Natural selection, which includes sexual selection, is the fact that some traits make it more likely for an organism to survive and reproduce. Population genetics describes natural selection by defining fitness as a propensity or probability of survival and reproduction in a particular environment. The fitness is normally given by the symbol w=1-s where s is the selection coefficient.
Atrial fibrillation with RVR (Rapid Ventricular Response) is a heart condition caused by irregular electrical activity that results in irregular contractions of the heart walls and its chambers. The heart (atrium), shakes with a rapid tremulous movement or makes fine irregular twitching movements, generally referred to as fibrillating.
Atrial fibrillation, often referred to as afib, is an irregular heartbeat, a rapid heartbeat, or a quivering of the upper chambers of the heart, called the atria. Atrial fibrillation is due to a malfunction in the heart's electrical system, and is the most common heart irregularity, or cardiac arrhythmia. For more detail about how the heart works and what happens to the heart when it is in atrial fibrillation, see About the Heart.