1. The great biological paradigm
NATURAL SELECTION
The great biological paradigm
© 2016 Paul Billiet ODWS

2. Observation 1: Exponential growth
Populations tend to produce more offspring than the environment can support.
© 2016 Paul Billiet ODWS

3. Thomas Malthus Essay on the Principle of Population (1798)
Populations in nature cannot continually increase. Sooner or later food supply is insufficient and famine stops further growth
Both Charles Darwin and Alfred Russel Wallace had read Malthus and understood the idea of exponential population growth.
© 2016 Paul Billiet ODWS

4. 1 pair of cockroaches could produce 164 000 million in 7 months
Cockroach population
© 2016 Paul Billiet ODWS

5. Observation 2: Zero growthK
Numbers
Time 1 2 3
The numbers of individuals in a population remain stable
In terms of population growth the population at its carrying capacity has zero growth.
© 2016 Paul Billiet ODWS

6. Deduction 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.
© 2016 Paul Billiet ODWS

7. Observation 3: Variation
Some of these variations are inherited by the offspring
The mechanism of the inheritance of genes was being worked out at this time, remained undiscovered by biologists until 1900
Darwin was however aware that sexual reproduction mixes variations to produce new combinations (recombinants).
© 2016 Paul Billiet ODWS

8. Neo-Darwinism The great synthesis of the 20th century
Mendel’s work was able to explain some of the patterns of inheritance through the mixing that occurs during meiosis and fertilisation
Darwin could not explain the origin of new variants
This had to wait until the 1920s and 1930s when work began on mutations after the discovery of radiation.
© 2016 Paul Billiet ODWS

9. Deduction 2: Survival of the fittest - Adaptation
There will be a struggle for survival between the members of the population
Individuals with advantageous variations will breed and produce more offspring.
© 2016 Paul Billiet ODWS

10. VISTA random Variations in living organisms… that are Inherited…
will be Selected…
through Time…
leading to Adaptations to changes in local environments.
© 2016 Paul Billiet ODWS

11. Natural selection in action
As generations pass by, the proportions of the alleles for the different variants will change, in favour of those that provide the best adaptations
Natural selection has been observed at work in populations of species over the past century
Examples include: pesticide resistance in insects, antibiotic resistance in bacteria, industrial melanism in moths, tolerance to heavy metals in plants.
© 2016 Paul Billiet ODWS

12. 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 favoured a long period of slow changes
Recently refined to include the possibility of rapid changes over a short period of time (punctuated equilibrium).
© 2016 Paul Billiet ODWS

13. Natural selection is not the only way
Whether fast or slow, observing the evolution of a new species is not easy in the lifetime of a scientist
That species evolve is a fact but that they evolve by natural selection is one theory
Other mechanisms exist that can also lead to the evolution of species (e.g. genetic drift).
© 2016 Paul Billiet ODWS

14. A revolutionary idea
The theory of natural selection is based on chance
It has no direction
There is no progressive “improvement” of organisms No guiding hand
Adam Smith “The Wealth of Nations” Laissez-faire economics
“Never say higher or lower” (Darwin) No organism is more evolved than any other.
© 2016 Paul Billiet ODWS

15. A persistent icon
© 2016 Paul Billiet ODWS