Evolution: Things You Must Know About Evolution

 

INTRODUCTION:

Evolution

(L, evolve - to unenroll) means the descent of a new form of the organism from the pre-existing ones. It is a gradual change, which is going on very slowly and for millions of years, which we can only imagine. Thus, the entire process of change includes events that have neither recognizable beginning nor end in time or space.

Evolution is a kind of gradual formation of new organisms from pre-existing primitive organisms through slow and steady changes. It is the progressive development of a simple organism to a complex organism over a long period. Thus, evolution can be defined as the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are evolved. Once life came into existence, it becomes continuous, forms of organisms over a long period. Evolution is proceeding even now and will continue in the future as well. It is a naturally occurring, slow continuous, and irreversible process to change.

EVOLUTION OF LIFE

According to scientists, life originated on the earth about 600 million years ago in the form of marine algae and primitive vertebrates. Gradually, the earth cooled, and there was an exactly spontaneous generation of life-how and exactly when we are not in a position to say with any degree of certainty. 

Life must be originated in the form of aquatic bacteria. All the living organisms on the earth which we see today have evolved from some of the other simple ancestors that lived on this earth in this past. Regarding the evolution of organisms on the earth, scientists have two views (hypothesis) which are as follows:

1. Special Creation By God:

According to this hypothesis, all the species of plants and animals were created by God. Some other scientists believed that all living organisms present on the earth were created by God in the beginning in their present forms. This hypothesis is not accepted nowadays due to a lack of evidence.

2. Organic Evolution

According to the theory of organic evolution, unicellular organisms evolved into simple organisms multicellular organisms like invertebrates, which in turn evolved into advanced multicellular vertebrates like fish. 

The fishes evolved into amphibians which gradually evolved into reptiles from which birds and mammals have evolved. The term 'organic evolution' was proposed by Charles Darwin, which means that the present complex life has evolved from an earlier simple form of life by gradual changes.

Organic Evolution is a very slow process and a human-life time or even all the human history is too short to witness it. According to this theory, new species are formed from the old ones. Evolution thus clearly shows that there are profound and continuous modifications going in the forms of life from generation to generation over a long period.

 It shows how things have become. What are they? How are things changing? According to evolution, there is no break in the chain of life and everything has originated from something of the past.

 The idea of evolution, called the theory of evolution, called the theory of evolution, is generally accepted but its principle is still an imperfect synthesis.

EVIDENCE IN FAVOR OF ORGANIC REVOLUTION

Various biological studies tell us that since their origin, living organisms have been changing their organisms to evolve into complex forms.

 Several common features of various kinds of organisms provide evidence in the favour of evolution. Evidence obtained from various studies reinforces the view that living organisms have evolved from common ancestors. The major evidence that supports the theory of organic evolution are mentioned below:

1. Evidence from the study of fossils.

                                                             Fig: Fossil of Archaeopteryx

Fossils are the petrified remains of ancient plants and animals or impressions left by them in rocks. A fossil is thus a relic of the past life - plant sedimentary rocks provide direct and most reliable evidence of evolution. Rocks formed in the strata in successive geological periods of the earth are bear particular types of fossils. The lower strata of rocks contain fossils of simple and primitive organisms whereas the upper strata contain fossils of advanced organisms. Regular graduation from simple to complex is noted in the fossils of the rocks This geological succession completely agrees with the concept of evolution. It is remarkable that fossils of anyone stratum bear close resemblance to those of the previous or next stratum. This makes clear that there is a relationship between the organisms that appeared through successive periods of geological history. Thus, fossils provide evidence that present animals and plants have evolved from the previously existing ones through the process of continuous evolution.

2. Evidence from the comparative morphology and anatomy.

Similarities in the morphological and anatomical characters among certain groups of plants and animals are very characteristics from the standpoint of evolution.

Organs similar in structure ad origin but different in functions are called homologous organs. Forelimbs of a man, for limbs of a horse, patagium of a bat, wings of a bird, the study is done, we can find the similar basic structures with some of the modifications according to the adaption. 

They contain almost similar types and numbers of bones, blood vessels, nerves, and muscles arranged in the same pattern. However, they perform various functions according to their habit and habitat.

                                                                                                       Source: shutterstock.com

                                                        Fig: Homology in the forelimbs

**Tap on the image for a clear view**

Organs having a similar function but different in structure and origin are called analogous organs, e.g. wings of insects, birds, penguins, patagium of a bat, etc. 

The analogous organs provide evidence for evolution by telling us that though they are not derived from common ancestors, they can still evolve to perform similar functions to survive and flourish under hostile environmental conditions. The analogous organs provide the mechanism for evolution.

3. Embryological shreds of evidence

Embryology is the study of the nature and development of embryos. It shows a great resemblance among certain groups of plants and animals. 

E.g. the embryos dicots look alike. Similar in the case with other groups of plants and animals.

The embryos of different vertebrates like fish, salamander, turtle, chick, rabbit, and human resemblance a lot in their early stage of development and it is very difficult to distinguish one from another.

 Later they differ from each other after their growth and development and finally change their respective animals according to their parental characters. It indicates that all these animals have evolved from a common ancestor.

The biogenetic theory (recapitulation theory ) states that an animal during its development repeats certain ancestral stages of its evolution. For example, the immature stage in the life cycle of the frog is a fish-like tadpole having gills and a tail. This shows that the frog has evolved from a fish-like ancestor.

Similarly, temporary non-functional teeth in birds' embryos indicate that birds have evolved from toothed reptiles. In this way, evidence from the study of embryos supports the theory of organic evolution.

4. Evidence from the study of vestigial organs.

Vestigial organs are those organs in organisms that are functionless and rudimentary now but used to function in their ancestors, e.g. coccyx, vermiform appendix, etc. 

Though the vermiform appendix, are muscles and tails are functionless in humans they are quite functional in the herbivores animals like cows, buffalo, etc.

 In, cattle, the vermiform appendix helps to digest cellulose. The ear muscles are used by the animals to rotate (move) their ears in the direction of the source of the sound. Similarly, the nictitating membrane is functionless in humans.

 It proves the animals hang vestigial organs and the animals that still use the organs may have evolved from common ancestors. Therefore, the presence of vestigial organs may have evolved from common ancestors. 

Therefore, the presence of vestigial organs is good evidence to support the theory of organic evolution.

5. Shreds of evidence from bridge organisms or connecting links.

Organisms having characteristics of two different groups are called bridge organisms or connecting links. Duck-billed platypus, Archaeopteryx, lungfishes, etc. are some examples of bridge animals. The structure and behavior of those organisms resemble the lower groups and partly the higher groups.

 

Archaeopteryx, a fossil bird of the Jurassic period, serves, as a connecting link between the reptiles and the birds. Like reptiles, it shows the presence of teeth in jaws, a long tail, with caudal vertebrates, each finger ending into a claw, and like birds, it has body feathers, forelimbs modified into wings, presence of four toes. 

These characteristics show that birds have evolved from reptilian ancestors. Similarly, duck-billed Platypus serves as connecting link between reptiles and mammals. It is oviparous and cold-blooded like reptiles and it has mammary glands, hairs, and other mammalian characteristics. 

Certain fishes having lungs ( lungfishes ) also serve as the connecting links between fishes and amphibians. Therefore, the shreds of evidence obtained from bridge animals support the theory of organic evolution.

6. Evidence from distribution organisms.

There are different types of living organisms in different parts of the earth. Organisms found in one part of the earth may or may not be found in the other parts.

 It has been seen that many allided species of plants and animals remain confined to a particular area. The explanation is that they sprang up from a common ancestor in that region and could not migrate owing to some barriers like high mountains, seas, deserts, etc. 

They changed themselves there according to the new environment. If one region is declined to form the other for a long time variation also occurs in the organisms and finally, new species of organisms are evolved.

THEORIES OF EVOLUTION

Evidence in favor of evolution testifies evolution is real. Several theories have been put forward from time to time to explain the process of the evolution of organisms on the earth. Historically, three great revolutionary theories have been put regarding the evolution of organisms on the earth by three scientists to explain eh mechanism of evolution. Those theories are as follows:

1. Lamarck's theory of evolution or Lamarckism

The first general theory of evolution was proposed by Jean Baptiste de Lamark ( 1744-1829 AD), a French anatomist and taxonomist. He was the first author to give a general theory of evolution.

 His theory was outlined in 1801 and published in 1809. He considered there had been a progressive development in form and structure. According to Lamarck, the development was important not as a direct cause of evolution but merely as an occasion for evolutionary changes affecting the shape and organization of animals. 

When an animal lives in a changing environment, it encounters new needs, which stimulate the animal to satisfy them. The important points of his theory are as follows:

           

            1. Environmental effect on organisms.

Due to the changed mode of life, certain physical changes need to originate in the animal, which is responsible for the modification of organs. 

This can be illustrated by several examples. A bird is driven by the necessity to seek its food in eater gradually assumed characters adapting to swimming, wading, or searching the water.

           

            2. The effect of use and disuse of organs.

Structural variations are due to functional needs. When a structure is used, its size increases and failure to use it results in its disappearance. This is called the theory of use and disguise of organs.

 Lamarck illustrated the disuse by the structure of a snake. He explained that in crawling along the grass, the snake's legs would be stretched repeatedly when passing through a narrow passage. 

The legs would interfere with crawling and would hinder the movement of the body. Thus, snakes lost their legs although they are found in other reptiles.

           

            3. Inheritance or transmission of acquired characters.

The advantages gained by every organism due to the structural changes caused by use or disuse are transmitted to the offspring.

 Thus, the progression or retrogression is transmitted to the offspring. This view was explained by the example of the giraffe as follows.

The horse-like ancestor of the Giraffe live in a new and arid environment, where the food available was only from the taller trees. The animal tried to reach up to the leaves.

 Though a conscious effort to reach up to the leaves, neck, and limbs lengthened. The small gain in length made during the lifetime of an individual was passed on to the progeny, who gained it more and more. Ultimately the long-necked giraffe with a long neck and legs developed.

        

    OBJECTIONS TO LAMARCK'S TO THEORY

There is no reliable evidence for Lamarck's theory and it has little support today. The main objection to this theory are:

• The main objection is in the assumption of the inheritance of acquired characters. The muscles of an athlete increase in strength and bulk extensive use but they are never transmitted from father to child.

•  Similarly, Lamarck's theory of inheritance of acquired characters was strongly criticized by Wiseman. 

• He cut the tails of white mice continuously for 21 generations. However, their property did not how any cut tails.

•  Thus, if his theory was a fact, then tallies mice should have been born, which did not happen.

•  The second objection to his theory is the fact that the conscious effort as evident in giraffe and other animals is hard to be expected in the plants.

•  The change in the structure of a body by use and disuse of organs is not manifest practically.

2. Darwin's theory of evolution of Darwinism

Charles Darwin (1809-1882), a famous English naturalist, put forward the next theory to solve the problem of evolution. He published his theory in the book " The Origin Of Species By Means And Natural Selections". 

His theory is popularly known as " Theory of the Natural Selection" or "Dwarinism".

Darwin went on the voyage of world exploration, in 1831, on a ship named H.M.S Beagle and studied the flora and fauna during his journey. He also studied the population theory of Malthus and then proposed his theory of natural selection.

Darwin's theory can be summarized under the following points:

1. Balance of Population Growth

Living Organisms have an enormous potential to produce a large number of offspring but the size of the population of any kind of organism remains more or less constant and they are found in a large number. For example, the fruits fly, Drosophila, completes the life cycle in 10 to 14 days and each female lays about 300 eggs.

 If all survive and breed there will be about 200000000 Drosophila in 40 to 50 days during the summer.

An elephant is a slow breeder. Darwin assumed that the elephant matures in 30 years and lives for 100 years: each female produces about 6 offspring.

 In 750 years, a single pair would rise to about 19 million offspring. Even though all species produce a large number of offspring, the population remains fairly constant due to the struggle between members of the same species of different species for food, space, and mate.

2. Struggle for existence

The population of most species remains constant because of the various checks.

 According to Darwin, several factors are responsible for keeping animals and plants more or less in limited numbers.

 The struggle for existence may be intraspecific, i.e. among the member of the same species, sharing a common environment. The organisms compete with one another for food, space, and mate.

 Similarly, the climate also plays an important role in determining the average number of species due to extreme heat, cold, heavy rains or droughts, and natural calamities. Thus, the struggle for existence removes many individuals.

3. Variation and Heredity

Since, the everlasting competition to survive, therefore, there is a variation of living beings. 

Some variations are suited to the environmental conditions, while others are not. 

According to Darwin, these variations are preserved and transmitted to the offspring, although no cause for this variation was assigned by him. 

Darwin knew nothing of chromosomes and genes, yet get realized that much of the variation in organisms is hereditary. Those variations which are unsuited to the particular condition are eliminated.

4. Survival of the fittest or natural selection

Darwin regarded that in the struggle for existence, the individual who is provided with favorable variation will lead, a more successful life would survive and propagate their offspring and others, and ultimately give rise to new forms. These new forms are better adapted to their surroundings.

 Animals that fail to develop suitable new variations will disappear and the organisms that develop suitable variations are selected by the nature.

 It is called survival of the fittest or natural selection. So, the elimination of less progressive organisms and selection of more progressive organisms by nature is called natural selection.

5. Origin of the Species

Darwin believed that the adaption of the survival to the environment may result in the formation of new structures, factors, or behaviors. These useful variations when transmitted to the offspring will tend to increase generation after generation until new species will develop.

 Thus, by modification with changes in the environment, it is regarded that the species have reached their present state of fitness.

EVIDENCE IN FAVOR IN DWARNISIM

i. In the past, there existed a large number of animals due to the scarcity of food, changed the climate, and struggle for their habitats only a few survived.

ii. Production of various domestic varieties of artificial selection is evident.

iii. Ancestors of horses were four-toed, from them modern horses have evolved by reduction of three toes, fibula, and ulna.

CRITICISM OF DWARNISIM   

i. According to Darwin's theory, only fluctuating variations are the cole cause of the origin of species.

 But it is doubtful whether these small variations are of any reak advantage in the struggle for existence to bring natural selection into play.

ii. Darwin's theory could not explain how variations arise.

iii. According to Darwin's theory, natural selection is the only factor to bring out evolution but not only complete favor for evolution.

iv. According to Darwin's theory, only useful variations are preserved and transmitted to the offspring. It is so, there would not be the selection of disused parts.

v. Darwin's theory of natural selection does not satisfactorily explain the occurrence of vestigial organs. According to the theory, vestigial organs are useless.

 Hugo De Vries Theory of Evolution

A Dutch botanist Hugo De Varies propounded a theory, in 1901-1903 A.D, explain the cause of evolution. He held that small variations which Darwin regarded as most important from the standpoint of evolution are only fluctuations around the specific type.

 These variations are not inheritable. Hugo De Vries held large variations appearing suddenly and spontaneously in the offspring are the cause of evolution. De Vries called these variations 'mutations'. His theory is also called "Mutation Theory".

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                                                      SUMMARY

1. Evolution is the process of gradual and orderly changes in organisms from a simple form to a complex one over a long period. It is a continuous but very slow process.

2. Organic evolution is a series of related and gradual changes in plants and animals which occur due to continuous variations.

3. The doctrine of evolution is supported by many pieces of evidence based on paleontology, comparative morphology, and anatomy, embryology, a study of vestigial organs and connecting links, etc.

4. Fossils are the relies upon dead plants and animals preserved in relics over a long period.

5. Vestigial organs are the organs present as reduced structure and functionless in the body, e.g. nictitating membrane, vermiform appendix, ear muscles of human beings, etc.

6. Homologous organs are organs having similar structure and origin but different in function.

7. The animals which show characters of two different groups of organisms are called bridge animals or connecting links.

8. Jean Baptiste de Lamarck was the first author to give a general theory of evolution.

9. According to Lamarck, the favorable variations caused due to use and disuse after a considerably, long period result in the evolution of new species.

10. According to Darwin's theory, the struggle eliminates the unfit individuals. The fit organisms possess some favorable variations and they survive and reproduce. This is called the survival of the fittest.

11. The selection of more progressive organisms and elimination of less progressive organisms by the nature is called natural selection.