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In population genetics, directional selection is a type of natural selection in which one extreme phenotype is favored over both the other extreme and moderate phenotypes. This genetic selection causes the allele frequency to shift toward the chosen extreme over time as allele ratios change from generation to generation.
[1] [2] The positions of the limbs (and the arms in particular) have important implications for directional terms in those appendages. The penis in the anatomical position is described in its erect position and therefore lies against the abdomen, hence the dorsal surface of the penis is actually anterior when the penis is flaccid.
The uncinate fasciculus is a bi-directional pathway between the temporal lobe and frontal lobe; it is traditionally considered to be part of the limbic system. [2] It has been proposed that the uncinate fasciculus allows mnemonic representations stored in the temporal lobe to interact with and guide decision making in the frontal lobe. [4]
The peacock tail in flight, the classic example of an ornament assumed to be a Fisherian runaway. Fisherian runaway or runaway selection is a sexual selection mechanism proposed by the mathematical biologist Ronald Fisher in the early 20th century, to account for the evolution of ostentatious male ornamentation by persistent, directional female choice.
By reducing the size of the opening, organisms achieved true imaging, allowing for fine directional sensing and even some shape-sensing. Eyes of this nature are currently found in the nautilus. Lacking a cornea or lens, they provide poor resolution and dim imaging, but are still, for the purpose of vision, a major improvement over the early ...
For improved clarity, the directional term palmar (from Latin palma 'palm of the hand') is commonly used to describe the front of the hand, and dorsal is the back of the hand. For example, the top of a dog 's paw is its dorsal surface; the underside, either the palmar (on the forelimb) or the plantar (on the hindlimb) surface.
The rostro-caudal axis of the human central nervous system (magenta in the diagram) makes a near 90° bend at the level of the midbrain and continues through the brain-stem and spinal cord. In human anatomy, the occipital lobes and the back of the head are posterior but not caudal to the frontal lobes and the face.
Schematic of the HPA axis (CRH, corticotropin-releasing hormone; ACTH, adrenocorticotropic hormone) Hypothalamus, pituitary gland, and adrenal cortex The hypothalamic–pituitary–adrenal axis (HPA axis or HTPA axis) is a complex set of direct influences and feedback interactions among three components: the hypothalamus (a part of the brain located below the thalamus), the pituitary gland (a ...