1. Evolution by Natural selection
Life Sciences Gr.12
Evolution by Natural selection
Together , Moving Gauteng City Region Forward

2. The Tree of Life All living things share a common ancestor.
movies\TreeOfLIfe.avi

3. What is evolution?
All things we see today arose from things that existed in the past but they look different because things change over long periods of time
Biological evolution refers to the changes that life forms have undergone over long periods of time – distinct from the evolution of the Earth itself.
Biological Evolution means that all present-day forms of life …
have descended from, and are related to, those that lived in the past
may look different because they became modified from one
generation to another
They look diff because things change over time…
Applies also to the galaxy and the universe

4. Hypothesis and Theory TERM DEFINITION EXAMPLE Hypothesis
A testable statement about the natural world that can be used to build more complex inferences and explanations.
The high incidence of HIV-AIDS is due to the lack of education about the causes of the disease
Theory
A well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.
The sun is the centre of the universe, around which all the planets revolve (Heliocentric theory)

5. Theories of evolution Erasmus Darwin (1731 – 1802)
Ideas on evolution that were proposed by Erasmus Darwin: All life developed from simple forms There are similarities amongst various organisms Artificial selection and metamorphosis showed how changes may have occurred
Jean Baptiste de Lamarck (1744 – 1829):
Two ideas of Lamarck in explaining evolution: Use and disuse Inheritance of modified characteristics
Alfred Wallace (1823 – 1913)
Natural selection as an explanation for evolution
Charles Darwin (1809 – 1882)
Charles Darwin's Theory of Evolution by Natural Selection

6. Use and disuse of organs
Lamarck’s theory
Use and disuse of organs
Changes in the environment create new needs that cause organisms to modify their existing organs to meet the need. Repeated use of the organ would cause it to enlarge and become more efficient. Disuse of a organ would cause it to degenerate
Inheritance of acquired characteristics
The modification an organism acquired during its
life time could be pass on to its offspring.

7. Lamarck’s Theory Cont...
Lamarck thought that if an animal acquired a characteristic during its
lifetime, it could pass it on to its offsprings.
Hence giraffes got their long necks through generations of straining to reach high branches.

8. How did Lamarck explained the long necks of giraffes?
- All giraffes had short necks originally
- Giraffes frequently stretched/used their necks to reach
- for leaves of tall trees
- necks become longer
- The long necks acquired in this way
could be passed on to the next generation /were inherited
Snake

9. Why Lamarck’s theory is not accepted by most life scientists today:
Acquired characteristics are not inherited/do
not cause any change to the DNA of an organism's
gametes (sperms or ova) OR
Organisms did not evolve because they want
to evolve
Lamarck believed in determinism (internal drive of
organisms to change)

11. Observations on which Darwin based his theory:
Charles Darwin
Observations on which Darwin based his theory:
Offspring of the same species produce a large number of offspring
Offspring show a great deal of variation
The offspring produced compete with each other for resources
Of the large number of offspring produced, only a few survive- ones with best adapted characteristics
Survival of organisms as a result of natural selection

12. Darwin’s theory of evolution by natural selection
Most species produce a large number of offspring in their life-times
Variation: Offspring of the same species show a great deal of variation
Competition: These offspring compete with each other for food, shelter etc.
Survival: Only a small number of offspring survive
Genetics: Organisms pass genetic traits on to their offspring. Later used the term survival of the fittest
Natural selection : Those organisms with the most beneficial traits are more likely to survive and reproduce.
Natural Selection from generation to generation caused the population to gradually change …
leading to the evolution of new species

13. How would Darwin account for the long necks of the giraffes?
As a result of genetic variation in the giraffe population
some giraffes have longer necks than others
Environmental change/competition for resources occurred causing those with shorter necks to die
and those with longer necks to survive
This is natural selection/survival of the fittest
The genes/genotype for longer necks
were passed on to subsequent generations most of which now have long necks

14. Sources of variation in populations.
Crossing over. (Prophase I)
Random arrangement of chromosomes. (Metaphase I)
Independent assortment/ random segregation/ random assortment
Chance/random fertilisation of gametes
Random mating
Mutation
OR meiosis

15. Sources of variation Explained
Crossing over in Prophase I of meiosis the exchange of genetic material, leads to new combinations of maternal and paternal genetic material in each new cell resulting from meiosis.
Random arrangement of chromosomes during metaphase allows different combinations of chromosomes.
Random fertilisation between different egg cells and different sperm.
Random mating between organisms within a species.
Mutation changes the structure of a gene or chromosome.

16. Finches of Galapagos Natural selection example 1 Leaves Buds and fruit
Insects
Larvae/worms
Ancestral seed-
eating species from
the mainland
Natural selection example 1

17. Natural selection
From a comparison of pictures A and B, describe the feature of the cacti that
have enabled them to survive long periods of hot, dry weather conditions.
Name the mechanism put forward by Darwin to explain his theory of
evolution that is illustrated in these diagrams.
Roots
Offspring A B C
The structural adaptation of cacti over time
Time

18. Natural selection:
Explain how Darwin would have used the example of the tortoises to explain speciation.
During continental drift
the tortoise populations were isolated/separated from the original population
on different islands
living under different environmental conditions
and through natural selection developed into new species
not able to interbreed after a period of time

19. Artificial selection
For ages, humans have been doing breeding experiments to develop organisms with a selected set of desirable characteristics. This is called artificial selection.

20. Differences between Natural Selection and Artificial Selection
The environment or nature is the selective force.
Humans represent the selective force.
Selection is in response to suitability
to the environment.
Selection is in response to satisfying
human needs.
Occurs within a species.
May involve one or more species (as in cross breeding).

21. Punctuated Equilibrium

22. Punctuated Equilibrium cont…
Evolution is not always gradual
as proposed by Lamarckism and Darwinism.
• Evolution involves long periods of time where species do not change or change very little (known as equilibrium). This alternates with (is punctuated by) short periods of time where rapid changes occur.
• New species are formed in a short period of time, relative to the long periods of no/little change.
• This is supported by the absence of transitional fossils ‘missing links’ indicating the period of rapid change.

23. SPECIATION
Species is a group of organisms that can interbreed and produce fertile offspring.
Geographical isolation and reproductive isolation mechanisms, isolate the gene pool of a species resulting with formation of new species.
Examples of reproductive isolation mechanisms:
Breeding at different times of the year
Species-specific courtship behaviour (animals)
Adaptation to different pollinators (plants)
Infertile offspring (e.g. mules)

24. Allopatric (geographic)
In allopatric speciation, species diverge because of a geographical barrier like, mountain , river, volcano, crack etc.
Original population
Initial step to speciation
Evolution of reproductive isolation
New distinct species
Barrier formation
In isolation

25. Describe how speciation occurs when a population becomes separated by a geographical barrier.
The population will split up into two groups
and each group adapts to the new environmental factors/ develops separately
each group undergoes natural selection independently
Develops differently
each group may become genotypically
and phenotypically different
which might prevent them from interbreeding/become reproductively isolated leading to the formation of a new species

26. How is the geological time scale used as evidence for evolution?
Macro-evolution refers to the changes that have led to the development of the larger taxa (groups) of life forms such as the different phyla, classes, orders, families and genera that we see in the fossil record.
Since these changes have taken place over millions of years, it has become more convenient for scientists to develop a geological time scale that show the history of life.

27. : Paleontology providing evidence for evolution
How do fossils provide evidence for evolution?
Fossils provide evidence of life forms that
existed in the past/that are now extinct

28. Evidence for evolution
Genetic evidence:
Scientists state that organisms are closely related
and are likely to have a common ancestor if they have:
–– Identical DNA structure
–– Similar sequence of genes
–– Similar portions of DNA with no functions and
–– Similar mutations (mitochondrial DNA)
Species that are closely related have a greater similarity to each other than distant species.

29. Evidence for Evolution
Cultural evidence:
Cultural evidence from studies of tools and
weapons, as well as language is also used to show similarities and differences between humans and African apes.