1. The great biological paradigm
NATURAL SELECTION
The great biological paradigm
© 2008 Paul Billiet ODWS

2. Observation 1: Exponential growth
Populations tend to produce more offspring than the environment can support
© 2008 Paul Billiet ODWS

3. Population Growth
1 pair of cockroaches could produce 164 million offspring in 7 months
Cockroach population
© 2008 Paul Billiet ODWS

4. Numbers of Individuals
Exponential Population growth
Numbers of Individuals
Time K 2 1
© 2008 Paul Billiet ODWS

5. Observation 2: Zero growth
The numbers of individuals in a population remain stable
In terms of population growth the population at its carrying capacity has zero growth
© 2008 Paul Billiet ODWS

6. Numbers of Individuals
Population growth at carrying capacity (k)
Numbers of Individuals
Time K 3 2 1
© 2008 Paul Billiet ODWS

7. If there is a stable population of 100 mice and each mouse pair can produce 12 babies.
What must happen to the extra babies?
They die.

8. Infrence 1 Competition There must be a struggle for survival
Some of the offspring produced in a generation do not survive.
Darwin identified competition as a major factor limiting population sizes
© 2008 Paul Billiet ODWS

9. Observation 3: Variation
Every species has slight differences between them; Variation.
The variation results in differences in the adaptations with in a population.
What are some examples of variation in an adaptation?
Where is information stored to determine how an organism grows and develops.
How could that information be changed?
© 2008 Paul Billiet ODWS

10. Inference 2: Survival of the fittest
There will be a struggle for survival between the members of the population
Individuals with advantageous adaptations will breed and produce more offspring
© 2008 Paul Billiet ODWS

11. Natural selection in action
Examples include:
pesticide resistance in insects,
antibiotic resistance in bacteria,
industrial melanism in moths,
tolerance to heavy metals in plants
© 2008 Paul Billiet ODWS

12. Different types of selection
Directional Selection
What most of us think about when we think about selection
An extreme value is selected for
Ex.Clear fur in what would become polar bears

13. Different kinds of selection
Stabilizing or Normalizing selection
The middle is selected for
Many examples here
Symmetry
Two eyes

14. Different kinds of selection
Disruptive selection
Extremes are selected for
Might be where the two sexes come from
Can lead to speciation (the creation of a new species).

15. The Origin of Species by Natural Selection
Darwin and Wallace argued that if natural selection proceeded for a long enough period of time it could bring about the evolution of new species
Darwin himself favored a long period of slow changes
Recently this has been refined to include the possibility of rapid changes over a short period of time (punctuated equilibrium)
© 2008 Paul Billiet ODWS

16. Genetic Drift
Any changes in the allele frequency in a population due to random chance
This is only possible in a small population
If there are five bugs of two colors
And three get killed this can dramatically change the future generations

17. The Bottleneck effect or The squished bug effect
If the population is small then random chance can play a big role in the ratio of population with no Natural selective forces.

18. The Bottleneck Effect
The actions of people sometimes cause bottlenecks in other species.
N. California elephant seal population reduced to individuals in the 1890s.
Current population > 30,000.
Variation drastically reduced – 24 genes with 1 allele. 18

20. Founder effect Related to genetic drift
If a small portion of a larger population is isolated
The groups allele frequency may be very different from the original population

21. Polydactyly
The Amish have a much higher frequency of Polydactyly with in there population due to the founder effect. The original immigrants who started the Amish communities had this genetic disorder and since they tend to only marry other Amish it has persisted in the population.

22. Convergent Evolution
Thylacine Thylacinus cynocephalus
Species exposed to the same selective pressure in different parts of the world tend to develop the same adaptations
Even though they may be completely unrelated
e.g. the placental wolf and the marsupial thylacine or Tasmanian wolf
Wolf Canis lupus
© 2008 Paul Billiet ODWS

23. Convergent Evolution

24. Divergent Evolution
Populations of a species that are separated and evolve under different selective pressures develop different adaptations as they diverge
They are usually geographically separated so that there is a barrier to the mixing of genes
© 2008 Paul Billiet ODWS

25. Mammal Forelimbs

26. Adaptive radiation
Species that populate a new area where there are vacant niches will diverge and specialise as they fill the vacant niches (e.g. Darwin’s Finches on the Galapagos)
© 2008 Paul Billiet ODWS