bonjour tous le monde
c'est quoi la selection naturelle négative et positive (je l'est trouvé dans ce texte mais je n'ai pas bien compris
The Major Types of Natural Selection
Natural selection can act in a wide range of different manners. The simplest and most relevant
for this course are listed below:
• Positive selection occurs when a beneficial mutation occurs in a population and
increases in frequency. Of course, if this mutationis increasing in frequency, then other
variation must be concurrently decreasing in frequency. The classic example of positive
selection is the spread of an antibiotic resistanceallele through a population of bacteria
that are being exposed to that antibiotic.
• Negative (purifying)[/B] selection is the opposite of positive selection. It occurs when a
detrimental mutation is selected out of a population. Since most proteins have
undergone millions or billions of years of evolution, most mutations that cause a change
to a protein coding sequence are believed to be detrimental. These detrimental mutations
will be purged from the population by negative selection. For any protein, the fraction of
mutations that are detrimental is directly related to the ‘evolutionary conservation’ of
that protein.
• Balancing or diversifying selection is selection that favours the maintenance of genetic
variation at a locus. While both positive and negative selection purge variation (either
selecting for or against variants), balancing selection actually maintains variation by
selecting for multiple genetic variants. The easiest way to imagine this happening is to
consider the case of multiple environments that select for different allelic forms of a
protein. For example, a receptor protein that is strongly beneficial when a pathogen is
present in a population, but does nothing but put aload on the system when there is no
pathogen.
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Envoyé par shmikkki 
