(By Dr. Girish Chandra)
In evolutionary terms, differences between closely related organisms are termed as variations. Thus differences between individuals of the same species, subspecies or race can be called variations and not differences between two genera, families or classes. Only heritable changes have evolutionary significance, since they only make the population change from generation to generation as the evolution goes on. Variations can be of several types and can be classified in different ways based on the type of character taken into account.
Kinds of variations
1. Group variations: When one population of a species differs from the other, it can be termed as group variation. For example, African population of man is black while European population is white. Insect populations show group variations even within a small range of distribution.
2. Individual variations: These are differences among the individuals of the same population. They are very important in taxonomic studies, when extent of variation within a deme is taken into account for comparing and assigning the population to a taxon.
Variations can also be classified in the following 3 categories based on the type of character considered:
1. Meristic variations: These are variations that can be counted in numbers. For example, man has 12 pairs of ribs but if some individual has 13 pairs of ribs, it will be called a meristic variation. Similarly some people possess 6 fingers instead of 5 due to trisomy, and a starfish may have 6 arms instead of the usual five.
2. Quantitative variations: Variations that can be measured in size or weight, such as tall versus dwarf animal, heavy versus light body, long tail versus short tail etc.
3. Qualitative variations: These include characters, which depict identification quality of an individual, e.g. presence or absence of spots, hairs, colour, stripes, specialized feathers etc.
Based on the continuity of a character the variations can be classified into the following two categories:
1. Continuous variations (=Clinal variations) (=Minus-plus variations): Variations which fluctuate above or below the average, with intermediate stages also found. For example, in a population some individuals are larger, some smaller and some intermediate. In Indian population, some people have lighter skin, some darker and all kinds of intermediate shades are also found.
2. Discontinuous variations: These variations deviate greatly from the average individuals. Major mutations and disruptive selection produce some individuals, which are distinct from the others. For example, white tiger, hornless calf and albino peacock or cow.
Based on the inheritance, variations can be classified into the following two types:
1. Somatic variations: These are also called somatogenic variations, which are produced in the body due to the effect of environment. They are not heritable as they are not due to genetic changes, as for example, muscles of a wrestler, or weak individuals under starvation conditions.
2. Genetic variations: They are germinal variations that occur in genes. They are blastogenic, heritable and produce phenotypic changes in the populations. If somatic variations persist for a longer time, they tend to become genetic and become important in evolution.
Some variations not classified in the above-mentioned ways are as follows:
1. Age variations: When young ones differ from the adults distinctly, as caterpillars differ from the adult butterfly or tadpole differ from frog, it is called age variation. These variations have adaptive value but not evolutionary significance, although they help in taxonomic identification.
2. Seasonal variations: When animals change their appearances in different seasons, mainly as climatic adaptation, they are called seasonal variations, e.g. winter fur in temperate animals like snowshoe hare, whose fur becomes white in winter and brown in summer. Seasonal variations are commonly found in insects, like butterflies, grasshoppers, bugs etc.
3. Habitat variations: In sedentary animals like sponges, corals, oysters as well as in plants, variations are produced due to local environmental influences of the particular habitat. Some of the mobile animals, namely grasshoppers, locusts and plant bugs also show habitat variations. Chameleon can change its colour according to the habitat.
4. Castes in social insects: Division of labour in social insects, e.g. termites, honey bees, ants and wasps creates castes which have specialized organs to carry out a particular job in the colony. Therefore we can see different types of individuals moving about in the same population.
5. Polymorphism: When different types of individuals occur in a single interbreeding population of a non-social animal, it is called polymorphism. It is very common among insects, like butterflies and beetles, which show dry and wet season forms.
Sources of variations
Somatic as well genetic variations can be produced in the following ways:
Environment: Environmental factors, namely, heat cold, rain, draught, food etc. influence populations drastically and normally cause somatic variations within a short period. These variations are generally not heritable and disappear if environmental conditions revert back. If tadpoles are fed on thyroid extract, they quickly metamorphose into small frogs, without growing in size.
2. Endocrine glands: Optimum activity and balance in the functioning of endocrine glands is very important for the normal growth of the body. Over or under functioning of pituitary, thyroid, adrenal etc. can produce variations in the body, which are again only somatic and not heritable. Such variations are also due to the environmental effects.
3. Blending inheritance: Darwin laid emphasis on blending inheritance as a major source of variation in animals, without understanding its genetic mechanism. Characters of both the parents merge in the first generation of offsprings, due to crossing over between homologous chromosomes in diplotene stage of meiosis. But the characters segregate in the subsequent generations according to Mendel’s laws, but still the variations are produced.
4. Mutation: Mutation is a change in the sequence of nitrogenous base pairs in DNA. For example, if AGC is a codon on DNA, its complementary codon on m-RNA will be UCG, which will code for the synthesis of amino acid serine. If a small mutation replaced Guanine with Adenine in the codon of DNA, the complementary codon in m-RNA will also change to UUG, which will now synthesize a different aminoacid, Leucine. This is a point mutation that will alter the relevant character on the body. Similarly, point mutation produces sickle-cell anemia in man, when codon CTT coding for glutamic acid is changed to CAT, which codes for valine.
Mutations can be produced by excessive environmental conditions or by radiation but most of the times they are caused naturally during the duplication of DNA, when new bases are synthesized. As these mutations are recessive, they remain hidden and do not express for a long time, and since they are genetic they play very important role in evolution.
Role of mutation in natural selection: Mutations may lead to formation of multiple alleles at a given locus. New alleles change the gene frequency in a population that upsets genetic equilibrium, causing microevolution.
In diploid organisms mutations are recessive and therefore remain hidden for a long time but keep spreading and accumulating in the population causing hybrid vigor (Heterosis) and variations, which help in natural selection.
In haploid organisms they affect the character immediately and expose it to natural selection.
Recombination of mutant genes takes place during crossing over, which makes the population heterozygous and much fitter when exposed to natural selection.
Due to the genetic drift, an advantageous mutation may be quickly fixed helping in natural selection or sometimes harmful mutations may be fixed, causing extinction of the population.
5. Chromosomal aberrations: This involves breaking and rejoining of a segment of chromosome during the prophase of meiosis, by deletion, duplication, inversion or translocation.
1. Deletion: When a small part of the chromosome breaks apart, generally during crossing over by the action of endonuclease and is lost. In man, a terminal deletion in chromosome 21 causes granulocytic leukemia and deletion of one arm of 5th chromosome causes Cri-du-chat syndrome in children. In Drosophila, deletion in X chromosome causes notched wings. Deletion causes abnormality in crossing over during pachytene stage of meiosis.
2. Duplication: When one gene is represented more than once, if deleted portion of one chromosome gets attached with another chromosome. The zygote will have 3 doses of genes and crossing over will be unequal. Bar-eye in Drosophila arises due to duplication of a small section of X chromosome.
3. Inversion: When a piece of chromosome breaks and joins at the same place after rotation, the sequence of genes is altered. It is most likely to occur in meiosis in germ cells if chromosomes form a loop. Chromosome breaks at center and gets reattached after reversing. This will cause abnormal synopsis and crossing over. There is not loss or gain of genes but sometimes the effect may be due to new position of the genes.
4. Translocation: This involves transfer of one gene block from one linkage group to another in non-homologous chromosomes. Most of the translocations do not produce any abnormality in carriers as deletion in one is balanced by the other. Fifty percent of the offsprings of carriers (heterozygotes) will be grossly abnormal. Translocations, like inversions, do not produce immediate effects but accumulate over long periods to introduce reproductive isolation in allopatric populations. In Drosophila 6 species have been produced by translocation from the ancestral Drosophila virilis, which has 6 pairs of chromosomes.
6. Aneuploidy: When the entire chromosome is either lost or duplicated, it is called Aneuploidy. In Drosophila the gene for white eye is X chromosome. When two X chromosomes fail to separate during oogenesis (non-disjunction), female offsprings with 3X chromosomes will be superfemales, which have low viability and are sterile, while some eggs will have no X chromosome at all. OY individuals do not survive because of the upset in gene balance.
In man non-disjunction in sex chromosomes influences secondary sexual characters. For example, trisomic XXX produces superfemales, while XXY causes Klinefelter’s syndrome. Monosomic (2n-1) XO causes Turner’s syndrome, while YO is non-viable. Down’s syndrome or Mongolism is caused due to duplication of chromosome No. 21 and it causes mental retardation, malformed ears, no sexual maturity and susceptible lungs. Aneuploidy causes abnormality in organisms but sometimes these abnormalities may prove to be useful in different environmental conditions.
7. Polyploidy: When there is duplication of the entire haploid set (genome) of chromosomes, due to abnormal mitosis or meiosis, leading to triploid or tetraploid organisms, if diploid gametes are produced. Such offsprings are normally sterile, but in plants owing to vegetative reproduction, they may be able to produce offsprings. In horticulture, polyploidy is induced by treating them with Colchicine (obtained from the seeds of Colchicum autumnale) to obtain better fruits and flowers. Polyploidy is not successful in animals, except in some parthenogenetically reproducing animals, viz. shrimps, isopods, moths, beetles, flies etc.
8. Hybridization: When isolating mechanisms between two species break down and they produce offsprings. Such offsprings are normally sterile but sometimes they can backcross with the parents to produce second generation of individuals (introgressive hybridization). For example, Raphanobrassica is produced by crossing Raphanus (Radish) and Brassica (Cabbage). Both have 18 chromosomes. Hybridization is a common mechanism that produces variation in invertebrates, particularly insects.