observes phenotypic differences. The combination of these four types of data allow population studies that can identify the type of selection occurring and quantify the extent of selection. However, a meta-analysis of studies that measured selection in the wild failed to find an overall trend for stabilizing selection. The reason can be that methods for detecting stabilizing selection are complex. They can involve studying the changes that causes natural selection in the mean and variance of the trait, or measuring fitness for a range of different phenotypes under natural conditions and examining the relationship between these fitness measurements and the trait value, but analysis and interpretation of the results is not straightforward. Examples The most common form of stabilizing selection is based on phenotypes of a population. In phenotype based stabilizing selection, the mean value of a phenotype is selected for, resulting a decrease in the phenotypic variation found in a population. Humans Stabilizing selection is the most common form of nonlinear selection (non-directional) in humans. There are few examples of genes with direct evidence of stabilizing selection in humans. However, most quantitative traits (height, birthweight, schizophrenia) are thought to be under stabilizing selection, due to their polygenicity and the distribution of the phenotypes throughout human populations. Birth Weight − A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious diseases more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher. The bell curve of the human population peaks at a birth weight that the newly born babies exhibit the minimum death rate. Plants Height − Another example of a trait, that might be acted on by stabilizing selection, is plant height. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease. Cacti Spine Number − Desert populations of spiny cacti experience predation by peccaries, which consume the fleshy part of the cactus. This can be prevented by increasing the