Natural Selection: A Two Step Process

Sources of Variation


Sexual Reproduction

A colored human karyotype, which is a way of depicting a collection of chromosomes in an organism.

Unknown source.

More Detail on Sources of Variation in Sexual Reproduction

Summary and Contrast

Depicts how chromosomes replicate in interphase I of meosis, to clarify where the paired chromosomes come from.

Upper part of Figure 13.5, page 231, Campbell's Biology, 5th Edition

Independent Assortment of Homologous Chromosomes

Shows the two possibilities that could result from independent assortment in a two-chromosome organism.

Figure 13.8, page 238, Campbell's Biology, 5th Edition

Crossing Over

Shows and explains how crossing over works, using two homologous chromosome pairs.

Top right quadrant of Figure 13.7, page 234, Campbell's Biology, 5th Edition

Random Joining of Gametes

Depicts many sperm trying to fertilize an egg, showing how random the process is.

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Preservation of Variation


An illustration of a heterozygous pair of homologous chromosomes.

Figure 14.3, page 241, Campbell's Biology, 5th Edition

Balanced Polymorphism

Modes of Selection

Depicts how a snail population may be changed by three different types of selection, depending on the circumstances.

Figure 23.11, page 441, Campbell's Biology, 5th Edition

Sexual Selection