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Summary Grade 12 Life sciences - Life Sciences (Biology)
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Grade 12 Life Sciences - Life Sciences (Biology)
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Grade 12 Life sciences - Life Sciences (Biology)
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EvolutionBiological evolution - genetic changes in a population (of living organisms) that are inherited over
successive generations due to natural selection and resulting in the formation of new species.
Macro-evolution is the development of new life forms or species from earlier life forms over many
generations.
Speciation forms the basis of macro-evolution where new species develop from existing species
over time.
Theory - well-substantiated explanation or hypothesis supported by evidence.
Evolution is considered a scientific theory, based on a series of hypotheses that have been tested
and verified.
The scientific theory of evolution emerged from different lines of evidence:
- Fossil records
- Modification by descent
- Biogeography
- Genetics
- Other forms of evidence
Fossil records
Scientists expect to find earlier, simpler fossils in the deeper, older rock layers and recent, more
complex fossils in the upper, more recent rock layers.
This indicates the order in which evolution occurred.
Archaeopteryx is a transitional fossil between reptiles and birds. It is said to be the earliest and
most primitive bird.
Modification by descent
Basic body plans of different plant and animal groups were modified to adapt to their different
environments.
The forelimbs of various vertebrates like amphibians, reptiles, birds and mammals look different
and perform different functions, but show the same basic body plan.
,Functions include digging, flying, running, swimming and climbing.
The basic body plan consists of the following bones:
- one proximal longbone (humerus)
- two distal bones (radius and ulna)
- number of wristbones (carpals)
- five metacarpals that form the palm
- phalanges (fingers)
This pattern is known as a pentadactyl limb.
Similar structures (eg. forelimbs of vertebrates) with the same
body plan that perform different functions are known as
homologous structures. (Cat and whale)
Thus homologous structures provide evidence of evolution because the similarities may be traced
back to a common ancestor.
The adaptations in the structure of the forelimbs develop independently in each group according
to the organism’s habitat and mode of life (eg. movement in air, water or trees).
Divergent evolution - organisms develop from a common ancestor, but follow different
evolutionary paths.
Convergent evolution - unrelated organisms develop structures for similar environments
independently of each other.
Body structures that perform the same function but differ in a common ancestor are known as
analogous structures. (Bat and butterfly)
Biogeography
The study of the distribution of existing (extant) and extinct plant/animal species in specific
geographical regions.
These studies prove that closely related species usually occur in the same geographical region
and they may share a common ancestor.
Flightless birds, such as the ostrich (Africa), emu (Australia), rhea (South America) and the extinct
moa (New Zealand), show similarities but belong to separate species.
,Scientists claim that flightless birds may have developed from a common ancestor, but were
separated geographically when Gondwanaland broke apart.
The flightless birds adapted to the changing environments.
Genetics
The study of inheritance.
Closely related organisms have more similarities in their DNA.
It shows how changes in genotype/phenotype are transferred to successive generations.
Other forms of evidence
Comparative biochemistry - the molecular composition of most living organisms is very similar,
confirming Darwin’s idea that all living organisms have a common ancestor.
Vestigial organs - Some organisms have organs/structures that, with disuse, have decreased in
size over evolutionary time. (Whales have no hind limbs, but have remains of pelvic girdles)
Comparative embryology - embryos of vertebrates with similarities at stages of development.
Development of the theory of evolution
- Erasmus Darwin
- Jean Baptiste de Lamarck
- Alfred Wallace
- Charles Darwin
Erasmus Darwin
The grandfather of Charles Darwin.
He proposed:
- that life on earth originated from a single common ancestor (a simple life form).
- there are similarities between different organisms which indicate that one species developed
from another over time.
- phenomena such as artificial selection in animals and metamorphosis in tadpoles shows
how changes occurred over time.
, Jean Baptiste de Lamarck
French naturalist, proposed his theory of evolution in his book Philosophie Zoologique.
Lamarck’s theory was based on two related ideas:
- The use or disuse of organs may cause the organs to increase or decrease in size or even
completely disappear.
- During their lifetime organisms acquire certain changes in characteristics that are inherited
by their offspring. This results in changes in populations and the formation of new species.
Lamarckism describes how an organism’s acquired characteristics are transferred to its offspring.
Lamarck’s Laws:
- ‘Law’ of Use and Disuse
- ‘Law’ of Inheritance of Acquired characteristics
Application of Lamarckism
According to Lamarck, giraffe ancestors had short necks.
Over generations these short-necked giraffes stretched their necks to reach the leaves on the top
branches for food. The stretching caused their necks to grow stronger and longer. This acquired
trait (long neck) was passed on to the next generations. As a result all modern giraffes have long
necks.
Lamarck believed that organisms possess an internal driving force - to strive for complexity and
perfection.
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