5.6 - Variation, change and evolution
Variation in a population
● Variation within a species can be genetic, environmental, or a combination of both
Phenotype, genotype and environment
● Phenotype (“appearance”) = genotype (one’s genes) + environment
Mutations and the origin of variation
● ***Mutation: origin of ALL genetic changes; rare & random change in genetic
material (inheritable)
○ Occurs when information coded in DNA of a gene is altered in some way
■ E.g.: One/more bases coding for amino acids may be altered - a
single base might be exchanged for a different one, such as adenine
(A) being swapped for guanine (G)
● If a codon is changed in such way, then the amino acid
specified by that codon might be different
● A change in just 1 amino acid, among the 100+
making up a protein, might cause a major change to its
shape and function; meaning that the mutant (突變體)
protein works less well/not at all
○ FOR THIS REASON, most mutations are harmful while
others have no effect on the protein and phenotype
● Mutations occur naturally and spontaneously (自發地), HOWEVER certain
factors can increase incidence (發生率) of mutations (such factors called
mutagens [誘變劑] - generate/“cause” a mutation)
○ Ionising radiation, e.g.: gamma rays/X-rays/UV light rays
○ Chemical mutagens, e.g.: chemicals in tobacco
How natural selection leads to evolution
● ***Evolution: natural changes in phenotypes within population, over several
generations (observation in which successive generations of living organisms seem
to change over time)
○ ***Theory of evolution - life began as simple organisms from which more
complex organisms evolved (rather than popping into existence)
1
, ● Darwin’s statements
○ Observation 1 — 2 parent organisms produce more offspring in their lifetime
than are required to replace the previous generation
○ Observation 2 — Numbers in plant and animal populations remain remarkably
constant - they do not continually increase
○ Conclusion 1 — Indicate that there is a “struggle for survival”/some
selection must occur - either for mates or for some environmental factor
——————————————————————————————————
○ Observation 3 — There is natural variation in a population of sexually
reproducing organisms - this variation is genetically determined
○ Observation 4 — Some individuals in the population inherit an advantageous
set of genes, giving them a phenotype that provides an advantage in a
particular environment. These individuals are more likely to survive as they
are better adapted to their environment and so pass on their genes to the
next generation
○ Conclusion 2 — Show how the individuals that are better adapted are more
likely to survive and breed and so pass on their genes to the next generation.
This is often described as “survival of the fittest” - and here “fittest” indicates
the individuals that survive long enough to produce the next generation
Natural selection and evolution in action
● .
● E.g.: peppered moths (due to environmental factors - recent)
○ Widespread in UK
○ Found in 2 forms - dark, light-coloured
○ During daytime, usually rest on trees’ bark (樹皮)
○ HOWEVER, in some parts of UK, as a result of industrial smoke pollution,
trees’ bark blackened → light-coloured moth variety could be seen
more easily by insectivorous (食蟲的) birds which feed on them (i.e.:
more prone to predators); whereas dark form less visible to predators
○ As a result, in polluted regions, light variety was more often eaten →
became rarer than dark form; in unpolluted regions, light variety
remained common
○ ***When smoke pollution stopped, tree trunks no longer darkened ⇒
situation reversed
2
Variation in a population
● Variation within a species can be genetic, environmental, or a combination of both
Phenotype, genotype and environment
● Phenotype (“appearance”) = genotype (one’s genes) + environment
Mutations and the origin of variation
● ***Mutation: origin of ALL genetic changes; rare & random change in genetic
material (inheritable)
○ Occurs when information coded in DNA of a gene is altered in some way
■ E.g.: One/more bases coding for amino acids may be altered - a
single base might be exchanged for a different one, such as adenine
(A) being swapped for guanine (G)
● If a codon is changed in such way, then the amino acid
specified by that codon might be different
● A change in just 1 amino acid, among the 100+
making up a protein, might cause a major change to its
shape and function; meaning that the mutant (突變體)
protein works less well/not at all
○ FOR THIS REASON, most mutations are harmful while
others have no effect on the protein and phenotype
● Mutations occur naturally and spontaneously (自發地), HOWEVER certain
factors can increase incidence (發生率) of mutations (such factors called
mutagens [誘變劑] - generate/“cause” a mutation)
○ Ionising radiation, e.g.: gamma rays/X-rays/UV light rays
○ Chemical mutagens, e.g.: chemicals in tobacco
How natural selection leads to evolution
● ***Evolution: natural changes in phenotypes within population, over several
generations (observation in which successive generations of living organisms seem
to change over time)
○ ***Theory of evolution - life began as simple organisms from which more
complex organisms evolved (rather than popping into existence)
1
, ● Darwin’s statements
○ Observation 1 — 2 parent organisms produce more offspring in their lifetime
than are required to replace the previous generation
○ Observation 2 — Numbers in plant and animal populations remain remarkably
constant - they do not continually increase
○ Conclusion 1 — Indicate that there is a “struggle for survival”/some
selection must occur - either for mates or for some environmental factor
——————————————————————————————————
○ Observation 3 — There is natural variation in a population of sexually
reproducing organisms - this variation is genetically determined
○ Observation 4 — Some individuals in the population inherit an advantageous
set of genes, giving them a phenotype that provides an advantage in a
particular environment. These individuals are more likely to survive as they
are better adapted to their environment and so pass on their genes to the
next generation
○ Conclusion 2 — Show how the individuals that are better adapted are more
likely to survive and breed and so pass on their genes to the next generation.
This is often described as “survival of the fittest” - and here “fittest” indicates
the individuals that survive long enough to produce the next generation
Natural selection and evolution in action
● .
● E.g.: peppered moths (due to environmental factors - recent)
○ Widespread in UK
○ Found in 2 forms - dark, light-coloured
○ During daytime, usually rest on trees’ bark (樹皮)
○ HOWEVER, in some parts of UK, as a result of industrial smoke pollution,
trees’ bark blackened → light-coloured moth variety could be seen
more easily by insectivorous (食蟲的) birds which feed on them (i.e.:
more prone to predators); whereas dark form less visible to predators
○ As a result, in polluted regions, light variety was more often eaten →
became rarer than dark form; in unpolluted regions, light variety
remained common
○ ***When smoke pollution stopped, tree trunks no longer darkened ⇒
situation reversed
2
