Download Types of Selection

Types of Selection
And their results.
Types of Selective Pressure

Directional Selection

natural selection favoring one of the extreme
variations of a trait.

Long bills in woodpeckers for finding food.
Normal
variation
Selecti
on for
longer
beaks
Types of Selective Pressure

Stabilizing Selection

natural selection that favors average individuals in a
population.


Small spiders can’t catch big insects
Big spiders get eaten by birds
Selection for
average size
spiders
Normal
variation
Types of Selective Pressure

Disruptive Selection

Individuals with either extreme of a trait’s variation are selected
for.


Dark limpets on wet, dark colored rocks
Light limpets on dry, light colored rocks
Selection for
light limpets
Normal
variation
Selection for
dark limpets
Types of Evolution

Directional


Over time a species adapts to its environment due to directional
selective pressures
Bird’s bones have become lighter and longer over time to aid in
flight and some insects look more and more like plants for
disguise.
Industrial Melanism
Industrial
revolution = smoke
from coal.
 Peppered Moth
used light colored
lichen as
camouflage.
 What happened?

Industrial Melanism

Directional
selection by
predation.
Types of Evolution

Convergent

Two species evolve similar adaptations even though they do
not share a recent common ancestor.




Sharks and dolphins have many similar traits but are NOT closely
related.
Dolphin ancestors did not have hydrodynamic shapes and shark
like pectoral fins, they had legs and bodies built for walking
Dolphin ancestors that looked more like sharks were more
successful in the ocean
Dolphins evolved some shark like features
Results of Evolution - Convergence

Analogous Structures

Convergent evolution causes
organisms to develop structures
with similar appearances to
meet the same need.

For example, insect and bird wings
probably evolved separately when
their different ancestors adapted
independently to similar ways of
life.
Results of Evolution - Convergence

Mimicry - A structural adaptation that enables one
species to resemble another species.


In one form of mimicry, a harmless species has
adaptations that result in a physical resemblance to a
harmful species.
Predators that avoid the harmful looking species also
avoid the similar-looking harmless species.
Toxic
Monarch
Butterfly
Harmless
Viceroy
Butterfly
Results of Evolution - Divergence

Divergent


A population of species is
separated by a
geographical barrier
On each side of the barrier
different selective
pressures cause the
species to evolve in
different directions.

Directional in different
directions or different
random changes.
Results of Evolution - Divergence

Adaptive Radiation

Species diverge to
fill unoccupied
niches in a new
environment.

Disruptive
selection or small
geographic areas
with different
directional
selection.
Honeycreepers of Hawaii
Extinct
mamo
Amakihi
Possible
Ancestral
Lasan finch
Crested
honeycreeper
Kauai
Niihau
Molokai
Oahu
Maui
Lanai
Akialoa
Kahoolawe
Akepa
Akiapolaau
Akikiki
Liwi
Hawaii
Apapane
Maui
parrotbill
Palila
Ou
Grosbeak
finch
Results of Evolution - Divergence

Endemics

Each island population undergoes different selective and
random evolution.
Results of Evolution - Divergence

Homologous Structures

Divergent evolution causes similar structures to
evolve to meet different needs
Crocodile
forelimb
Whale
forelimb
Bird
wing