Heredity and Evolution

Introduction

     Heredity is the transmission of characteristics, physical or mental from parents to offspring, that is from one generation to the successive generation.

Meaning of Heredity

     Heredity may be defined as the transmission of characteristics through generations. The characteristics include all physical, physiological and psychological characteristics in organisms. All these characteristics are called the traits.

Mendel's Work

     Gregor Mendel was the first to carry out scientific studies on transmission of characteristics from the parent to the offspring. He selected garden peas as they showed many contrasting traits that were easy to track through the generations.

Monohybrid Cross

     In the first experiment, Mendel considered only one trait at a time. Some of the traits he considered were the height, flower colour, shape of the seed, etc. First he ensured that he had pure-bred tall and pure-bred short plants by selecting seeds from plants that had been self-pollinating for many generations.

Physical Basis of Heredity

     According to Mendel, the genotype is made of certain structures called factors. The factors controlled the inheritance of all traits. These factors are present in pairs. The factors are now called genes. Thus the physical basis of heredity are the genes or factors. The different expressions of the same genes are called alleles. Each trait may be represented with an alphabet. If the alphabet T represents height, T represents tallness and t represents shortness. If the letter R represents the colour of the flower, R represents red and r represents white.

Laws of Inheritance

     According to this law, all the traits are separate entities or units by themselves. Their inheritance is controlled by factors, now called genes.

Chromosomes and Chromosome - Theory of Inheritance

     Waldeyer is credited with the discovery of chromosomes in the year 1888 within the nucleus. He observed them as darkly stained bodies. However, it was the American William Sutton who noticed the similarities between the behaviour of the mendelian factors or genes and that of the chromosomes during meiosis.

Genetic Material

     The genetic material is a nucleoprotein called chromatin. It is made up of DNA and protein.

Structure of Chromosomes

     The chromatin material just before cell division forms into chromosomes. Each chromosome is made up of two longitudinal strands called the chromatids.

Sex Determination

     During the formation of gametes, the females will have only one type of gametes, all with one X chromosome. However, the males will produce two types of gametes or sperms, half with X chromosome and half with Y chromosome. Thus the sex of a zygote is determined by which male gamete fuses with the female gamete.

Variation

     Variation may be defined as differences among individuals of a species. All around us we see different organisms belonging to different species. Within each species too, no two individuals are identical to each other. Asexual reproduction produces identical offspring called clones and identical twins are seen in sexual reproduction.

Types of Variations

     They are somatic and non-inheritable. They are caused by the influence of the environment upon the somatic cells.

Significance of Variations

     Variations provide the species a way to survive and adapt to the changing environmental conditions. Since there are variations, nature can choose the best suited individual and allow the species to multiply further. Thus, natural selection is possible only because of variation.

Organic Evolution

     Evolution may be defined as a gradual development of more complex species from pre-existing simpler forms. It is an extremely slow process and has occurred over millions of years. Evolution that has occurred is clear because of the evidences of fossils, etc. However, the mechanism of evolution has not satisfactorily been explained.

Theories of Organic Evolution

     According to Lamarck, continuous use of a part results in it being well-developed and disuse of a part over a long period of time will result in its degeneration. For example, giraffes were forced to extend their necks and stretch their legs to reach higher vegetation over a period of time.

Evidences of Evolution

     Inspite of many evidences, studies and theories, we are still not aware of how the evolution actually occurs. The search for this knowledge continues.

Palaeontology

     Study of fossils is called palaenotolgy. Fossils are defined as preserved remains of a living organism that existed on earth a long time ago. Fossils are formed by the preservation of the remains of the organisms that existed in the earlier days.

Geological Distribution

     The organisms show particular patterns of distribution on earth. The distribution of the species give an indication of how the earth was earlier. It is believed that the earth was a single mass called the Pangea. It consisted of two large continental masses called Laurasia and Gondwanaland that were interconnected by land bridges.

Comparative Embryology

     Haeckel (1834-1919) formulated that 'ontogeny recapitulates phylogeny'. This means that the developmental stages that an organism goes through repeat the evolutionary history of the group to which the organism belongs.

Comparative Study

     The common features show that an evolutionary process exists and there is a link between the different groups.

Summary

     All these evidences point to the fact that evolution occurs. The exact mechanism of evolution, however, is not yet conclusively proven. But constant study and research in the above fields is continuing for answers.