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Population Genetics
Chapter 13 – Part 1
Variation
 Variation means differences between individuals
within a species or population.
 A population is considered to be all the organisms of
the same species found within a specific geographic
region at a particular time.
 A group of organisms consisting of similar individuals
capable of exchanging genes or interbreeding and
producing viable offspring.
Types of Variation
There are many different types of variation that can
exist within a population, including:
 Structural
 Biochemical
 Physiological
 Behavioural
 Developmental
 Geographic
Structural Variation
 Structural variation is seen in
many different species.
 For example: the picture on
the right shows four types of
Galapagos finches that Charles
Darwin studied.
 Each of these finches have a
distinct beak shape and come
from different islands. These
are structural variants.
 An example of biochemical
Biochemical
Variation
variation is in the colouring of
fur in cats.
 Colouring is determined by
whether a pigment is produced
or not.
 If a pigment is not produced,
then the cat will be white.
 However if a pigment is
produced then there will be
colour present.
 The production of pigment is by
specific biochemical pathways
involving enzymes.
Physiological Variation
Physiological variations may include any of the following:
 Ability to taste chemicals
 Ability to detect odours
 Distinguishing between colours
 Differences in hormones
Behavioural Variation
 Behavioural variations are differences in the behaviours
expressed by the members of a species.
 For example: there has been research into the
behaviours of various populations of chimpanzees.
Researchers have found distinctly different behaviours
in each of the populations that they studied. This was
especially seen in their nesting behaviours.
Developmental Variation
Developmental variations refer to the changes between
young immature members of a population/species and the
older mature individuals.
Geographic Variation
 Geographic barriers can stop members of a species
from moving in between regions. This isolation can
cause some species to differentiate over time.
 Isolation may be caused by:
 Mountains
 Rivers
 Oceans
 Human development
 E.g.: Chimpanzees and Bonobo Chimpanzees live
in the Congo Basin in Central Africa. Separation
between these species occurred about one million
years ago by the formation of the Congo River.
Monomorphic vs Polymorphic
 Traits within a populations may be
monomorphic or polymorphic.
 Monomorphic refers to traits were there is
no variation in a population. E.g.: the
colouring on galahs.
 Polymorphic refers to traits which have
two or more variants. E.g.: human eye
colour, height, etc.
Discontinuous or Continuous
Variation
 Discontinuous variation refers to traits which are
discrete.
E.g.: a person may be A, B, AB or O blood type
 Continuous variation refers to traits which occur on a
continuum.
E.g.: a person’s height, or their skin colour.
Discontinuous Variation
Blood Type
A
B
Percentage in
Australia
38
10
O
AB
49
3
(Red Cross Australia, 2012)
Continuous
Variation
Continuous
or Discontinuous
Variation
 Discontinuous variation refers to traits which are
discrete.
E.g.: a person may be A, B, AB or O blood type
 Continuous variation refers to traits which occur on a
continuum.
E.g.: a person’s height, or their skin colour.
Causes of Phenotypic Variation
 There are many causes of phenotypic variation.
 Some of these include:
 Genetic factors including mutation
 Environmental factors (including food sources and
predators)
Causes of Phenotypic Variation
Phenotypic Variation = Genetic Variation +
Environmental Variation
VP = V G + V E
Causes of Phenotypic Variation
 Phenotypic variation may be caused by many different
factors, not just genetics.
 The environment can have a large impact on whether
certain traits are displayed or not and to what degree.
The External Environment
 The external environment can have an impact on the
phenotype of individuals in a population.
 E.g.: the flower colour of hydrangeas is determined by the
acidity of the soil they are growing in.
The Internal Environment
 The internal environment of organisms can also cause
variation.
 This may involve hormones which causes distinct
variation seen between the sexes.
Inherited Factors Cause
Variation
 The most obvious causes of
variation are those traits that
are inherited from the parents
of an individual (i.e. genetic).
 These inherited traits
distinguish individuals from
others, as they have a distinct
combination of traits.
Variation Due to One Gene
(Monogenic)
 Monogenic inheritance refers to the case where only
one gene determines a phenotype, usually with two
alleles, one being dominant and the other recessive.
 E.g.: ABO Blood Groups
Variation Due to Polygenes
 Polygenic inheritance refers to characteristics that are
determined by the interaction of genes at several
different loci
 This can be on different chromosomes or at different
places on a single chromosome.
 Polygenic traits usually show continuous variation.
 E.g.: height, eye colour
Mutations: Source of New Genetic
Variation
 Gene mutation allows the
introduction of new alleles
into a population.
 This means that the
population will have
increased genetic variation.
Genes in Populations
Gene Pool
 The entire genetic information
found in a population is called
the gene pool.
Allele Frequencies in a Gene
Pool
 The proportion of alleles in a gene pool is referred to as
the ‘allele frequency’. Allele frequencies tend to stay
the same within a population.
 Over time allele frequencies may change depending on
other factors, including environmental influences,
selection, migration and chance events.
 The allele frequencies are: p = most common allele,
and q = least common allele. The allele frequencies
add up to 1.0
Selection
 Survival of the Fittest
 Individuals with an advantageous phenotype tend to be
more successful than their peers reproductively.
 This means that they contribute more offspring to the
next generation than others.
 When these traits have a genetic basis, selection can
increase the prevalence of those traits, because
offspring will inherit those traits from their parents.
Selection Acts on Phenotypes
 Selection acts on phenotypes.
 This is because the phenotype is what an individual
expresses.
 If the traits they are displaying are advantageous, then
they are selected for.
Genetic Fitness Varies in Different
Environments
 A phenotype which may be advantageous in
one environment may not be as
advantageous in another.
 An example from your text book discusses
individuals that carry the sickle-cell anaemia
allele in malaria prone regions of Africa
compared with North America.