Populations of some species are prone to extinction for two reasons

1. The landscapes they live in are patchy (making dispersal between populations difficult).

2. Environmental conditions often change in a rapid and unpredictable manner.

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#XL - T Zoology

Postulate of Darwin

Geometric increase 

Limited food & Space

Struggle for existence 

Variation 

 Natural selection

Inheritance of useful characters

Speciation

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#XL - T Zoology

Darwin’s evolutionary theory - Common descent

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#XL - T Zoology

Gradualism is Darwin’s proposition that the differences between even radically different organisms have evolved by small steps through intermediate forms, not by leaps

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Natural selection was Darwin’s brilliant hypothesis, that accounts for adaptations, features that appear “designed” to fit organisms to their environment.

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Alfred Russel Wallace - Who was collecting specimens in the Malay Archipelago, had independently conceived of natural selection

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Populational change is Darwin’s hypothesis that evolution occurs by changes in the proportions (frequencies) of different variant kinds of individuals within a population.