Today’s Objective:
• Identify how natural selection can create new species.
Can be found in the book:
Pg. 404 - 413
NATURAL SELECTION
• There are three different types of natural selection:
stabilizing, directional, and disruptive.
• Stabilizing selection is a natural selection that
favors average individuals in a population.
Evolution will
not occur
Middle sized Siberian Huskies are selected for
Stabilizing
Selection
• Example: human birth weight. Babies of low weight lose heat
more quickly and get ill from infectious disease more easily,
whereas babies of large body weight are more difficult to deliver
through the pelvis
NATURAL SELECTION
• Directional selection occurs when natural selection
favors one of the extreme variations of a trait.
• This type of selection can lead to rapid evolution of
a population.
Examples: Directional Selection
• Peppered Moths: as the environment changes,
so do the traits that are fit for the new
environment.
• In the case of the moths, the forests changed
from light to dark and selection moved in the
direction of darker moths
• Antibiotic Resistance
• Pesticide Resistance
NATURAL SELECTION
• In disruptive selection, individuals with either
extreme of a trait’s variation are selected for.
• This results in eventually having no intermediate
form of a trait, and leading to two separate species.
What type of selection?
• Tortoise neck length
– Short grasses, for short-necked tortoises
– Tall grasses, for long-necked tortoises
– No grasses for average-necked tortoises, so
over time, they are selected against
Disruptive Selection
What type of selection?
• Lizard body size:
– Large lizards are easily seen by predators, but
smaller lizards cannot run as fast to escape the
predators
– Mid sized lizards are most fit in the environment, so
they survive and reproduce more often, changing the
allele frequencies in the population
What type of selection?
• Anteater tongue length:
– Anteaters with long tongues are most fit
because of the depth of the nests of the
termites they eat.
So…what is a species?
– A population whose members can interbreed &
produce viable, fertile offspring
– Being reproductively compatible is a key
component
Distinct species:
songs & behaviors are different
enough to prevent interbreeding
Eastern Meadowlark Western Meadowlark
Sturnella neglecta
Sturnella magna
SPECIATION
• The evolution of new
species, a process called
speciation.
• This occurs when members of
similar populations change so
much from each other that they
no longer interbreed to produce
fertile offspring.
SPECIATION
• In nature, physical barriers
can break large populations
into smaller ones.
• Geographic isolation occurs
whenever a physical barrier
divides a population and over
time they change and become two
different species.
SPECIATION
SPECIATION
SPECIATION
EVIDENCE
• Most of the evidence for evolution is indirect, coming
from sources such as fossils and studies of anatomy,
embryology, and biochemistry.
BIOCHEMISTRY
Scientists believe that the fact that ALL LIVING
THINGS have A,T,C, and G in their DNA and all use
the same coding for proteins means we are all
related in some way.
One form of evidence in the unity of life…..
Anatomical evidence
Science sees structural similarities as evidence that
organisms evolved from a common ancestor.
Crocodile
forelimb
Bird
wing
Whale
forelimb
Homologous
parts are similar
in structure, but
may be very
different in
specific function.
Structural features with a common evolutionary origin
are called homologous structures.
The body parts of organisms that do not have a common
evolutionary origin but are similar in function are called
analogous structures.
Analogous parts
are very different in
structure, but
perform similar
functions.
ANATOMY
• Vestigial structure—a body structure in a present-day
organism that no longer serves its original purpose, but
was probably useful to an ancestor.
Video clip
EMBRYOLOGY
• An embryo is the earliest stage of growth and
development of both plants and animals.
• Embryos of different species have similar
pharyngeal pouches and tails.
EMBRYOLOGY
• Scientists believe the shared features in the
young embryos of different species suggest
evolution from a distant, common ancestor.
Pharyngeal
pouches
Pharyngeal
pouches
Tail
Fish
Tail
Reptile
Bird
Mammal
Chicken
Rat
Turtle