CLASS WORK

Class X-D&XH​​​       BIOLOGY

16/01/2014

Evolution

Evolution – (more precisely organic evolution) is the constant process of gradual irreversible changes occurring in the organisms since the origin of life that has given rise to enormous variety of complex organisms on this earth.

                                                   ‘How Evolution Occurs’

An llustration to show variations in a population - A group of twelve red beetles live on green leaves of some bushes and reproduce sexually, so are likely to develop variations. Let us consider that crows are the natural predators of these beetles. They regulate the population of beetles by eating them and so fewer beetles are available for sexual reproduction There  are following possibilities. 

First Situation – There is a colour variation during sexual reproduction and one beetle develops green body colour instead of normal red colour. This beetle can inherit this variation in colour on to its progeny so that all its offsprings are green in colour. Crows cannot easily locate green coloured  beetles on the green leaves of bushes but continue to eat red beetles which are easily located on green leaves by them. As a result, number or green beetles in beetle population increases in each generation.

Second Situation – Another colour variation arises in beetles during sexual reproduction and in the progeny, one beetle develops blue body colour instead of normal red. This beetle can also pass this colour variation to its progeny so that all itsoffsprings are blue coloured beetles. Crows can detect both red and blue colouredbeetles easily on green leaves of bushes and eat them regularly. As the population of beetles expands, initially there are few blue coloured beetles among the majority of redcoloured beetles. As this point, an elephant comes by and stamps on the bushes where the beetles live. Consequently, most of the beetles get killed. By chance, the few beetles that survived were blue coloured. The beetle population again slowly expands and will contain mostly blue coloured beetles over a period of time.

Third Situation – As the population of beetles increase, the bushes suffer from a disease, the availability of food for beetles decreases. Expanding population of beetles is now poorly nourished. The weight of beetles decreases but after few years as the plant disease is eliminated and enough food is available for the beetles they gain weight come back to their normal size.

Conclusions

1- In situation 1, ‘Natural selection’ is directing the evolution in beetle population .Greencoloured beetles in the population are adapted to fit better in their environment than the red beetles. This type of variation give a survival advantage

2- In situation 2, colour change gave no survival advantage. Instead it was a matter of accidental survival that changed the common characteristic of the resultantpopulation.So in small populations accidents can change the frequency of some genes in a population even if these give no survival advantage. The phenomenon is called genetic drift and it provides diversity without any adaptations. ​

3- In situation 3 decrease in the body weight due to starvation is an acquired trait in the beetles during their life time. It will not bring any change in the DNA and, therefore, this trait cannot be inherited. And so cannot direct evolution.  When food became abundant after few generations the beetles gained weight.

                                             Acquired and Inherited Traits

Acquired Traits – Certain traits are acquired by organisms in their life time. It involves changes in the non-reproductive tissues caused by environmental factors. It will not bring any change in the DNA. The changes in the non-reproduction tissues cannot be passed on to the DNA of the germ cells hence such acquired changes are not inherited by the progeny over generations.

Examples

1. Decrease in the body weight of beetles due to starvation is an acquired trait by the beetles during their lift time

2. Tailless mice (if tail is removed by surgery) cannot produce tailless progeny

Inherited Traits – These traits are controlled by specific genes and are passed on from one generation to another. Any alteration in the DNA (genes) will be passed on through germ cells to the progeny resulting in variations in them.

Examples

1. Formation of green beetles in the population of red beetles  

2. Skin colour, eye colour, form of hair are few examples of inherited traits in human beings

3. height of plant, form of seed, colour of flower etc are examples of inherited traits in pea plant.

Class Work

Q.1 Give differences between inherited and acquired traits.

Q.2 “Only variations that confer an advantage on an individual organism will survive in a population”. Justify this statement.

Q.3 What are the different ways in which individuals with a particular trait may increase in a populations?

Q.4 Why are traits acquired during the life-time of an individual not inherited?

Q.5 Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?

                                                                                                                                                      SEEMA MISRA