In biology, Darwinian fitness or simply fitness of a biological trait describes how successful an organism has been at passing on its genes. It is different from physical fitness wherein the latter is concerned about the physical wellbeing of an organism. In Darwinian fitness, it is about the suitability of an organism to reproduce offspring. The more likely that an individual is able to survive and live longer to reproduce, the higher is the fitness of that individual. There are two ways through which fitness can be measured: (1) absolute fitness and (2) relative fitness. Absolute fitness pertains to the fitness of an organism based on the number of offspring that a fit organism would reproduce in its lifetime and that its offspring would reach reproductive age. Relative fitness is a standardized absolute fitness. It is a measure of biological fitness wherein the reproductive rate (of a genotype or a phenotype) is relative to the maximum reproductive rate (of other genotypes or phenotypes) in a given population. It can be measured by absolute fitness divided by the average number of offspring in a particular population. It is expressed as wrel.