8.6
Selection
Selection and speciation
Aims
You have already studied various types of speciation and selection but this worksheet will
introduce you to another type of selection and several types of speciation. This should allow
you to broaden your knowledge of the processes that contribute to the development of new
species.
Types of selection
New species are formed over time from existing species. Organisms become isolated due to
various mechanisms and differential selection pressures mean that allele frequencies within
those populations change until eventually a new species is formed. Selection pressures do not
always cause new species. Sometimes the distribution of phenotypes in a population changes.
You should have learnt already about directional selection and stabilising selection. Another
type of selection, disruptive selection, is outlined below.
Disruptive selection is a form of selection that will favour two different extremes of a
characteristic. This normally means that those with traits that lie between these extremes are
not selected and become less common. This creates a situation called balanced
polymorphism, when two distinct forms are present in the population. Disruptive selection
normally occurs in an environment that is constantly changing or fluctuating.
Two main types of isolation occur during speciation: geographical isolation and reproductive
isolation. The mechanisms that contribute to reproductive isolation prevent gene flow
between populations that may or may not be geographically isolated. They are the reason that
closely related species that live in the same habitat may not reproduce successfully. They also
often appear after two species have become geographically isolated and this is the reason
that, even when organisms from the two separated populations are reunited, they fail to
reproduce. Summaries of reproductive isolating mechanisms can be seen in the table below.
Mechanism
Summary
temporal
Different species may have different breeding seasons or may carry out
mating activity at different times of the day or year.
hybrid sterility
Although offspring are produced (hybrids), these are sterile and cannot
produce further offspring.
ecological
Organisms may occupy different habitats within the same location.
gamete
mortality
This is when the sperm and the egg do not meet due to either the sperm
failing to reach the egg or being able to penetrate the egg.
hybrid
inviability
When mating occurs, fertilisation takes place but the embryo does
not develop.
behavioural
Different organisms may have different mating rituals such as dances or
displays. This are normally very species specific.
structural
This usually occurs when the sexual organs of the organisms are
not compatible.
AQA Biology A2 Stretch and challenge © Nelson Thornes Ltd 2009
1
8.6
Selection
Questions
1 What is the definition of a species?
2 Give an example of a selection pressure that could cause directional selection.
3 A fluctuating environment can cause disruptive selection and a stable environment causes
stabilising selection. What type of environment is likely to cause directional selection?
4 Summarise two types of reproductive isolation.
5 Draw the shape of graph that you might expect to see after disruptive selection has occurred.
6 Two of the reproductive isolation mechanisms in the table occur after mating and fertilisation has
occurred (post-zygotic). Which two mechanisms are post-zygotic? Explain your answer.
7 Why are post-zygotic isolation mechanisms inefficient for the organisms involved?
8 Why are post-zygotic isolation mechanisms more of a problem for rare organisms?
AQA Biology A2 Stretch and challenge © Nelson Thornes Ltd 2009
2