Top 2 Theories Explaining the Causes of Evolution

The following points highlight the top two theories explaining the causes of evolution. The theories are: 1. Lamarckism or Theory of Inheritance of Acquired Characters 2. Evolutionary by Natural Selection (Darwinism).

Theory # 1. Lamarckism or Theory of Inheritance of Acquired Characters:

Jean Baptiste Lamarck (1744 – 1829), a French biologist formulated the first theory of evolution (Fig. 27. 10). His systematic study made him convinced that species were not constant but rather they were derived from pre-existing species by modifications and changes. To account for this he devised “the theory of inheritance of acquired characters”.

The theory states at:

(i) Living organisms and their component parts tend to increase in size continuously.

(ii) Production of a new organ results from a new need and from the new movement.

(iii) If an organ is in continuous use it will tend to develop in progressive way, but those organs which are in little use will tend to degenerate, e.g., fishes living in deep dark caves were blind presumably because of disuse of their eyes, frogs were considered to have developed webbed feet by stretching their toes in swimming.

Lamarck presumed that Giraffe developed long neck to secure food from tall trees (Fig. 27.11). Wading birds developed long neck and long beaks.

(iv) Inheritance of acquired characters.

Modifications thus produced during the life of the individuals would eventually find their way into hereditary material and would then be transferred to springs or in other words acquired characters are inherited. Thus the changes will be preserved over a long period of time and new types of organisms develop from the ancestral types.

Lamarck presumed that the changes in the climate, geographical conditions and food requirements were main causes for the development of new organs and disappearance of older ones, he existence of vestigial organs could not be explained unless it is admitted that because of continuous disuse they are gradually degenerating.

Objections:

Lamarck’s theory found little support and was rejected on the following grounds:

(i) His first principal suggestion that the organs have tendency to increase in size, although illustrated in many groups of animals is far from universal truth. Many organisms do not show any tendency towards gigantism and in many cases size reduction has been a prominent feature.

(ii) The second principle that new organs arise to fulfill new needs of organisms is incorrect, since the desire of an animal alone cannot lead to the formation of new organs. The idea that organs in continuous use will tend to develop in a progressive way and those in little use will tend to degenerate, appears meaningless in practice and has been disapproved by many experiment.

(iii) The idea that the acquired characters are inherited has not been demonstrated experimentally, e.g., the powerful arm muscles of blacksmith are not inherited by his child. The germinal material during the developmental processes in plants and animals cannot be easily modified by environmental modifications.

August Weismann (1892), the author of the “Theory of Continuity of Germplasm” suggested that reproduction in animals was accomplished not by the ordinary body cells (somatoplasm) but by germplasm which was transmitted essentially unchanged from generation to generation.

Neo-Lamarckism:

Although Lamarck’s doctrine of inheritance of acquired characters was strongly refuted and rejected, yet a few biologists accepted the theory in modified way. The prominent among them were cope (1840 -1897), Giard (1846 -1908), Packard Spencer and Mc Bridge.

According to Neo-Lamarckism the adaptation is the universal feature of living beings. It arises as a result of interaction between the structure, function and environment. The changed or fluctuating environmental conditions can alter the habit and structure of the organisms.

So, the organisms acquire new adaptation in response to new environmental conditions and consequently variations among plants and animals may result. These variations become established gradually in the heredity of the race.

This is the modified version of Lamarckism or Neo-Lamarckism because it does not follow the general perfecting tendency in evolution and stresses mainly on the direct action of environment on organic structure. According to some Neo Lamarckians, the fur development on the skin of some animals as adaptation against cold weather is the consequence of changed environment from warmer to colder state.

On the other hand, if the environment comes to the normal state again, the fur would also disappear. Neo-Lamarckians have discarded natural selection as the sole mechanism of evolution. They are of the opinion that the interaction between the structure, function and environment is the only cause of evolution. Today no evolutionist supports the Neo-Lamarckism.

Theory # 2. Evolutionary by Natural Selection (Darwinism):

Charles Robert Darwin (1809 – 1882) was born in England (Fig. 27.12). His father Robert was an eminent physician. After his early education, Darwin studied medicine at the University of Edinburg, but he was of academic taste. Then his father advised him to become clergyman which he liked.

He also studied Theology in Cambridge University, later on he developed interest in natural history and in collection of natural objects. John Steven Henslow, a botanist at Cambridge developed in him interest in plants. He was a keen observer of plants, animals and geological specimens on the advice of Dr. Henslow. After graduation, he accepted job of naturalist aboard the HMS Beagle exploration ship for five years.

The ship Beagle left Plymouth on December 27, 1831 and visited several Islands of Atlantic ocean, coasts of South America and some Islands including Galapagos in South Pacific.

During that voyage period he observed and collected an enormous amount of materials and he was impressed with the striking adaptations found in the animals and plants he collected and with the progressive changes in the different species of given genus.

In Galapagos Island he observed several species of individual genera which resembled other species of the mainland. This observation led Darwin to reason that the species may become modified. When he returned home in 1836, he spent two years in writing a book. The Voyage of the Beagle.

He was married in 1838 and later Darwin couple were blessed with two daughters and five sons. Darwin acted as secretary of Geological Society from 1838 to 1841. While he was working on the concept of evolution, Darwin was surprised to receive from Alfred Russel Wallace, another English naturalist, then in Malaya, an essay for his opinion.

The essay contained all the salient points of the theory that Darwin was giving shape. Darwin first hesitated in giving any comment on Wallace but idea on the insistence of the geologist Charles Lyell and the Botanist Joseph Hooker, Darwin prepared an abstract of his conclusion for joint publication with Wallace.

Both these observers of nature read their paper in 1858, before Linnaean Society, London. Darwin published his first book, The Origin of species in 1859.

Darwin in his life time wrote more than a dozen books besides numerous articles and letters. The publication of the theory of evolution caused great furor as the basic ideas contained in that were against the ideas of the Bible. But progressive missionary supported him.

Darwin and Wallace’s theory of evolution are based essentially on three observations and two conclusions which are as follows:

Observation 1:

Observation free from environmental pressure. Every species tends to multiply in geometric progression. In other words, a population doubling its number in one year should increase four times its size in the second year and eight times in the third year.

Simple plant of Amarathus may form 20, 00, 000 seeds. Mushroom in one breeze may scatter two billion spores and so on. Thus the number offsprings produced by plants and animals is always greater than that which could actually survive in nature.

Observation 2:

The size of the population remains constant over long period of time.

Conclusion I:

Competitions or struggles for existence. Not all the sperms and eggs will form zygotes, not all zygotes will develop into adults and not all adults will survive and reproduce.

If there is over production of species, there must start a struggle for existence among members of same species specific competition) or among members of different species (interspecific competition) or against the environment changes (environmental struggle).

Observation 3:

Variation:

Not all members of a species are alike (i.e., there exist considerable individual variations in them).

Conclusion II:

Survival of the fittest:

In the struggle for existence, therefore, individuals having several favourable variations will have better chance of survival and less fit species will be destroyed by the physical or biotic factors. This is known as “natural selection”.

Thus, according to Darwin and Wallace, environment is the principal cause of natural selection. Under the prolonged and continued selective influence of the environment, the organisms accumulate many new favourable variations. The variations probably caused evolution of new species from ancestral species.

In order to explain the inheritance of characters, Darwin proposed The Hypothesis of Pangenest in 1866 which assumed that all the organs and perhaps all the cells in the body of an individual produced minute particles or pangenes which carried information about them.

The pangenes travelled from the organs through blood stream to the grounds where they were assembled to form sex cells a gametes—the eggs and sperms.

The gametes, therefore, contained pangenes from each and every organ and they transmited them to the next generation. In this way, environmental changes would produce modified organs which, in turn, would produce modified pangenes that would transmit the change to the next generation.

Criticisms:

Galton (1875) on the basis of several experimental proofs refuted the hypothesis of pangenesis.

Many objections were raised against the theory of natural selection:

Some of them are as follow:

(i) Natural selection is regarded as a cause of the evolution of new species but it is not the sole cause because it is now known that varieties arise due to mutations. Moreover, the mutation-depend purely on chance. Further, the variations of small magnitudes were not taken into consideration in the explanation of natural selection.

(ii) Darwin’s theory did not explain as to how the variations arose in the individuals.

(iii) It is not always the case that useful variations are selected. A useful organ can develop to such an extent (over-adaptation) that some time it may be lethal for the individuals. An example of over-adaptation is found in Yucca flowers in which stigmas are so peculiarly shaped that special insect can only affect natural pollination in them. In absence of that moth or insect Yucca flowers fail to form seeds.

If natural selection had operated favourably on useful variations then non-useful variations should not have been selected. This fact is opposed to Darwin’s theory. It is also probable that over-adaptation might have been an important factor in the extinction of many kinds of plants and animals during the course of organic evolution.

(iv) It has not been explained that the useful and non-useful variations are inherited;

(v) New variation would be lost by ‘dilution’ as the individuals bred with others. Although the phenotypic expression of a gene may be altered when it exists in association with certain other genes, yet the gene itself is not altered and is transmitted to successive generations as such.

(vi) Darwinism indirectly accepts the Lamarckian principle of inheritance of acquired characters which cannot be explained in the light of recent concept of genetics.

Neo-Drawinism:

This is modification of Darwinism in light of cytogenetics. The main contributors of this theory are T.H. Huxley, S. Wright, Th. Dobjhansky, Karl Gegenbaur, Ernst Haeckel, Weismann and G.L. Stebbins Darwin’s theory was criticised for several defects. It did not explain the origin of variations and could not distinguish somatic (non-heritable) from germinal (heritable) variations.

Darwin failed to explain why even harmful variations were selected or preserved leading ultimately to extinction of the species e.g., horns of Irish elks. The supporters of this theory believed that natural selection has accounted every-thing that has undergone evolution.

Weismann and his supporters discarded Darwin’s theory except its principal element of natural selection. This made clear-cut distinction between somatic and germinal variations in their theory of germplasm but they could not appreciate the role of mutations in evolution. Darwin was of the opinion that the mutation resulted mainly by single force, i.e., natural selection.

According to Neo-Darwinism adaptations result from multiple forces and the natural selection is one of them. The theory also holds that the characters are not inherited as such but there are character determinant which control the development of the particular characters.

The ultimate character results due to the interaction of the determinants as well as the interaction between organisms and the environment during development.

This theory gives better mechanism of evolution and states that:

(i) The sources of variations are mutations and gene recombinations through sexual reproduction.

(ii) Chromosomal organization and variation affect genetic linkage and produce variations in gene pool of cross fertilizing populations.

It is remarkable that variations only provide the raw material and not the direction of evolution which is the function of natural selection. The limits to which the natural selection can guide the population are set by reproductive isolation only. Neo-Darwinism has received great support from recent studies of population genetics.

De Vries Mutation Theory:

Hugo De Vries (1884 -1935), a Dutch botanist, propounded the mutation theory in 1901 to explain the origin of new species in the living organisms (Fig. 27.13). According to this theory, new species arise not by gradual accumulation of small variations in the organisms over a long stretch of time but by appearance of some permanent sudden and unpredictable changes (mutations).

Fully established new species may continue to live side by side with the parental forms.

In the course of his studies on the evening primrose (Oenothera lamarckiana) De Vries marked the sudden appearance of new characters. Those newly developed characters were inheritable. He called such sudden hereditary changes as mutations and believed that some of his mutants produced in a single step were actually new species.

Mutations are some time called discontinuous variations because they are not connected by a series of intermediate forms. In this way, Darwin’s theory differs basically from the mutation theory of evolution because the theory of natural selection emphasized the importance of continuous variation in forming an unbroken gradually merging series.

Oenothera lamarckiana has 14 chromosomes but De Vries found that some of the offsprings had 15, 16, 20, 22,24, 27,28, 29 and 30 chromosomes. The new individuals differed in flower size, shape, arrangement of buds and size of seeds, etc. To those new abnormal types, De Vries called elementary species. He believed that those differences were due to mutations and they furnished sufficient materials for evolution.

De Vries concluded his theory in the following ways:

(i) New elementary species appear suddenly and attain full constancy at once.

(ii) The same new species are produced in large number of individuals. This increases the chance of selection by nature.

(iii) Mutability is something fundamentally different from fluctuating variability.

(iv) Mutations take place in nearly all directions and may involve any character.

Evolution is based upon the genetic variations and changes in gene frequency. The ultimate source of all genetic variations is mutation whether produced due to change in the molecular organization of genes or in the linear sequence of genes on the chromosomes or in the number of chromosomes. Recombination is another source of genetic variation.

Some interesting facts highlighting the role of mutation in evolution are outlined below:

1. Mutations may be spontaneous or induced. A mutation arising in a natural population is spontaneous. Haemophilia in man, white eyes and vestigial wings in Drosophila, resistance of bacteria to penicillin and resistance of flies to DDT etc. are spontaneous mutations.

2. Mutations may be induced by X-rays, Gamma-rays, UV-rays and chemicals like mustard gas, formaline and colchicine.

3. Mutations arising spontaneously are not directed by environment. The environmental influence can affect the mutation rate but cannot cause a particular mutation to take place. The environment plays important role in the selection of mutants.

4. Mutations are relatively, though not always, persistent. A mutant gene may later mutate again into a new allele or it may change back to original condition.

5. Generally, mutations are deleterious and rarely they are advantageous but only useful mutations are selected. The organisms with useful mutations are better adapted species in course of evolution.

6. Beneficial mutations spread through population rapidly. If they are dominant, they are selected at once resulting in rapid evolutionary change. Recessive mutant genes spread very slowly.

7. The chromosomal mutations are important in genetics of the individuals. Such mutations result in an increase in the amount of genetic variabilities which are important from evolution point of view. Addition of new genes by insertion into the individual chromosome may sometimes be important in promoting evolutionary change.

Criticisms:

(i) The mutations observed by De Vries in oenothera were actually due to chromosomal rearrangement and segregation, rather than the gene mutations. What De Vries called as new species were actually hybrids of the old species.

(ii) The mutation theory does not give any proper direction to the evolution unless natural selection operates on them.

(iii) It is well known that most of mutations are not useful but usually lethal and, therefore, they would eliminate the species rather than propagate it.

(iv) If mutations are only accidents then mutant animals may not be necessarily adapted to their environments.

(v) Genetic studies have shown that small variations produced mutations and gene recombination’s provide the raw material on which natural selection operates. The natural selection acts to short out individuals with favourable variations from population for survival in the struggle for existence and only the favoured individuals of the population lead to the origin of new species in course of evolution.

Modern Synthetic Theory of Evolution:

The various views on evolution have now been merged into one broad coherent theory, the modern synthetic theory.

The modem theory of evolution is a synthesis of Mendel’s and Darwin’s observations. In this theory, Mendelian population is described as object of evolution.

The heritable variations which appear among the individuals of such a population either by sexual recombination’s or by mutations are described as the raw materials of evolutionary process, and natural selections acting on the heritable variations of population have been described as mechanism of evolution.

The principles involved in the evolution of new species according to modem theory, may be grouped under two main heads:

1. Isolation.

2. Hybridization.

1. Isolation:

In order to bring new species into existence, population should necessarily be segregated into smaller units. This segregation of population into small units is referred to as isolation.

Isolation may be brought about by geographical barriers, such as, high mountains or large bodies of waters over which plants cannot pass (geographical isolation) or by factors restricting or eliminating interbreeding or natural population or subsequent reproduction of the hybrids between them {reproductive isolation). By isolation, plants reach in different types of environments.

The isolated species develop in divergent lines and become quite distinct. Mutations also arise in the isolated species. Such changes result new races and eventually new species in the population. Evolution is essentially a change in the genetic compositions of populations.

To be ranked as separate species two populations must differ in many genes. It is now obvious that if one population has to become different from another, then the two must be distantly separated over a longer period of time so as to prevent unrestricted interbreeding.

2. Hybridization:

Natural selection in the modem view of evolution is a creative force. The species hybridization is believed to be important in the production of new forms of plants and animals, although the role which it plays is generally believed to be less important than isolation.

Hybridization is possible between two different sexually reproducing species. Sexual reproduction involves not only the fusion of two kinds of gametes but also the entire mechanism of independent assortment of homologous chromosomes and segregation and recombination of genes which comprise the essentials of Mendelian laws of heredity in its modem meaning.

The combination of genes from different individual increases the number of variations upon which the natural selection operates. Sexual reproduction does not induce struggle for existence or survival of the fittest in the population. Organisms usually struggle rather very rarely.

Indeed animals prefer to avoid struggle and similarly the plants have never been seen engaged in the struggle. Moreover, natural selection does not eliminate the unfit individuals from population. The fit individuals may be the mightiest in the population, but they might happen to be sterile so the natural selection cannot operate upon them.

Evolution at Molecular Level:

The recombination is of great importance in the evolution. Single mutational change may be lost or passed on without great impact on population but if its effect is modified and enhanced by recombination an unending contribution to variation is begun.

Variation is the raw material for evolutionary change. Recombination is its principal source, says Savage, J.M., of the University of Southern California. Mutation alone has relatively little effect on variation without the pervasive impact of recombination.

Gene mutations, although often expressed in visible characteristics or physiological activity of the organism, are ultimately caused by rearrangement, addition or deletion of specific nitrogenous bases in DNA.

Changes in the order of bases result in the modifications in the polypeptide chain produced under the control of gene and these modifications will result in production of modified enzyme or the protein or suppression of some enzyme feature.

The modified enzyme may ultimately express itself by affecting a change in the physiology of development of the organism. Since a change in one of the bases may theoretically occur at any point along the length of one strand of DNA and incredible number of mutations are possible.

Some mutations involve either addition or deletion of one or many bases along the DNA strand. The possibility for mutation along the extraordinarily long DNA molecules that form the axis of chromosome provide more than adequate basis for variation and, therefore, raw materials for future evolutionary changes.