Biology Today — December 2017

C. Biogeographical Evidences

• Biogeography is the pattern of distribution of animals and plants in different parts of the Earth. It shows the evolutionary
  inter-relationship between living organisms.


• On the basis of distribution of plants and animals on the Earth, A.R. Wallace divided the Earth into six biogeographic realms:
  Palaearctic, Oriental, Ethiopian, Australian, Nearctic and Neotropical. These realms are separated from one another by major
  barriers like sea, deserts, mountains.


• It is believed that around the carboniferous period or slightly earlier, all the present day continents were in the form of a single
  big land mass called pangaea. Later on, due to various geological changes, tectonic movements, huge land masses broke off
  and drifted apart from one another. These land masses became different continents that got separated from one another by
  barriers, e.g., sea which prevented the free movement of organisms. Each of the continents formed had different environmental
  conditions, hence plants and animals also evolved differently.

Biogeographical evidences can be discussed under following headings:

— Discontinuous distribution of closely related species

• Sometimes closely related similar species exist at widely separated places without any representative in intervening territory.
  This is called discontinuous distribution. Two specific examples of discontinuous distribution are given below :

(a) Alligators. They occur only in South-eastern United States and Eastern China. The North American continent was connected
with East Asia in early Coenozoic. The alligators were distributed over the entire region. But due to certain barriers, the
alligators of two regions were separated for long time and developed some mutations. Therefore, these alligators are somewhat
different but they are related species of the same genus.

(b) Lung fishes. During early stages of continental drift, South America, Africa, Antarctica and Australia were interconnected.
Later on they were separated. Antarctica was shifted to a far away place. Now the lung fishes are only found in South America,
Africa and Australia.

— Restricted distribution

• When some unique organisms are confined in some parts of the world, due to their habitat isolation, it is called restricted distribution.


• For example, monotremes or egg laying mammals occur only in Australia. Marsupials, the pouched mammals, are exclusively
  found in Australia, New Zealand and South America.


• This restricted distribution of monotremes and marsupials is explained on the basis that Australia, New Zealand and South
  America were once continuous with the mainland of Asia, but got separated in late Cretaceous Period before the appearance
  of carnivorous Eutherian mammals. Placental mammals being more adapted, eliminated monotremes and most marsupials on
  Asian mainland. But the primitive mammals of Australia survived as placental mammals could not reach there because of no
  land connections. Placental mammals evolved in Asia along different lines and in South America while Australia marsupials
  also diversified along different lines.

— Adaptive radiation (Divergent evolution)

• Development of different functional structures from common ancestral form is called
  adaptive radiation. Homologous organs show adaptive radiation or divergent evolution.


• Examples :
  (i) Darwin’s finches of Galapagos island : Finches found on different islands of
  Galapagos Island have different types of modified beaks due to different feeding
  habits, but had common ancestor. Common ancestral seed eating ground finches
  radiated to different geographical areas and adapted different feeding habits, so
  developed different types of beaks.
  (ii) Australian marsupials : Marsupials (pouched mammals) survived, flourished and
  evolved along different lines by divergent evolution.
  (iii) Adaptive radiation in mammals : Adaptive radiation in mammals is based on limb
  structure and type of locomotion. The primitive common ancestor was a land animal,
  like modern shrews, with short five-toed plantigrade (walking type of locomotion)
  limbs with no specific specialisation. From this stem mammal, various modern types
  of mammals have evolved by the modification of limbs and other structures adapted
  to a wide variety of habitats.

Woodpecker

finch

Insectivorous

tree finch

Cactus ground finch

Vegetarian

tree finch

Seed-eating

ground finch

Fig.: Darwin’s finches

Warbler finch