Hypotheses of Evolution

〈一〉 The hypothesis of Lamarck --- The use and disuse of organs

Lamarck believed that organs of living things can become well-developed for repeated uses, and the property of the well-developed organs could be transmitted to the next generation. Eventually, the appearance and function of this organ will be deviated from the original one and becomes a new character. Finally, the organism collected these new characters and became a new species.
If the organ was not used for a long time, it would become poorly developed and degenerated. Eventually, it may become a vestigial organ or totally disappear. A new character and a new species is thus formed.

To take giraffes as example, according to the hypothesis of Lamarck, it was said that there were giraffes with long necks and short necks at the very beginning. They fed on grasses on the ground. When the grasses on the ground vanished, they needed to eat the leaves on the bushes. When leaves on lower bushes were cleared, they needed to extend their necks to the higher bushes for leaves. So, their necks became longer and longer and took the shape of giraffe today.

The point of Lamarck's hypothesis that was poorly accepted by scientists is that the acquired characters can be transmitted to the next generation. Few scientists could accept this point. furthermore, after the development of genetics, this point had become fatal in the world of biologists.

〈二〉The hypothesis of Darwin --- Natural selection

Darwin believed that there should be some difference among individuals of the same species. If they lived in the same environment, some of them would be more suitable for living than the other. If the competition was strict, some of them would be erased out. Only the one which was the most fitted for survival could be left. This was the natural selection. If natural selection occurred on various factors, eventually, the individuals left would be quite different from the extincted species at the beginning. This hypothesis included 4 suppositions. The hypothesis cannot miss any of them. They were

(1)	Individuals of the same species can have deviations.
(2)	Individuals of the next generation would resemble their predecessor.
(3)	Descendents of an individual could become more and more and could grow to the stage of mature for reproduction.
(4)	As a result of competition, some of the descendents can live better and survive better.

According to these 4 suppositions, two points of conclusion can be obtained.

(1)	The winner in the competition can survive and reproduce. They can transfer their sucessful character to the next generation and make individuals in the next generation more fit to live and much successful in competition.
(2)	After some time, those which belonged to the same group at the very beginning would become distinct groups. The difference between these two groups can even be enough to be characterized as two distinct species.

Take giraffes as example again, according to the hypothesis of Darwin, there could be two or more types of giraffes at the very beginning. Some with longer necks and some with shorter necks. When they cleared away all the herbs on the ground, they started to consume the leaves on bushes. When leaves on lower bushes were vanished, they needed to consume the leaves on higher bushes. Then, only those with long necks could get the food and survive. Those with short necks would be starved to die. The giraffes with longer necks would breed successfully. This character was then transferred to the next generations. All younger individuals had the characteristics of long necks. So, they were guarenteed successful in further competition. Eventually, we can have giraffes with long necks today. This is the hypothesis of natural selection.

(3) The Neo-Darwinism

By the time of Lamarck and Darwin, the theory of genetics was not yet born. After the establishment of genetics by the work of Mendel, some theories of genetics were added to the hypothesis of Darwin and strengthened the hypothesis of natural selection. Then, it was called the Noe-Darwinism. The hypothesis of Neo-Darwinism consisted of the following elements.

(1)	Individual difference; the difference is due to the different hereditory factors carried by the individuals.
(2)	The production of new traits; In the process of reproduction, some hereditory traits could be changed. This is called mutation. Mutation can induce the formation of new traits.
(3)	Struggle for survival.
(4)	Natural selection; Organisms in nature were not only screened by the nature, but also by some artificial means. This enabled the shift of the traits.
(5)	Isolation; No matter the natural isolation or artificial isolation, individuals with variation could not have exchange of hereditory factors. As variations and isolation continued, the difference enlarged. Eventually, they would become different species.

(25.05.2006)