Lesson 3 Natural Selection Flashcards
Microevolution
The change in the gene frequencies within a population
over time.
Evolution within a species
Natural Selection 1
The process by which the characteristics of a population of organisms
change because individuals with certain heritable traits survive
specific local environmental conditions.
Adaptation is necessary for natural selection. Species that are well
adapted to their environment survive and reproduce.
Natural selection operates on the variation that is already present in
a species.
Natural Selection 2
It CANNOT create new
structures or processes.
It DOES NOT result in
new and improved
species
It DOES produce better
suited species to an
environment
Variation
Natural selection can only occur if there is
variation amongst members of the same
species.
Sources of variation include mutation, meiosis
and sexual reproduction.
Mutation – the original source of
variation, new alleles are produced by
random mutation
Meiosis – every gamete produced by
meiosis is likely to carry a different
combination of alleles due to crossing
over and random assortment.
Sexual Reproduction – the offspring are a
combination of the alleles of two
individuals.
Variation in
Populations
The expression of a trait should
follow a normal distribution
pattern.
Adaptation
Adaptations are characteristics
that make an individual suited to
its environment and way of life.
These include structural, behavioural
and physiological adaptations.
Some variations are better suited
to the environment.
Better suited variations lead to
differential survival and
reproduction.
Differential Reproduction
Most species produce more offspring than the environment
can support.
This leads to competition for resources and not every
individual will obtain enough to survive and reproduce.
Individuals that are better adapted to their environment
will be better able to pass on their alleles to the next
generation.
This, over time will lead to a shift in the allele frequencies
within a population.
Stabilizing Selection
Favours individuals with the “average”
expression of the trait
Selects against individuals with extreme
expressions of the trait.
Ex. Human birth weight
Until recent medical advancements,
infants that were born too small
tended not to survive while larger
infants died during child birth
Directional Selection
Favours individuals with one extreme
expression of the trait.
Selects against individuals with
average expressions as well as the
other extreme.
Ex. Camouflage in Walking Stick insects
Walking sticks are born in an array
of colours.
Colours that blend into their
surroundings are selected for
while all other colours are
selected against.
Disruptive Selection
Favours individuals at both
extremes of the trait.
Selects against individuals with the
average expression of the trait.
Ex. Limpets (marine organisms)
Have shell colours that range
from white to very dark
brown
Dark limpets attach to dark
rocks and are less visible to
predators
Light limpets attach to light
rocks and are less visible to
predators
Intermediate colours are all
visible and are consumed by
predators (selected against)
