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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
phenotypes
 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
pressure

The allele is only common where it
provides an overall advantage to the
individual
 This establishes a relationship between
mutations and evolution:
I.
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
inheritable
 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
competition

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
pollination


Pgs 556-564
Finish up lab
HW questions