Evolution and Classification

Question 1 Define the term analogous organs, vestigial organs and homologous organs?

Question 2 Why is artificial selection beneficial?

Question 3 Why are acquired characters not inheritable?

Question 4 Differentiate  between artificial and natural selection?

Question 5 How can you explain the existence of analogous organs?

Question 6 What is the most reliable evidence for evolution and why?

Question 7 What is embryology? How does embryology provide evidence for evolution?

Question 8 State biogenetic law?

Characteristics of organisms

It refer to the details of external and internal appearance or behaviour that distinguish them from one another.

These characteristics of organisms also form the basis for the classification of organisms. By identifying hierarchies of characteristics between species, we can work out the evolutionary relationships of the species that we see around us.

The more characteristics two species have in common, the more closely they are related. The more they are related, the more recently they will have had a common ancestor.

 

Let us take an example of a brother and sister who are closely related as they have common ancestors i.e., their parents. Sister and her first cousin are also related but lesser in degree than brother and sister. This is because cousins have common ancestors in the form of grand parents and not the parents. Classification of species is a reflection of their evolutionary relationship.

There are some basic characteristics that are shared by most organisms. Body of animals and plants is formed of microscopic units called cells.

Organisms can be classified into two broad groups, i.e., eukaryotes and prokaryotes on the basis whether their cells possess well defined nucleus and other cell organelles or not. Eukaryotes can be grouped into unicellular and multicellular organisms on the basis of whether they are made of single cell or large number of cells.

Organisms even can be classified into plants and animals on the basis of photosynthetic property. Multicellular organisms can be further be grouped on the basis of levels of body organization.

In sponges, cells function more or less independently without organizing themselves into tissues. So, sponges have cellular level of organization.

In coelenterates, the cells are aggregated into tissues but the tissues do not form organs. This produces tissue level of organization.

From platyhelminthes (flat worms) onward to the most complex mammals, all animals have organ-system level of organization.

This gradual change from simple to complex organization is evolution.

Plants also show progressive change from simple to complex body organization supporting evolution.

 

Evidences

Enormous types of organisms that exist on the earth at present and also the fossils have large number of common features which provide evidences in favour of organic evolution and help us in tracing evolutionary relationships. The present day diverse forms of living organisms have evolved from common ancestors.

These evidences have been categorized as:

(1) Morphological and Anatomical Evidences,

(2) Embryological Evidences, and

(3) Palaeontological Evidences

Morphological and Anatomical Evidences

These evidences are based on the similarities in the external and internal features of the different kinds of organisms.

These features are:

(1) Homologous organs,

(2) Analogous organs, and

(3) Vestigial organs.

(1) Homologous organs

The organs which perform different functions in different species but have similar basic structure and similar embryonic origin are called homologous organs.

There are number of organs in different groups of animals or plants which have similar basic design but are used for different purposes by the possessors.

For example: the forelimbs of a frog, a lizard, a bird and a human being show similarity in basic structure. These organs perform entirely different functions :

(a) the forelimbs of a frog help the animal to prop up the front end of the body at rest, and also act as shock absorbers when the animal lands on the ground after a leap.

(b) the forelimbs of a lizard are modified for creeping movements

(c) the forelimbs of a bird are modified for flying purposes,

(d) the forelimbs of a human being are used for grasping.

 

Forelimbs of a frog, a lizard, a bird and human being are, therefore homologous organs. The basic similarity in the forelimbs of these different vertebrates indicates that all these vertebrates had common ancestry, i.e., they have evolved from a common ancestor who had five digited or pentadactyle limbs (basic plan of limbs).

The pentadactyle limbs of the ancestral vertebrate became modified according to the special needs of the subsequent generations during the course of evolution.

The modification in the limbs in these vertebrates include :

(1) reduction in number of digits,

(2) fusion of certain bones, and

(3) external form.

Plants also have homologous organs.

For example: a thorn of Bougainvillea plant  and a tendril of Passiflora plant are homologous organs. Both these perform different functions but have similar basic structural design i.e. both are modified branches.

(2) Analogous Organs

The organs which are quite different in fundamental structure and embryonic origin but perform same function and may superficially look alike in entirely different species are called analogous organs.

The wings of an insect and a bird are analogous organs. It is so because both these organs in entirely different animals perform similar functions, i.e., they are used for flying in the air. However, they are very different in structure.

(a) An insect wing is a fold of membrane which is supported with few muscles.

(b)Wings of a bird are formed of limb bones covered with flesh, skin and feathers. 

The insects and birds have evolved from separate ancestral populations, but perform similar functions.

The wing of a bat (mammal) and the wing of a bird are analogous organs.

(a) Wings of bats are skin folds stretched mainly between elongated fingers.

(b) The wings of a bird are feathery covering all along the arm. The basic designs of the wings of bat and the bird, their structure and components are very different. They only look similar because both are adapted for flying but their origins are not common.

Evolution of Eyes: Eye is a complex organ of sight in animals. Flat worms, Palanaria have very simple eyes which are just eye-spots to detect light. Even these rudimentary structures in flat worm provide advantage to the animal to ensure its survival. From this basic design, more complex eyes later evolved in different organisms. Eyes in insects, octopus, other invertebrates and all vertebrates including human beings have different structures and also have separate evolutionary origin.

The eyes of insects, octopus and all vertebrates are analogous organs which have developed bit by bit over generations in their own way as adaptation for similar function.

Different organisms having organs with entirely different structures, can adapt to perform similar functions for their survival under adverse environmental conditions. Similarity developed in distantly related groups of organisms (e.g. insects and birds) as an adaptation for the same function, is called convergent evolution or analogy.

 

(3) Vestigial Organs

The organs which occur in reduced form and are useless to the possessor, but are homologous to the fully developed, functional organs in the ancestors or related forms are called vestigial organs.

Vermiform appendix of the large intestine and nictitating membrane in the eye of human beings are vestigial organs.

(a) Vermiform appendix is a blind tube at the end of caecum of large intestine in certain mammals including human beings. In herbivorous mammals , it is a large structure and serves for the digestion of cellulose contents of the food by bacterial action. In human beings, the habit of taking soft and cooked food has reduced the need for bacterial digestion. The appendix has consequently reduced and serves no useful role. 

(b) Nictitating membrane (third eyelid) is a merely small, pinkish, triangular flap in the inner angle of each eye in human beings. It has become reduced and nonfunctional in the course of time because the cornea of the eye in human beings is cleaned and protected by the upper eye lid. Nictitating membrane is well developed and functional in birds to provides protection to the eye. 

These organs were fully developed, functional and necessary in ancestral forms. However, these are gradually disappearing in the living forms due to change in their mode of life.

Embryological Evidences

Embryology refers to the study of development of an embryo of organism from fertilized egg to young one.

The early embryos of all the vertebrates resemble in shape and structure. Similarity among the early embryos shows that all the vertebrates have evolved from a common ancestor.

Ernst Haeckel proposed a law which is popularly known as the biogenetic law. This law states that an organism repeats its ancestral history during its embryonic development.

Ontogeny is the life history of an individual animal whereas phylogeny is the evolutionary history of the race of the animals.

The biogenetic law states that an animal in its individual embryonic development from egg to
adult repeats or recapitulates in condensed form the stages through which its ancestors
have passed in the course of their evolution.

 

Palaeontological Evidences

Palaeontology is the study of past life based on the fossil record.

If the body or some of its parts remain in an environment/medium that does not let it decompose completely, the body or its parts will eventually, be preserved or alternately leave the impressions.

Fossils are the remains or impressions of the dead animals and plants that lived in the remote past.

Formation of Fossil

Some 100 million years ago some invertebrates died on the sea bed and got buried in the soil or sand.More soil or sand was carried by rain into the streams and rivers and ultimately into the oceans, where it settled at the bottom layer after layer. The layers so deposited changed into rock due to the presence of the water above and also due to chemical reactions.

Millions of years laters, dinosaurs living in the area died and their bodies too got buried in mud. The mud also got compressed into rock in the course of time over the rock containing the earlier invertebrate fossils.

Again millions of years later, bodies of horse-like creatures died in the area and got fossilized in rocks above the earlier formed rocks. The rocks formed in this manner at the ocean floor are known as sedimentary rocks. Millions of years later, say, erosion by water flow exposed the horse-like fossils. If we dig deeper, we would come across older and older fossils.

Various kinds of fossils have been unearthed from sedimentary rocks. These include many fossil invertebrates (e.g. Ammonites, Trilobites), fossil fish (Knightia), dinosaur’s skull of Rajasaurus (from Narmada valley) etc. The fossils also provide evidence for evolution.

Age of fossils

There are two ways to know the age of fossil:

(1) Relative: If we dig the earth to locate fossils, the fossils we find closer to the surface are more recent than the fossils we find in deeper layers.

(2) Date the rocks or fossils: Dating the rocks or fossils is done by working out the ratio of uranium to lead in a rock or fossil. It is so because radioactive uranium decays spontaneously to lead at a constant and known rate. This property is used to determine the age of rocks or fossils.

 

Artificial Selection

Artificial selection is the process by which man selects traits useful to him for improving the qualities of domesticated plants and animals.

Man selects the individuals having the desired traits and separates them from those which do not possess such characters. The selected individuals are interbred. This process of artificial selection, when repeated for many generations, produces a new breed with desired traits. Wild forms are modified through artificial selection.

Example: If cows with high milk yield are desired, the animal breeders monitor the milk yield of a large number of cows and select those which produce a large quantity of milk. The calves of the high milk-yielding cows are interbred to get the next generation of calves. After repeating this process for a number of generations, a breed of high milk-yielding cows is produced.

Importance of Artificial Selection

(1) By artificial selection, animal breeders have been able to produce improved varieties of several domestic animals such as dogs, horses, pigeons, poultry, cows, goats, sheep and pigs from their wild ancestors.

(2) Plant breeders have obtained improved varieties of useful plants, namely, wheat, rice, sugarcane, cotton, pulses, vegetables, fruits and ornamental (flowering and foliage) plants. Many crop plant species such as cabbage, kohlrabi, kale, broccoli, cauliflower etc. have been produced by plant breeders from a common wild mustard species by selective breeding. 

Artificial Selection	Natural selection
It is an artificial process	It is a natural process
It is controlled by man on a limited scale in specific labs.	It is conducted by nature on a vast scale all over the world.
The result is predetermined	The result is unpredictable
Traits selected for improvement are beneficial to man	Traits selected for evolution are beneficial to the species.