EVOLUTION OF NEW SPECIES BY MUTATION AND ISOLATION

A) Mutation theory of organic evolution

Mutation is a change in genetic material resulting in a new stable characteristic which can be inherited by offspring. Hugo De Vries, a Dutch botanist propounded the mutation theory of organic evolution in 1901 after many years of scientific work with evening primrose plants. He discovered that out of 50,000 evening primrose, that 800 of these plants showed traits (variations) not present in their parent plants. He concluded that mutation occurred in the 800 primrose plants and the new traits (variations) found in the 800 plants were transmitted to the offspring.

From this study Hugo De Vries further concluded that mutations similar to primrose plants are frequently occurring in other organisms too and that change by mutation forms the basis for evolution.

 

Mutation

Mutation is the sudden changes in genetic makeup (DNA) either in gene or chromosome resulting in a new characteristic or trait that can be inherited. Mutation occurs during meiosis when the sperm and eggs (ova) are being formed. When mutation occurs, it will lead to the production of offspring with marked differences in appearance from other members of the population of the species. An organism arising from mutation is called mutant. Mutations provides raw materials for organic evolution through natural selection of new species. A mutation which provides an organism with beneficial trait (variation) for adaptation to its environment will survive the struggle for existence. This trait is inherited by the organism’s offspring and gradually the trait will spread through the entire population leading to formation of new species.

 

Types of mutation

There are two types of mutations namely: gene mutations (points mutations) and chromosome mutations.

 

Gene mutations 

Gene (point) mutations are sudden changes in the structure of DNA (gene code) at a particular point or area of the chromosome. Gene mutations occurring in the reproductive cells are inherited by all the cells of the offspring while somatic (body) mutations occur only in the somatic cells and are only inherited by the cells produced by mutant cells by mitosis. Somatic mutations are not transmitted to offspring since reproductive cells are not involved. Somatic mutations are transmitted in plants with vegetative reproduction, hence there are many new varieties of plants.

Somatic mutant cells may give rise to tumour which may become benign without affecting other cells or become malignant and may invade healthy cells, a condition referred to as cancer.

 

Chromosome mutations 

Chromosome mutations are caused either by breaking off or fragmentation of chromosomes during meiosis. The following may happen to the break off or fragmented part.

It may

1. be lost (deletion).
2. be attracted to another end of another chromosome (translocation).
3. be fused to another chromosome resulting in duplication.
4. turn through an angle of 180° and rejoin the chromosome (inversion).

 

Causes of mutation

Mutation may be caused by frequent exposure to

1. X-rays.
2. ultra-violet sun light.
3. cosmic radiation.
4. chemical mutagens usually present in cigarettes.

Minor gene mutations occur frequently with no or little characteristics (traits) expressed on the organism. Mutant genes are generally recessive in heterozygous offspring in which other gene is normal. For example, if one gene in a pair of genes AA mutates to a, the mutant characteristic will not appear in the heterozygous offspring (Aa). However, if mutant gene continues to increase in the population, a situation will arise in which two recessive mutant genes (aa) will pair, resulting in the production of a homozygous offspring with a new characteristic.

If the mutant recessive genes in the homozygous offspring are lethal, then organism stands a chance of possible elimination.

Major mutations are usually lethal (harmful) to the organism even leading to its death or serious deformity. For example, the gene mutation responsible for the production of chlorophyll in green plant is harmful since it will make the plant unable to manufacture food through photosynthesis and consequently it will lead to the death of the plant.

 

B) Evolution of new species by isolation

If members of the same species in populations are isolated or separated by natural barriers and prevented from interbreeding for a long period (thousands of years), new species will evolve as their genes may undergo mutation. The isolated species would not share the mutant genes with the species of other populations. Distinct variations would occur in the species of each separated population.

Possible natural barriers are mountain ranges, deserts, oceans, seas, lakes and rivers. With sea or ocean as a natural barrier, there is little or no chance of plants and animals of the species to interbreed.