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SAC 2 (Revision)
Evolutionary relationships
Focus of the task
Key knowledge:
• Change in populations: gene pool, allele frequencies,
selection pressures, genetic drift
• Natural selection as a mechanism of evolution
• Geological time: scale, relative and actual dating
techniques
• Patterns of evolution: divergent, convergent, allopatric
speciation, extinction
• The development of evolutionary theory
• Evolutionary relationships: conservation of genes,
phylogeny, mitochondrial evolution
Section A: Darwin’s finches
Charles Darwin
• Came up with the theory of evolution by
means of natural selection.
• Famous for the Galapagos finches that he
observed on the Galapagos islands- various
species of finches (different features- beak
shapes)
Darwin’s investigation
• Darwin investigated and observed many different species
and noticed variations.
• He wondered how the variations came about? HE noticed
similarities.
• Darwin concluded that:
– Differences (genetic variation) between individuals
determine how well they will survive (fitness).
– Traits that increase the possibility of an organism’s survival
are passed onto offspring (heritable).
Natural Selection in a Nutshell!
• As populations of living things expand, generation by generation, they will
inevitably run into limits: limits on food, space or the right kind of habitat
• These natural pressures limit or determine which individuals are able to
survive and reproduce (i.e. fitness levels)
• Not all individuals in a population are exactly alike
• Some will have traits that give them an advantage in surviving, mating &
passing on their traits to the next generation
• These differences are known as variation (i.e. genetic variation)
• A change in any aspect of the environment can suddenly turn what had
been just another variation or variant into either an advantage or a
disadvantage
• If a selective pressure (i.e. change in the environment), acts against or upon
the differences between the individuals in a population, you get natural
selection, which, through time, can cause a population to evolve (change)
Natural Selection as the mechanism
for EVOLUTION
• Natural selection = 'survival of the fittest’
• It is the process by which nature selects better adapted
individuals for more successful reproduction.
• Natural selection operates on individuals, but it is the
population that evolves!
VARIATION
SELECTION
EVOLUTION
occurs after
MANY
GENERATIONS
VARIATION
SELECTION
RESULT
Divergent Evolution
• Related organisms (species
with the same ancestor)
become more and more
DIFFERENT until they are a
different species!
Convergent Evolution
• Unrelated organisms
(species with different
ancestors) evolve similar
adaptations in response to
their environment.
Relatedness
Techniques for determining
relatedness
• DNA-DNA hybridisation
Section B: Evolutionary
Relationships Investigation
Cladograms
Homologous & Analogous Structures
Homologous- similar structure
• Same general structure but different functions, e.g.
wing of a bird, wing of a bat, leg of crocodile, flipper of
whale & arm of human
• Organisms with homologous structures have a
common ancestor.
Analogous- similar function
• Not necessarily similar in basic structure, e.g. a fly’s
wing and a bat’s wing.
• Organisms with analogous structures have different
ancestors.
Absolute Dating
Absolute age defines the actual age of the
things.
Radioisotope Dating:
• works on the fact that radioactive isotopes
decay (lose their radioactivity) in a predictable
way, over periods of time.
• The half life of a radioisotopes can be used to
determine the age of a sample (rock or fossil)
Absolute Dating
Types of Radioisotopes:
• Different radioisotopes have different half lives and are thus
useful for dating different types of fossilised remains.
• Carbon-14 works for fossils up to 40,000 years (there is
100% breakdown after 40,000 years)
• Older fossils may be dated using different radioisotopes
(i.e. Potassium-Argon has a half-life of 1.3 million years).
Relative Dating
Relative age refers to the age of something
when compared to another thing.
• Depends on stratification: the process of
formation of layers in sedimentary rocks