Biology Today — December 2017

• This shows that heart in all the groups of vertebrates has same basic structural plan which proves their inter-relationship and
  common ancestry. The gradual complexity of heart in the vertebrate series is due to:
  (i) transition from aquatic to terrestrial mode of life
  (ii) change of mode of respiration from branchial to pulmonary
  (iii) for separation of oxygenated and deoxygenated blood.


• This confirms that the heart of vertebrates have changed in the course of evolution of mammals from fishes. Similarly, the brain
  of different vertebrates are homologous organs but show gradual complexity from fishes to humans.

Sinus

venosus

Conus

arteriosus

Truncus

arteriosus

Systemic

arches

Systemic

arch

Pisces Amphibia

V

V

A RV RV

RA LA RA LA

L LV

Bird and Mammal

Pulmonary

arch

Pulmonary

arch

Septum

Interven-

tricular

septum

Reptile

Fig.: Comparative anatomy of heart of vertebrates

• Legs and mouthparts of insects, thorns of Bougainvillea and tendril of Cucurbita are
  also homologous organs.


• Homology seen amongst molecules is called molecular homology. Example, proteins found in blood of man and ape are
  similar. The phylogeny of an organism can be traced by using the base sequence in nucleic acids and amino acid sequence of
  proteins in related organisms.

(II) Analogous organs

• Structures which are different in their basic structure and developmental
  origin but appear similar and perform similar function are analogous
  organs. This relationship between structures of different groups of
  animals of separate lineage due to their similar function is called
  analogy or convergent evolution or adaptive convergence.


• Examples :


• Insect and bird wing – These are different in basic anatomical
  framework and origin, e.g., insect-wing is formed from
  integument while the bird-wing is a modified forelimb,
  but are analogous organs as both are flat structures and
  are adapted for flight. Even the wings of bird (modified
  forelimb) and wings of bats (patagia of skin) are analogous
  structures.


• Both tracheae of an insect and lungs of vertebrates
  are adapted for respiration but are not homologous as
  tracheae are ectodermal in origin while the lungs are
  endodermal in origin.


• Potato tuber and sweet potato – Both are
  underground fleshy structures adapted for storage of
  food and vegetative reproduction but are analogous structures as potato tuber is an underground stem while sweet potato
  is a fleshy adventitious root.

(III) Connecting links

• The living organisms having the characters of two different taxonomic categories are called connecting links.


• Examples : Protopterus (Lung fish) – It is a connecting link between bony fish in having paired fins, dermal scales, lateral
  line system and gills and amphibians in having internal nares, lungs and 3-chambered heart.

Fig.: Homologous structures :

thorns and tendrils in plants

Thorns of

Bougainvillea

Thorn

Tendrils of

Cucurbita

Tendrils

Fig.: Wings of an insect are analogous to wings of bird

Insect wing Bird wing

Forewing

Hind

wing Humerus

Radius

Ulna Feathers

Carpals

Carpometacarpus

Fig.: Potato showing analogous similarity with

sweet potato as both are starchy food items

Underground stem Underground root