Pp 134 - 142
Define evolution.
 Evolution is the cumulative
change in the heritable
characteristics of a population
over time.
 Not only does species evolve
over time, but also new species
can arise by evolution from
pre-existing ones.
 Variation within a species is as
result of different selection
pressures operating in
different parts of the world,
yet this variation is not so vast
to justify a construct such as
race having a biological or
scientific basis
Evidence for Evolution
 It is difficult to prove that
all organisms on Earth are
as result of evolution, but
there is a lot of evidence
that supports the theory of
evolution, including; fossil record,
 selective breeding of
domesticated animals &
 homologous structures.
Fossil Records
 fossils show changes in
organisms over time
 fossilized organisms are
different from those that
are existing today, but
they also share features
with existing organisms
(i.e. have homologous
structures) suggest
common ancestry;
 fossil show intermediate
stages in evolution of
groups i.e. there are
missing link fossils
Selective Breeding
 man has selectively breed animals
and plants for thousands of years.
 the breeds of animal that are reared
for human use are clearly related to
wild species & in many cases
domesticated animals can still
interbreed with their wild relative
 domesticated breeds have been
developed by selecting desirable
traits, and breeding from them
 the striking differences in the
heritable characteristics of
domesticated breeds provides
evidence that species can evolve
rapidly
Homologous Structures
 Homologous structures are
structures that have developed
from a common ancestor , sharing
same fundamental plan but
performing different functions
 The pentadactyl limb is an
example of a homologous structure
found in groups of organisms.
 the forelimbs of all tetrapods (such
as amphibians, reptiles, birds,
mammals) have the same basic
pattern of 5 metacarpals and 5
phalanges arising from the same
embryological structures because
development is determined by
many shared genes from a common
a ancestor
 in each case the bones are modified
and adapted to the function of the
limb such as digging, swimming,
flying, grasping etc.
Populations tend to over-produce
 Many species often
produce more offspring
than can be supported by
the environment in which
they live i.e. more than the
carrying capacity
 For example, female frogs
lay hundreds of eggs but
only a handful survive to
adulthood and plants often
produce hundreds more
seeds than necessary to
propagate the species.
Consequence of the potential
overproduction of offspring
 The population produces more offspring
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than the carrying capacity of the
environment can support
Overproduction results in struggle for
existence i.e. offspring compete for
limited resources (intraspecific
competition)
Some individuals have characteristic (or
combination ) that give them a
competitive advantage.
These individuals are consequently 'fitter'
in terms of freedom from disease, food
availability etc.
These individuals are more likely to
successfully reproduce (offspring survive)
Through inheritance of the genes for
these advantageous characteristics the
frequency of these characteristics become
greater in the next generation.
Members of a species show variation
 Populations of a species
show variation i.e.
differences in phenotypes
 Variation maybe
discontinuous e.g. blood
groups or continuous e.g.
skin colour
 Discontinuous variation
usually indicates the
condition is controlled by
one to two genes while
Continuous variation
normally indicates a
polygenic condition or
multiple alleles
How sexual reproduction
promotes variation in a species.
 segregation of alleles
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during meiosis
crossing over in prophase I
of meiosis
random orientation i.e.
assortment of homologues
at metaphase I
fertilization by chance, one
of many male gametes
fertile the ovum
Large number of possible
different gametes is 2n (223)
genes (alleles) combining
from two parents
How natural selection leads to
 Theory of evolution by natural
evolution
selection originally advanced by
Darwin & Wallace
 based on observations that
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overproduction of offspring leads to
struggle for survival
variation exists within the
population
some varieties are better adapted
than others
best adapted individuals survive,
reproduce and pass on their
characteristics (genes)to the next
generation i.e. they are “selected for”
with time, advantageous variants
become more frequent in the
population
evolution is change in species (allele
frequency) with time
evidence that species have evolved
include observed evolution such as
multiple antibiotic resistance
Examples of evolution in response
to environmental change
 examples of evolution in
response to
environmental change
includes:
 antibiotic resistance in
bacteria
 melanism in peppered
moth & ladybugs
 heavy metal tolerance in
plants
 beak size in Darwin’s
finches
Antibiotic Resistance
 evolution is the process of
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cumulative change over time
the theory of evolution in
response to environmental
change was proposed by Darwin
& Wallace
populations tend to grow
exponentially
more offspring than the
environment can sustain are
produced resulting in struggle
for survival
but the populations still remain
constant over time
individuals in populations show
heritable variation due to
mutations & sexual
reproduction
 individuals in a population may
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have traits that are better suit
them to the environment e.g.
resistance to antibiotics in
bacteria
resistance to antibiotics in
bacteria is heritable i.e. it can be
passed from one generation to
the next
individuals with antibiotic
resistance will survive when
antibiotic is applied
they will reproduce and leave
more offspring with resistance to
antibiotic
the population will tend to
accumulate the adaptation of
antibiotic resistance
therefore, the population will
evolve with time
Evolution of beak size in Darwin’s finches
 evolution is the process of
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cumulative change over time
the theory of evolution in
response to environmental change
was proposed by Darwin &
Wallace
populations tend to grow
exponentially
more offspring than the
environment can sustain are
produced resulting in struggle
for survival
but the populations still remain
constant over time
individuals in populations show
heritable variation due to
mutations & sexual reproduction
 individuals in Darwin’s finches
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population show variation in beak
size between weak long beaks &
strong short beaks
change in environment; weather
dried up, nuts become more
common than fruits
individuals with short strong beaks
got more food, survived &
reproduced i.e. They had survival &
reproductive advantage
they reproduced & passed the genes
for strong beaks to their offspring
with time population will tend to
more finches with short strong
beaks
therefore, the population will
evolve with time
Revision Questions
 Define evolution
[1]
 Outline the evidences for
evolution
[6]
 Explain the consequence
of the potential
overproduction of
offspring
[3]
 Explain how sexual
reproduction promotes
variation in a species [6]
 Explain how natural
selection leads to
evolution
[8]
 Giving 2 examples,
explain how evolution
occurs in response to
environmental change [9]