Short notes on the Evidences from Comparative Morphology & Anatomy

(a) Homologous Organs:

They are organs which have similar internal structure and similar origin. Homologous organs may appear different externally and functionally in different organisms, e.g.

i. Leaves may be simple, compound, reduced to scales or modified into spines and tendrils.

ii. Stems may be modified into tendrils (Passiflora), thorn (Bongainvillea, Duranta), cladode (Ruscus) phylloclade (Opuntia).

Differences in homologous organs are examples of divergent evolution/adaptive radiation. Occurrence of similar types of biomolecules in various groups of organisms is called ‘Molecular homology’, e.g. RUBISCO in plants.

(b) Analogous Organs:

They are structurally different organs which get modified to perform similar function. Analogous organs having similar external appearance are called ‘homoplastic’, e.g. photosynthesis in wild pea (stipules), Australian Acacia (petiole), and Ruscus (cladodes).

(c) Vestigial Organs:

They are imperfectly developed nonfunctional organs which are fully developed and functional in related and ancestral forms. The vestigial organs are not required by organisms due to alternate development, e.g. Leaves are reduced to scales in Ruscus and Asparagus. Pistil is reduced in ray florets of sunflower.

(d) Atavism:

It is reappearance of an ancestral trait which is presently absent or vestigial.

(e) Connecting Link:

They are intermediate forms between two groups and show the possible path of evolution.

Evidences from Embryology:

(a) Australian Acacia shows transitional stages between bipinnate leaves and phyllodes in the seeding state.

(b) Juvenile protonema state resembling filamentous algae occurs in mosses.

(c) Presence of ciliated sperms in Cycads and Ginkgo though they are carried by pollen tubes indicating resemblance with bryophytes/pterido- phytes where ciliated sperms swim in external water to reach female sex organs.

Recapitualation Theory/Biogenetic law:

Earnst Haeckel (1868, 1874) formulated biogenetic law or recapitualation theory which states that ontogeny (developmental history of an individual) repeats phylogeny (developmental history of the race). The occurrence of ancestral traits in the embryo in called palaeogenesis. Evidences from Physiology/Biochemistry

Living beings possess similar types of biochemicals, biochemical reactions and body function, e.g., aerobic organisms perform Krebs’ cycle having similar types of enzymes from Clamydonionas, Amoeba to human beings and Peepal tree. Hormones of vertebrate are similar in structure and function. Chlorophyll ‘a’ is universal in eukaryotic autotrophs as well as blue green algae. Evidences from Genetics

Accumulation of mutations produce new varieties and races e.g. Porto santo rabbits, ancon sheep, dwarf wheat, red sunflowers etc. Hybridisation and induction of polyploidy hars given rise to new plants like Triticale, Raphanobrassica.

Evidences from Taxonomy:

Depending upon their resemblances and differences, living beings are divided into groups e.g. Monera, Protista, Fungi, Plantae and Animalia. Kingdom Plantae has four divisions while Animalia has a number of phyla. The common characters present in a species, genus, family, order, class or phylum indicate a common ancestry while the difference amongst them indicate evolution or modifications.