natural selection at the level of the allele biston betularia sickle cell anemia
TRANSCRIPT
Industrial melanism
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
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
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
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?
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 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 occure.g. different flowering times or breeding seasons
SawFly (Tenthredo livida)
© 2008 Paul Billiet ODWS
Behavioural
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
Snow geese (Chen caerulescens)
© 2008 Paul Billiet ODWS
Mechanical/anatomical
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
Fertilisation
© 2008 Paul Billiet ODWS