BIOE 109 Evolution Summer 2009 Lecture 2- Part II Formulation of theory of natural selection.

BIOE 109 EvolutionSummer 2009

Lecture 2- Part IIFormulation of theory of natural selection

Darwin’s “Five Theories”


1. Evolution per se


1. Evolution per se

• the world is steadily changing and populations of organisms are transformed over time.


1. Evolution per se


2. Common descent


1. Evolution per se


2. Common descent • every group of organisms has descended from a common ancestor.


1. Evolution per se


2. Common descent • every group of organisms has descended from a common ancestor.

• all species can ultimately be traced to a single origin of life on earth.

• this “dethroned” man from the pinnacle of “scala naturae”

Darwin’s tree of life

Tree of Life: the first-known sketch by Charles Darwin of an evolutionary tree describing the relationships among groups of organisms.

“Life had been originally breathed into a few forms or into one” (Origin, page 490)

Caricatures of Darwin…

London Sketch Book, 1874Hornet Magazine, 1872


3. Multiplication of species



• this process is now called “speciation”.




• Darwin’s view similar to what is now called allopatric speciation.





4. Gradualism





4. Gradualism

• most evolutionary change occurs slowly.





4. Gradualism


5. Natural selection





4. Gradualism


5. Natural selection

• this was Darwin’s mechanism for how evolutionary change occurred. • very few supported this explanation.

Similarities between Lamarck’s and Darwin’s theories


1. Lineages change over time - “evolution” occurs.



2. A continually changing world drives evolutionary change.



2. A continually changing world drives evolutionary change.

3. The rate of change is slow (gradualism).

Differences between Lamarck’s and Darwin’s Theories


Lamarck Darwin

Inheritance soft hard


Lamarck Darwin


Extinction no yes


Lamarck Darwin


Extinction no yes

Orthogenesis yes no


Lamarck Darwin


Extinction no yes

Orthogenesis yes no

Common descent no yes


Lamarck Darwin


Extinction no yes

Orthogenesis yes no


Speciation no yes


Lamarck Darwin


Extinction no yes

Orthogenesis yes no


Speciation no yes

Role of environment “creates” “sorts” variation variation

The Darwinian Revolution


Darwin’s theory challenged the most basic beliefs of his day:



1. The belief in a constant world of limited age.




2. The belief in a world designed by a wise and benign creator.





3. The belief in the immutability of species.






4. The belief in the unique position of man in creation.






4. The belief in the unique position of man in creation.

5. The belief in the philosophy of essentialism.

Darwin’s formulation of the principle of natural selection


Example of a syllogism


Example of a syllogism – “a form of reasoning

in which a conclusion is drawn from two or

more given, or assumed, propositions”.


Fact 1: Natural populations have large excess reproductive capacities.


Fact 1: Natural populations have large excess reproductive capacities. Fact 2: Population sizes generally remain stable.


Fact 1: Natural populations have large excess reproductive capacities. Fact 2: Population sizes generally remain stable. Fact 3: Resources are limiting.


Fact 1: Natural populations have large excess reproductive capacities. Fact 2: Population sizes generally remain stable. Fact 3: Resources are limiting. Inference 1: A severe struggle for existence must occur in nature.

Robert Thomas Malthus (1766-1834)

An essay on the principle of population, 1798


Fact 4: An abundance of variation exists among individuals of a species.


Fact 4: An abundance of variation exists among individuals of a species. Fact 5: A proportion of this variation is heritable.


Fact 4: An abundance of variation exists among individuals of a species. Fact 5: A proportion of this variation is heritable. Inference 2: Genetically superior individuals will outsurvive and/or outreproduce others.


Fact 4: An abundance of variation exists among individuals of a species. Fact 5: A proportion of this variation is heritable. Inference 2: Genetically superior individuals will outsurvive and/or outreproduce others. Inference 3: Over many generations,evolutionary change occurs in the population.

Evolution by natural selection can be defined as:

“changes in the relative frequencies

of different genotypes (genes) in a population

because of differences in the survivorship and/or

reproduction of their phenotypes”.

Objections to evolution by natural selection:

1.Objections from essentialists- selection can nevertouch the underlying “essence” of species

2. Objections from creationists




3. Ambiguity of the term “selection”- it implies someone who selects (is “nature” a replacement of God?)





Survive

Fittest

Who survives?





Survive

Fittest

Who survives? Who is the fittest?





4. Probabilistic nature of evolution





4. Probabilistic nature of evolutionStep 1: genetic variation is created





4. Probabilistic nature of evolutionStep 1: genetic variation is createdStep 2: variation is sorted due to process of selection






5. Objections to methods used- inductive vs. hypothetico-deductive






5. Objections to methods used- inductive vs. hypothetico-deductive

6. Absence of proof

BIOE 109 Evolution Summer 2009 Lecture 2- Part II Formulation of theory of natural selection.

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