Download reproductively separated

Isolation and speciation
Key words:
Natural selection and evolution
Gene pool
Explain how natural selection can lead to a change in allele
frequencies (evolution) within a gene pool.
Allelic
frequency
• Variation (due to mutation, meiosis + random fertilisation of gametes)
• Advantageous allele
• Individuals survive and reproduce
• Advantageous allele passed on
• Repeats over generations
• Allele becomes more common in the gene pool= change in allele
frequency
Types of selection and conditions they occur?
• Stabilising- eliminates extremes of the phenotype range; constant
environmental conditions
• Directional- favours one extreme of the phenotype; environmental
change
• Disruptive- favours both extremes of the phenotype; an
environmental factor takes 2 or more distinct forms (e.g.
temperature)- most important in bringing about evolutionary change.
Environmental factors affect the probability of a gene being passed on and its frequency
within the gene pool
Speciation
The evolution of a new species from
existing ones.
Definition of species?
• similar characteristics
• breed
• fertile offspring
Members of a species are
reproductively separated from other
species.
How new species are formed:
Bullet point:
• Within a species are a number of
populations
• A population becomes separated
from the others
• Undergoes different mutations
• Different environments
• Different selection pressures
• Change in allele frequency
• Populations unable to interbreed
Diagram:
2 forms of speciation
Allopatric
2 populations geographically
separated
Physical barrier- ocean, river,
mountain range, desert
If environmental conditions vary
Different selection pressures
Change in allele frequencies
New species formed
Rich questions:
- Explain what happens to cause
speciation.
- How do the mechanisms of
reproductive isolation differ in
allopatric and sympatric speciation?
Sympatric
A population in the same area
becomes reproductively
separated.
e.g. apple maggot fly
http://media.hhmi.org/biointeractive/fil
ms/OriginSpecies-Lizards.html
Genetic drift
• Small populations
• Few alleles
• Those passed on increase in
frequency in gene pool
• Population changes rapidly
• Genetic drift greater
• More likely that speciation will
occur
http://nortonbooks.com/college/biology/ani
mations/ch16a01.htm
• Large populations
- How is genetic drift fundamentally
different to natural selection?
- Why does genetic drift only have
noticeable effects in small populations?