Biology Today — December 2017

EVOLUTION

• Evolution means an orderly change from one condition to another. It is believed that chemical evolution was replaced
  by organic or biological evolution. Organic evolution is a process of cumulative change of living populations and in the
  descendant populations of organisms. Therefore evolution also refers to genetic changes in populations of organisms through
  time that lead to differences among them.


• The term was proposed by Charles Darwin that signifies “Descent with modification” i.e., the present complex plants
  and animals have evolved from earlier simpler forms of life by gradual changes.
  EVIDENCES OF EVOLUTION


• The doctrine of organic evolution is supported by many types of evidences.

A. Morphological and Anatomical Evidences

• These are the proofs of inter-relationship between living organisms of different groups, based on the comparative studies of
  external and internal morphology (structure). These are of following types:
  (I) Homologous organs


• The structures which are different in appearance and perform different functions but have similar basic structure and
  developmental origin are called homologous organs.


• Examples:
  (a) Forelimbs of vertebrates like seal, bird, bat, horse, man, cat’s paw, etc., look different and perform different functions.
  Forelimbs Seal Bird Bat Horse Man
  Appearance Flipper Wing Support patagium Elongated Thumb opposite to fingers
  Function Swimming Flying Flying Running Grasping


• But even then all these are homologous organs because:


• These are built on same pentadactyl plan (5-digited) though modified differently to perform different functions and involve
  reduction in the number of digits, fusion of some bones, etc.


• These show similar arrangement of structures like bones,
  blood vessels, muscles, nerves, etc. The bone arrangement
  reported in these forelimbs is as under:
  Part of forelimb Name of bones
  Upper arm
  Forearm
  Wrist
  Palm
  Fingers

Humerus

Radius and Ulna

Carpals

Metacarpals

Phalanges

• These bones are derived from the same part of body.
  
  
  • This relationship between the structures having common
    basic plan and similar developmental origin is called homology, although these structures might have diverged due to their
    specific function. Presence of homologous organs confirm:
    (i) Common ancestry and inter-relationship.
    (ii) Divergent evolution (same structures have become different in different organisms due to different adaptations) due to
    adaptive radiations.
  
  
  
  


• Similarly, the forelimbs of whale, bat, cheetah and human are homologous organs.

(b) Comparative anatomy of heart of vertebrates

• Heart in different vertebrates are similar not only in their position (on ventral side of anterior part of body cavity), general
  organisation (formed of chambers) and even microscopic structure (formed of cardiac muscles) but also show gradual
  complexity in its internal structure from fishes to mammals. It is 2-chambered, venous and branchial with single circulation in
  fishes; 3-chambered and arteriovenous pushing mixed blood in amphibians; structurally 3-chambered but with incompletely
  partitioned ventricle in the reptiles; and 4-chambered with completely partitioned ventricle in the birds and mammals.

Fig.: Forelimbs of vertebrates as homologous organs

Human

Humerus

Phalanges

Radius

Ulna

Carpals

Metacarpals

Cat Whale Bat