1. University of Sindh Jamshoro
Name: Rimsha Pahore
Group: Group B
Assignment topic: Theories of Evolution
2. Evolution and Theories of Evolution
Evolution is the process of change through time. It is the process by which modern
organisms have descended from ancient organisms. The concept of evolution is as
old as human civilization according to Anaximander’s theory the man was born in
the form of a fish, than migrated on the land by removing body scales. According
to another theory the theory of Aristotle, he arranged all kinds of animals and
plants in a systematic manner and explained that the living organisms of the past
were incomplete they passed through the process of changes and evolved in the
The development of scientific thoughts in 18th
century solved the problem
of evolution of new species. In this connection Lamarck and Darwin contributed in
remarkable way and presented their theory of evolution.
Lamarck’s Theory of Evolution
Using fossils records as a guide Lamarck was able to develop three states.
1) Influence of Environment.
2) Use and disuse of Organs
3) Inheritance of Acquired character.
Lamarck (1744-1829) was a French biologist. He
presented his theory of evolution in 1809. It is an
important theory of evolution and also called theory of
inheritance of acquired characters.
3. 1) Influence of environment:-
According to Lamarck environment plays an important role in the development of
new characters in the body and then in the process of evolution. When an organism
migrated to any other place and lives in a particular environment for a long time,
this environment has great influence on them.
Same time this influence is not visible, because it requires a very long period, their
needs and habits gradually changed. These changes cause of use or disuse of
certain organs of the body.
2) Use and Disuse of organs
It is also an important point in the process of evolution, which according to
Lamarck some organs become stronger and well developed. These organs are
widely used by living organisms, while disuse organs will become gradually
weaker and as a result of this disuse these organs sometimes may be completely
Giraffe has long neck and fore limbs but the ancestors of the giraffe were short
sized. They migrated to such places where there were no grasses on ground.
4. They were compelled to feed on the leaves of the trees, to reach the leaves they
continuously stretched their neck and forelegs. The process was repeated and
changed characters inherited in many generations, so the neck and fore limb
Lamarck’s often used the webbed feet of duck and geese as an example for how
using an organ strengthens it. He said some birds need to go to the water to get
food and spread their toes when they strike the water, so by stretching their toes to
swim through water these birds stretched the skin in between the toes with the time
large membrane are formed for better swimming.
3) Inheritance of Acquired character:-
By the use and disuse of certain organs some new characteristics are developed in
the body of organism called acquired characteristic. These characters are
transferred from one generation to another generation. After many generations the
changes are accumulated to such extent that a marked alteration occurs in the body
and it results in the development of a new species is called evolution.
5. Weismann’s theory (Germinal Continuity theory)
To experiment Lamarck theory August Weismann 1892 a German biologist
perform experiments on mice he cut the tails of mice for 21 successive generations
but every generation was found with tail of full length, the acquired character was
not transmitted, so Weismann strongly opposed and reject Lamarckian theory
based on his experiments
Weismann’s theory also proved that the environment can change the somatic cells
but the modification of somatic cells can not alter the genetic material in the germ
cells in this way the acquired characters are not transmitted to offspring.
Darwin Theory of Evolution (Theory of Natural Selection)
Darwin’s theory is based on four factors.
1) Over production
2) Struggle for Existence
3) Variations and survival of the fittest
4) Natural selection and origins of species.
Although the experimental evidence proved the first
two point of Lamarck theory, the third point
inheritance of acquired characters however did not
seem to be operating in the process of evolution.
Charles Darwin 1809-1882 was an English Biologist. He
explained the theory in his famous book Origin of
Species by natural selection. His theory is based on
accurate observations, intensive studies and prolonged
experiments for over 20 years.
6. 1) Over Production:-
According to Darwin, it is a matter of common observation that all
organisms are very fertile and high reproductive rate, for example paramecium
can produce 600 times per year. If all the individuals survived and continued to
be divided, their total bulk would exceed that of earth.
In the same case plants like orchid plant produces many thousands seeds, a
mushroom produce million spores if all spores or seeds develop into new plant
only one plant would have covered the earth, but due to evolution it is not
2) Struggle For Existence
The over production results in competition for space, food, light and
air this is called struggle for existence, due to struggle for existence the number
of species remain almost stationary or within limits. The struggle between
living organisms is of different types.
Intra specific struggle when struggle between similar species.
Inter specific Struggle when struggle between different species.
Environmental Struggle when struggle against the extreme forces of drought,
rain, cold and lightening.
During this struggle under different conditions many individuals are killed or
eliminated and only smaller number reaches at maturity and reproduces.
3) Variations and Survival of fittest:-
During the process of struggle for existence certain changes are developed in
the body of living organisms called variations. The individual who show
suitable variations continue to exist and reproduce them, it is known as survival
of fittest while those individuals who show unfavorable variations cannot
survive anymore they either migrate to other places or gradually disappear. The
advantage variations are passed down from generation to generation.
7. 4) Natural Selection and Origins of species:-
As we here discussed that species that have favorable variation start to
reproduce and those who don’t have favorable variations killed so this is known
as natural selection.
The individuals which have favorable variations live to maturity and transmit
their favorable variations to their offspring; this process continues generations
after generations for a long time. These variations are accumulated to such
extent that a new species formed, the structure of the body is modified. It is
known as origin of species.
For example Darwin observed many varieties of finches in the Galapagos Islands,
he visited within less than just two decades the finches have evolved smaller beaks
and most of these changes happened within one generation, all these varieties had
evolved from the original seed eating finches.
8. Devries Mutation theory;-
Mutations are the changes in the germ cells, which are produced suddenly and not
in continuous manner. According to devries small changes do not play any role in
evolution, because due to small changes new species cannot develop, but big
sudden changes mutations can cause species, which indicate the process of
Example:- Devries performed his experiments on evening prime rose plants,
according to his experiments he observed sudden new characters in plants after
hybridization. These characters also transferred into new generations. He got eight
new species of prime rose. Devries explained that original prime rose plant had 14
chromosomes, but due to mutations 15-30 chromosomes developed in new species
which showed different size and shape of seeds and fruits of prime rose.
Objections of Mutation theory:-
According to morgon mutations provide raw material for evolution, these are not
purely responsible for evolution, but some other forces and factors are also
involved. According to some scientist mutations are sometimes very small and
unobserved so they cannot take part in process of evolution.
Devries was a biologist of Denmark country he
presented his theory about evolution in 1901, known as
mutation theory. According to this theory new species
are evolved due to sudden and discontinuous changes
called mutations, so this theory is called mutation
9. Micro Evolution and its factors
Micro evolution refers to change in the allele frequencies that occur over time
within a population, means the descendant is clearly of the same type as the
ancestor. This change happens over a relatively short amount of time compared to
changes termed in macroevolution.
This change is due to four different processes mutation, selection natural or
artificial, gene flow and genetic drift.
The example of micro evolution works by interbreeding of the same species of
animals like dogs have descended from wolves and are able to breed between very
diverse breeds. This occurs all over the world with same species.
A mutation is the random change in the DNA sequence within a gene or
chromosome of a living organism. They can also be described as copying errors
10. within the DNA. Mutations can be spontaneous (random) but they can also be
triggered by other many internal and external factors such as toxins, chemical
substances or radiations.
Many mutations are neutral they can neither harm not benefit the organism, but
they can also be deleterious mutation can affect the characteristic of the organism
and in turn reduce their susceptibility to several illness another one beneficial
mutations can lead to reproductive success and adaptability of an organism to its
2) Natural Selection:-
Natural selection dictates organisms with favorable traits are more likely to survive
and reproduce, and pass on their genes (alleles) into next generations.
Black peppered moths were the example of natural selection; researchers
recognized that the light moths no longer worked in the dark forest. Dark moths
live longer in dark forest so they have more time to breed, but in the last 50 years
most industrial countries have significantly reduce their pollution so the number of
dark moths are drooping as the forest become cleaner,
11. Gene Flow (Migration):-
In population gene flow or migration also known as allele flow is the transfer of
genetic material from one population to another.
For example some beetles with brown genes immigrated from another population,
or some beetles carrying green genes emigrated.
Genetic drift refers to changes in allele frequencies of a gene pool due to chance.
For example a certain type of plant can produce blue or yellow flowers, during a
fire many yellow flowers are destroyed and now since blue is the dominant allele,
the plant reproduce plants that only producing blue flowers.
12. Macro Evolution and its factors
Macro evolution involves variations in allele frequencies at or above species level
basically in macro evolution new species formed. The history on larger scale, so
instead of focusing on an individual species, a macro evolutionary lens might
requires that we zoom out on the trees of the life. Like the origin of mammals, the
evolution of terrestrial animals from aquatic.
Macroevolution encompasses the greatest trends and transformations in evolution
such as origin of mammals and the radiations of the flowering plants, however it’s
not necessarily easy to see macroevolution history however we reconstruct the
history of life by using all available evidence geology, fossils, and living
There are two hypotheses theories to explain how groups diverge.
An illustration shows how Macroevolution occurs.
13. The Phyletic gradualism theorizes that most speciation is slow, uniform and
gradual when evolution occurs in this mode, it is usually by the through a process
called Anagenesis. It is the process in which a species gradually accumulating
change, eventually becomes sufficiently distinct from its ancestral form without
branching or splitting.
In Evolutionary biology, punctuated equilibrium is a theory that proposes that once
a species appears in the fossil record, the population will become stable, showing
little evolutionary change for most of its geological history. This state of little or no
morphological change is called stasis. When significant evolutionary change
occurs, the theory proposes that it is generally restricted to rare and geologically
rapid events of branching speciation called cladogenesis.
Cladogenesis is the process by which a species splits into two distinct species
rather than one species gradually transforming into another. From one ancestor
different species produces and from that ancestor another different species so on.
Both of these theories work together nowadays, even many scientists believe that
evolution may occur by both of them working together.