WEIT 2012 Chap5

THE ENGINE OF EVOLUTION

What is natural selection ?

Everywhere in nature, animals seem to fit perfectly their environement, as if they had been designed to, and early naturalists believed that this perfection was God's product. It was Charles Robert Darwin (1809-1889), who the first used the terms natural selection to describe the key process of Evolution.
In his book The Origin, he defined the natural selection as a mindless and materialistic process, gradual and nonrandom, by which some genotypical and phenotypical traits become either more or less common in a given population.

He came to this definition by realizing two things:
• The different species produce an excessive number of offspring. As the ressources are limited, they can't all survive.
• In the diverse populations, individuals vary in many heritable traits.

From these observations, he came to the fact that in a polpulation, the individuals who have the biological traits the more adapted to the environment, which fit the best their environment will be more likely to survive to predators, to feed themselves, or to find a mate, etc... As a result, and from the first observation, these individuals will be able to produce more offspring than the others, and to colonize the environment in few generations. This is what he called natural selection. Be more precise here. The definition of natural selection is very important and should be clear for you and for the other students--Pierre.brawand 28 mars 2012 à 13:43 (CEST)

Is natural selection based on chance ?

While learning Darwinism, many people understand that "everything happens by chance", but it is wrong.
A species can neither decide to evolve, nor to wait and hope to evolve. Natural selection or adaptation and through it, evolution, happens because of necessity.
To evolve is a necessity, in order to maintain equilibrium in the Nature, for example between predators and preys as the asian hornets and the japanese honeybees. Adaptation to environment is inevitable, because the Nature needs equilibrium.

Three linked factors are involved into the process of adaptation, which can't take place if one of these conditions isn't fulfilled.

• Variability: Individuals of a same population have to show some phenotypical differences, because adaptation can't appear from nowhere.
• Heritability: These variations have to be heritable, it means that a proportion of these differences have to have a genetic basis. The individuals have to have difference in their genotype, in order for these variations to be passed on to the next generation.
• Probability of leaving offspring: The last aspect of natural selection or adaptation is that these genetic variations must affect positively an individual's probability of leaving offspring, in order for the variation to be transmitted to the next generation.

too short--Pierre.brawand 28 mars 2012 à 13:43 (CEST)

Why is natural selection a very long process?

...

What about evolution without selection ?

When sexual reproduction occurs, one genes of each parents isn't transmitted to the offspring. If the family has only one child, one allele of each parents will be lost, but it can also be lost with several child, if the same gene is transmitted to all the children. Each generation, it is the same, the genes can be lost. This process is called random sampling. A gene can either disappear or become fixed in the population. This is genetic drift.
Genetic drift, as natural selection make a genetic change that allows to speak about evolution. But natural selection is way more powerful than genetic drift:

• As seen before, natural selection is a non random process that change the frequency of the alleles, regarding how useful they are. It gets rid of harmful ones and replaces them by raising the frequency of beneficial ones.
•On the other hand, genetic drift is a random process, so if the frequency of an allele changes, it is regardless of its utility. Son the evolution caused by the genetic drift is not an adaptation; it can't build an eye or a wing, things allowed by natural selection.

Genetic drift may causes an other problem. In small populations, random sampling may go in the opposite direction of natural selection, making a harmful allele becoming more frequent, instead of destroying it.
Genetic drift probably had a role in the evolution, most of all in the evolution of small populations. It shows that there can be evolution without natural selection, but there are only few examples.

What is artificial selection ?

Artificial selection is a process which has the same result that natural selection but through a shortest way. It is a process that consists of a transformation of a plant or an animal into an ameliorated and more adapted form in order to satisfy human's desires in a shortest time than nature selection can.
The most obvious example is the evolution of dogs. All the species have different shapes, temperaments, colours, and sizes however they descend from a single ancestral species which is the Eurasian gray wolf. Of course they evolved because of natural selection over thousand of years but they can't be as different as they are now only through this natural selection. Some dogs for example can run faster than others or are bigger than others. This is not due to natural selection but to artificial selection. For example, because people needed faster dogs in order to win some competitions, breeders changed some traits of the dogs in order to give a size that permits it to run faster than is ancestor.
It shows that artificial selection is a shortest way to obtain what humans want and what nature can make only over thousand of years.

What are the advantages of artificial selection?

Natural selection is very advantageous for humans.

• It is a process that can make life easier. For example, breeders can change the traits of their animals and plants in order to have best harvests and more productive animals in a shortest time. They also need to choose which traits they want to modify to win more time (and consequently more money)...

What are the disadvantages of artificial selection?

Artificial selection can have dramatic results because it is not 100% safe: it can undergo mutations...

Why some people dislike artificial selection ?

...

This is not the easiest chapter of the book but it is maybe the most important to explain the mechanisms... There is a lot to do here in order to have a complete picture of the chapter. Try to organize yourself in order to share the work efficiently. OK?--Pierre.brawand 28 mars 2012 à 13:46 (CEST)

---

• retour à Evolution_3BIbDF10-11_2011/12
  
• retour à Accueil