beings have a long evolutionary history. Present day man is a
product of evolutionary changes in pre-human ancestors who were
arboreal in habit.
- Family Hominidae: Humans belong to the only living species of
this family, namely, homo sapiens.
- Parapithecus: 36 million year old fossil. It was unearthed
from a place called Fayum in Egypt. It is believed to be
a kind of monkey.
- Ramapithecus: It was named so because it was found in the Siwalik hills of India in the year 1910.It is
12 -14 million years old fossil. It is believed to be an ape.
study of fossils to determine the structure and evolution of
distinct animals and plants is called paleontology.
fossil record shows that the evolution of man has taken place from
pliocene period through the following progressive changes:
- From quadrupedal to full erect posture with eyes facing in
- Increase in size of cranium.
- Reduction in brow ridges.
- Reduction in jaws.
- Change from U-shaped to crescentic dentition.
- Development of an apposable thumb for better grip of hand.
- Development of power of communication.
Important characteristics of Dryopithecus:
- It was of the size of a rhesus monkey.
- The body was slender.
- The tail was absent.
- It had a relatively smaller head.
- The face showed projecting snout, prognathism (projecting
jaws) was not very prominent as in modern apes.
- They had 32 teeth like modern apes.
- The canines were smaller than in apes.
- Incisors were straight like that in Hominidae and not
slanting as in apes.
- The teeth were arranged in a U-shaped pattern, as in apes.
- They lacked brow-ridge, which is one of the important
characteristics of apes.
- They had a comparatively well-developed brain.
- The thumb of Dryopithecus is comparatively longer than in
- The pelvic girdle and vertebral column suggest that
Dryopithecus was not capable of walking erect. It probably was
Australopithecus is the earliest known Hominid.
It is considered as a connecting link between ape and man as it
showed both ape-like and man-like characters. Example:
- Jaws and teeth larger in size than in man.
- Chin was absent.
- Eyebrow ridges were large.
- Skull was small with a cranial capacity of 500-700 ccs.
- Foramen magnum was ventral in position, which indicate
they had bipedal gait like man.
- The incisors and canines were uniform in size and similar
to those of a man.
Types of Autralopithecus:
Early Australopithecus were of two types, namely, A. ramidus and A.
- They were 1 to 1.5 metres in height.
- They were fully bipedal.
- Their snout was prognathous.
- The average size of the brain was 400 ml.
- They had parallel rows of teeth like those in apes.
- They had fairly large and sharp canines.
- The knee joint and femurs showed modifications for walking
They are three kinds of later Australopithecus, namely:
- Australopithecus africanus, also called gracile.
- Australopithecus robustus, also called robust
- Australopithecus boisei, also called superrobust
- They are comparatively small, i.e., 4.5 feet in height.
- The face was only slightly prognathous.
- They have less massive jaws.
- They are without sagittal crest (sagittal crest is a bony
ridge along the top of the skull running from front to back. It is
used for attachment of muscles.)
- They are slender and hence called gracile.
- They are comparatively larger, i.e., 5 feet.
- The face was even lesser prognathous than the A.
- They have heavy jaws.
- They have sagittal crest.
- They have reduced incisor and canine teeth.
- They are robust or tough and hence called A. robustus.
- They are very robust and hence the name.
- Rest of the features are the same as A. robustus.
Theory of Natural Selection
- Overproduction or Prodigality of Production
All organisms have a capacity to reproduce, which is beyond their
necessity for progenies. Organisms increase in number in geometric
progression, i.e.,, by successive multiplication.
- The Struggle for Existence
In nature, the number of any species is constant in spite of the high
capacity of reproduction. The limited supply of food, shelter, etc.
leads to competition among the progenies. This is called the
struggle for existence.
The struggle for existence is of 3 types, namely:
- Inter-specific Struggle: It is the struggle between members of
different species. Example, Struggle between deer and horses for
- Intra-specific Struggle: It is the struggle between members of
the same species. Example, Struggle for grass between members of
the same species.
- Environmental Struggle: Natural selection shows a struggle
among all the living organisms against the environmental odds such
as extremes of temperature, light and water.
Variations are differences between individuals belonging to same
species, which are not due to difference of age or sex. Variations
in a population may be environmental or genetic. Mutations or
crossing over during meiosis, give rise to genetic variations.
Genetic variations may be adaptive or non-adaptive. During sexual
reproduction recombination takes place which results in variations.
There are 3 types of variations, namely:
- Morphological variations
- Physiological variations
- Behavioural variations
Variations may be beneficial or harmful.
Variation: Colouration of butterfly that makes it difficult to be
noticed by the predators.
Variation: Insufficient secretion of insulin.
- Natural Selection
According to Darwin, individuals with useful variations survive in
the struggle for existence and those with harmful variations are
Favourable adaptive variations are selected by nature.
This according to Darwin is called natural selection or the
principle of the survival of the fittest.
Natural selection brings about changes.
- Creates new species.
- Make organisms more and more adaptable to their
Mutations are the basic cause of variations. Any change in
the genetic material that is passed on to the next generation constitutes
Mutations are of two types, namely:
Genetic mutations are most common and they are recessive.
of Natural Selection
- Differential reproduction: Differential
reproduction results from non-random mating of individuals. It
results in natural selection.
Natural selection has 2 aspects:
- Differential Survival: This refers to the different rates of
survival of individuals having different variations.
- Differential Fertility: This refers to differences in the rates
Evolution gives rise to new species. This is called speciation.
Speciation is the result of differential reproduction and isolation.
is a group of potentially interbreeding populations (group of
physically, biochemically and behaviourally similar organisms) that
are reproductively isolated from other such groups.
totality of all genes and their alleles in a population constitute a
gene pool. A species has its gene pool. A population also has its
gene pool of a species is a closed one since it cannot receive genes
from outside its members. On the other hand, the gene pool of
population is an open one because it can receive new genes from
members of other populations of the same species. A population in
which the gene pool changes, undergoes evolution.
thus occurs when the open gene pool of a population (or a group of
populations) becomes a closed one. Separate closed gene pools of
daughter species are formed out of the gene pool of the original
species during the process of speciation.
- Stages of Speciation
Formation of populations
Formation of races or subspecies
isolation of population or group of populations is very essential
for the formation of new species. This type of speciation is called
geographic or allopatric speciation and is the most common type of
speciation especially in animals.
other type of speciation is called the quantum or sympatric
speciation, which is rare in animals. It results due to the sudden
appearance of chromosomal mutations like polyploidy in plants.
Sympatric speciation does not require geographic isolation.
characteristic that is advantageous to a particular organism or population
with reference to an environment is called an adaptation. Thus,
adaptations are genetically determined characteristics which
increase the chances of survival and reproduction of an organism.
Adaptations may be fossorial, cursorial, aquatic, arboreal, volant,
cave, desert, reproductive, physiological, etc.
regard to the type of environment there are 4 types of adaptations:
- Adaptations to physical environment
- Adaptations to biotic environment
- Intraspecific adaptations
- Internal adaptations
Animals with intermediate characters between two major groups of animals
are called connecting links. These connecting links further support the
concept of common ancestory. Example,
Fishes ® Lung Fish ® Amphibia
Amphibian ® Seymouria ® Reptiles
Reptiles ® Archaeopteryx ® Birds
Modern horse has evolved over a period of 60 million years, from 11"
in height to around 60" in height. Following are the various stages
in the evolution of horse.
Eohippus ® Mesohippus ®Merychippus ® Pliohippus and Equus
Recombination: In sexually reproducing
organisms, at the time of gamete formation during meiosis, exchange of
genes between certain segments of homologous chromosomes takes place,
resulting into new combination of genes in the offspring. This is known
as recombination. Recombination is a source of variation.
Crossing over: The exchange of segments between
the non-sister chromatids of homo-logous chromosomes is called crossing
Industrial melanism: Industrial melanism occurs
due to a change in the environment caused by industrial revolution.
Reproductive isolation: The inability of an individual
belonging to a particular species to produce fertile offspring due to
geographical isolation, is called reproductive isolation. Reproductive
isolation leads to the origin of new species.