Short guide to writing an abstract

An abstract must contain the essential elements of the report: it must tell the readers what problem was addressed and the most important results that are being reported. Usually it also indicates something about the importance of the work, such as what it contributes to a long-standing area of research. It may also tell which methods were used, but usually not in detail (unless a new method is part of the report).

A useful formula for constructing abstracts is 1) an opening sentence that indicates what the general area of research is, often as a general statement of an important process or problem; 2) a second sentence that shows where within that area the present report belongs; and 3) something like "Here we present..." or "We have measured/tested..." or "To test this hypothesis, we...". This is followed by an outline of the main results and then a concluding sentence saying why this is so important.

Here are two examples: 

Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma
Fischer B and Mitteroecker P. 2015: PNAS 112:5655-5660. 
http://www.pnas.org/content/112/18/5655.full

Compared with other primates, childbirth is remarkably difficult in humans because the head of a human neonate is large relative to the birth-relevant dimensions of the maternal pelvis. It seems puzzling that females have not evolved wider pelvises despite the high maternal mortality and morbidity risk connected to child- birth. Despite this seeming lack of change in average pelvic morphology, we show that humans have evolved a complex link between pelvis shape, stature, and head circumference that was not recognized before. The identified covariance patterns contribute to ameliorate the “obstetric dilemma.” Females with a large head, who are likely to give birth to neonates with a large head, possess birth canals that are shaped to better accommodate large-headed neonates. Short females with an increased risk of cephalopelvic mismatch possess a rounder inlet, which is beneficial for obstetrics. We suggest that these covariances have evolved by the strong correlational selection resulting from childbirth. Although males are not subject to obstetric selection, they also show part of these association patterns, indicating a genetic–developmental origin of integration.

On the Origin of Species by Natural and Sexual Selection 
van Doorn GS, Edelaar P and Weissing FJ. 2009: Science 326:1704-1707.
http://science.sciencemag.org/content/326/5960/1704

Ecological speciation is considered an adaptive response to selection for local adaptation. However, besides suitable ecological conditions, the process requires assortative mating to protect the nascent species from homogenization by gene flow. By means of a simple model, we demonstrate that disruptive ecological selection favors the evolution of sexual preferences for ornaments that signal local adaptation. Such preferences induce assortative mating with respect to ecological characters and enhance the strength of disruptive selection. Natural and sexual selection thus work in concert to achieve local adaptation and reproductive isolation, even in the presence of substantial gene flow. The resulting speciation process ensues without the divergence of mating preferences, avoiding problems that have plagued previous models of speciation by sexual selection.