Compare and contrast directional selection and disruptive selection and provide an example of each

1. SIMILARITIES
Both directional and disruptive selection are types of natural selection. There are three types of natural selection, these are: stabilizing, directional and disruptive selections. Natural selection refers to the mechanism of organisms' evolution, which favor those that are more adapted to their environments. Such living organisms tend to live long, up to reproductive stage and give birth to many offspring. 

2. DIFFERENCES
Directional selection theory states that an extreme trait is always favored over other traits and this causes the allele frequency to shift over a period of time to favor the extreme trait. This implies that, a favored trait will be expressed as the most beneficial frequency in a population. The disruptive selection on the other hand is bimodial in nature and it favors two extreme traits in a population. This type of selection selects against the average individuals in the population by selecting traits at both extremes, thus leaving very few individuals in the middle. 

3. EXAMPLES
An example of directional selection is the length of the giraffe neck. Natural selection favored those giraffes, which have longer necks and can reach more food on high trees while those will shorter necks were wipe out, because they have restricted access to foods.
 An example of disruptive selection is the beak size of finches. The study on these birds was carried out by Darwin. He observed that, because the majority of seeds found in the environment of the birds were either large or small, both finches with large and small beaks were favored in that environment.

Environmental science studies the interactions between the physical, chemical, and biological components of the environment, including their effects on all types of organisms. earth science (also known as geoscience), is an inclusive term for all sciences related to earth (geology, meteorology, oceanogr maphy, etc).  although environmental and earth science cover essentially the same material, environmental science places greater emphasis on the biological realm, while earth science places greater emphasis on the physical realm.    environmental and earth science study the interactions of four major systems or “spheres” (figure 8.6).  the  geosphere  consists of the core, mantle and crust of the earth.the  atmosphere  contains all of the earth’s air and is divided into troposphere, stratosphere, mesosphere, thermosphere and ionosphere.the  hydrosphere  contains all of the solid, liquid and gaseous water on earth, extending from the depths of the sea to the upper reaches of the troposphere where water is found.  ninety-seven percent of the hydrosphere is found in salty oceans, and the remainder is found as vapor or droplets in the atmosphere and as liquid in ground water, lakes, rivers, glaciers and snowfields.  the  biosphere  is the collection of all earth’s life forms, distributed in major life zones known as biomes:   tundra, boreal forest, temperate deciduous forest, temperate grassland, desert, savannah, tropical rainforest, chaparral, freshwater, and marine.although the four systems have their unique identities, there is substantial interaction between them.  environmental scientists study the effects of events in one sphere on the other spheres. for example, a volcanic eruption in the geosphere may cause profound direct and indirect effects on the hydrosphere, atmosphere and biosphere as follows: example 1 (volcano)    on may 18, 1980, mount saint helens, in the state of washington, erupted.  this event altered the surrounding environment, and provided scientists with an opportunity to study the effects of volcanic eruptions on the geosphere, hydrosphere, atmosphere and biosphere.  such studies are vital because volcanic eruptions will continue to occur, and will have increasing impact on humans as people continue to settle lands closer to dormant volcanoes. the following are but a few of the myriad of interactions resulting from a volcanic 

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