NATURAL SELECTION AT THE
LEVEL OF THE ALLELE
Sickle cell anemia
Biston betularia
Industrial melanism
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The melanic allele has a selective
advantage in polluted environments
More individuals with the melanic allele
are reproduce
The next generation will have a higher
proportion of the melanic allele
compared to the other (speckled)
In successive generations it will become
predominant until it is said to be fixed in
the population
© 2008 Paul Billiet ODWS
Recessive alleles can hang on
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The melanic allele is dominant (M)
If it is present it will be expressed and
selection acts on it immediately
The speckled allele is recessive (m) so it
will be carried by heterozygotes (Mm)
It may remain “hidden” in the population
form many generations
Thus the melanic allele may not become
completely fixed
© 2008 Paul Billiet ODWS
Balanced and transient
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Industrial melanism is an example of transient
polymorphism where one allele replaces another
Sickle cell anaemia is an example of balanced
polymorphism
Two alleles are advantaged in zones infested by
malaria
The sickle allele gives protection to malaria but
can lead to a fatal blood disease
The normal haemoglobin allele permits normal
transport of oxygen but gives no protection to
malaria
© 2008 Paul Billiet ODWS
From one species to another
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An accumulation of many advantageous
alleles
Eventually individuals with different alleles
can no longer breed together
Separate species are formed that are
genetically incompatible
Many generations are needed
Therefore, long periods of time are
needed
© 2008 Paul Billiet ODWS
But what is a species?
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A group of individuals that breed together freely
in nature to produce fully fertile offspring
Does forced mating count?
If populations are geographically separated it is
not possible to test this definition
If populations are separated in time it is not
possible to test this
e.g. fossils in different strata
Some species only show asexual reproduction
A potentially interbreeding population having
a common gene pool
© 2008 Paul Billiet ODWS
Mechanisms of speciation
Isolation of a population
so that it cannot breed
freely with others is
necessary
 Geographic
 Ecological
 Behavioural
 Mechanical/anatomical
 Physiological
 Genetic
Madagascar Google earth
Ringtailed lemurs
(Lemur catta)
© 2008 Paul Billiet ODWS
Geographic
Fragmentation of the range
 Changes in climate
isolate populations on mountain tops
cause a rise in sea level creating islands
 Geological changes which raise mountains or
create new seaways
Migration
 Migration of a population to a new area
 If the population is small it may not represent the
gene pool of the parent population left behind
(the founder effect)
© 2008 Paul Billiet ODWS
Ecological
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Populations can become
isolated within the range of
the parent population
Differences in food
preferences may develop
in a part of the population
that stop them from
breeding freely
Seasonal isolation may
occur
e.g. different flowering
times or breeding seasons
© 2008 Paul Billiet ODWS
SawFly (Tenthredo livida)
Behavioural
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Parts of a population
may develop a
preference for a
particular variety
They may not mate with
any other
e.g. The snow goose
blue forms tend to mate
with blue forms and
white forms tend to
mate with white forms
© 2008 Paul Billiet ODWS
Snow geese (Chen caerulescens)
Mechanical/anatomical
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Genetalia or floral parts
may be incompatible
Pin and thrumb primroses (Primula vulgaris)
© 2008 Paul Billiet ODWS
Physiological
Fertilisation may be prevented
by:
 Failure of the gametes to be
attracted to one another
 The sperm cell receptors of
the oocyte may be
incompatible with the
acrosome
 Pollen tubes cannot find or
penetrate the embryo sac in
flowers
© 2008 Paul Billiet ODWS
Fertilisation
Genetic
Hybrid inviability
 Hybrid offspring die
 Hybrid infertility
Hybrids survive but are
incapable of producing
gametes
Zedonk
Liger
© 2008 Paul Billiet ODWS