Download Evolutionary Concepts

Evolutionary Concepts
Types of Selection
Types of Evolution
Rate of Evolution
Natural Selection & Populations
Review:
 Natural selection can affect the
distribution of phenotypes in populations
 If a particular version of a trait makes
individuals better adapted, then that
version of the trait will become more
common in the population over time.
Overview of Selection Types (select animation tab at the top of the box)
Stabilizing Selection

Occurs when
individuals with
the intermediate
phenotype are
better adapted
than individuals
with either
extreme version
of the phenotype
Stabilizing Selection, cont’d

Example: infant
birth weight in
humans tends to be
around 7 lb – birth
weights significantly
higher or lower than
this put baby and /
or mom at danger
For humans, both very small and
very large babies have higher
mortality (or lower survivorship) –
as shown in the graph above (red
line, right “y” axis).
•Some of this data is based on studies conducted during the first
half of the 20th century.
•More recent studies show that this trend toward stabilizing
selection appears to be ending; why might that be?
Directional Selection

Occurs when individuals with one
extreme version of the phenotype are
best adapted to the environment
Directional Selection, cont’d
Example: Average
beak depth and
overall body size in a
population of finches
was larger in the
years following a
drought (1977)
because of the
changes in which
seeds were available
Animation
Directional Selection, cont’d

Ex: When DDT is
sprayed to stop
the spread of
malaria or West
Nile, mosquitoes
with a high
resistance to DDT
are favored – so
the resistance
becomes more
common.
Disruptive Selection

Occurs when individuals with either extreme
version of the phenotype are better adapted
than individuals with the intermediate phenotype
Some scientists
think that this type
of selection can
lead to the
formation of two
distinct species
(speciation)
Disruptive Selection, cont’d

Ex: British land
snails live in field
and forest. Lightbanded shells are
better camouflage
in fields, while dark,
unbanded shells are
better in the forest.
Snails with
“inbetween” shells
are not successful
in either habitat.
Short Animation
Types of Evolution

Convergent Evolution – two or more
species become more similar over time
Divergent Evolution – two or more
species become more different over time
Convergent Evolution
Becoming more similar
 Happens when species face similar
environmental pressures
 The similar structures that result are
considered to be analogous structures;
they serve a similar function in different
organisms with no common ancestry
convergent evolution video clip

The falcon, bat, and pterodactyl at left
have different evolutionary ancestors;
however, all developed similar wings to
meet similar environmental conditions /
pressures.
Below, the marsupial mammals on
the bottom row – found only in
Australia – closely resemble placental
mammals found on other continents.
Divergent Evolution



Species that used to be similar become
increasingly different
Happens when species (or populations)
face different environmental pressures
Can stem from adaptive radiation –
when one or a few populations / species
diversify to fill different ecological roles
Rate of Evolution

Two
conflicting
hypotheses:
• Gradualism
• Punctuated
Equilibrium
Gradualism

evolution occurs continuously at a slow,
steady rate
Punctuated Equilibrium

there are long stretches of time with little
or no evolutionary change, interrupted by
(relatively) brief periods of significant /
drastic change