Variation and selection
▪ Variations are differences in characteristics shown by organisms
belonging to the same natural population or species.
▪ Two types of variations: continuous and discontinuous.
Discontinuous Variations (qualitative variations)
▪ Traits that exhibit a limited form of variation.
▪ Variations that have clear-cut differences with no intermediates
between them.
▪ Examples: tongue-rolling, sex (gender), blood groups in humans, ear
lobe adherence, diastema, widow’s peck.
▪ Controlled by one or two genes. Little or no environmental influence.
Depicted using pie charts, histograms.
Continuous variations (Quantitative inheritance)
▪ Traits that show many intermediates between two extremes.
▪ Examples: height, body weight (body mass), skin colour, shape of
leaves, flower shape, intelligence.
▪ Produced by a combined effect of polygenes and environmental
factors such as diet, health, temperature, water.
▪ Represented using bell-shaped curves (normal distributions/gaussian
curves).
Reading assignment: Discuss how various factors influence continuous
variation traits.
Selection
▪ Process by which those organisms which appear physically,
physiologically and behaviourally better adapted to the environment
survive and reproduce; while those organisms not so well adapted
either fail to reproduce or die.
▪ Divided into artificial and natural selection
▪ Selection depends upon the existence of phenotypic variation within
the population and it is part of the mechanism by which species adapts
to its environment.
▪ Increase in population leads to limitation in environmental factors
such as food, light, water.
▪ Organisms exhibiting traits that give them competitive advantage will
obtain the resources, survive and reproduce.
▪ Other organisms may die before reproducing.
▪ Both the environment and population size may operate together to
produce a selection pressure.
▪ Selection pressure can increase or reduce the spread of an allele within
a population.
Stabilising selection
▪ It occurs when phenotypic features coincide with optimal
environmental conditions (e.g. an organism is camouflaged) and
competition is not severe.
▪ It eliminates extremes in all populations.
▪ For example, there is an optimal height among living organisms,
stabilising selection operating through breeding potential will eliminate
people with smaller or longer heights.
Directional selection
▪ Operates in response to gradual changes in environmental conditions.
▪ It operates on the range of phenotypes existing within the population
and exerts selection pressure which moves the mean phenotype to one
phenotype extreme
▪ Promotes evolutionary change by producing a selection pressure
which favours the increase in frequency of new alleles within the
population.
▪ Directional selection forms the basis of artificial selection where the
selective breeding of phenotypes showing the desirable traits increases
the frequency of those phenotypes within the population.
Disruptive (diversifying) selection
▪ Fluctuating conditions within an environment (e.g. associated with
season) may favour the presence of more than one phenotype within the
population.
▪ Selection pressures acting from within the population as a result of
increased competition may push the phenotypes away from the
population mean towards the extreme of the population.
▪ This can split a population into two subpopulations.
▪ If gene flow between the subpopulations is prevented, each population
may give rise to the appearance of different phenotypes within a
population.
▪ This phenomenon is called polymorphism.
▪Within a species, organisms with different phenotypes (ecotypes), may
show adaptations to particular environmental conditions
▪ The gradation of characteristics along a geographical range is a
phenotypic response to climate and/or edaphic (soil) variables and is
known as a cline.
The three types of selections are graphically depicted in the next slides
Artificial selection
▪ Selection in which humans exert a directional selection pressure.
▪ It leads to changes in allele and genotype frequencies within a
population.
▪ It is an evolutionary mechanism which gives rise to new breeds,
strains, varieties, races and subspecies.