Download Natural selection

Selection

Aims:

Must be able to recall the types of selection,
with examples.

Should be able to describe examples of each
form of selection and their possible effects.

Could be able to explain how selection acts
on allele frequencies in populations.
Selection - Competition:

Members of a
population compete
with each other for:

Members of a
population are also
exposed to competition
from other species to:

Living space

Predation

Energy supplies
(food/sunlight)

Parasites

Disease causing
organisms

Suitable mates
Selection Acts on Phenotypes
In a polymorphic population living under certain
environmental conditions, different phenotypes may have
different survival and reproductive rates.
A phenotype that makes the greatest contribution to the gene pool in
the next generation has a higher fitness value and is said to be “at a
selective advantage”
The phenotype that makes a lesser contribution is termed less fit
and is said to be “selected against”
Selecting Agents:
The agent that causes these differences to occur between
phenotypes is the selecting agent.
Examples?
• Physical agents (climate change, food shortage)
• Biological agents (infectious disease, predation)
• Chemical agents (soil or water pollutant, herbicide,
pesticide)
Selection: Two Kinds.

Population living in the wild


Natural Selection
Population living in captivity

Artificial Selection
Natural Selection

Populations of sexually
reproducing organisms consist
of varied individuals, with
some variants leaving more
offspring than others.
AA
Aa
aa
Aa
AA
Aa
AA

Differential success in
reproduction (differential
fitness) is called natural
selection.
Aa
AA
Aa
Aa
aa
Aa
AA
aa
aa
Aa
Aa

Natural selection is responsible
for most evolutionary change
by selectively altering genetic
variation through differential
survival and reproduction.
AA
Aa
AA
Selection pressures may reduce
the frequencies of certain alleles
Natural Selection
 Natural selection acts on phenotypes by:
AA
Aa
 Reducing reproductive success of
phenotypes poorly suited to
prevailing conditions - alleles
become less common in gene pool.
aa
Aa
AA
Aa
AA
Aa
AA
Aa
 Enhancing the survival and
Aa
reproductive success of phenotypes
well suited to the prevailing conditions alleles become more common
in the gene pool.
Aa
Aa
aa
AA
aa
Aa
AA
Aa
 Natural selection therefore changes
the composition of a gene pool increases the probability favorable alleles
will come together in the same individual.
aa
AA
Phenotype with allele combination
aa is more vulnerable to frost.
Modes of Natural Selection

Natural selection changes allele frequencies in populations, but it
does not produce the “perfect organism”.

Rather than developing new phenotypes, it reduces the frequency of
phenotypes that are less suited to the prevailing conditions (e.g.
increased frequecy of heavy frosts).

Traits (e.g. skin color, height) that
are under polygenic control show quantitative variation in the
phenotype. Natural selection acts
on this variation.
Stabilising Selection

After selection, and for
some generations later,
there is a reduction in
the amount of variation.
Frequency
Before selection there
is a broad range of
variation in the
population:
Eliminated
Eliminated
Frequency

Retained
Variation in phenotype
Activity
 Answer
questions from page 317 in
Biozone book.
Stabilizing selection
against extremes in birth
weight range results in
most births between 3-4kg.

The histogram shows
percentage of births in
each weight class. The red
line shows associated %
mortality.

Modern medical
intervention is reducing
this selection pressure by
increasing the survival.
Percent of births

Percent mortality
Stabilising Selection in
Human Birth Weights
Birth weight (kg)
Selection against low birth
weight babies with poor
organ development
Selection against high birth
weight (large babies) due to
childbirth complications
Activity
 Answer
questions from page 323 in
Biozone.
Peppered Moths
 The peppered moth, Biston betularia,
occurs in two forms (or morphs):
 The mottled or gray form is well
camouflaged and less conspicuous
(to predators) against the lichencovered bark of trees in unpolluted
regions.
 The dark melanic forms are
conspicuous in such environments as
their body shape stands out against
the background.
Gray or mottled form of the peppered moth
Biston betularia; camouflaged on lichen
 With the onset of the Industrial
Revolution in England, the air quality
declined, killing off lichen and resulting
in a marked increase in the relative
frequency of the dark moths.
 In a polluted environment,
directional selection favored the
melanic forms.
Melanic or carbonaria form of the peppered
moth Biston betularia ; conspicuous on lichen,
but camouflaged on soot-covered vegetation
Peppered Moths

Industrial areas
In the 1940s and 1950s, coal
burning was still intense
around the industrial centers
of Manchester and Liverpool.

During this time, melanic
forms remained dominant
in these regions.

In the rural areas further
south and west of the
industrial centers, the gray
forms increased dramatically.
Key to Frequency Graphs
Gray or
speckled form
Melanic or
carbonaria form
Non-industrial areas
Frequency of
peppered moth
forms in 1950
Peppered Moths
 With the decline of coal burning factories and the Clean Air Acts in
cities, the air quality improved between 1960 and 1980.
 Sulphur dioxide and smoke levels dropped to a fraction of their
previous levels.
 This caused the proportion of melanic peppered moths to plummet…
Frequency of melanic peppered moth
related to reduced air pollution
 Now, with cleaner air,
selection is increasingly
in favor of the
gray form.
Melanic Biston betularia
Summer
smoke
Winter sulfur
dioxide
Stages of Natural Selection
Variation exists in the phenotypes of a population.
1.

There are either brown or yellow shelled snails
There is a selective pressure.
2.

A predator arrives in the habitat
Phenotypes with a higher fitness level pass on genes.
3.

Those that are brown are camouflaged and are more likely to
survive
Over several generations the ratio of phenotypes changes in
favour of the phenotype with the higher fitness value.
4.

Browns breed and increase the occurrence of brown offspring.
Activity
 Answer
questions from pages 321 and
322 in Biozone book