Download Speciation

Theory of Evolution
• A change in the allele
frequencies of a population
from one generation to the
next
Gene Pool
• All the different alleles in the gene
pool of a population at a given time.
Processes of evolution
• There are several processes by
which the allele frequencies can be
changed.
1. Natural Selection
A process resulting in the survival of
those individuals in a population that
have the genotypes that make them
better adapted to the environment.
These individuals are able to reproduce
and pass these features on to the next
generation. Those less well adapted
tend to be eliminated.
• Understanding Evolution - Inheritance
and Change - Natural selection in action
- Understanding Evolution.wmv
Darwin’s theory of natural
selection
OVERPRODUCTION
• Populations tend to produce too many
offspring for the environment to support
which means that some will die.
GENETIC VARIATION
• The current conditions of the
environment tend to favour particular
genetic variations within the population.
NATURAL SELECTION
• These individuals have a greater ability
to survive and/or be healthy enough to
reproduce
INHERITANCE
•
Those that do reproduce pass their
“successful” genes on to the next
generation .i.e. their genetic variations
are inherited.
REPRODUCTIVE SUCCESS
• The best suited leave the most
offspring. Each new generation will
contain more offspring with the better
suited traits, and less with the
unfavourable traits.
Modes of Action
• Natural selection has three modes of action:
1. Stabilizing selection
2. Directional selection
3. Disruptive selection
Number
of
Individuals
Small
Large
Size of individuals
a. Stabilizing Selection
• Acts against extremes and favours the
intermediate.
Number
of
Individuals
Small
Large
Size of individuals
b. Directional Selection
• Favours variants of one extreme.
Number
of
Individuals
Small
Large
Size of individuals
c. Disruptive Selection
• Favours variants of opposite extremes.
Number
of
Individuals
Small
Large
Size of individuals
Genetic drift:
The changes in allele frequencies
that occur in populations as a result of
RANDOM changes such as infertility or
sudden death of a breeding individual.
These changes in allele frequencies are
significant (show up) in SMALL
POPULATIONS, but are not so
significant in large populations.
Founder Effect
• The result of starting a new population
with a low number of individuals
(founders) so that their gene pool may
not contain the same proportion of
alleles as the parent population.
Gene Flow
The gain or loss of alleles from a
population by the movement of individuals or
gametes.
• Immigration increases genetic diversity
• Emigration decreases genetic diversity
Mutation:
Change in an organism’s DNA that
creates a new allele.
This is the only way new information can
enter a population.
This will increase genetic diversity.
Speciation
• The formation of a new species.
Species
• A group of actually, or potentially,
interbreeding organisms that are
reproductively isolated from other
groups. They produce fertile
offspring.
• Question 1 What is a species.docx
Allopatric Speciation
• Occurs when a group becomes isolated from
the ancestral population by a physical barrier
(geographical isolation.)
• The allele frequency of this group may not
represent the original and may be acted
upon by different selection pressures so
that, over time, they become
reproductively isolated so a new species
is formed.
Stage 1
Parent population moves into a new
environment
• New niches may open up or groups
may wander away from each other due
to competition for resources
Stage 2
Geographical isolation of parts of the
population
May be unable to return to main
population due to the formation of
physical barriers due to climate change or
natural disasters.
Stage 3
Different selection pressures
Will favour some phenotypes over others
leading to better reproductive success in
these individuals (natural selection). This
could lead to the formation of a subspecies.
Stage 4
Reproductive isolation
Over time, isolated subspecies may
undergo further genetic and behavioural
changes that ensure the populations
remain reproductively isolated. Thus a
new species is formed.
Even if they are reunited, they will not be able to
interbreed (an example of allopatric leading to
sympatric speciation)
Examples of allopatric speciation
• Kaka
Nestor meridionalis
• Kea
Nestor notabilis
• Kakapo
Stigops habroptilus
Ancestors of this group became
isolated from the remaining parrots
when New Zealand broke away from
Gondwana about 82 mya, resulting in a
physical separation of the two groups.
This is led to allopatric speciation.
(See later notes for speciation that occurred later in their
development)
Break up of Pangea
• http://highered.mcgrawhill.com/sites/0072402466/student_view
0/chapter19/animations_and_movies.ht
ml#
Sympatric Speciation
• Speciation that takes place in
geographically overlapping populations
with no physical barriers separating
gene pools.
There are two types:
• Niche isolation
• Instant speciation by polyploidy
Niche Isolation.
•
•
This happens in environments with
“micro-habitats”. Organisms living in
micro-habitats within an area rarely
come in contact with each other even
though they are not physically
separated.
Can also occur through organisms
becoming excluded from the main
population because they are unable to
compete so they establish themselves
in a different micro-habitat.
• Lack of gene flow between these
two populations and the build up of
genetic variation over time can
result in reproductive isolation.
http://evolution.berkeley.edu/e
vosite/evo101/VC1eSympatric
.shtml
Bird lice
• Pigeons are able to groom their back
feathers, but cannot reach to groom
their heads. Some lice phenotypes
were better suited to survive in the back
feathers because they were faster
movers and flatter. They had long legs
enabling them to hold onto the feather
barbs. These features helped them to
escape grooming and so were selected
for in this area. Fatter, slow moving lice
could only survive on the head.
b. Polyploidy
(Polyploids have more than 2 sets of
chromosomes)
Autopolyploidy occurs through nondisjunction in all chromosomes during
meiosis and gives instant reproductive
isolation from the parent (Instant
speciation). Any offspring , if formed at
all, will be sterile.
Because in polyploidy the sex determining
mechanism is disturbed animals rarely
achieve new species status this way.
Plants, however, can reproduce
vegetatively and some can self- fertilise,
so can achieve a breeding population.
– In allopolyploidy the offspring, called
“hybrids”, are usually infertile because
they have different chromosome
numbers and types that do not match in
meiosis.
– However, if the hybrids undergo nondisjunction during meiosis, this restores
chromosome numbers that do match
and so the species can become fertile
again (called amphiploidy).This can
cause sympatric speciation.
Allopolyploidy
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Species A
2n = 4
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
Figure 24.9
Normal gamete
n=3
• Question Recent Speciation in
Salsify.docx
• Question Leucogene Phylogeny.docx
Speciation animationsummary
• http://bcs.whfreeman.com/thelifewire/co
ntent/chp24/2402001.html
CLINES
• A continuous gradation in a phenotypic
characteristic within a species,
associated with a change in an
environmental variable across a region
e.g. altitude or temperature
• In some clines the difference in
appearance at the extremes of the cline
prevents mating and so a new species
can form.
Tomtits
• North Is.
South Is.
Ring species
• A special type of cline that has a circular
or looped geographical distribution
resulting in the two ends of the cline
overlapping. Adjacent populations can
interbreed but not the two ends of the
ring.
• Question 1 What is a species.docx