19.1 On the Road To Speciation
Species means "kind," as in "a particular kind of duck."
Through the history of any species, micro evolutionary processes have been introducing changes in the gene pools of its populations. Remember, particular alleles give rise to the heritable traits of populations. As an outcome of mutation, genetic drift, gene flow, and natural selection, different alleles can increase or decrease in frequency, or even disappear over time.
Given the variation in phenotypic details, perhaps we should move past the details, down to a more basic function that units members of a species and isolates them to all other species.
Reproduction as a defining function in species is central to the biological species concept.
"Species are groups of interbreeding natural populations that are reproductively isolated from other such groups. They belong to the same species because they can contribute to a shared gene pool.
19.2 Reproductive Isolating Mechanisms
a. Certain isolating mechanisms take effect before or during fertilization. Because they prevent mating or pollination between genetically divergent populations, hybrid zygotes cannot form. The same isolating mechanisms operate to prevent interbreeding of individuals of different species.
b. Temporal Isolation
Temporal isolation results from interpopulational differences in the timing of reproduction. For most animals or plants, mating or pollination is a seasonal event that is over rather quickly, sometimes in less than a day. Even closely related species may be isolated from each other if their reproductive cycles do no coincide.
.The cicada, a type of insect that spends nearly all of its life underground in immature form, feeding on juices from plant tissues. One species matures, emerges, and reproduces every 13 years. The other species does this every 17 years. Only once in 221 years do they release gametes at the same time.