Types of selection

DIRECTIONAL SELECTION 

Favoured trait is at one extreme end of the range of traits 

Results in phenotypes at one extreme being selected for and those at the other extreme being selected against 

• Environmental change occurs
• Phenotypes that are suited to the new environment are more likely to survive
• Individuals that fall to one of the extremes either side of the mean are more likely to survive and breed
• So the gene will be passed on to the next generation so the mean will move in the direction of these individuals. 

Antibiotic Resistance Example (antibiotics and penicillin) 

• Spontaneous mutation occurs in the allele of gene making a new protein 
• The new protein was enzyme that broke down penicillin ~ penicillinase
• The mutation was passed on down generations as more survived to pass it on
• The population of the antibiotic resistant bacteria became greater than the non- resistant bacteria
• Population's normal distribution curve shifted in the direction of having a greater resistance  to penicillin 

STABILIZING SELECTION

Selected against extreme phenotypes and favours the majority that have well adapted to their environment 

• Environmental conditions remain stable 
• Individuals with phenotypes closest to the mean are favoured 
• They are more likely to pass their alleles onto the next generation 
• Eliminated phenotypes at the extremes

Human Birth Weights Example 

• Small babies - struggle to fight off infection and stay warm (selected against)
• Large babies - are too big to be delivered naturally (selected against)
• Middle babies - survive! (selected for / favoured)
• Survival rates for babies have been higher for those in the middle weight range, around the mean - which helped stabilized the average birth rate 
• The population's characteristics are being preserved rather than changed