NATURAL SELECTION AT THE LEVEL OF THE ALLELE

Biston betulariaSickle cell anemia

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

GeneticHybrid inviability Hybrid offspring die Hybrid infertility

Hybrids survive but are incapable of producing gametes

Liger

Zedonk

© 2008 Paul Billiet ODWS