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Summary Sheet
Variation
Natural selection can only occur if there is variation amongst members of the same
species
 The Theory of evolution and natural selection in 1830s. One of the
observations which Darwin based the theory of evolution by natural
selections is variation. Typical populations vary in many respects.
 Natural Selection depends on variation within populations
Sources of Variation
Mutation, meiosis and sexual reproduction cause variation between individuals in a
species.
 Mutation – original source of variation
 Meiosis – new combinations of alleles by breaking up the existing
combinations in a diploid cell.
 Sexual reproduction – fusion of male and female gametes.
 Only source of variation is mutation.
Adaptations
Adaptations are characteristics that make an individual suited to its environment
and way of life.
 Characteristics develop over time and thus that species evolve.
 Theory adaptions develop by natural selection, not with the direct
purpose of making an individual suited to its environment.
 Characteristics do not develop during the lifetime of one individual.
 Characteristics that do develop during a lifetime are acquired
characteristic and cannot be inherited.
Overproduction of Offsprings
Species tend to produce more offspring than the environment can support.
 Living organisms vary in the number of offspring they produce.
 Relatively low breeding rate – Bucorvus Leadbeateri
 Fast breeding rate – Cocos nucifera
 There is an overall trend in living organisms for more offspring to be
produced than the environment can support. Darwin pointed out that this
will tend to lead to a struggle for existence within a population:
o Competition for resources and not every individual will obtain
enough to allow them to survive and reproduce.
Differential survival and reproduction
Individuals that are better adapted tend to survive and produce more offspring
while the less well adapted tend to die or produce fewer offspring.
 Chances play a part in deciding which individual survives and reproduces.
 Characteristics of an individual also have an influence.
 The less well adapted individuals tend to die or fail to reproduce.
 The best adapted tend to survive and produce many offspring.
Inheritance
Individuals that reproduce pass on characteristics to their offspring
 Variation between individuals can be passed on to offspring. It is heritable
 Variations in behavior can be heritable.
 Due to differences in their genes, results for difference in behavior and
characteristics.
 Not all features are passed on to offspring.
 Acquired characteristics are therefore not significant in the evolution of a
species.
Progressive Change
Natural selection increases the frequency of characteristics that make individuals
better adapted and decreases the frequency of other characteristics leading to
changes within the species.
 Better-adapted individuals survive they can reproduce and pass on
characteristics to their offspring.
 Less adapted have lower survival rate and less reproductive success.
 Increase in the proportion of individuals in a population with
characteristics that make them well adapted.
 Characteristics of the population gradually change- this is evolution by
natural selection.
 Major evolutionary changes are likely to occur over a long time period
through many generations.
Application 1 – Galapagos Finches
Changes in beaks of finches on Daphne Major
 Darwin visited the Galapagos Islands and collected specimens of small
birds, which were finches. There are 14 species in total. Darwin observed
the sizes and shapes of the beaks as well as their diet.
 Variations in the shape and size of beaks are mostly due to genes, though
environment ahs some effect. The proportion of the variation is due to
genes, which is called heritability.
 One of the objections to the theory of evolution by natural selection is that
significant changes cause by natural selection have not been observed
actually occurring. It is unreasonable to expect huge changes to have
occurred in a species, even if it had been followed since Darwin’s theory
was published.
Application 2 – Antibiotic Resistance
Antibiotic resistance is due to genes in bacteria and so it can be inherited. The
mechanism that causes antibiotic resistance to become more prevalent of to
diminish.
 The evolution of multiple antibiotic resistance occurred rapidly due to:
o Widespread use of antibiotics, treating diseases and in animal
feeds used on farms
o Bacteria can reproduce rapidly
o Populations of bacteria are huge, increasing the chance of a gene
for antibiotic resistance being formed by mutation.
o Bacteria can pass genes on to other bacteria using plasmids.
Natural Selection and antibiotic resistance
The following trends have become established:
 Resistance appear within a few years
 Resistance to the antibiotic spreads to more and more species of
pathogenic bacteria.
 In each species the proportion of infections that are cause by a resistant
strain increases,
The development of antibiotic resistance is therefore an example of evolution. It
can be explained in terms of the theory of natural selection.