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
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