The most important mechanism of evolution
NATURAL SELECTION
How does Natural selection change the genetic makeup of a population
by favoring the alleles that contribute to the reproductive success and selecting against those that are harmful
Natural selection does not eliminate alleles directly, Instead….
Individuals with the best phenotypes are more likely to survive long enough to reproduce, those with poorly suited phenotypes are are less likely to survive to reproduce
What are the 3 modes of Natural Selection
are distinguished by their effects on the phenotypes ina population
- Directional selection
2. Disruptive selection
3. Stabilizing
How are the 3 modes of natural selection distinguished
by their effects on the phenotypes in a population
DIRECTIONAL SELECTION
one extreme phenotype is the fittest and the environment selects against the others
eg. dark tree trunk with white wing moths
mrsa
DISRUPTIVE SELECTION
( DIVERSIFYING SELECTION)
2 or more extreme phenotypes are fitter than the intermediate phenotype
STABILIZING SELECTION
( NORMALIZING SELECTION)
Extreme phenotypes are less fit than the intermidiate
It tends to reduce variation in apopulation and is therefore more common in stable unchanging environment
BALANCED POLYMORPHISM
multiple alleles of a gene persists indefinately in the population at more or less constant frequencies. ( even harmful ones)
Circumstances where natural selection maintains BALANCED POLYMORPHISM
Heterozygous advantage
HETEROZYGOUS ADVANTAGE
A circumstance of balanced polymorphism when an individual with 2 different alleles for a gene ( heterozygote) has geater fitness than those whose alleles are identical.
Heterozygotes maintain harmful recessive allele in apopulation evenif homozygous recessive individuals have greatly reduced fitness
Heterozygotes with disease have a reproductive edge than homozygotes (why natural selection does not remove an harmful allele in heterozygotes)
