State and explain the “law of independent assortment” in a typical Mendelian dihybrid cross.

OR

(a) How do the observations made during moth collection in pre- and postindustrialized era in England support evolution by Natural Selection ?

(b) Explain the phenomenon that is well represented by Darwin’s finches other than natural selection.

Law of independent assortment: This law states that, the alleles of two or more genes get assorted into gametes independent of each other. The allele received for one gene does not influence the allele received for another gene.

In a Dihybrid cross, Mendel crossed varieties of pea plants that differ in two characters and each character was controlled by two alleles of a gene.

He crossed a plant having round and yellow seeds(YYRR) with a plant having green and wrinkled seeds(yyrr).

In the F1 generation he observed:

9 plants - Round yellow seeds

3 plants - Round green seeds

3 plants- wrinkled yellow

1 plant - wrinkled green

In this he found, that Round yellow and Green wrinkled are parental combinations, while Round green and yellow wrinkled are new combinations, which means that the two characters are inherited independent of each other and hence produced new combination.

This explains the Law of independent assortment.

OR

(a): Natural selection is the key concept of Darwin’s theory of Evolution. The organism which adapt well in the environment will be selected by the nature and will survive more in nature.This is called Natural selection.

Industrial melanism is an example of the Natural selection of a particular form of an organism in an environment which has changed due to the ill effects of pollution created by industrial activities.

An observation supporting evolution by natural selection comes from England.

Pre-industrialisation- In 1850, in collection of moths it was observed that, there were more white wings moths on trees than dark winged moths. Due to less pollution, the trees had light coloured trunk due to lichen. On the light background white-winged moths were not visible.

Post-industrialisation - In the collection carried out in the same area after industrialisation in 1920 there were more dark-winged moths, the proportion was reversed, and this was due to air-pollution.

So, nature selected only those moths which were better suited.

(b): The phenomenon well represented by Darwin’s finches other than natural selection is Adaptive Radiation.

In this, all species deriving from a common ancestor, have overtime successfully adapted to their environment through natural selection. Previously, the finches occupied the South American mainland but somehow managed to occupy Galapagos islands, over 600 miles away, they occupied an ecological niche with less competition.

As the population began to grow in these conditions, intra-specific competition became a factor, and resources were squeezed and could not sustain the population of the finches for long due to natural selection, and the changes in the gene pool, the finches became more adapted to the environment.As the competition grew, the finches managed to find new ecological niches, that would have less competition and allow them and their genome to be continued.

Over the long term, the original finch species may have disappeared, but by diversifying, would stand a better chance for survival.All in all, the finches had adapted to their environments via natural selection, which in turn, has allowed the species to survive in the longer term, the prime directive of any species.