Introduction

The process and result of evolution differ geographically as no population of organism is distributed universally, neither the factors that influence adaptations are identical everywhere.Evolution of population depends on local circumstances that it faces over a period of time.These circumstances include physical and biological factors.Their effects may be hereditary and mediated by natural selection.

The distribution of a species or group of organisms is influenced by the time and place of its origin.Evidently,the results of evolution are expressed in the form of differential regional faunas and evidenced by the fossils records that indicate past regional changes,i.e,the geographical and geological distribution.

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ASPECTS OF ANIMAL DISTRIBUTION

There are two aspects of distribution of life on Earth:

1.Distribution of Animals in Space:-

It deals with the study of distribution of animal groups or animals over the earth’s surface whether on land or in water.

This can be divided into two types,

1.Geographical distribution:-

Deals with horizontal distribution of animals on land and fresh water in different continents and on different islands.

2.Bathymetric distribution:-

Deals with vertical distribution of animals in space both on land and in water.

It includes three realms.

(a) Holobiotic or distribution of animals in sea, i.e. marine realm.

(b) limnobiotic or distribution of animals in fresh water realm.

(c) Geobiotic or distribution of animals in terrestrial realm.

2. Distribution of Animals in Time or Geological Distribution:-

Distribution of animals in the past earth history. It can be studied only through fossils.

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ZOOGEOGRAPHICAL DISTRIBUTION

What is Zoogeography?

Animal geography or Zoogeography is concerned with the distribution of all the animals,invertebrates and vertebrates,the terrestrial and aquatic over the whole world.

There are nearly 10,00,000 species of animals.Most commonly the geography of land and fresh water vertebrates is taken into account,which constitutes just about 2% of the total animal strength.

The distribution of animals is called Fauna.Distribution of animal can be studied at three levels:geographical distribution over the whole world,regional distribution is selected segments of the world and local distribution which includes geographical distribution of species in relation to each other and in relation to ecology and evolution.The Zoogeography covers the distribution of animals over the whole world.

The area of distribution of animals is represented on map which shows the range of the family is called a distribution map.The map can be plotted depicting the distribution of a particular group or species or it represents the distribution of all the animals.The former are called simple maps and the latter as compound maps.

PATTERNS OF ANIMAL DISTRIBUTION

The animals exhibit specific patterns of distribution over the globe:

1. Cosmopolitan or Continuous distribution:- animals are found all over the world. e.g.- rats, bats, hawks, cukoos.

2. Discontinuous distribution:-animals are distributed in two or more widely separated geographical areas. It may occur in several small areas in the same continent or in two different continent of the same or different hemispheres. e.g.-dipnoi fish, peripatus.

3. Bipolar distribution:-a no.of species are found to inhabit only the Artic and Antarctic waters with no representatives in the intermediate oceans.such distribution is called bipolar distribution and this characteristic as bipolarity. e.g.-lampta, clione.

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What is Endemism?

The phenomenon of restricted distribution to a small area associated with some geographical or ecological factors is called Endemism.

Among endemics, some species exhibit very localised distribution are called local endemics.Sometimes mutants appear and vanish without being able to compete with parental species is called Pseudo-endomics.Some species may show a restricted distribution but cover large areas in course of time is called expanding or progressive endemics.Some old species may be restricted to a small region because of a severe decline in their population is called contracting or retrogressive endemics.

Endemicspecies…

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The first attempt to divide the world into zoogeographical regions was made in 1857 by P.L.Sclater.His division was based upon the different kinds of birds found in different regions.In 1876, A.R.Wallace supported Sclater’s division and they divided the world into six geographical regions.

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P.L. Sclater

(Map showing six geographical Realms)

The map of different Geographical Regions is shown below:

Zoogeographical Realms (Faunal realms)

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In 1887, Heilprin suggested that Palaearctic and Nearctic regions had many zoological features in common. These two regions were combined into Holarctic Region. Thus, he recognised following five regions:

Wallace’s Classification is given below:

Robinson’s Classification is the latest and most accepted.H. Robinson has presented following geographical classification:

1. Palearctic Region :-

It is the Largest region among six geographical realms. It Includes Europe, Asia and North Africa apart from the South andSouth-east Asia.

The Example of theanimals of the Palearctic Region are:-

Siberian tiger,

Grey wolf,

Polar bear,

Norway rat.

2. Nearctic Region :-

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Includes most of North America, Greenland and Iceland.

Example from Nearctic Region :-

37 mammal families-peccary , polar bear, pronghorn antelope,musk ox , porcupine.

3. Ethiopian Region :-

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Includes Africa, South of the Sahara and the adjacent island of Madagascar.

Examples from Ethiopian region:-

It includes 52 mammal families (most of all regions) mountain gorilla, African elephant, giraffe, aardvark, numerous lemur spp. (Madagascar), many viverrids (civets)

4. Oriental Region (Sclater’s India):-

Includes Southern Asia from Baluchisthan and to Myanmar, South- east Asia and some of the islands in the Indonesian region.

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Examples from Oriental Region:-

50 mammal families

- Malay tapir, Indian tiger, water buffalo, Indian elephant

5. Neotropical Region:-

Includes whole of South America and most of Central America.

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Examples from Neotropical Region :

50 mammal families

- Sloth, howler monkey, tapir, capybara

6. Australian Region:-

Includes Australia, Tasmania, New Zealand, New Guinea and Eastern islands of Indonesia.

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Examples from Australian Region:-

28 mammal families

- wombat, kangaroo, bandicoot, echidna

WALLACE’S LINE AND WEBER’S LINE

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Wallace, while studying the fauna of Malaya Archipelago, found that the fauna of Bali and Lombok separated by a distance of about twenty miles was markedly different from one another. This inspired him to study the fauna of other islands of this region like Celebes, Kei island, Aru, Timor, Moluccas and New Guinea and separated them into Oriental and Australian regions. In 1863, he drew a line on the map separating these islands into two regions. This line is popularly known as Wallace’s line after the name of its originator.

The Wallace’s line runs between Philippines and Moluccas in North, between Borneo and Celebes South- west and between Bali and Lombok Southwards.

Weber drew another line more eastwards separating Moluccas from Celebes and Kei islands from Timor. This line is known as Weber’s line.

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Fig (a).Representation of course of Wallace’s line and Weber’s line

The Wallace’s line and Weber’s line are given in Fig (a).

BARRIERS TO DISPERSAL

Barriers may be defined as the factors which hinder in the normal distribution of animals.

This may be either vast tracts of territory inhospitable to species or narrow bands of environment that may act as narrow fences, preventing species from migration to new regions.

e.g., Himalayan mountain ranges act as effective barrier between India and Eurasia and doesn’t permit migration of animals from India to Eurasia or vice versa, or a thick forest act as a barrier for the dispersal of land animals or a desert doesn’t permit the entry and dispersal of forest animals.

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The Fig (b).shows the different types of barriers to the distribution of animal.

Fig (b). Different types of Barriers affecting the migration of animals

Conclusion:

Zoogeography is helpful in understanding the Evolution. The increase in number of animal by reproduction makes them to distribute in all direction.Dispersal continues until barrier is reached. It concerned with the distribution of all the animals,invertebrates and vertebrates,the terrestrial and aquatic over the whole world. The distribution of animals is called Fauna. Distribution of animal can be studied at three levels:geographical distribution over the whole world,regional distribution is selected segments of the world and local distribution which includes geographical distribution of species in relation to each other and in relation to ecology and evolution.The Zoogeography covers the distribution of animals over the whole world.

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