When it's not everted, it's just an empty sac like a deflated lungso it probably couldn't exert much force on anything. Evolution has no purpose. Repp, M. This type of selection often drives speciation. Researchers investigated females of Phymaturus and Liolaemus lizards and discovered a structure that was previously undescribed.
The Encyclopedia of Snakes.
International Zoo Yearbook Sex Dev. Alligators That Are Always Erect. Horm Behav. Consequently, females can store sperm internally  for as long as five years, or possibly longer. Specifically, the hemipenes of squamata are found to develop on the posterior side, while the paired genitals of non-squamata amniota develop on the anterior side.
Relatively few snakes are venomous. That could be helpful in case antivenin treatment is needed. Back to the Elk Grove Patch. Eliminate shelter.
Some alleles may be more likely to be passed on with alleles that confer a beneficial phenotype because of their physical proximity on the chromosomes. Amarello, C. Snakes do so much that is good for humans, yet humans by and large hate and fear snakes. Key Terms directional selection : a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction disruptive selection : or diversifying selection a mode of natural selection in which extreme values for a trait are favored over intermediate values stabilizing selection : a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value.
Asymmetry in hemipenis size and usage in gartersnakes Thamnophis sirtalis. The handicap principle states that only the best males survive the risks from traits that may actually be detrimental to a species; therefore, they are more fit as mating partners.