1. Natural Selection
The Mechanism of Evolution

2. Natural Selection – Darwin’s and Wallace’s Big Idea
Both Darwin and Wallace proposed natural selection as an explanation for how evolution occurs
- Why is Evolution by Natural Selection
so strongly associated with Darwin?
- He published The Origin of Species
a lengthy and well written
explanation of natural selection

3. The Foundations of Natural Selection
Observations of the natural world – the Beagle voyage
The writings of Thomas Malthus- population grows faster than food – leads to competition
Uniformitarianism – proposed by James Hutton
- the processes that shape the earth are
constant and ongoing at a slow uniform rate
- they are too slow to be seen in a human lifetime
but add up to major changes over geologic time

4. The Foundations of Natural Selection continued
Artificial selection- people have chosen variations in animals and plants to emphasize and produced plants and animals that are beneficial to humans

5. Artificial Selection

6. Genetic Variation: a key to natural selection
Mutation
Occurs randomly -1 in 100,000 replications - creates new alleles
- must occur in sex cells to be passed on
- can be harmful, helpful or have no effect

7. Genetic variation the key to evolution - continued
Migration – new individuals join a population and bring in new genes
Genes can move into or out of a population by migration

8. Variation a key to selection
Recombination
Meiosis-crossing over
Sexual Reproduction

9. Natural Selection All species have genetic variation
The environment provides challenges to
an individual ‘s ability to reproduce
More individuals are born than can survive creates competition to survive
Individuals who are better suited to the environment produce more offspring
The traits of individuals who are better suited to the environment increase in a population

10. Natural Selection in a Nutshell
Differences in phenotype leads to differences in the success rate of organisms to reproduce.
Those phenotypes which are better suited to their environment, have more reproductive success
Natural Selection acts on phenotype to change the gene pool

11. Natural Selection
all of the genes in a population are referred to as the population’s gene pool
Over time, the selection of phenotypes,
changes the frequency with which alleles occur in a population’s gene pool
- the alleles for advantageous traits become more common, while alleles for harmful traits become less common

12. Natural Selection Leads to Adaptations
Adaptations are traits that enhance the survival and reproductive success of an organism

13. 3 Types of Natural Selection: Directional Selection
Selects for one of the extreme phenotypes
- directional selection occurs in response to a change in the environment
that gives a competitive advantage to a particular phenotype

14. Examples of Directional Selection
Bacteria developing resistance to antibiotics
The beak sizes of ground finches on Daphne Island

15. 3 Types of Natural Selection: Stabilizing Selection
Selects for the average phenotype and against the extreme phenotypes
Occurs when the environment is stable for long periods
Example birth weight of babies.

16. Stabilizing Selection

17. 3 Types of Natural Selection: Disruptive Selection
Selects against the average and selects both extremes.
Example: African Seed Crackers

18. African Seed Crackers Birds feed on two types of seeds one
large, one small.
Birds with average
size bills can’t eat
either type efficiently
And so aren’t common
in the population

19. Genetic Drift: Another Force for Change
Genetic Drift = Changes in a population’s gene pool that happen by chance
- its effect is most profound in small
populations
- it reduces genetic variation in these
- this could reduce a population’s ability to survive environmental change

20. Genetic Drift Continued
An organism can be well suited to its environment can be removed by chance
The result: Organisms
that aren’t as well
adapted can survive to
reproduce by chance
- this shifts the gene pool
of the population

21. Genetic Drift
Once an allele is removed from a population, it is unlikely to return

22. Genetic Drift – The Bottle-neck effect
Something happens to catastrophically reduce a population’s size for at least
one generation
Dramatically changes allele frequencies
in a small population

23. Bottle necked species
Elephant seals hunted to near extinction – 20 individuals
- which 20 survived? The best adapted or the least desireable?
Was their survival random luck?
-now number 30,000 but all individuals are descended from the 20 survivors
-little genetic diversity this population has a reduced ability to survive an environmental challenge

24. Founder Effect
Occurs when a small population migrates to a new area or becomes isolated
Limited gene pool – over time some generally rare traits become more common
- Huntington’s disease in South Africa
- Amish communities
FE most commonly recognized in genetic disorders – occurs in other traits as well
When the Affrikans setteled S.Africa their original set of genes had a disproportionate number of Huntington’s alleles – since it was a small isolated population Huntingtons is much more common there than eleswhere

25. Genetic Drift as a force for change
In small isolated populations - genetic drift could allow otherwise rare alleles to become common
- this could cause the isolated population to diverge from the main population and over time become a new species