Natural Selection and

Speciation

Stabilizing Selection• favors the average individual• reduces variation in a population

Examples• birth weight in humans• Siberian husky- muscle mass • “living fossils”- organisms that

have remained unchanged for millions of years (sharks, crocs)

Birth Weight

Directional Selection• favors one of the extreme variations• leads to rapid evolution of a population

ExamplesPEPPERED MOTHS PESTICIDE/ANTIBIOTIC RESISTANCE

Disruptive Selection• favors both extreme variations

Examples• Darwin’s finches

–small beaked birds could eat small seeds

–large beaked birds could eat large seeds

–medium beaked birds could not eat either seed efficiently

                

         

                

         

                

         

STABILIZING DIRECTIONAL DISRUPTIVE

Divergent Evolution• occurs when closely related

species evolve to look differently• populations change to adapt to

different environments• adaptive radiation- one species

evolves into several different species

Darwin’s Finches

Hawaiian Island Honeycreepers

Convergent Evolution• occurs when species of

different ancestry evolve to look alike–occurs b/c species occupy similar environments

• analogous traits

Examples

• whales and fish

• birds and butterflies

• marsupial and placental mammals

Speciation

• evolutionary formation of a new species