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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Evolution Page 1

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1.

 

 

  1. Human beings have a long evolutionary history. Present day man is a product of evolutionary changes in pre-human ancestors who were arboreal in habit.
  2. Family Hominidae: Humans belong to the only living species of this family, namely, homo sapiens.
  3. 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.
  4. 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.

 

 

2.

 

 

  1. The study of fossils to determine the structure and evolution of distinct animals and plants is called paleontology.
  2. The 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 front.
    • 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.

 

 

3.

 

 

  1. DRYOPITHECUS
    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 apes.
    • The pelvic girdle and vertebral column suggest that Dryopithecus was not capable of walking erect. It probably was quadrupedal.
  1. AUSTRALOPITHECUS
    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:

    Ape-like Characters:
    • 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.

Human-like Characters:

    • 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. afarensis
Australopithecus Afarensis
Main Characteristics:

    • 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 erect.

They are three kinds of later Australopithecus, namely:

    • Australopithecus africanus, also called gracile.
    • Australopithecus robustus, also called robust Australopithecus.
    • Australopithecus boisei, also called superrobust Australopithecus.


Australopithecus Africanus

    • 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.


Australopithecus Robustus

    • They are comparatively larger, i.e., 5 feet.
    • The face was even lesser prognathous than the A. africanus.
    • 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.


Australopithecus Boisei

    • They are very robust and hence the name.
    • Rest of the features are the same as A. robustus.

 

 

4.

Darwin's 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 grass.
    • 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
    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.

4.      Beneficial Variation: Colouration of butterfly that makes it difficult to be noticed by the predators.

5.      Harmful Variation: Insufficient secretion of insulin.

6.      Neutral Variation:

  • Natural Selection
    According to Darwin, individuals with useful variations survive in the struggle for existence and those with harmful variations are wiped out.
    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 surroundings.

 

 

5.

Sources of Variation

 

Mutations are the basic cause of variations. Any change in the genetic material that is passed on to the next generation constitutes mutation.

Mutations are of two types, namely:

  1. Genetic mutations
  2. Chromosomal mutations
    Genetic mutations are most common and they are recessive.

 

 

6.

Role of Natural Selection

 

  • Variation
  • Differential reproduction: Differential reproduction results from non-random mating of individuals. It results in natural selection.
  • Speciation

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 of reproduction.

 

 

7.

Speciation

 

Evolution gives rise to new species. This is called speciation.
Speciation is the result of differential reproduction and isolation.

  • Species is a group of potentially interbreeding populations (group of physically, biochemically and behaviourally similar organisms) that are reproductively isolated from other such groups.
  • The 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.
  • The 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.
  • Speciation 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

    Stage 1
    Formation of populations

    Stage 2
    Formation of races or subspecies

    Stage 3
    New species
  • Geographic 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.
  • The 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.

 

 

8.

Adaptation

 

  • Any 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.
  • With regard to the type of environment there are 4 types of adaptations:
    1. Adaptations to physical environment
    2. Adaptations to biotic environment
    3. Intraspecific adaptations
    4. Internal adaptations


Connecting Links
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

 

9.

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.

10.

Crossing over: The exchange of segments between the non-sister chromatids of homo-logous chromosomes is called crossing over.

11.

Industrial melanism: Industrial melanism occurs due to a change in the environment caused by industrial revolution.

12.

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.

 

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