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

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

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

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

Observation 2: Zero growth K 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

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

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

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

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

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

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

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

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

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

A persistent icon © 2016 Paul Billiet ODWS