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Vanessa Couldridge
Richard Knight
Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/
http://en.wikipedia.org/wiki/Image:Darwin%27s_finches.jpeg
Evolution: Selection and
Speciation
Forms of Selection
Different selective pressures will cause a
population to respond in different ways

The values of the trait under selection will
change according to the selective pressure

There are three different forms of selection:

Stabilizing selection

Directional selection

Disruptive selection
Frequency

Trait value
Stabilizing Selection
Mean values of the trait are favoured

It is the most common form of selection

Example: In seabirds, individuals with an
average body
size are less
likely to die in
a storm than
those with
large or small
body sizes
Frequency

Trait value
Directional Selection
One extreme of a trait is favoured over the
other extreme

Leads to a shift in the mean trait value

Example: The long neck of the giraffe evolved
as giraffes with
longer necks
were better
able to find
food
Frequency

Trait value
Disruptive Selection
Both extremes of a trait are favoured

Leads to a splitting of the population over time

Can produce polymorphic forms

Example: The African finch Pyrenestes ostrinus
has
a
bimodal
distribution
of bill size
Frequency

Trait value
Speciation

Speciation is the process whereby a new
species arises from an ancestral species

It requires reproductive isolation

There are three major types of speciation,
depending on the degree of
geographical separation:
Speciation

Allopatric speciation

Parapatric speciation

Sympatric speciation
event
Allopatric Speciation

A new species is formed in complete geographic isolation from the
ancestral species

Some extrinsic barrier splits a species into two populations and
prevents the two groups from mixing and interbreeding with each
other

The populations continue to evolve and diverge from one another
in isolation

Eventually they diverge to such an extent that if they were to reencounter one another, they would no longer be able to interbreed
Original population
Physical barrier
New species
Allopatric Speciation: Example

Formation of the Grand Canyon caused an
ancestral population of squirrels to split into
two groups that could no longer interbreed
Kaibab Squirrel
Abert Squirrel
Parapatric Speciation

Speciation with incomplete geographic isolation

Individuals are more likely to mate with their nearest
neighbours than with individuals at the opposite end of
the geographic range of the population

Often results in a cline – gradual change in adjacent
populations over the species’ range

Varying selective pressures over the geographic range
can lead to speciation
Original population
New niche entered
New species

Pollution from mines has
led to contamination of the
soil in part of the range of
a species of grass

Plants growing on
contaminated soil have
evolved a tolerance for
heavy metals and also
flower at a different time to
plants growing on
uncontaminated soil
http://en.wikipedia.org/wiki/Image:AnthoxanthumOdoratum.jpg
Parapatric Speciation: Example
Ring species

Occur when the geographical distribution of a
species is circular in shape with clinal variation

Each population is slightly different to its
neighbouring populations, but can still
interbreed with them

When the “ring” closes,
the two populations at
each end of the
distribution are
sufficiently different that
they cannot interbreed
Ring species: Example

Adjacent populations can
interbreed, except for the
herring gull and the lesser
black-backed gull
http://en.wikipedia.org/wiki/Image:Rings_species_example.png
Larus gulls have a circular distribution around North Pole
http://en.wikipedia.org/wiki/Image:PT05_ubt.jpeg

Sympatric Speciation

Speciation in the complete absence of any geographical
barriers to gene flow

A new species arises from within the same geographical
area of the ancestral species

May occur if there are two or more morphs in the
population and assortative mating leads to reproductive
isolation between the morphs, eventually resulting in
speciation
Original population
Genetic polymorphism
New species

Apple maggot flies in the same geographic area
feed and mate on either hawthorn (native host)
or apple (introduced host)

Males and
females mate,
and females lay
their eggs, on the
host they grew
up on

The two races do
not interbreed
http://www.msu.edu/course/lbs/144/f04/graphics/2_flies.jpg
Sympatric Speciation: Example
Adaptive Radiation
Example:
Darwin’s
finches of the
Galapagos
Islands
Cactus
finch
Woodpecker
finch
Sharp
beaked
ground
finch
Cactus
eater
Seed
eater
Small tree
finch
Insect
eater
Ground finches

Occupy
different
niches
Warbler
finch
Crushing bill

Rapid
evolution of
many new
species from
a single
ancestor
Tree finches

Fruit
eater
Medium
ground
finch
Large tree
tree
Vegetarian
tree finch
Small
ground
finch
Large
Ground
finch

Hybridization can lead to instant speciation

Polyploid hybrids are unable to mate with
diploid parental types

Example:
Tetraploid
hybrids of two
species of gila
monster are
reproductively
isolated from the
parent species
http://inst.sfcc.edu/~zoo/images/Gila%20Monster.jpg
Hybridization (Polyploidy)

Evolution is a gradual process

Species continue to adapt at a
uniform rate throughout time

There is no clear demarcation
of the point where one species
becomes another species
http://www.don-lindsay-archive.org/creation/snails_big.gif
Time
Phyletic gradualism
Character

Long periods of stasis
interrupted by rapid bursts
of change tied to speciation
events

Most adaptation occurs
immediately after speciation

Time
Punctuated Equilibrium
Character
Example: A study of fossil Pliocene snails at
Lake Turkana showed that species showed no
change for long periods followed by bursts of
rapid change associated with fluctuating lake
levels

Prevent different species from interbreeding

Prezygotic or premating isolating mechanisms:

Ecological isolation

Temporal isolation

Physiological isolation

Mechanical isolation

Behavioural isolation
(e.g. male calls attract
females of the same species in frogs)
http://en.wikipedia.org/wiki/Image:Litoria_chloris_calling.jpg
Isolating Mechanisms
Isolating Mechanisms
Postzygotic or postmating isolating
mechanisms:

Gamete mortality

Zygote mortality


Hybrid unfitness
or breakdown
Hybrid infertility
or sterility
(e.g. mule)
http://en.wikipedia.org/wiki/Image:Bg06.jpg
