Evidences from Biogeography

The study at the geographic distribution of plants and animals is called biogeography. Biogeographers explain the reason of distribution of organisms in different regions. Biogeographic studies show that life in different parts of the world has distinctive evolutionary histories. Biogeographers explain following factors:

I. Geographical barriers: They explain how similar groups of organisms can live in different places. These places are separated by impenetrable barriers. For example, native cats live in most continents of the earth. But they cannot cross open oceans. They have common ancestry. But they have undergone millions of years of independent evolution. Therefore, they have developed differences.

2. Evolutionary adaptations in the isolated organisms: Biogeographers also explain that the plants and animals are separated by geographical barriers. They were very different in spite of similar environments. For example, the animals that inhabit Australia and Tasmania are very different from animals in any other part of the world. The major native herbivores of Australia and Tasmania are kangaroos (Macropus). Members of the deer and cattle perform the same role in the other parts of the world. Similarly, the Tasmanian wolf (tiger) (Thylacinus cynocephalus) is extinct now. It was a predatory marsupial.

3. Life on islands: The biogeographers explain the presence of only few and unique resident species in oceanic islands. They write island colonization and subsequent evolutionary events. These evolutionary events are different from the ancestral mainland groups. Charles Darwin explained this distribution during his observation in Galapagos.

Biogeographical regions

biogeographers divide the world into six major biogeographic regions. these regions are:

I. Ethiopian regions: It includes most of African continent. It is

separated from the Palearctic region by Arabian Desert Sahara.

  1. Palearctic region: It includes most of the Europe. China, and Russia. It is separated from the Oriental regions by Himalayan Mountains.
  2. Oriental regions: It includes sub Himalayan region, Pakistan. India etc.
  3. Australian region: It is composed of continent of Australia. It is separated from the Oriental region by deep ocean channel.
  4. Neacrtic region: It is composed of USA, Canada etc. It is separated from the Neotropical region by mountains of Southern Mexico.
  5. Neotropical region: It is composed of most of the South America.

Evidences from Paleontology

‘Hie study of fossil is called paleontology. Fossil provide direct evidence of evolution. Fossils are evidence of plants and animals that existed in the past. These plants and animals incorporated into the earth’s crust. Sediments quickly cover them. It prevents scavenging and reduces the supply of oxygen. Thus decomposition occurs slowly. Fossilization is mostly occurs in aquatic or semi aquatic environments. Thus the fossil record is more complete for those groups of organisms living in or around water. Some gaps may be present in the fossil record. But paleontology produces nearly complete understanding of many evolutionary lineages. Paleontologists estimate that the earth is about 4.6 billion years old.They have also used the fossil record to describe the history of life on the earth.

Evidences from Comparative Anatomy

Similarities among the structures of animal occur due to common evolutionary origin. Comparative anatomy is branch of zoology. It is fundamentally based on relationship of structures among organisms. Comparative anatomists study the structure of fossilized living animals. They look for similarities that could indicate close relationships. They study three types of structure:

(a)   Homologous structures: Structure4 ‘derived from common ancestry are called homologous structure. For example, vertebrate appendages have a common arrangement of similar bones although some of them perform different functions. This similarity in appendage structure indicates that the vertebrates evolved from a common ancestor.



(b)    Analogous structures: The structures having different origin but performs similar functions are called analogous structures. Analogous structures are produced due to convergent evolution. Convergent evolution occurs when two unrelated organisms adapt to similar conditions. It causes superficial similarities in structure. For example. the wing of a bird and the wing of an insect are both adopted for flight. Rut they are not homologous. These structures are analogous.

(c)   Vestigial structure: The functionless structures in the body are called vestigial structures. Organ i sins often retain structures that have lost their usefulness. These structures are vestigial structures. They are poorly developed. For example. boa constrictors (a reptile) have minute remnants of hind limb (pelvic) hones left. Such remnants are clear indications of evolution.

Evidences from Molecular Biology

Molecular biology has provided a lot of important information about evolutionary relationships. The animals also have homologous biochemical processes like homologous structures.Structure and function are based on the genetic DNA molecule. Related animals have similar DNA derived from their common ancestor. DNA carries the codes for proteins. Thus related animals have similar proteins. The zoologists extract and analyze the structure of proteins from an tissue with the help of latest laboratory techniques. They also compare the DNA of different animals. They look for dissimilarities in the structure of related proteins and DNA. It gives constant mutation rate. In this way they estimate the time since divergence from a common ancestral molecule.

Evidence from Comparative Embryology

Closely related organisms pass through similar stages during their embryonic development. For example. all vertebrate embryos develop gill pouches on the sides of their throats. The fishes, frogs. snakes, birds, humans, and all other vertebrates have similar embryonic stages’ of development. Their embryos have more similarities than the differences. These vertebrates diverge more and more with the progress of development. ‘thus they develop the distinctive characteristics of their classes. ‘Hie gill pouches develop into gills in fishes. These embryonic structures modified into other structures in terrestrial vertebrates. For example. gill pouches modified into eustachian tube in humans. Eustachian tubes connect the middle ear with the throat. Comparative embryology develops homology among structures like gill pouches. These structures are changed in later development and their common origin is not fully apparent. Thus it becomes difficult to compare their fully developed forms.



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