Selection.
5. SELECTION
 Darwin realized that perceptual selection existed in nature in the form of Natural
selection.
 Natural selection is a continuous process in a gigantic scale of screening for all
the living matter.
 Nature/environment selects living organisms with better suited inherited traits to
survive and reproduce.
 Offspring inherit these better traits, and as a whole the population improves for
that particular environment
 Natural selection depends on the environment, acts on existing heritable variation.
6. SPECIATION
 Over many generations, favorable adaptations (in a particular environment)
gradually accumulate in species.
 The selected organisms will give rise next generation. In this way, the variation s
successfully transmitted to the succeeding generation.
 The operation of Natural selection over many genes might produce descends which
differ from their ancestors.
 Eventually, accumulated changes become so great; the result is a new species.
 In this way, two or more species may arise from single ancestral stock.

The simplified evolutionary mechanism as proposed by Darwin.
1. Reproduction will tend to leave more offspring than their peers, causing the traits to
increase in frequency over generations.
2. Resources are limited in nature
3. Organisms with heritable selective traits are favored for survival.
4. Causes populations to become adapted, or increasingly well-suited, to their
environments over time.
5. Changes that allow an organism to better adapt to its environment will be selected.
6. Organisms change over time as a result of changes in heritable physical or behavioral
traits resulting in speciation.
A3. MUTATION THEORY
Oenothera lamarckiana
 A Dutch botanist, HUGO
DE VRIES worked on the
plant, evening primrose.
 Oenothera lamarckiana
 According to Hugo De Vries,
New species are not formed
by continuous variations,
but by sudden appearance
of variations, which he
named as mutations.
HUGO DE VRIES
Hugo de Vries experiment on Oenothera lamarckiana
 O. Lamarckian (evening primrose) was self-pollinated and its seeds were allowed to
grow, majority of F1 plants were similar to the parents, but a few were different
plants.
 The different plants were also self-pollinated and when their seeds were sown, the
majority of the plants were similar to the parents while a few were still more different
plants and this continued generation after generation.
 These plants appeared to be new species; Hugo de Vries suggested from his
experiments that new types of inherited characteristics may appear suddenly without
any previous indication of their presence in the race.
 A new type much longer than the original type was obtained “O. gigas”
 He concluded that, evolution is a discontinuous and jerky process, occurs by
mutation.
Hugo de Vries experiment on Oenothera lamarckiana

According to Hugo De Vries,
 Hugo de Vries states that, Mutations are heritable and persist in successive
generations.
 Mutations are random and directionless while Darwin‟s variations are small and
directional.
 According to Darwin evolution is gradual while Hugo de Vries believed that mutation
caused species formation and hence known as saltation (single step large mutation).
SALIENT FEATURES OF MUTATION THEORY
1. Mutations or discontinuous variations are the raw material of evolution.
2. Mutations appear all of a sudden. They become operational immediately.
3. The same type of mutations can appear in a number of individuals of a species.
4. Mutations are inheritable.
5. Mutations appear in all conceivable directions.
6. Useful mutations are selected by nature. Lethal mutations are eliminated.
However, useless and less harmful ones can persist in the progeny.
7. Accumulation of variations produces new species. Sometimes a new species is
produced from a single mutation.
8. Evolution is a jerky and discontinuous process.
NOTE: Mutation is random, minute and irreversible, measureable change in the gene of an
organism or a cell, with the necessary implications that any phenotypic manifestation of
the change may potentially or actually be shown by all descendant organisms or cells.
SUPPORTING EVIDENCES FOR MUTATION THEORY
CRITICISM ON MUTATION THEORY
 Natural mutations are not common
 Oenothera lamarckiana of Hugo de Vries was not a normal plant but a complex
heterozygous form with chromosome aberrations.
 Ancon Sheep is a
short legged variety
which appeared
suddenly in
Massachusetts in
1791.
 A single mutation
can give rise to a
new variety and
even species of
plants, e.g.,
Delicious Apple,
Cicer gigas, Noval
Orange, Red
Sunflower.

A4. NEO-DARWINISM / SYNTHETIC THEORY OF NATURAL SELECTION
(MODERN THEORY OF EVOLUTION)
 Darwin‟s theory of natural selection was accepted.
 The strong supporters of Darwinism were Wallace, Huxley, Haeckel, and Weismann.
 Darwin‟s theory lacked an input of modern concepts of genetics and the mechanisms
how characters appear and persist in a population.
 In the light of recent researches the theory was modified. Several experimental
evidences have gone in favor of Darwinism.
 Based on those facts and statistical data a synthetic theory of evolution was
proposed.
 This is modified theory of Darwinism. This is called Neo-Darwinism.
 The Synthetic theory emerged by the synthesis of the original idea given by Charles
Darwin and addition of new knowledge of genetics, population dynamics, statistics,
and heredity to the theory.
According to Neo-Darwinism the following factors operate for the formation of new
species.
1. Variations
2. Mutations
3. Natural selection
4. Genetic drift
5. Isolation of species.
Over production, struggle for existence and universal occurrence of variation will take
place as usual. But in the synthetic theory the formation of variations and mutations were
discussed with experimental evidence for evolution which Darwin was unable to explain.
1. VARIATIONS:
 During Darwin‟s time little was known about genetic variations (During Meiosis and
crossing over synapsis will take place, resulting in regrouping of genes. Because of
which genetic variation will appear in offsprings)
 The exchange of chromosomal segments between two chromosomes will result in
aberrations, will become heritable variations.
 The sets of chromosomes will increases or decrease. This is called ploidy. Because of
this polyploidy heritable variations will arise they will be carried to number of
generations.
 This may result in the origin of new species.
2. MUTATIONS:
 Any change in the nucleotide sequence of DNA and if one pair of nucleotides is
replaced mutations will arise. These mutations are called point mutations.
 These are caused spontaneously in nature. They can also be brought by induction of
mutagens (Mustard gas, x-rays, gamma rays, electric shocks, temperature shocks).
 These mutations are rare, sudden and heritable. They may be harmful or beneficial.
Because of these sudden mutations new species are formed.

3. NATURAL SELECTION:
 Natural selection includes aft forces both physical and biotic factors and determine
how and in what direction an organism is to change.
 Natural selection has no favoritism. But it is obvious that the organisms which are
suited for environmental conditions will survive over power in the force of
competition. Because of this better survivors are retained in the nature.
4. GENETIC DRIFT:
 In small interbreeding population heterozygous gene pairs will tend to become
homozygous.
 Because of this, disadvantage characters may be expressed and those organisms will
be weeded out.
 They operate in small populations of Islands. This genetic drift will provide a way to
determine the line of evolution due to isolation.
 Isolation (physiological or geographical Isolation) is very important part in evolution.
The effects of natural selection in different environments will give different species. Thus
the old Darwin’s concept is re-organized with experimental proofs, New-Darwinism was
proposed.
Examples
1. The industrial melanic moth:
 Biston betularia, the industrial melanic moth, is a gray colored moth that perfectly
camouflages on tree trunks covered with lichenin England and escapes predation
by birds.
 With industrial revolution in England in the middle of 19th century, lichens on tree
trunks got killed due to smoke belching out of factories.
 Tree trunks were now bare and dark and made the light gray moth prominent to
the predatory birds.
 Now natural selection favored dark coloured moths, which could camouflage on
bare tree trunks. Since the moth has only one generation in a year, in less than 50
generations, the natural selection replaced gray population with black population.
2. Resistance in mosquitoes and houseflies:
 DDT was used extensively, sometimes by airplanes over large areas.
 Initially it killed 99% of mosquito population but at the same time put a lot of
pressure on the surviving individuals to mutate.
 Mutant resistant strains survived DDT application and became the parents of the
next generation.
 Natural selection preserved the resistant populations and eliminated the
susceptible ones.
Natural selection is a phenomenon that forces the species to keep improving generation
after generation so that they remain in the fittest state to survive in a particular
environment. Random genetic changes provide raw material that causes variations and
gives natural selection a chance to operate.

A5. EVOLUTION AT MOLECULAR LEVEL: NUCLEOTIDE SEQUENCE
MOLECULAR EVOLUTION
 Molecular evolution is a basic process of changes in DNA & RNA nucleotide sequence
by substitution of 1 nucleotide to another during evolutionary time.
 Molecular evolution is commonly reported by point mutation i.e., base substitution.
The base substitution is extremely slow process, takes millions of years for base
change and its selection.
 Genome sequencing is used for reconstructing the phylogenetic tree to understand
the evolutionary history. It is also used for molecular evolutionary studies in
formulating/estimating the rate of evolution.
RATE OF MOLECULAR EVOLUTION
 We have to compare the sample DNA nucleotide sequence to common ancestor DNA
i.e., to compare 2 sequences that have decent from the common ancestor.
 If any sequence of length (N) differ from each other at „n‟ site, then the proportions of
difference is referred as the Degree of Divergence (haemming distance)
 If degree of divergence is small, then chance is to have less than 1 substitution at any
site is negligible.
 If degree of divergence is “0”. Then, there is no difference between observed and
actual sequence.
 If degree of divergence is more, then observed number of difference will not match the
actual difference. May be due to multiple hits (more than one mutation at the same site)
MOLECULAR EVOLUTIONMUTATIONS
Mutation in nucleotide evolution can be off 2 types;
1. Synonymous mutation: Synonymous mutation is the mutation that changes
nucleotide sequence, but results in production of same amino acid, due to
degeneracy property of genetic code. Therefore, Structure or function protein remains
unaltered.
2. Non-synonymous mutation: Non-synonymous mutation is the mutation that
changes nucleotide sequence, such that nucleotide sequence codes different amino
acid. Thus, alters the protein structure and function.
 Therefore, it‟s important to account the number of synonymous and non-synonymous
base substitution.
 Rate of nucleotide evolution in coding will be different from non-coding region.
 Therefore, we must check which region in genome sequence is used for evolutionary
studies.
Factors effecting rate & pattern of nucleotides substitution modify protein structure.
Structural constrain should not affect the protein function. Stronger functional
constrain lower is it rate of evolution, and vice-versa. The alternative nucleotide‟s which
evolve site without affecting their gene product and function is found to be more favored
by natural selection. On the other hand, Non-synonymous changes prove to improvise
functional constrains by changing nucleotide sequence and altering amino acid &
protein function. Increase in non-synonymous changes increase in functional constrain.

A6. MOLECULAR PHYLOGENETIC TREE
PHYLOGENETIC TREE
Phylogenetic tree or evolutionary tree is also called DENDROGRAM. It is a graphical
diagram showing the evolutionary relationships of the group of organisms which
originated from common ancestral form (i.e., the history of an organism lineages as they
change through the time). It implies that different species have aroused from common
ancestors, which are connected by passage of genes along the branches of the
phylogenetic tree.
PHYLOGENETIC TREE TERMINOLOGY
Ancestor Present day species
 The ancestor is the tree “trunk”
 Organisms that have arisen from
it are placed at the ends of a tree
“branches”
 The organisms that are alive today
are the leaves of the tree.
In this phylogenetic tree;
 A and B more closely related to
each other than either is to C.
 A, B and C form a CLADE, that is
a sister group to clade composed
of D and E.
 A, B, C, D and E is known as OTU
- Operational Taxonomical Unit.
The distance of one group from another group indicates the Degree of relationship; that
is closely related groups are located on branches close to one another. If we could trace
the history back down the branches of the tree of the life, we could encounter their
ancestors who lived millions and millions of years ago. The new lineages generally retain
many of their ancestral features, which are then gradually modified and supplemented
as Novel traits that help them to better adjust to given environment they live in.
TYPES OF PHYLOGENETIC TREE
1. ROOTED TREE
A single node is designated as root and it
represents a common ancestor with a
unique path leading from it through
evolutionary time to any other node.
2. UNROOTED TREE
Unrooted tree represents the phylogeny
without the root node. It specifies the inter
related nodes, but says nothings about the
direction in which evolution occurred.

MOLECULAR PHYLOGENETIC TREE
 Molecular phylogenetics is the branch of phylogeny that analyzes genetic, hereditary
molecular differences, predominately in DNA sequences, to gain information on an
organism‟s evolutionary relationships.
 Molecular phylogenetics is a fundamental aspect of bioinformatics.
 The similarity of biological functions and molecular mechanisms in living organisms
strongly suggests that species descended from a common ancestor.
 Molecular phylogenetics uses the structure and function of molecules and how they
change over time to infer these evolutionary relationships.
 The Molecular phylogenetic tree is the phylogeny which represents the
comparative molecular data (obtained by molecular phylogenetic analysis)
correlating ancestral linages and present day species.
 Molecular data that are in the form of DNA or protein sequences can also provide very
useful evolutionary perspectives of existing organisms because, as organisms evolve,
the genetic materials accumulate mutations over time causing phenotypic changes.
 Genes are the medium for recording the accumulated mutations; they can serve as
molecular fossils. Through comparative analysis of the molecular fossils from a
number of related organisms, the evolutionary history of the genes and even the
organisms can be revealed.
PHYLOINFORMATICS
 The development and use of computational and an array of bioinformatics tools, the
ability to analyze large data sets in practical computing times, and yielding an
optimal or near-optimal solutions with high probability are being possible.
 In response to this trend, much of the current research in phyloinformatics (i.e.,
computational phylogenetics) concentrates on the development of more efficient
heuristic approaches.
 There are several bioinformatics tools and databases that can be used for
phylogenetic analysis such as PANTHER, P-Pod, PFam, TreeFam, and the PhyloFacts
structural phylogenomic encyclopedia.
 Each of these databases uses different algorithms and draws on different sources for
sequence information, and therefore the trees estimated by PANTHER, for example,
may differ significantly from those generated by P-Pod or PFam.
 As with all bioinformatics tools of this type, it is important to test different methods,
compare the results, then determine which database works best (according to
consensus results) for studies involving different types of datasets.

A7. SPECIATION
SPECIES
 Species is the group of interbreeding individuals with genetic and morphological
similarities, which are reproductively isolated and differ from other such groups.
 The biological species concept defines a species as a group of individuals living in one
or more populations that can potentially interbreed to produce healthy, fertile
offspring.
KEY CHARACTERISTICS OF SPECIES
 Each species will have its own habitat. Every individual shall try and adapt to the
conditions revealing in the environment.
 Every species have the ability to form a new species (Speciation).
 As breeding is restricted within species, Gene flow is concealed to species.
SPECIATION
 Speciation is the process by which new species form.
 Speciation is the process by which new-distinct species descendants from an
ancestral population due to the interference of geographic, physiological, anatomical,
or behavioral factors that prevent previously interbreeding populations from breeding
with each other.
 Speciation occurs when a group within a species separates from other members of its
species and develops its own unique characteristics.
 Speciation involves reproductive isolation of groups within the original population
and accumulation of genetic differences between the two groups.

CLASSIFICATION OF SPECIATION
 Speciation can take place in two general ways;
1. PHYLETIC SPECIATION: A single species may change over time into a new form
that is different enough to be considered a new species. This process is known as
anagenesis.
2. TRUE SPECIATION : More commonly, a species may become split into two groups
that no longer share the same gene pool. This process is known as cladogenesis.
 Most of the evolutionary biologists classify speciation based on two major methods;
1. Allopatric speciation
2. Sympatric speciation
SPECIATION
PHYLETIC
SPECIATION
one species at a time
AUTOGENOUS
PHYLETIC
SPECIATION
one species
replaces other
species
ALLOGENOUS
PHYLETIC
SPECIATION
two species cross
results in species
TRUE
SPECIATION
one species gives rise
to 2/ more species
INSTANTANEOUS
SPECIATION
speciation at one stroke;
mutation favoured by
natural selection
GRADUAL
SPECIATION
slow process;
every individual
adapts to
environment
ALLOPATRIC
SPECIATION
Speciation at
different location by
isolation
GEOGRAPHICAL
ISOLATION
Physical isolation
SYMPATRIC
SPECIATION
Speciation at same
location
BIOLOGICAL
ISOLATION
1. Premating isolation
2. Postmating isolation

A8. METHODS OF SPECIATION; ALLOPATRIC AND SYMPATRIC
 New species form by speciation, in which an ancestral population splits into two or
more genetically distinct descendant populations. Speciation involves reproductive
isolation of groups within the original population and accumulation of genetic
differences between the two groups.
 In allopatric speciation, groups become reproductively isolated and diverge due to a
geographical barrier. In sympatric speciation, reproductive isolation and divergence
occur without geographical barriers—for example, by polyploidy
ALLOPATRIC SPECIATION
 allo meaning other and patric meaning homeland; involves geographic separation
of populations from a parent species and subsequent evolution.
 In allopatric speciation, organisms of an ancestral species evolve into two or more
descendant species after a period of physical separation caused by a geographic
barrier, such as a mountain range, rockslide, or river.
 Sometimes barriers, such as a lava flow, split populations by changing the landscape.
Other times, populations become separated after some members cross a pre-existing
barrier. For example, members of a mainland population may become isolated on an
island if they float over on a piece of debris.
 Over time, the populations may become genetically different in response to the
natural selection imposed by their different environments.
 Once the groups are reproductively isolated, they may undergo genetic divergence.
That is, they may gradually become more and more different in their genetic makeup
and heritable features over many generations.
 Genetic divergence happens because of natural selection, which may favor different
traits in each environment, and other evolutionary forces like genetic drift.
Case study: squirrels and the Grand Canyon
 The Grand Canyon was gradually carved out by the Colorado River over millions of
years. Before it formed, only one species of squirrel inhabited the area. As the canyon
got deeper over time, it became increasingly difficult for squirrels to travel between
the north and south sides. As the canyon deepened, it acted as geographic barrier to
squirrel populations on either side. Two squirrel species evolved as a result of
allopatric speciation.
 Eventually, the canyon became too deep for the squirrels to cross and a subgroup of
squirrels became isolated on each side. Because the squirrels on the north and south
sides were reproductively isolated from one another due to the deep canyon barrier,
they eventually diverged into different species.
 Harris's antelope squirrel evolved on the south side of the Grand Canyon and the
white-tailed antelope squirrel evolved on the north side of the Grand Canyon.

 Allopatric speciation is speciation that happens when two populations of the
same species become isolated from each other due to geographic changes. Speciation
is a gradual process by which populations evolve into different species. A species is
itself defined as a population that can interbreed, so during speciation, members of a
population form two or more distinct populations that can no longer breed with each
other.
Steps of Allopatric Speciation
1. A geographic change separates
members of a population into more
than one group. Such changes
could include the formation of a new
mountain range or new waterway, or
the development of new canyons, for
example. Also, human activities
such as civil engineering,
agriculture, and pollution can have
an effect on habitable environments
and cause some members of a
population to migrate.
2. Different gene mutations occur and
build up in the different populations
over time. The different variations of
genes may lead to different
characteristics between the two
populations.
3. The populations become so different
that members of the different
populations can no longer breed
with each other anymore if were
they to be in the same habitat in the
same time. If this is the case,
allopatric speciation has occurred.
The following diagram represents an
experiment on fruit flies where the
population was forcibly separated and
the two groups were fed a different diet.
After many generations the flies looked
different and preferred to mate with flies
from their own group. If these two
populations continued to diverge for a
long time, they could become two
different species through allopatric
speciation.

SYMPATRIC SPECIATION
 Sym meaning same and patric meaning homeland; involves speciation occurring
within a parent species remaining in one location.
 In sympatric speciation, organisms from the same ancestral species become
reproductively isolated and diverge without any physical separation.
 Species are kept distinct from one another by prezygotic and postzygotic barriers.
These barriers keep organisms of different species from mating to produce fertile
offspring, acting before and after the formation of a zygote, respectively. These
barriers maintain the reproductive isolation of species.
 There are several ways that sympatric speciation can happen.
Sympatric speciation without polyploidy
 This speciation phenomenon most commonly occurs through polyploidy, in which an
offspring or group of offspring will be produced with twice the normal number of
chromosomes. Where a normal individual has two copies of each chromosome
(diploidy), these offspring may have four copies (tetraploidy). A tetraploid individual
cannot mate with a diploid individual, creating reproductive isolation.
Sympatric speciation without polyploidy
 Sympatric speciation may take place when subgroups in a population use different
habitats or resources, even though those habitats or resources are in the same
geographical area.
One classic example is the North American apple maggot fly.
North American apple
maggot fly
 As the name suggests,
North American apple
maggot flies, can feed and
mate on apple trees. The
original host plant of these
flies, however, was the
hawthorn tree.
 It was only when European settlers introduced
apple trees about 200 years ago that some flies in
the population started to exploit apples as a food
source instead.
 The apple maggot fly is thought to have evolved
through sympatric speciation from its ancestor, the
North American maggot fly. This example of
sympatric speciation occurred through habitat
differentiation: apple maggot flies began to prefer
apple trees as host plants, whereas their ancestors
used hawthorn trees.
 In this way, the population was effectively divided
into two groups with limited gene flow between
them, even though there was no reason an apple
fly couldn't go over to a hawthorne tree, or vice
versa.
 As they diverge, the groups may evolve traits that act as prezygotic and/or postzygotic
barriers to reproduction. For instance, if one group evolves large body size and the
other evolves small body size, the organisms may not be physically able to mate- a
prezygotic barrier, if the populations are reunited.
 If the reproductive barriers that have arisen are strong-effectively preventing gene
flow- the groups will keep evolving along separate paths. That is, they won't exchange
genes with one another even if the geographical barrier is removed. At this point, the
groups can be considered separate species.

A9. PREMATING AND POST MATING ISOLATING MECHANISMS
 The biological properties of organisms that prevent interbreeding between the
different species are called reproductive isolating mechanisms (RIMs).
 The reproductive characteristics which prevent species from fusing.
 Reproductive isolating mechanisms are commonly called as Biological isolation.
 The two types of reproductive isolating mechanisms are;
1. Premating isolating mechanisms
2. Post mating isolating mechanisms
PREMATING ISOLATING MECHANISMS
 It is also referred as Pre-zygotic isolating mechanisms.
 Premating isolation prevents species from fusing.
 Isolating mechanisms are particularly important in the biological species concept, in
which species of sexual organisms are defined by reproductive isolation.
TYPES:
1. Temporal isolation:
Individuals of different species do not mate because they are active at different times
of day or in different seasons.
2. Ecological isolation:
Individuals mate in their preferred habitat, and therefore do not meet individuals of
other species with different ecological preferences.
3. Behavioral isolation:
Potential mates meet, but choose members of their own species.
4. Mechanical isolation:
Copulation is attempted, but transfer of sperm does not take place.
POST MATING ISOLATING MECHANISMS
 Post-mating RIMs reduce the viability or fertility of hybrids or their progeny.
 The mechanism is also referred as Post-zygotic RIMs.
 Post-mating isolating mechanisms are the result of developmental or physiological
differences between the members of two species after mating;
1. Gametic incompatibility:
Sperm transfer takes place, but egg is not fertilized. Gamete isolation is the physical
or chemical incompatibility of gametes of two different species. If the gametes lack
receptors to facilitate fusion, they cannot form a zygote. An egg may have receptors
only for the sperm of its own species
2. Zygotic mortality:
Egg is fertilized, but zygote does not develop. Zygote mortality is a mechanism when
the zygote dies soon after its formation.
3. Hybrid unviability:
Hybrid embryo forms with reduced viability. The offspring of parents of two different
species is known as a hybrid. It dies before reaching sexual maturity.
4. Hybrid sterility:
Hybrid is viable, but resulting adult is sterile. The hybrid fails to reproduce sexually.
For example, the mule is a sterile hybrid between a male donkey and a mare, a hinny
is a sterile hybrid between a stallion and a female donkey.
5. Hybrid breakdown:
First generation (F1) hybrids are viable and fertile, but further hybrid generations (F2
and backcrosses) may be unviable or sterile.
The micro-evolutionary forces will eventually give rise to the macro-evolutionary patterns
**********

Evolutionary genetics - Theories,

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