Sex change in plants and animals: a unified perspective

Asexual reproduction allows plants to reproduce without growing flowers, attracting pollinators, or dispersing seeds. Offspring are genetically identical to the parent and produced without the fusion of male and female gametes.

From Water to Land

In most organisms, sex is determined by the ratio of X and Y chromosomes. However, in some organisms, such as Drosophila and C.elegans, sex is determined by the ratio of the number of X chromosomes to the number of sets of autosomes. The Y chromosome in Drosophila is active but does not determine sex. It contains genes responsible for the production of sperms in adult flies.  
Normal male Drosophila has a ratio of one X chromosome to two sets of autosomes. In contrast, normal female...

Overview

Speciation is the evolutionary process resulting in the formation of new, distinct species—groups of reproductively isolated populations.

Speciation describes the formation of one or more new species from one or sometimes multiple original species. The resulting species are discrete from the parent species, and barriers to reproduction will typically exist. There are two primary mechanisms, speciation with and without geographic isolation—allopatric and sympatric speciation, respectively.