NATURAL SELECTION &
DARWINS THEORY OF EVOLUTION
Learning Outcome
explain why variation (as a result of mutation, meiosis and sexual
reproduction) is important in natural selection
explain, with examples, how environmental factors act as forces of
natural selection
state that variation and competition lead to differential survival of,
and reproduction by, those organisms best fitted to the
environment
give examples of environmental factors that act as forces of natural
selection
explain the role of natural selection as a possible mechanism for
evolution
give examples of artificial selection such as in the production of
economically important plants and animals
Natural Selection
It is the process by which
organisms that are better
adapted to their
environment survive and
breed while those less
well adapted fail to do so.
A.k.a. survival of the
fittest
Individuals that
are adapted will
survive to
reproductive age
and will pass on
their favourable
alleles to the next
generation, for
them to produce
viable offspring.
Over many
generations, the
individuals with
beneficial alleles are
more likely to
survive to breed
(increase in
frequency over time)
and therefore
increase in number
at the expense of
Forces of Natural Selection
1) Mutation
Spontaneous and random change in gene or
chromosomal structure
Results in variation within the same population
May cause a new variety to be introduced
Forces of Natural Selection
2) Environmental (abiotic) factors
Resource availability e.g. habitat, etc.
Sudden random changes in climate e.g. ice
age
Extreme weathers e.g. heatwave, typhoon,
etc.
Human influence e.g. deforestation,
industrialisation
Geographical access
Continental drifts
Before the industrial revolution in Britain, most
peppered moths were of the pale variety and were
camouflaged against the pale birch trees.
Moths with a mutant black colouring were easily
spotted and eaten by birds. This gave the white
variety an advantage, and they were more likely to
survive to reproduce.
Airborne pollution in industrial areas blackened the
birch tree bark with soot. This meant that the mutant
black moths were now camouflaged, while the white
variety became more vulnerable to predators.
This gave the black variety an advantage, and they
were more likely to survive and reproduce. Over time,
the black peppered moths became far more numerous
in urban areas than the pale variety.
Continental Drifts
Forces of Natural Selection
3) Biological factors
Resource availability e.g. Food, habitat,
mates, etc.
Predation
Disease
So how do these forces result in
natural selection?
Mutation results in the formation of a new variety and
brings about variation within the population. These
mutations can be inherited and passed from one
generation to the next.
Varieties within the same population will compete with
one another and only organisms with beneficial
adaptations will be able to survive and reproduce to
pass on their favourable genes to the next generations.
Environmental selective pressures will select these
individuals which are successful at adapting i.e.
differential reproduction.
Over time, multiple rounds of natural selection will
change the allele frequency within a population. This
will give rise to new varieties that are distinctly
Principles of Natural Selection
https://youtu.be/vnktXHBvE8s
Summary of Natural Selection
There is genetic variation within a population (which can
be inherited)
There is competition for survival (populations tend to
produce more offspring than the environment can support)
Organisms with beneficial adaptations will be more suited
to their environment and more likely to survive to
reproduce and pass on their genes
Environmental selective pressures lead to differential
reproduction
Over generations there will be a change in allele frequency
within a population (evolution)
New species introduced
Case Study on Natural Selection
Sonoran Desert
Located in the south-western part of the
USA
Spans across a few states as well as
encroach into Mexico
Rock Pocket Mouse
Group Activity
Get into groups of 3 to 4 and collect
a set of illustrations
Step 1: count the no. of mice in
each location
Step 2: arrange the illustrations in
chronological order
Step 3: explain why you have
arranged the illustrations in this
manner
Step 4: Video
http://
media.hhmi.org/biointeractive/interactivevideo/pocketmousequiz/index.html
Lets Discuss
Why are some mice light-coloured
and some mice dark-coloured?
Does fur colour confer any selective
advantage or disadvantage?
What role does rock pocket mouse
play in the desert food web?
What can explain the differences
among the illustrations?
Step 5: Complete the Table
Step 6: Complete the Bar Graph
Fitness
Describes how good a genotype is at leaving
offspring in the next generation relative to how
good other genotypes are at it.
If brown beetles consistently leave more offspring
than green beetles because of their colour, brown
beetles is therefore said to have a higher fitness.
In Summary
Variation is brought about by mutation
Adaptability of a strain or variety of organism
varies with environmental factors
Nature selects variety that is able to survive
and reproduce, ensuring its continuity
Fitness is a relative concept
Artificial Selection
Aka selective breeding of plants or animals to
produce desirable trait(s)
Humans select and intentionally cross individuals
or strains/varieties with desirable traits to
produce the ideal offspring
Directly influenced by human, less so by
environment
Often for commercial purposes
Crops e.g. wild mustard, rice, etc.
Livestock or pets e.g. cattles, dogs, etc.
Selective Breeding in Vegetables
Artificial Selection in Poodles
Evolution occurs
when natural
selection causes
changes in relative
frequencies of
alleles in the gene
pool.
It is the process by
which the present
complex forms of
living organisms
have arisen from
ancestral forms.
Is There Variation in Finches?
A Lab Activity
Does natural selection increases variation in a population?
How about evolution?
