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Fact

Pathway

◦ Establishment of genealogical relationship (“tree thinking”)

◦ Common descent

◦ Multiplication of species viasplitting or budding

Mechanism

◦ Natural selection and other mechanisms

Diversity could be explained using a natural mechanism which was also sufficient to explain good (and bad) design.

Thus, design was apparent and did not require a designer.

“Evil” is a meaningless concept when considering the natural world. Death and extinction are part of the natural process. Moral evil (i.e. human evil) has natural roots.

“I think Darwin's idea of natural selection is the best idea anybody ever had, ahead of Newton, ahead of Einstein. What it does is it promises to unite the two most disparate features of all of reality. On the one side, purposeless matter and motion, jostling particles; on the other side, meaning, purpose, design. Before Darwin these were completely separate realms. After Darwin we can see how they all fit together into a single big picture.”

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Five facts

Three inferences

Potential exponential increase of populations (“superfecundity”)

Source: Thomas Malthus, William Paley, observation

Observed steady-state stability of most populations

Source: observation

Limitation of resources

Source: observation

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“The power of population is so superior to the

power of the earth to produce subsistence for

man, that premature death must in some shape or

other visit the human race. The vices of mankind

are active and able ministers of depopulation.

They are the precursors in the great army of

destruction; and often finish the dreadful work

themselves. But should they fail in this war of

extermination, sickly seasons, epidemics,

pestilence, and plague, advance in terrific array,

and sweep off their thousands and tens of

thousands. Should success be still incomplete,

gigantic inevitable famine stalks in the rear, and

with one mighty blow levels the population with

the food of the world.“

Essay on the Principle of Population, 1798

Linear Growth

Exponential Growth

MalthusianCatastrophe

Food Production

Population Growth

Struggle for existence among individuals

Author of inference: Malthus

Variation between individuals

Source: animal breeders & observation

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Heritability of much of this variation

Source: animal breeders

Some of this variation is advantageous in certainenvironments therefore differential survival, i.e. natural selection, will occur.

Inference held by Darwin& Wallace.

(Also held by Wells, Blyth & Matthew. See next lecture)

Over many generations, provided selection pressure is maintained, evolution will occur

Inference unique to Darwin & Wallace

If there are variations, and

If these are inherited, and

If one variant is more suited to some task that the others, and

If that task directly affects survival and therefore reproduction,

Then selection will result in evolutionary change

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Evolution by natural selection need not occur, however, if evolution does occur it will be according to certain “rules”

Natural selection acts on individuals but its consequence occur in populations. It does not, however, work for the good of the species.

Fitness is a relative concept depending on the environment and the population.

No need to be able to predict long-term course of evolution. Evolution is analogous to a poker tournament.

Natural selection is not forward looking and does not lead to perfection or necessarily progress. Adaptations need not be “perfect” in any sense.

Natural selection is non-random.

Random

Variation

Non-Random

Selection

A: X is intricate and well suited to a task T

W1: X is a product of intelligent design

W2: X is a product of random physical forces

Paley claims that the likelihood of W1 given Aexceeds that of W2, i.e. P(A|W1) >> P(A|W2)

A: X may or may not be intricate or well-suited to a task T.

W1: X is a product of intelligent design

W2: X is a product of a non-random natural mechanism

Darwin claims that the likelihood of W2 given A exceeds that of W1, i.e. P(A|W2) >> P(A|W1)

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“A celebrated author and divine has written to me that „he has gradually learnt to see that it is just as noble a conception of the Deity to believe that He created a few original forms capable of self-development into other and needful forms, as to believe that He required a fresh act of creation to supply the voids caused by the action of His laws.‟”

Not in originating natural selection

But in seeing what natural selection was potentially capable of doing

But where was the evidence for natural selection?

1. Is the population variable?

2. Is some of the variation among individuals

within the population heritable?

3. Do individuals vary in their success as

surviving or reproducing?

4. Are survival and reproduction non-random?

5. Did the population change over time?

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“This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called 'natural selection', or the preservation of favouredraces in the struggle for life.”

Principles of Biology 1:444 (1864)

Herbert Spencer

Suggested by Wallace and first used by Darwin in 5th edition (1869):

“I have called this principle, by which each slight variation, if useful, is preserved, by the term natural selection, in order to mark its relation to man's power of selection. But the expression often used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient.”

“fittest” = “most suited”

Darwin worried about losing “the analogy between nature's selection and the fanciers [i.e. breeders].”

“I am convinced that [it] has been the main, but not exclusive means of modification.”

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Part I – The Descent or Origin of Man

Part II – Sexual Selection

Part III – Sexual Selection in Relation to Man and Conclusion

Male / Male Competition

◦ Weaponry for fighting and display

Female Choice

◦ Even if male wins, he might not be chosen.

◦ Indicators of fitness and condition

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“Descent with modification through natural selection”

Transmutation of Species

Common Descent

„Struggle‟ with biosphere & other organisms

MechanismNatural Selection

Sexual Selection

Lamarckism