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Patterns of Selection
Last day we looked at how mutations provide a
continuous supply of new genetic variations, which
can be inherited and expressed as different
 Today: how do these factors relate to natural
selection: genetic variation, competition within
populations and diverse environments
This allele is different from the normal gene for
hemoglobin by having a single base-pair mutation
 Homozygous individuals are severely affected
 Heterozygous “
“ mildly affected but are
much more resistant to malaria than those with the
normal hemoglobin
 Carriers of the allele are at a disadvantage in areas
where malaria does not occur, while they are favoured in
areas where malaria is common
 Therefore they are much more likely to survive and pass
on their genes to the next generation
 In this case the environment provides the selection
The allele is only common where it
provides an overall advantage to the
 This establishes a relationship between
mutations and evolution:
Harmful mutations are frequent, but
are selected against and thus these
mutant alleles remain rare
II. Beneficial mutations though rare,
when selected for, accumulate over time
Genes provide the source of variation
but not the selective forces
Occurs when the most common phenotypes within a
population are the most favoured by the
environment==conversely any trait that deviates from this
is selected against
 Example: human birth weights are variable and partially
 3 kg is the norm for a human baby
 Babies that weigh less are often premature, and those
that weigh more often have complications affecting theirs
and their mother’s survival
 Natural selection has eliminated the extremes, so that
today most babies are near the ideal weight
By far the most common form of selection
Occurs when the env’t favours individuals with more
extreme variations of the trait, which could result in
an observable change in the population
Humans can cause directional selection as
well=salmon have large populations and short
generation times
Occurs when the env’t favours individuals at the
extreme ends of the trait, as opposed to the
intermediate variations
They depend on soft seeds and
hard seeds for food
 Finches with small bills are efficient
feeding on the soft seeds, and those
with large bills are able to crack the
hard seeds
 Important because it forms
distinctive forms within a population
that may become isolated breeding
populations with separate gene pools
Favours the selection of any trait
that confers an advantage in terms of
the mating success of the individual
 This is associated with sexual
dimorphism: which is the physical
(often extreme) differences in the
appearance of males and females
 The most common forms of sexual
selection are the results of female
mate choice and male to male
Females can chose based on physical
traits, colouration, or behavioural traits
such as courtship displays and songs
 Sometimes males develop features that
enable them to establish and defend a
territory from other males=sometimes
detaining the females
 How would you be able to tell these are
not env’tal selective pressures?
 Both sexes would possess the features
 MHC=sweaty shirts
Some features are a compromise between mating
and remaining conspicuous to predators==bright
colours and song
 Sometimes leads to extremes==runaway
selection==stalk eyed fly
% favouring long
% favouring short
Female Line of Origin
Sexual diversity is not limited to just animals
Plants do not select mates but they do need to
attract suitors to assist in pollination
 Flowers and scents are the most obvious examples
of sexual features that have evolved==maximize
Pgs 556-564
Finish up lab
HW questions