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GRADE 12 BIOLOGY NOTES - DNA
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Introduction Evolution: the change in the characteristics of a species over several generations
and relies on the process of natural selection
Hypothesis: an idea formulated to explain observations. It must be testable as the
idea has not been proved
Theory: after a hypothesis has been tested and proved, it becomes a theory. This
is well-supported with evidence
Evidence for the theory of evolution
Fossil record
Modification by descent
Genetics
Biogeography
Fossil records
Older fossils (deeper layers) show evidence of simpler fossils and recent fossils
(upper layers) include more complex fossils
The age of rock strata is determined relatively by comparing the layer order
(deeper layers deposited before shallower layers)
As well as by determining the absolute age of the rock layer (e.g. by radiometric
dating)
Modification by descent
Organisms living today descended from earlier ancestors, who were modified, or
physically changed, through the course of their descent. Homologous structures
support this evidence.
Equivalent structures with the same body plan that perform different functions
are called homologous structures
The different adaptations develop independently according to the species habitat
and mode of life
E.g. All tetrapods (four limbed animals) look different and use their limbs for
different functions.
However the structural arrangement of humerus: radius and ulna: carpals:
metacarpals and phalanges are the same in all groups.
Genetics
Closely related species have more similarities in their DNA
The greater the sameness in DNA sequences between two species, the more
recently they share a common ancestor
Mutation provides the source of variation necessary or evolution to occur
Changes in genotype and phenotype are transferred to successive generations
, Gene pools of populations can change over time and lead to speciation.
Biologists study the DNA sequences of different organisms to determine their
evolutionary relationships
Biogeography
The study of the distribution of extant and extinct species
It is seen that closely related species usually are found in the same geographical
region, suggesting a common ancestor in that region.
Many geographical regions become isolated by barriers, preventing populations
from spreading from one region to another e.g. mountain ranges, oceans and
deserts.
These barriers often also cause the formation of new species
Variation
Species: group of similar organisms that are able to interbreed and produce fertile
offspring.
Population: the sum of organisms of the same species occupying the same
habitat at the same time and having the ability to interbreed.
Variation: small differences that exist between individuals of the same species or
population
Sources of variation
Mutations: change the structure of a gene -> altered genotype -> altered
phenotype
Variation due to meiosis
- Crossing over: during P1, the homologous chromosomes swap a segment of
DNA
- Random arrangement of chromosomes: during M1 and M2, chromosomes
arrange randomly at the equator of the cell
Random fertilization of gametes: egg and sperm cells produced by meiosis are all
different so random fertilization results in different combinations of genetic
material in offspring
Continuous and discontinuous variation
Continuous variation
- There are a range of different phenotypes for a particular characteristic. E.g.
height
- Brought about by many genes working together (polygenic inheritance)
- Environment can affect the appearance of characteristics
- When plotted on a bar graph – you see a bell-shaped, normal distribution
curve is produced.
Discontinuous variation
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