Where, though, do these hereditary variations come from?

This was the great gap in Darwin’s theory, and he never filled it. The problem was serious because, according to the prevailing belief in blending inheritance, variation should decrease, not increase.

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#XL - T Zoology

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λ represents the finite rate of population change over some time interval, often a year

λ = N1/N0

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Populations grow when R0 or λ>1, populations decline when R0 or λ<1, and are at equilibrium when R0 or λ=1

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The simplest method to measure population density is to visually count the number of organisms in a population in a given habitat.

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Quadrats - A square frame often, but not always, measuring 50 × 50 cm. 

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Pitfall traps set into the ground catch wandering species such as beetles and spiders. These have rain covers to prevent the animals from drowning