Natural selection can occur with or without environmental change. Show
Directional selectionDirectional selectionDirectional Selection: If thicker-shelled oysters are more resistant to breakage than thinner-shelled oysters, crabs will be less able to prey upon them, and thicker-shelled oysters will be more likely to survive to reproduce. Stabilizing selectionStabilizing selectionStabilizing Selection: Continuing our oyster example, very light-colored or very dark-colored oysters might be more frequently preyed upon by shore birds, simply because they are more obvious on the oyster bar; as a result, the intermediate hues become more common. Disruptive or Diversifying selectionDisruptive selectionDisruptive
Selection: This might happen in shallow water among rocks. Light-colored oysters are
more crypticCryptic Coloration: Kin selectionKin selectionKin Selection: Sexual selectionThe evolutionary fitness of an organism not only depends upon its ability to survive but also its ability to reproduce. To reproduce, an individual must obtain a mate and produce viable
offspring. Natural selection favors traits that maximize the ability of an individual to compete for and attract mates, and/or the ability to produce offspring — this is called sexual selectionSexual Selection: Sexual dimorphismFemales of the Cumberlandian Combshell have a low bulge along the edge of the shell that accommodates enlarged gills for brooding larvae; males have more streamlined shells. Sexual dimorphismSexual Dimorphism: Some freshwater pearl mussels also exhibit sexual dimorphism, but obviously not to visually attract a mate. The endangered Cumberlandian Combshell (Epioblasma brevidens) of the American Midwest is a medium-sized (to 2 inches) mussel with an oval to quadrate shell. Female shells have a low bulge along the edge of the shell that accommodates the enlarged gills in which it broods its larvae; males lack this bulge and are more streamlined. In this example, females that have a larger bulge (= larger gills) might produce more offspring, ultimately leading to a population with females that have larger bulges and larger gills. Mating rituals and other complex behaviorsOctopus cyanea on a reef in Kona, Hawaii. As with appearance, males that have the most attractive mating ritual potentially win the right to mate with the female. Birds provide the best examples of complex mating rituals, often involving dances, vocalizations, and fluffed up feathers. Over time, the rituals most preferred by the females will become more common. There are no good examples of mating rituals among bivalves, but we can look to their relative, the octopus. The Day Octopus (Octopus cyanea), found in the South Pacific and Indian Oceans, has an elaborate mating ritual that signals his mating intent to the female. The ritual involves arm waving and skin darkening as the male approaches the female. If the male is positively received, the female is inseminated by the placement of spermatophores into her oviduct through the use of his modified arm. The female then lays thousands of eggs inside her lair. In this way, successful male rituals are passed down to offspring; males whose displays fail to attract a mate do not produce offspring. Find more information about the Day Octopus here. The Wavy-Rayed Lampmussel (Lampsilis fasciola), of the American Midwest, has an elaborate lure display. Watch a video produced by North Carolina State University here. Freshwater pearl mussels have an interesting behavior of another sort related to reproduction. Female mussels brood their larvae, called
glochidiaGlochidium: Male-male competitionThe Common Octopus, Octopus vulgaris, is found worldwide in tropical and semitropical waters from nearshore shallows to as deep as 200 meters (over 650 feet). It is one of the most studied invertebrate animals. In species with males that battle over rights to mate with females, such as elephants and deer, the male that wins a fight because he is the strongest, most dominant, or most intelligent will win the right to mate with the female. Over time, the features that allow the males to win (larger tusks, larger antlers, larger body size) will become more common. We again look to the octopus for a molluscan example. Octopus males are very aggressive when fighting over females. They are seen wrestling and rolling around on the sea floor. In some cases, they have been observed strangling an opponent by cutting off the flow of water to its gills. You can read the 2008 interview with Dr. Christine Huffard, University of California at Berkeley, that details this behavior here. You can also watch a video of the Indian Ocean octopus, Abdopus aculeatus, fighting in the wild over a female here, or another battle (showing some dramatic color changes on the part of the two octopuses) here. When selection favors intermediate phenotype and act against extreme phenotype is called as?Stabilizing selection
When individuals with intermediate phenotypes are favoured and extreme phenotypes are selected against, the selection is said to be stabilizing. (See the left column of the figure.) The range and distribution of phenotypes then remains approximately the same from one generation to another.
What type of selection favors intermediate phenotypes?The only choice that would favour intermediate phenotypes of a species over extreme phenotypes within a particular population is stabilizing selection. Stabilizing selection will select against extreme phenotypes of a particular species.
What is natural selection against extreme phenotype called?In population genetics, directional selection, is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype.
What type of selection would favor individuals of intermediate rather than extreme sizes?Stabilizing selection
occurs when natural selection favors the intermediate states of continuous variation. Over time, the intermediate states become more common and each extreme variation will become less common or lost.
|