EVIDENCES OF ORGANIC EVOLUTION

Palaeontological Evidences

• Study of fossils is called palaeontology.
• According to Charles Lyell, Fossils are impression or remains of hard parts of life-fo rms found in rocks.
• Rocks form sediments and a cross-section of Earth's crust indicates the arrangement of sediments one over the other during the long history of Earth. Such types of rocks are called as sedimentary rocks.
• Mostly fossils are found in sedimentary rocks.
• Different-aged rock sediments contain fossils of different life-forms who probably died during the formation of that sediment.
• A study of fossils in different sedimentary layers indicates the geological period in which they existed.
• Some of them represent extinct organisms (e.g., Dinosaurs).
• The study shows that life-forms varied over time and certain life forms are restricted to certain geological time spans.

New forms of life have arisen at different times in the history of Earth. i.e. evolution has taken place.

1.0Determination of the age of fossil

There are different ways of determining age of fossils

1. Relative method: If we dig into the Earth and start finding fossils it is reasonable to suppose that the fossils we find closer to the surface are more recent than the fossils we find in deeper layers.

2. Using radioactive elements: Rocks contain some radioactive elements that decay and convert into their more stable form. This decay takes place at a constant rate for each radioactive element irrespective of the environment condition. e.g., Carbon dating.

Archaeopteryx is the connecting link between reptiles and aves.

2.0Connecting Links

Some organisms possess characters of two separate groups called as connecting links, which proves that members of higher groups have evolved from the lower group. Examples:  

• Virus - Between living and non-living
• Euglena - Between plants and animals
• Neopilina - between Annelida and Mollusca
• Peripatus - between Annelida and Arthropoda
• Balanoglossus - between Non-Chordata and Chordata
• Chimaera - between Cartilaginous and Bony fishes
• Protopterus (Lung fish) - between Pisces and Amphibia
• Seymauria - between Amphibia and Reptilia
• Archaeopteryx - between Reptilia and Aves
• Platypus and Echidna - between Reptilia and Mammalia 

3.0Evidences from Comparative Morphology and Anatomy

Similarities and differences are found among organisms of today and those that existed years ago. Such similarities can be interpreted to understand whether common ancestors were shared or not.

These similarities are of two types

(A) Homology (B) Analogy

(A) Homology

The organs which have common origin, embryonic development and same fundamental structure but perform similar or different functions are called as homologous organs and this phenomenon is called homology.

Examples of homologous organs

(i) Forelimbs of mammals - Whales, bats, cheetah and human (all mammals) share similarities in the pattern of bones of forelimbs. Though these forelimbs perform different functions in these animals, they have similar anatomical structure. 

Hence, in these animals the same structure developed along different directions due to adaptations to different needs. This is divergent evolution and these structure are homologous. Homology indicates common ancestry.  

(ii) Thorn of Bougainvillea and tendril of Cucurbita both are modification of axillary bud.

Thorn of Bougainvillea and tendril of Cucurbita

(iii) Hearts of vertebrate

Fish – 2-Chambered Heart

Frog – 3-Chambered Heart

Reptile - 3-Chambered Heart 

Bird - 4-Chambered Heart

Mammal -4-Chambered Heart

(iv) Brains of vertebrate

v)  Mouth parts of insects

Cockroach - (Biting & chewing)

Honey bee - (Chewing & lapping)

Mosquito - (Piercing & Sucking)

In each of these insect's mouth parts comprise labrum, mandible, maxilla etc.

(vi) Testes in male and ovaries in female 

(vii) Potato and Ginger - Both are modified shoot

(viii) Radish and Carrot - Both are modified roots

Molecular homology - Homology found at molecular level, can also be termed as evidence from biochemistry.

Similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry. Composition and structure of protoplasm, enzymes, hormones, DNA, blood in chordates is also almost same. It shows that organisms shared ancestors in recent or distant past.

For example, the plasma proteins found in the blood of man and apes are similar, ABO blood grouping is present in apes also.

WHAT IS ADAPTIVE RADIATION/ADAPTIVE DIVERGENCE

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation. 

Examples:

(1) Darwin's finches - During the journey of Galapagos Islands, Darwin observed an amazing diversity of creatures. Of particular interest, small black birds later called Darwin's Finches amazed him. 

Galapagos island is situated near south America which is a group of 22 smaller islands.

He realized that there were many varieties of finches at Galapagos island. All the varieties, he conjectured, evolved on the island itself. From the original seed-eating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches.

Variety of beaks of finches that Darwin found in Galapagos Island

(2) Australian Marsupials - A number of marsupials, each different from the other evolved from an ancestral stock, but all within the Australian island continent.

(3) Placental Mammals - A number of placental mammals have evolved from a common ancestral type in other parts of world also. Placental mammals in Australia also exhibit adaptive radiation.

(B) Analogy

The organs which have different origin and fundamental structures but perform similar functions are called analogous organs and this phenomenon is called as analogy.

When different structures evolve for the same function due to the similar habitat or adaptations this is called convergent evolution. Analogy doesn't indicate common ancestry.

One can say that it is the similar habitat that has resulted in selection of similar adaptive features in different group of organisms but toward the same function. 

Examples : 

(i) Wings of butterfly and birds - They are not anatomically similar structures though they perform similar functions i.e. used for flying.

ii) Eye of the octopus and of mammals. 

(iii) Flippers of Penguins and Dolphins.

iv) Sweet potato (root modification) and potato (stem modification).

(v) Sting of bee and scorpion.

(vi) Chloragogen cells of Earthworm and liver of vertebrates. 

Flippers of Penguins and Dolphins

Sting of bee and scorpion  

Wings of butterfly and birds

Sweet potato (root modification) and potato (stem modification)

4.0Convergent evolution or Adaptive convergence

• When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution.

Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be 'similar' to a corresponding marsupial. They show convergent evolution. 

e.g. Wolf (placental) and Tasmanian wolf (marsupial).

Evidences from Embryology 

• In 1866, Ernst Haeckel proposed 'Biogenetic law' which explained “Ontogeny recapitulates phylogeny”
• Ernst Haeckel law was based on the observation of certain features during embryonic stage common to all vertebrates that are absent in adult. For example the embryos of all vertebrates including humans develop a row of vestigial gill slits just behind the head but it is functional organ only in fish and not found in any other adult vertebrates.

Common features of vertebrates 

It means an organism shows its ancestral adult stages during its embryonic development. In other words, embryos of advanced species pass through stages represented

by adult organisms of more primitive species.

• It shows that all organisms have common ancestry. 
• Interestingly, Von Baer (1828) had disproven the 'Biogenetic law' before Haeckel invented it. He observed that embryos never pass through the adult stages of other animals.

Examples: 

(1) The tadpole (larva of amphibians) resembles with fishes. This indicates origin of amphibians from fishes.

Tadpole (Larva)

(2) During the development of heart in higher vertebrates like birds & mammals, it initially exhibits the 2– chambered state same as fishes. Later on, it develops into 3– chambered as in amphibians & reptiles and finally in the last embryonic stages it becomes 4– chambered as present in the adults. This proves that all vertebrates have evolved from common fish like ancestors and also that both birds & mammals have evolved from reptiles.

(V) Evidences from Biogeographical Distribution

• The study of geographical distribution of animals and plant species in different parts of Earth is called as biogeography.
• On the basis of fauna and flora Alfred Russel Wallace divided the whole world into six biogeographical regions called realms. 
• In South America, mammals resembling horse, hippopotamus, bear, rabbit, etc. were present. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna.
• In prehistoric time Australia was a part of Asian continent. After the evolution of prototherians from reptiles Australia got separated from mainland of Asia. Later on eutherian mammals evolved in Asia which were carnivores in nature and they destroyed prototherians and marsupials from Asia but pouched mammals (marsupials) of Australia survived because of lack of competition from any other mammal.