Evolutionary Genetics

EVOLUTIONARY GENETICS
A Presentation by
Dr. N. Sannigrahi, Associate Professor,
Department of Botany,
Nistarini College, Purulia,
D,B. Road, Purulia ( W.B) India, 723101

EVOLUTION
CONTENTS
 Evolution- Definition & Scope
 Theories of evolution
 Lamarckism
 Darwinism
 Ingredients of evolution
 Neo-Darwinism
 Succession & Fossil Records
 Gradualistic Model of Evolution
 Punctuational Model of evolution
 Fixation of Mutant alleles
 Random Drift
 Molecular drive
 Genetic Variation-Phenotypic Variation

EVOLUTION
 Evolution is the process of heritable change in populations of organisms
over multiple generations. Evolutionary biology is the study of this process,
which can occur through mechanisms including natural selection, sexual
selection and genetic drift. Although the work of Charles Darwin (see the
entry on Darwinism is usually the starting point for contemporary
understandings of evolution, interestingly, he does not use the term in the
first edition of On the Origin of Species, referring instead to “descent with
modification”. In the early-mid 20th century, the “modern synthesis” gave
birth to population genetics which provided a mathematization of
Darwinian evolutionary theory in light of Mendelian genetics .This yielded
a prevalent—probably the most prevalent—understanding of evolution as
“any change in the frequency of alleles within a population from one
generation to the next”. Note, however, that this definition refers to
evolution only in a micro evolutionary context and thus doesn’t reference
the emergence of new species (and their new characteristics), although it is
intended to underlie those macro evolutionary changes.

THEORIES OF EVOLUTION
 Although the evolution in biology is still a paradox and it is an appetizer of
thought of natural scientists to explore the magic of the reality behind the
diversity of life, the following theories are basically talked about in this
domain as below:
 Lamarckism,
 Darwinism,
 Mutation theory ,
 Neo-Darwinism.
 LAMARCKISM THEORY- Propounded by Jean Baptiste Lamarck
(1744-1829) basically comprising of the four postulates as follows:
 1. Living organisms and their organs tend to increase in size during
evolution,
 2. New organs would develop if this development was needed for its
survival,
 3. The organs frequently used will develop more while the organs not
being used will tend to become weaker.

THEORIES OF EVOLUTION
 4. The modification thus produced due to the use and disuse in the traits of
an organism will be inherited and would accumulate in time.
 The long neck of the Giraffes was exemplified in this regard and later on
this ‘theory of the inheritance of acquired character’ was severely criticized
and mostly rejected although the important variations may be brought
about by the environment in this regard deserve mentioning. This theory is
now only of academic interest without any rigid followers.
 B. DARWINISM
 Darwinism is a theory of biological evolution developed by the English
naturalist Charles Darwin (1809–1882) and others, stating that all species
of organisms arise and develop through the natural selection of small,
inherited variations that increase the individual's ability to compete,
survive, and reproduce. Natural selection causes populations to become
adapted, or increasingly well-suited, to their environments over time.
Natural selection depends on the environment and requires existing
heritable variation in a group.

B. DARWINISM
 over production (Rapid Multiplication):
 All organisms possess enormous fertility. They multiply in geometric ratio.
Some examples are cited below:
 Limited Food and Space: Despite of rapid multiplication of all types of
species, food and space and other resources remain limited. They are not
liable to increase.
 Struggle for Existence: The struggle for existence can be of three types.
 (i) Intraspecific Struggle: It is the struggle between the individuals of the
same species because their requirements like food, shelter, breeding places,
etc. are similar. Many human wars are the examples of intraspecific
struggle. Cannibalism (eating the individuals of its own species) is another
example of this type of struggle.
 (ii) Interspecific Struggle: It is the struggle between the members of
different species. This struggle is normally for food and shelter. For
example, a fox hunts out a rabbit, while the fox is preyed upon by a tiger.

B. DARWINISM
 iii) Environmental Struggle: It is the struggle between the organisms and the
environmental factors, such as drought, heavy rains, extreme heat or cold,
earthquakes, diseases, etc. Thus, climate and other natural factors also help in
restricting the number of individuals of particular species.
 4. Appearance of Variations: Except the identical twins, no two individuals
are similar and their requirements are also not exactly the same. It means there
are differences among the individuals. These differences are called variations.
Due to the variations some individuals would be better adjusted towards the
surroundings than the others.
 Adaptive modifications are caused through the struggle for existence.
According to Darwin, the variations are gradual (continuous) and those which
are helpful in the adaptations of an organism towards its surroundings would be
passed on to the next generation, while the others disappear.
 Natural Selection or Survival of the Fittest:
 The organisms which are provided with favorable variations would survive,
because they are the fittest to face their surroundings, while the unfit are
destroyed. Originally it was an idea of Herbert Spencer (1820-1903) who used
the phrase ‘the survival of the fittest’ first time. While Darwin named it as
natural selection.

B. DARWINISM
 It is to be noted that only survival of the fittest is not enough. But
organisms should also adapt or change themselves according to the
changed conditions of the environment as environment is always changing.
These, therefore, will survive more and hence are selected by nature.
Darwin called it natural selection and implied it as a mechanism of
evolution. Alfred Wallace a naturalist who worked in Malay Archepelago
had also come to similar conclusions around the same time.
 6. Inheritance of useful variations: The organisms after getting fitted to
the surroundings transmit their useful variations to the next generation,
while the non-useful variations are eliminated. Darwin could not
differentiate between continuous and discontinuous variations. In this
respect, Darwin agreed with Lamarck’s views, because according to
Darwin acquired characters which are useful to the possessor could be
inherited.
 7. Speciation (Formation of new species):
 Darwin considered that useful variations are transmitted to the offspring
and appear more prominently in succeeding generations. After some
generations these continuous and gradual variations in the possessor would
be so distinct that they form a new species.

B. DARWINISM
 These factors are the "forces of evolution." There are four such forces:
mutation, gene flow, genetic drift, and natural selection.
 MUTATION: Hugo de Vries believed that mutation causes evolution and
not the minor heritable variations which was mentioned by Darwin.
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). On the basis of above observations,
Hugo de Vries (1901) put forward a theory of evolution, called mutation
theory. The theory states that evolution is a jerky process where new
varieties and species are formed by mutations (discontinuous variations)
that function as raw material of evolution. The salient features of mutation
theory are:. 1.Mutations or discontinuous variations are the raw material of
evolution.2. Mutations appear all of a sudden. They become operational
immediately.
 3. Unlike Darwin’s continuous variations or fluctuations, mutations do not
revolve around the mean or normal character of the species.
 4. The same type of mutations can appear in a number of individuals of a
species.

MUTATION
 5. All mutations are inheritable.
 6. Mutations appear in all conceivable directions.
 7. Useful mutations are selected by nature. Lethal mutations are eliminated.
However, useless and less harmful ones can persist in the progeny.
 8. Accumulation of variations produce new species. Sometimes a new
species is produced from a single mutation.
 9. Evolution is a jerky and discontinuous process.
 Points in Favor of the Mutation Theory:
 (1) Mutations are actually the source of all variations and hence fountain
head of evolution.
 (2) Mutation theory can explain both progressive and retrogressive
evolution.
 (3) As the ratio of mutations is not the same in all individuals and their
parts, mutation theory can explain the occurrence of both changed and
unchanged forms
 (4) A number of mutations have appeared in the past.

MUTATION
 Points against the Mutation Theory (Criticism of the Mutation
Theory):
 (1) Oenothera lamarckiana of Hugo de Vries was not a normal plant but a
complex heterozygous form with chromosome aberrations.
 2) Most of the mutations are negative or retrogressive.
 (3) Mutations are generally recessive while traits taking part in evolution
are usually dominant.
 (4) Mutation theory cannot satisfactorily explain the development of
mimicry, mutual dependence of flowers and pollinating insects.
 (5) This theory does not explain the role of nature.
 Significance of Hugo de Vries’ Mutation Theory:
 This Theory gives direct attention to the mutations. But later on it was
thought that evolution cannot occur by mutations alone. Natural selection
and isolation of mutants were also essential for evolution.

GENE FLOW
 Gene flow is also called gene migration. Gene flow is the transfer of
genetic material from one population to another. Gene flow can take place
between two populations of the same species through migration, and is
mediated by reproduction and vertical gene transfer from parent to
offspring. Plant populations experience gene flow by spreading their pollen
long distances. Animals experience gene flow when individuals leave a
family group or herd to join other populations. The flow of individuals in
and out of a population introduces new alleles and increases genetic
variation within that population. Mutations are changes to an organism’s
DNA that create diversity within a population by introducing new alleles.
Some mutations are harmful and are quickly eliminated from the
population by natural selection; harmful mutations prevent organisms from
reaching sexual maturity and reproducing. Other mutations are beneficial
and can increase in a population if they help organisms reach sexual
maturity and reproduce.

GENE FLOW
 An important evolutionary force is gene flow: the flow of alleles in and out
of a population due to the migration of individuals or gametes. While some
populations are fairly stable, others experience more movement and
fluctuation. Many plants, for example, send their pollen by wind, insects, or
birds to pollinate other populations of the same species some distance
away. Even a population that may initially appear to be stable, such as a
pride of lions, can receive new genetic variation as developing males leave
their mothers to form new prides with genetically-unrelated females. This
variable flow of individuals in and out of the group not only changes the
gene structure of the population, but can also introduce new genetic
variation to populations in different geological locations and habitats.
Maintained gene flow between two populations can also lead to a
combination of the two gene pools, reducing the genetic variation between
the two groups. Gene flow strongly acts against speciation, by recombining
the gene pools of the groups, and thus, repairing the developing differences
in genetic variation that would have led to full speciation and creation of
daughter species.

GENETIC DRIFT
 For example, if a species of grass grows on both sides of a highway, pollen
is likely to be transported from one side to the other and vice versa. If this
pollen is able to fertilize the plant where it ends up and produce viable
offspring, then the alleles in the pollen have effectively linked the
population on one side of the highway with the other.
 GENETIC DRIFT
 Genetic drift is a mechanism of evolution in which allele frequencies of a
population change over generations due to chance (sampling error).
 Genetic drift occurs in all populations of non-infinite size, but its effects are
strongest in small populations.
 Genetic drift may result in the loss of some alleles (including beneficial
ones) and the fixation, or rise to 100%100%100, percent frequency, of
other alleles.
 Genetic drift can have major effects when a population is sharply reduced
in size by a natural disaster (bottleneck effect) or when a small group splits
off from the main population to found a colony (founder effect).
 Unlike natural selection, genetic drift does not depend on an allele’s
beneficial or harmful effects. Instead, drift changes allele frequencies
purely by chance, as random subsets of individuals (and the gametes of
those individuals) are sampled to produce the next generation

GENETIC DRIFT
 Every population experiences genetic drift, but small populations feel its
effects more strongly. Genetic drift does not take into account an allele’s
adaptive value to a population, and it may result in loss of a beneficial
allele or fixation (rise to 100%100%100, percent frequency) of a harmful
allele in a population.
 The founder effect and the bottleneck effect are cases in which a small
population is formed from a larger population. These “sampled”
populations often do not represent the genetic diversity of the original
population, and their small size means they may experience strong drift for
generations. Genetic drift has also a fundamental role in the neutral theory
of molecular evolution proposed by the population geneticist Motoo
Kimura. In this theory, most of the genetic variation in DNA and protein
sequences is explained by a balance between mutation and genetic drift.
Mutation slowly creates new allelic variation in DNA and proteins, and
genetic drift slowly eliminates this variability, thereby achieving a steady
state. A fundamental prediction of genetic drift theory is that the
substitution rate in genes is constant, and equal to the mutation rate.

NATURAL SELECTION
 Natural selection is a mechanism of evolution. Organisms that are more
adapted to their environment are more likely to survive and pass on the
genes that aided their success. This process causes species to change and
diverge over time. Natural selection is the process through which
populations of living organisms adapt and change. Individuals in a
population are naturally variable, meaning that they are all different in
some ways. This variation means that some individuals have traits better
suited to the environment than others. Individuals with adaptive traits—
traits that give them some advantage—are more likely to survive and
reproduce. These individuals then pass the adaptive traits on to their
offspring. Over time, these advantageous traits become more common in
the population. Through this process of natural selection, favorable traits
are transmitted through generations.
 Natural selection can lead to speciation, where one species gives rise to a
new and distinctly different species. It is one of the processes that drives
evolution and helps to explain the diversity of life on Earth.

NATURAL SELECTION
 Darwin and other scientists of his day argued that a process much like
artificial selection happened in nature, without any human intervention. He
argued that natural selection explained how a wide variety of life forms
developed over time from a single common ancestor. Darwin did not know
that genes existed, but he could see that many traits are heritable—passed
from parents to offspring.
 Mutations are changes in the structure of the molecules that make up genes,
called DNA. The mutation of genes is an important source of genetic
variation within a population. Mutations can be random (for example,
when replicating cells make an error while copying DNA), or happen as a
result of exposure to something in the environment, like harmful chemicals
or radiation.
 Mutations can be harmful, neutral, or sometimes helpful, resulting in a
new, advantageous trait. When mutations occur in germ cells (eggs and
sperm), they can be passed on to offspring
 If the environment changes rapidly, some species may not be able to adapt
fast enough through natural selection.

NEO-DARWINISM
 Neo Darwinism is a cohesive evolutionary theory that integrates Darwinian
evolution and Mendelian genetics. This is also known as The Modern
Synthetic Theory of Evolution. According to Neo Darwinism, the driving
force of speciation is only inheritable genetic variants. The Modern
Synthesis (or “Neo-Darwinism”), which arose out of the reconciliation of
Darwin’s theory of natural selection and Mendel’s research on genetics,
remains the foundation of evolutionary theory. It is often regarded as
‘synthetic theory’ of evolution. Different workers like Fisher, Dobzhanksi,
Haldane, Wright, Meyer, Stebbins advocated the theory and they have
attempted to explore
 1. the mode of the origin of genetic variation,
 2.the origin and the role of reproductive isolation,
 3.the occurrence and role of natural selection,
 4. identification And elucidation of the effects of other evolutionary forces
like random drift, molecular drive etc. The shifting balance theory as
proposed by Wright encompasses polymorphism, pleiotropy, the relation
between genotype and fitness and population structure etc. to explain it.

SUCCESSION & FOSSIL RECORD
 The remains of the organisms embedded in the rocks are fossils and carbon
dating is the most convenient tool to explore the paradox of evolution. The
fossil depicts the following pictures . The progressively more and more
complex life forms have successively appeared beginning with the simplest
one . The fossil records are sporadic and incomplete and it do not show the
relationships between the predecessors and successors although the scanty
examples can speak a little bit about it. The intermediate life forms linking
in between two groups may be rather few in number since they would be
relatively unadapted as they were still evolving. Some favors the global
upheavals . These geological upheavals may confuse the fossil record
making it difficult to detect the relationships in between the two organisms.
 Thus, the fossil; records and its inadequacy is one of the serious concern to
explore the paradox of the biological evolution in this blue planet.

GRADUALISTIC MODEL OF EVOLUTION
 As far as Darwinism is concerned, evolution is perceived to be a gradual
process of accumulation of genetic changes leading to the formation of new
species as a part of the speciation process and this concept of slow rate of
continuous evolution is called gradualism and the phenomenon is called
microevolution.
 The adaptations are expected to arise due to natural selection and
conversely , they would disappear if the selection is removed.
 The loss of adaptation in the absence of selection is evident in many
findings.
 The occurrence of random mutations has, therefore the mating ability is the
important concern in this regard.

PUNCTUATIONAL MODEL OF EVOLUTION
 This model of evolution speaks about the macroevolution that includes the
process of the formation of new species, genera and higher taxa by the gross
exchanges by hybridization and polyploidy. The gradual accumulation of
changes is also accepted here like that of microevolution. This abrupt changes
have taken place over relatively short geological periods account for the
appearance of new large taxonomic groups Most individuals of such population
will die but some variants may survive in the hostile environment and it
undergoes rapid change in their features in the face of extreme selection
pressure imposed upon them..
 Thus the Punctuational model has all the essential features of the gradualistic
model, bit it places a far greater emphasis on the role of the genetic drift,
Further, the selection in this model is more extreme and less refined than that of
the gradualistic model.
 Random mating among individuals of the population spreads new genes
throughout the population rather rapidly.

FIXATION OF MUTANT ALLELES
 Mutant is random but after mutation if new alleles or through gene duplication
coupled with new genes arrives in the population to be homozygous in due
course. This replacement is done by three mechanisms-
 Selection,
 Random drift,
 Molecular drive.
 SELECTION- it is the differential rates of reproduction of various genotypes.
Genetic load play a very important role either due to physical death or genetic
death. Genetic load of a population due to mutation producing inferior alleles –
mutational genetic load or segregation of generating inferior genotypes-
segregation genetic load. Natural selection tend to favor a beneficial mutant
allele causing gradual increase in its frequency, perhaps, till it replaces its
nomination allele in the population. Such a selection however, impose a
considerable genetic load on the species. If selection is operating on several
loci, the genetic load may become so great as to cause the extinction of the
species.

RANDOM DRIFT
 Genetic drift is one of the basic mechanisms of evolution. Random
fluctuation in gene frequencies of a population due to chance or sampling
error is the major concern of the change of the gene frequency in a
particular population. This is called genetic drift or random genetic drift or
random drift. In each generation, some individuals may, just by chance,
leave behind a few more descendants (and genes, of course!) than other
individuals. The genes and other genetic elements of the next generation
will be those of the “lucky” individuals, not necessarily the healthier or
“better” individuals. That, in a nutshell, is genetic drift. It happens to ALL
populations — there’s no avoiding the vagaries of chance. It does not occur
in the same direction in all the generations; its direction may change from
one generation to the next generation. Genetic drift may cause gene
variants to disappear completely and thereby reduce genetic variation. It
could also cause initially rare alleles to become much more frequent, and
even fixed. Consider a population of rabbits with brown fur and white fur,
white fur being the dominant allele. Due to genetic drift, only the brown
population might remain, with all the white ones eliminated. A couple with
brown and blue eyes has children with brown or blue eyes.

RANDOM DRIFT
 Genetic drift is a mechanism of evolution in which allele frequencies of a
population change over generations due to chance (sampling error).
 Genetic drift occurs in all populations of non-infinite size, but its effects are
strongest in small populations.
 Genetic drift may result in the loss of some alleles (including beneficial
ones) and the fixation, or rise to 100%100%100, percent frequency, of
other alleles.
 Genetic drift can have major effects when a population is sharply reduced
in size by a natural disaster (bottleneck effect) or when a small group splits
off from the main population to found a colony (founder effect).
 Unlike natural selection, genetic drift does not depend on an allele’s
beneficial or harmful effects. Instead, drift changes allele frequencies
purely by chance, as random subsets of individuals (and the gametes of
those individuals) are sampled to produce the next generation.
 When the genetic drift occurs that causes the chance of fluctuation in gene
frequencies of small random mating population.

RANDOM DRIFT
 The Random genetic drift hypothesis called ‘neutralist hypothesis’ or ‘non-
Darwinian hypothesis’ is supported by the occurrence of large amounts of
protein and enzyme polymorphism in almost all the species study.
 The polymorphic alleles will never be fixed , the maintenance of
polymorphism itself at so many loci would incur an enormous and
intolerable genetic load.
 The selection of is definitely responsible for the maintenance of
polymorphism although some mutations may be neutral under certain
environments but they may have selective value under certain conditions.

MOLECULAR DRIVE
 Very often, a rapid replacement of some nucleotide sequence by some
nucleotide sequence take place and the process of the fixation of genetic
variants in a population through molecular process is called molecular
drive.
 There are three mechanisms of molecular dri9ves- unequal chromatid or
chromosome exchange, gene conversion and the gain and the loss of DNA
transposon or transposable elements or jumping genes.
 The families of genes and non-coding sequences within a population may
be fixed as a consequence of molecular drive and this drive may be random
and directional and would not incur a genetic load itself.
 The unusual patterns of gene fixation due to molecular drive are proposed
to permit the origin of biological novelty and the species discontinuity
which are not accomplished by selection or genetic drift.

GENETIC VARIATION-PHENOTYPIC VARIATION
 Genetic variation in a population is the key features of in a population and it
can be visualized at several levels-
 Phenotype,
 Chromosomes ,
 Proteins and enzymes electrophortic variation,
 Amino acid sequence of specific proteins,
 Nucleotide sequences of specific gene,
 Nucleotide sequences of the entire genome.
 PHENOTYPIC VARIATION
 A numbers of species show remarkable similarity in their phenotypes, till they
carry large variety of gene mutations .e.g. 17% of the flies in the natural
population of Drosophila persimilis are free of lethal mutations
 The existence of various chromosome rearrangements widespread in many
species like grasshoppers, snails, Datura, Drosophila etc.

GENETIC VARIATION-PHENOTYPIC VARIATION
 Various species ranging from man to Drosophila may exhibit
polymorphism for 28% in man to 81% in Drosophila.
 The remarkable uniformity for various characters of the members of
species is due to canalization of the development of those characters
associated with the fitness. Canalization is the phenomenon by which the
development of a character is normally unaffected by the common varities
in the genotype as well as environment although it is disrupted by
environmental shocks and mutant genes.
 Genotypes with increased or decreased canalization may be selected; such
selection is called canalization selection.
 Genetic homoeostasis is the property of population to resist the efforts to
reduce the genetic variability beyond a certain level due to the particular
gene frequencies established for a population over a long period through
natural selection. Obviously, these gene frequencies confer the optimum
fitness on the population.
 The necessity for the maintenance of certain levels of heterozygosity to
ensure normal development is the another cause of genetic homeostasis.

HOPE, YOU HAVE ENJOYED THE JOURNEY

ACKNOWLEDGEMENT
 References:
 Google for images,
 Different WebPages for content,
 Principles of Genetics- Basu & Hossain,
 A textbook of Botany (Vol III) Ghosh, Bhattacharya, Hait
 Fundamentals of Genetics- B.D. Singh,
 A Textbook of Genetics- Ajoy Paul
 DISCLAIMER:
 This presentation has been made to enrich open source of information
without any financial interest. The presenter acknowledges Google for
images and other open sources of knowledge to develop this PPT.

Evolutionary Genetics

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