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THE STRUGGLE FOR
SURVIVAL
Ms. Muneer
Darwin’s Theory
Darwin summarized natural selection in these
words.
“can we doubt (remembering that many more
individuals are born than can possibly survive)
that individuals having any advantage, however
slight, over others, would have the best chance of
surviving and procreating their kind?”
Darwin’s Theory of Natural Selection is
based on some basic observations

Observation 1
 Individuals
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Observation 2
 Some

within a species vary in many ways.
of this variability can be inherited
Observation 3
 Every
generation produces far more offspring than
can survive and pass on their variations

Observation 4
 Populations
of species tend to remain stable in size
Natural Selection – 3 Principles
1.
2.
3.
Members of the same species compete with each
other for survival
Individuals with more favorable variations are
more likely to survive and pass them on. Survival
is not random
The favorable variations will become more
common. This is Natural Selection.
Environment Causes Selection
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In England in the early 1800’s , light coloured lichen
covered many surfaces, and the peppered moth
population was almost entirely of light coloured
lichen
Than came the industrial revolution and because of
the pollution, the colour of the trees changed
By the late 1800’s the lichen was killed and
replaced by black soot from coal burning.
By 1920, peppered moth population was almost
entirely composed of black soot individuals
Peppered moth
Black-bodied peppered moth
White-bodied peppered moth
Camouglaging
The light-colored moth (below the bark's scar) is nearly invisible on this
pollution-free tree, camouglaging it from predators
Source of Variation
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Darwin admitted that a weakness in his theory was
that he could offer no explanation for the source of
variation.
Six years after Darwin, Gregor Mendel presented
his work on garden peas.
Now we understand mutation and recombination is
the source of new inheritable variations.
Key Terms

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Natural Selection: the way in which nature favours
the reproductive success of some individuals with in
a population over others
It is the survival of the fittest – the organisms that
are best able to adapt to the environment will
survival and reproduce.
Artificial Selection – directed breeding of
animals/plants that exhibit a particular trait
Mechanisms of Evolution

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Gene pool is the complete set of all alleles
contained within a species or population
Not all evolutionary changes are the result of
natural selection
Evolution can occur due to catastrophic events
Mutation is the ultimate source of variation in an
individual’s gene pool
Random Change

Evolution defined in genetic terms as any change
in gene (and allele) frequencies within a
population or species.
Key factors in a Change to a Gene Pool
1.
2.
3.
4.
5.
Small populations - chance fluctuations cause
change
Non-random mating opportunities; preferred
individuals pass on their alleles in greater numbers
than others
Genetic mutation; new alleles are created or
alleles are changed
Migration - removes alleles from the population
Natural Selection; individuals with certain alleles
have greater reproductive success than others
increasing the relative frequency of their alleles
Genetic Drift

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When populations are small – chance can play a
significant role in altering allele frequencies
Change in the genetic makeup of a population
resulting from chance is called genetic drift
Genetic drift can lead to a fixation of alleles
Genetic Drift in Small Populations
Genetic Drift in Small Vs Large
Populations
Bottleneck Effect

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When a severe event results in a drastic reduction in
numbers, a population may experience a bottleneck
effect
A very small sample of alleles survive to establish a
new population
Relative frequencies may differ from original
population
Northern Elephant Seal Population
Bottleneck Effect
Founder Effect

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When a few individuals from a large population
leave to establish a new population
The allele frequencies of the new population will
not be the same as the original population
Polydactyly
Gene Flow
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When organism migrate, leaving one population
and joining another, they alter the allele frequencies
of both
In this way, genetic information is shared between
populations.
Unlike genetic drift, gene flow tends to reduce
between populations.
Natural Selection
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Small populations that result from a bottleneck or
founder effect are also subject to the effects of genetic
drift
This increases the chances that their gene pool will
differ from that of the original population
Although genetic drift and bottlenecks can be important
in some cases, natural selection is usually the major
driver behind changes that result in the evolution of a
significant adaptation
Natural selection is the only mechanism known that is
able to shape a species to its environment
Mutation
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Mutations are the only source of additional genetic
material and new alleles
Any mutation in a gamete can be passed on to later
generations
Since mutations are random changes they are more
likely to be neutral or harmful than beneficial
Inheritable Mutations
Neutral: a mutation that has no immediate effect on an
individual fitness or reproductive success
Harmful: a mutation that reduces an individual’s fitness
Beneficial: a mutation that gives an individual selective
advantage, increased reproductive success
Fitness – general term referring to lifetime reproductive
success of an individual
Gene Duplication
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Gene duplication occurs when unequal crossing over
during meiosis results in an additional copy of one
or more genes being inserted into a chromosome
Source of new genes
Pseudogenes
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Pseudogenes are genes that are duplicated and
later lose their ability to be transcribed - they
provide strong evidence for evolution
vestigial gene that no longer codes for a functioning
protein
The human genome contains thousands of
pseudogenes
Pseudogene

GULO gene is an example of pseudogene. This
gene is necessary for the production of vitamin C.
Human and most primates have defective GULO
genes
http://creation.com/images/pdfs/tj/j21_3/j21_3_118-127.pdf
How Common are Mutations?
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There is evidence of a rate ranging from 1 in
10,000 cell divisions
In humans, a gene coding for one enzyme used in
glycolysis occurs in a single functioning copy and 20
non-functioning pseudocopies