Download Assignment # Natural Selection As A Mechanism For Evolution

Assignment #
Natural Selection As A
Mechanism For Evolution
Mrs. McCarthy
Biology
Wednesday, May 24, 2017
I. Natural Selection- The process by which
populations change in response to their
environment. This causes the frequency of
certain alleles in a population to increase
or decrease over time.
II. Three types of natural selection
A. Directional Selection- natural selection
that favors extreme variations of a trait
1. Example: In giraffes necks those with
longer necks could reach the leaves in the
high trees and thus passed down their
genes
2. Allele frequencies in the gene pool tend
to move in one direction
B. Stabilizing selection- natural selection
that favors average individuals in a
population
1. Example: Small Mice- low frequency
Large Mice- Low Frequency
Medium Mice- High Frequency
2. Allele frequencies in the gene pool tend
to decline on the extremes
C. Disruptive Selection- natural selection that
favors individuals with traits at either extreme
1. Example:
–
–
–
–
Normal crab species are tan to dark brown
Crabs living in sandy areas are tan
Crabs living in muddy areas are dark brown
These two colors of crabs become different species
2. Example- Squirrels are normally brown
– An earthquake changes the climate of an area
– Some squirrels turn white
– Some squirrels turn black
3. Allele frequencies in the gene pool tend to
favor the extremes.
II. Speciation- the process of making new
species
A. Geographic isolation- a species is
separated by a physical barrier
1. A species is divided by some sort of
physical barrier and can no longer
interbreed
2. Over time, the species adapt to each
environment and become two distinct
species
B. Reproductive Isolation- occurs when
formerly interbreeding organisms can no
longer mate
C. Adaptive radiation- the evolution of
species from one common ancestor to fill
a variety of niches
1. Darwin’s finches diverged from a single
seed eating finch into several different
species
D. Hybridization- mating two genetically
similar species to create a new sterile
species
1. Mule- horse and donkey
2. Liger- Lion and Tiger