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Chapter 17: Variation and
Evolution
Leaving Certificate Biology
Higher Level
Evolution
• Evolution is the way in which living
organisms change genetically to
produce new better adapted forms of
life over a long period of time
Evolution (cont.)
• Evolution occurs over a long period of
time as a result of variation within species
caused by sexual reproduction and
mutations
– Sexual reproduction is the main source of
variation within a species
– Mutation is a change in the sequence of
bases in DNA – caused by loss, gain or
swapping of bases within DNA
Evolution (cont.)
• Sexual reproduction causes variation by:
– Independent assortment of chromosomes into
gametes at meiosis
– Crossing over (swapping of alleles between
homologous chromosomes) during meiosis
– Fertilisation – combining of a homologous
chromosome from the gamete of one
individual with the homologous chromosome
from a gamete of another individual
Evolution (cont.)
• Mutations cause variation by:
– Spontaneous mutations that occur during
replication of DNA and where the cell does not
repair it – they are totally random, vary rare and
not caused by any one factor
– Mutagens are chemicals that speed up the
process of spontaneous mutations – all
mutagens are potential carcinogens (cancercausing chemical)
• Examples of mutagens include: asbestos, benzene,
radiation (UV light, X-rays, gamma rays, cosmic rays,
α-particles, neutrons), arsenic, acrylamide, human
papilloma virus, Helicobacter pylori, hepatitis B and C
viruses
Evolution (cont.)
• Types of mutations:
– Gene mutations: change in the sequences of
bases within a single gene causing the resulting
protein to be non-functional
• Common genetic disease caused by gene mutations
include: sickle-cell anaemia, cystic fibrosis,
haemophilia, and albinism
– Chromosome mutations: changes in the
structure or number of chromosomes of cells
• A common example of a chromosome mutation is
Down’s Syndrome (name after the physician who
described this condition) where the individual has 3
number 21 chromosomes
Darwin and Wallace Theory of
Natural Selection
• Darwin worked in the Galapogos Islands
• He made three observations:
1. Organisms overbreed
2. Population remains constant
3. There is limited resources for organisms
• As a result of these observations he made two
conclusions:
1. There is variation amongst members of the same
species
2. Those organisms with the most beneficial variations
(adaptations) will survive and pass on those
characteristics to their offspring
Speciation
• Speciation is where accumulated genetic
changes over many millions of years that
result in an organism being better adapted
to its habitat results in the formation of
organisms that can no longer interbreed
with each other and a new species is
formed
Evidence for Evolution
• Comparative anatomy:
– Involves studying the anatomical (structural)
links between organisms of separate species
– Pentadactyl limbs of different vertebrate
animals have a similar pattern of arrangement
(i.e. five digits). But there is no need for this to
be the case if evolution hadn’t occurred (i.e.
wings of insects are completely different
structurally from that of a bat’s). Thus the
conclusion is that all variations of the
pentadactyl limb arose through natural
selection (evolution) from a common ancestor
Pentadactyl limb
Evidence for Evolution
• Fossil Records:
– A fossil is the preserved remains of an
organism or part of an organism or an
imprint related to an organism
– Fossil records support evolution because:
• They are very old (dated by carbon dating and rock
depth measurement) and show how organisms have
been slowly changing and adapting over the
millennia
• More recent fossils show increased complexity
when compared to older fossils e.g. the height of the
horse has increased over a 60 million year period
• Many fossils are of organisms now extinct but have
modern-day ‘relatives’
Dinosaur fossil
Evidence for Evolution
• Comparative Embryology:
– Study of structure of early embryos of
organisms of different species
• Embryos of species as diverse as fish, tortoise,
chick, rabbit, and human all have a similar
structure at the early developmental stage
• This shows evidence for evolution as this
developmental mechanism must have been come
from a common ancestor
Early embryos of the following
animals show similar structure