Organic Evolution

Heredity refers to the passing of traits or characteristics through genes from one generation (parent) to the other generation (offspring). This heredity is very evidently seen in sexual reproduction, as the variation of characteristics that are inherited is high.

During the sexual reproduction process, variation occurs due to some error in DNA copying. Variation is important because it contributes to evolution and forms the basis of heredity. variation is caused due to positive gene mutations, the interaction of genes with the environment and various combinations of genetic material. Remember that variation can occur through asexual reproduction process too. But these variations are not very noticeable.

Types of variation:-

-Variation are of two types:

(i) Somatic Variation

-It is also known as acquired traits.

-It is neither inherited nor transmitted

-It takes place in the body cell

(ii)Gametic Variation

-Also known as inherited traits

-Inherited as well as transmitted

-Takes place in the gametes/reproductive cells.

Mechanism of Organic Evolution:

• This theory was proposed by Hugo de Vries based on his work on evening primrose (Oenothera Lamarckian).  
• Large differences arising suddenly in a population are called mutations.
• Actually, mutations are sudden changes of genetic material (DNA) and hence all are inheritable.  
• In addition to recombination, the mutation is another phenomenon that leads to variation in DNA.  
• The mutation is a discontinuous source of variations and provides raw material for evolution.  
• According to Hugo de Vries it is a mutation that causes evolution and not the minor variations (heritable) that Darwin talked about. 
  
• Mutations are random and directionless while Darwinian variations are small and directional.  
• Evolution for Darwin was gradual while de Vries believed mutation caused speciation and hence called it saltation (single step large mutation). 

Criticism of Mutation Theory:

1. Natural mutations are not very common as Hugo de Vries thought.
   
2. Mutations are normally recessive and harmful, while the characters taking part in evolution are usually dominant.

ACQUIRED AND INHERITED TRAITS:-

EVOLUTION:-

Evolution is a gradual and continuing process of change that occur over a period of time, due to slight variations in genetic compositions as well as changes in environment, leading to formation of new species.

In the first situation, a colour variation arises during reproduction, so that there is one beetle that is green in colour instead of red. This beetle, moreover, can pass the colour on to its progeny, so that all its progeny beetles are green. Crows cannot see green-coloured beetles on the green leaves of the bushes, and therefore cannot eat them. What happens then? The progeny of green beetles is not eaten, while the progeny of red beetles continues to be eaten. As a result, there are more and more green beetles than red ones in the beetle population.

In a second situation, again, a colour variation arises during reproduction, but now it results in a beetle that is blue in colour instead of red. This beetle can also pass the colour on to its progeny, so that all its progeny beetles are blue. Crows can see blue-coloured beetles in the green leaves of the bushes as well as they can see red ones, and therefore can eat them. What happens initially? In the population, as it expands, there are a few blue beetles, but most are red. But at this point, an elephant comes by, and stamps on the bushes where the beetles live. This kills most of the beetles. By chance, the few beetles that have survived are mostly blue. The beetle population slowly expands again, but now, the beetles in the population are mostly blue. It is obvious that in both situations, what started out as a rare variation came to be a common characteristic in the population. In other words, the frequency of an inherited trait changed over generations. Since genes control traits, we can say that the frequency of certain genes in a population changed over generations. This is the essence of the idea of evolution.

In the first case, the variation became common because it gave a survival advantage. In other words, it was naturally selected. We can see that the natural selection is exerted by the crows. The more crows there are, the more red beetles would be eaten, and the more the proportion of green beetles in the population would be. Thus, natural selection is directing evolution in the beetle population. It results in adaptations in the beetle population to fit their environment better. In the second situation, the colour change gave no survival advantage. Instead, it was simply a matter of accidental survival of beetles of one colour that changed the common characteristic of the resultant population. The elephant would not have caused such major havoc in the beetle population if the beetle population had been very large. So, accidents in small populations can change the frequency of some genes in a population, even if they give no survival advantage. This is the notion of genetic drift, which provides diversity without any adaptations.

In a third situation, as the beetle population begins to expand, the bushes start suffering from a plant disease. The amount of leaf material for the beetles is reduced. The beetles are poorly nourished as a result. The average weight of adult beetles decreases from what it used to be when leaves were plentiful, but there is no genetic change occurring. After a few years and a few beetle generations of such scarcity, the plant disease is eliminated. There is a lot of leaf food. At this time, what would we expect the weight of the beetles to be?

The average weight of the beetles increases again.