Download Tax and Evol 6 Speciation

L6 Types of Natural Selection:
Stabilizing
Organisms that represent extreme departures from
the “normal” are removed from the population.
(Usually by miscarriage)
1. 
Tends to make the population more alike, elimination
“bad” genes from the pool.
L6 Stabilizing
Eg.
Cystic fibrosis
Small pelvis
Baby birth weight
Downs syndrome
L6 Natural Selection types
2. Disruptive: The opposite of stabilizing: When
extreme variations increase at the expense of
“normal” . Usually occurs when a population has been
separated.
L6 Disruptive (leads to speciation)
Eg.
River separates a population
Predation pressure favours
extremes not the norm
L6 Eg. butterflies
Do Not Copy!
L6 River Disruption
L6 Directional Selection
3.  Directional Natural selection: This is when an
extreme trait is selected for again and again.
Driving the trait in one direction. Eg
u  Black bears in Europe
u  Shrank in size during interglacial (warm) periods
u  Grew in size during ice ages
L6 Directional Image………..
L6 Directional ex. 2
L6 Disruptive selection causes….
There are several different reasons why a
population may become split and new species are
formed.
¨  This process is called speciation or the creation of a
new species.
¨ 
L6 Reasons for disruption
¨ 
Reproductive Isolation is the reason for new
species to form. This isolation can come in many
forms
¤  Premating
Factors. ie factors that prevent mating
n  Geographical
isolation
n  Habitat Preference
n  Behavioural Isolation
n  Different mating seasons
n  Size or structure problems
L6 Geographical Isolation
¨ 
A population of squirrels are split by a huge
canyon. Eventually a new species may arise.
L6 Habitat preference
¨ 
If a variation within a species allows them to live in
a slightly different location then mating with the
original group will stop!
L6 Behavioral differences
¨ 
Changes in behaviour patterns can turn a mate off
and prevent mating from occuring. Many animals
have elaborate mating rituals.
Video: Bird of
paradise beginning
of jungles in planet
earth
L6 Mating Seasons
¨ 
If a population of swans flies north 2 weeks early
then mating with the main group of swans is no
longer possible.
L6 Size and structure
¨ 
Chihuahua’s and St. Bernards are the same species
yet they cannot mate with each other for obvious
size reasons.
L6 Post mating factors
¨ 
Sometimes mating can occur but problems arrive
later. Thus mating occurs but not offspring.
¤  Eg.
Sterile offspring. Horses and donkeys=mule
¤  Miscarriges preventing success of pregnancy
Lion
Tiger
L6 Sympatric and Allopatric Speciation
Sympatric speciation is when new species arise even
though they live in the same geographic area.
¨  Allopatric speciation is when species form because
populations are geographically separated
¨ 
L6 Summary
¨ 
New species can be produced due to a variety of
factors affecting a starting population. Some of
these factors prevent mating (pre mating factors)
while others occur after mating has happened (post
mating factors)
?
Do Not Copy!
L6 Assignment
Do Evolution Review Sheet
¨  Watch Video Evidence for evolution BBC I think
¨  Do Darwin’s Finches Case Study (use clickers)
¨