Download Types of Natural Selection

Types of Natural Selection
Natural Selection and Genetic
Equilibrium
• Synthetic Theory of Evolution: Darwin’s ideas
about natural selection plus our modern
understanding of genetics.
• According to the Synthetic Theory of Evolution,
natural selection disturbs genetic equilibrium.
• As a result, allele frequencies will change.
• In this way, natural selection determines which
adaptations are beneficial to a species.
• There are three main types of natural selection
Directional Selection
• Directional Selection: An extreme phenotype
(very long or very short etc.) is a favorable
adaptation.
• Ex. Neck length in giraffes, very long necks are
favorable.
• Selects in the direction of one extreme phenotype
Stabilizing Selection
• Stabilizing Selection: Average phenotype is a favorable
adaptation. Extreme phenotypes are unfavorable.
• Operates most of the time in populations
• Ex. Mouse size: too big use too much energy to keep warm;
too small can’t burrow. Average is just right
• Limits evolution by keeping allele frequencies relatively
constant
Disruptive Selection
• Disruptive Selection: Most rare type of natural selection. Two
opposite extreme phenotypes are favorable over average
phenotype
• Ex. Some crabs show a continuous range of color from light tan
to dark brown. The light tan provides camouflage in the sand.
Dark brown provide camouflage in mud. Colors in between –
no camouflage
• Over time this could lead to speciation
Summary