WEIT 2012 Chap1

WHAT IS EVOLUTION

What is the definition of Evolution?

Evolution is a gradual process in which the biological populations change into a different and usually more complex or better form. Life on Earth began with a universal common ancestor. It changed gradually, taking several millions of years. This universal common ancestor, named LUCA (Last Universal Common Ancestor), branched out over time, throwing off many new and diverse species. This mechanism of evolutionary change is called natural selection. When you break that statement down, you find that it really consists of six components.

What are the six components of the statement of evolution?

The six components are evolution, gradualism, speciation, common ancestry, natural selection, and nonselective mechanisms of evolutionary change. Let see each of these components closer.

What is evolution?

Evolution is the fact that a population of organisms undergoes genetic change over time. Individual organisms do not evolve. The differences are based on the changes into the DNA, which originate as mutations. All the populations evolve but not at the same rate. The theory of evolution doesn't predict that population will constantly be evolving, or how fast they will change when they do. It depends on the evolutionary pressures they experience.

• See Appendix 3

What is gradualism?

Gradualism is the fact that it takes many generations to produce a substantial evolutionary change. The evolution of new features can take over hundreds or thousands (even millions) of generations. A species evolve faster or slower as evolutionary pressures wax and wane. Also, when the natural selection is strong, evolutionary change can be fast. But once a species becomes well adapted to a stable habitat, evolution often slows down. Many living species share fundamental traits such as biochemical pathways that we use to produce energy, our standart four letters DNA code, and how that code is read and translated into proteins. This tells us that every species goes back to a single common ancestor (who had those traits and passed them on to its descendants).

What is speciation?

Speciation, also called splitting, is a very slow process wich explains how a single ancestral species can split in several descendant species. More precisely, it's the appearence of slight differences between two different population of the same species and the splitting really occurs when the differences become so important that the two population can no longer interbreed. Species themsleves cannot split and don't have to. This process doesn't happen quite often, but enough to explain us the great diversity of the terrestrial flora and fauna. Since it doubles the number of opportunities for future speciation for a species, speciation might increase exponnentially the number of species, but more than 99 percent of the species did actually extinct without leaving any descendant.

What is common ancestry?

Common ancestry is the flip side of speciation. We can always look back in time, using either DNA sequences or fossils, and find descendants joining at their ancestors. Every pair of species shares a common ancestor sometimes in the past. To express the evolutionary relatedness among species through time, we use The Tree of Life. Tree which has been first proposed by Darwin in the 19th century. This Tree of Life could be done by looking at physical traits. Now we have the ability to make a Tree of Life even more precise with the knowing of DNA.

• See Appendix 2

What is natural selection?

Natural selection is a process which requires that individuals of a species vary genetically in their ability to survive and reproduce in their environment. In a more detailed way, if individuals within a species differ genetically from one another, and some of those differences affect the abilities of an individual to survive and reproduce in its environment, then in the next generation the “good” genes that lead to a higher survival and reproduction level will have relatively more copies than the “not so good” genes. Over time, the population will gradually become more and more suited to its environment as helpful mutations arise and spread through the population, while deleterious ones are weeded out. Ultimately, this process produces organisms that are well adapted to their habitats and way of life.

Can we consider Evolution as a theory?

First of all, we have to make clear what is a theory. "A theory is a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena." So in science, a theory is much more than only a supposition about how things are: it is a well-thought-out group of propositions meant to explain facts about the real world. Bec

• see Appendix 1

References

• http://dictionary.reference.com/browse/theory

don't you think that you could bring some examples to illustrate this chapter which is a summary of the main points discussed in the book. The other option that would be even better would be to make LINKS with the other chapters (the other wiki pages)...--Pierre.brawand 30 mars 2012 à 10:57 (CEST)

Examples?--Pierre.brawand 25 avril 2012 à 18:28 (CEST)

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