Darwin’s Tea Party The Theory of Evolution by Natural Selection



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Darwin’s Tea Party


Biological Evolution

  • Definition:

  • “Change in the genetic composition of a population during successive generations (due partly to natural selection) and resulting in the development of new species” (Wesbter’s).



Biological Evolution

  • The ancestors of the horse, for example, have undergone considerable change in physical size and proportions over the last 60 million years.



Biological Evolution

  • The adult size, shape, bones, internal organs and many other physical and behavioural features have changed.

  • The original Eohippus was thus a different “type of animal” (species) than the present day horse (Equus)



Evolution and Genes

  • Such changes in body parts are the result of changes in genes.

  • When nature “selects” the “fittest” for survival and reproduction, it is also selecting the fittest combination of genes to survive and reproduce.

  • These come “packaged” in a creature’s body.



Evolution and Genes

  • Thus Biological Evolution happens through the transmission of genetic characteristics from one generation to the next.

  • Individuals who are the most genetically “fit”, survive to reproduce (called natural selection or “survival of the fittest”) and pass on their fit characteristics.



Evolution and Genes

  • During Darwin’s time not much was known about genetics.

  • But Darwin did know that variation existed.

  • That was enough to establish that in every species only those individuals that had favourable variations would be more fit and thus live on and reproduce.



Evolution by Natural Selection

  • Natural selection is not the same as evolution

  • Natural selection is one cause of biological evolution.

  • It is like an “engine” that drives evolution on.



Evolution by Natural Selection

  • “Natural selection” is a metaphor which Darwin compared to “artificial selection” (the breeding of plants & animals).

  • In “artificial selection”, humans “select” which individual plant or animal will reproduce.

  • In natural selection, “nature” does this “selecting”.



Natural & Artificial Selection

  • In fact, Darwin begins his Origin by discussing the artificial selection (breeding) of pigeons.

  • In this example, we can see how three radically different looking breeds of pigeon (Pouter, Jacobin and Fantail) have been produced by artificial selection from the original Wild Rock pigeon.



Natural & Artificial Selection

  • Varieties of bananas produced by artificial selection



Natural & Artificial Selection



Natural Selection

  • How does it work?



Natural Selection is like a machine with 3 parts:

  • 1. Variation

  • The tendency for individuals in a species to differ slightly from their parents and from each other.



1. Variation



1. Variation

  • The tendency for individuals in a species to differ slightly from their parents and from each other.



1. Variation



1. Variation



1. Variation



1. Variation

  • The exact causes of variation were discovered only after Darwin’s death. The discovery was made by the Austrian monk Gregor Mendel, the father of modern genetics.



1. Variation

  • One cause of variation is sex - the combining of the genetic material of male and female to create offspring. (Also called sexual recombination)

  • Another cause was mutation - accidents to the genetic material

  • Variation is NOT caused by population increase or by the environment or by use/disuse, etc…



1. Variation

  • Variation means there are different degrees of fitness among individuals in a population of the same species.

  • Some happen to be born with advantageous variations (giving them added fitness) and others not.



2. Geometrical Population Increase



2. Geometrical Population Increase

  • The tendency for a population of any given species tends to outstrip the available resources necessary to keep it alive.



2. Geometrical Population Increase

  • The “principle of population” was first expressed by the British political economist, Reverend Thomas Malthus.



2. Geometrical Population Increase

  • In the human world, Malthus said, population tends to increase at a “geometric” rate while resources increase only at an “arithmetic” rate.



2. Geometrical Population Increase

  • Given this geometric rate of increase, part of the human population is bound to be poor or even die.



2. Geometrical Population Increase



2. Geometrical Population Increase

  • This “struggle for survival” would leave only the “fittest” individuals to survive (those with favourable variations) and reproduce

  • Thus:

  • GPI + V = NS (natural selection also called “the survival of the fittest”).



2. Geometrical Population Increase

  • By “fit” Darwin meant not just the “strongest” but those best adapted to the environment

  • These would tend to leave more offspring behind than the less fit.



3. Deep Time



3. Deep Time

  • Deep time is the theory that the earth – and life on it – have been around for much, much longer than previously thought.



3. Deep Time

  • Given sufficient time (deep time) natural selection will have “weeded out” so many unfavourable variations and accumulated so many favourable variations that a new species will have developed from a previous ancestor.



3. Deep Time

  • Evolution by Natural Selection thus required millions of years to transform small changes into big ones.

  • But many naturalists believed that the Earth was only about 6,000 years old.



3. Deep Time

  • The Irish Archbishop James Ussher (1581-1656) calculated the age of the earth according to what he believed the Bible indicated.



3. Deep Time

  • According to Ussher the universe began on Sunday, 23 October 4004 BC at 9:00 AM.

  • This was not enough time for evolution to have occurred



3. Deep Time

  • But by the 19th century, the science of geology confirmed that the earth was actually millions of years old.

  • Key was geologist Charles Lyell’s book Principles of Geology which Darwin had read on the Beagle.



3. Deep Time

  • Thanks to Lyell and the science of geology, the earth – and life on it – were proven to be much older than anyone previously thought

  • For Darwin deep time was a key to acceptance of his evolutionary theory which required a huge time scale.



3. Deep Time: The Grand Canyon



3. Deep Time: The Grand Canyon



3. Deep Time



3. Deep Time

  • Another way of visualizing deep time

  • Note the many periods of glaciation

  • Note the changes in the shape and position of continents



Summary: Natural Selection

  • Geometrical Population Increase (GPI)

    •  causes “struggle for survival” (meaning only some individuals will live and reproduce)
  • Variation (V)

    • causes differences in fitness b/w individuals
    • THUS:
    • GPI + V = “survival of the fittest” (natural selection)


Summary: NS + Deep Time



Cast of characters



The End

  • Written & Directed by

  • Gabriel Tordjman




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