Download Speciation - Deans Community High School

Chapter 18
Speciation
Gene Pool
The total of all the different genes in a population is known as the gene pool.
The gene frequency is the frequency of occurrence of an allele of a gene in a
population (relative to all the other alleles at the same locus).
If a population is large (and mating is random) then gene frequencies usually remain
constant from one generation to the next. This stability is known as genetic
equilibrium.
The process of evolution depends on changes occurring in the gene pool
(otherwise things continue as before)
Alteration to the gene pool results from the following processes. Make notes
using p 130 Torrance.

Mutation

Natural selection

Gene migration

Non-random mating

Genetic drift

Chance
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Chapter 18
Speciation
Organisms within a population may become isolated in different groups which no
longer share a common gene pool. They can no longer exchange genes because
some barrier prevents them interbreeding. This barrier may be

Geographical

Ecological

Reproductive
Use p131-132 Torrance ‘New Higher Biology’ to give examples of each of the above
isolating mechanisms. (barriers to reproduction).
Ensure you understand and remember the steps which lead to speciation (Fig 18.1)
and realise that separate species result when natural selection affects each subgroup in a different way so that over a long period of time the groups become so
genetically distinct that even if they are put together they can no longer interbreed
and produce fertile offspring. They have evolved to become separate species.
p132 Define endemic
Read through examples on p 132 – 133
Sorbus & European wren
P133 Make a note on the Interruption of gene flow
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