Meth and the Brain. 1  Meth releases a surge of dopamine, causing an intense rush of pleasure or prolonged sense of euphoria. 2  Over time, meth destroys dopamine receptors, making it impossible to feel pleasure. 3  Although these pleasure centers can heal over time, research suggests that damage to users' cognitive abilities may be permanent.
Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral contained in nacre, with a microstructure inherited from the shell.
On each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the amount of organisms. Group A is the original population and Group B is the population after selection. Graph 1 shows directional selection, in which a single extreme phenotype is favored. Graph 2 depicts stabilizing selection, where the intermediate phenotype is favored over the extreme traits. Graph 3 shows disruptive selection, in which the extreme phenotypes are favored over the intermediate. Stabilizing selection (not the same thing as negative selection) is a type of natural selection in which genetic diversity decreases and the population mean stabilizes on a particular trait value.