Definition from (NZQA) exam

Natural selection is where the best suited individuals have a greater chance of reproductive success. The survival of the species is promoted.

Individuals with more suited / better adapted phenotypes will compete more favorably than others that are less suited and are more likely to reproduce, passing on their favorable alleles.

Favorable alleles will increase in frequency within the population.

Directional selection 

is a type of natural selection in which the phenotype (the observable characteristics) of the species tends toward one extreme rather the mean phenotype or the opposite extreme phenotype. Directional selection is one of three widely studied types of natural selection, in addition to stabilizing selection and disruptive selection. In stabilizing selection, the extreme phenotypes gradually reduce in number in favor of the mean phenotype, while in disruptive selection, the mean phenotype shrinks in favor of extremes in either direction. 

Conditions Leading to Directional Selection

The directional selection phenomenon is usually seen in environments that have changed over time. Changes in weather, climate, or food availability can lead to directional selection. In a very timely example connected to climate change, sockeye salmon have recently been observed shifting the timing of their spawn run in Alaska, likely due to rising water temperatures. 

In a statistical analysis of natural selection, directional selection shows a population bell curve for a particular trait that shifts either further left or further right. However, unlike stabilizing selection, the height of the bell curve does not change. There are far fewer "average" individuals in a population that has undergone directional selection.

Human interaction can also speed up directional selection. For example, human hunters or fishermen pursuing quarry most often kill the bigger individuals of the population for their meat or other large ornamental or useful parts. Over time, this causes the population to skew toward the smaller individuals. A directional selection bell curve for size will show a shift to the left in this example of directional selection. Animal predators can also create directional selection. Because slower individuals in a prey population are more likely to be killed and eaten, directional selection will gradually skew the population toward faster individuals. A bell curve plotting species size will skew toward the right when documenting this form of directional selection. 

Disruptive selection 

is a type of natural selection that selects against the average individual in a population. The makeup of this type of population would show phenotypes (individuals with groups of traits) of both extremes but have very few individuals in the middle. Disruptive selection is the rarest of the three types of natural selection and can lead to the deviation in a species line.

Basically, it comes down to the individuals in the group who get to mate—who survive best. They are the ones who have traits on the extreme ends of the spectrum. The individual with just middle-of-the-road characteristics is not as successful at survival and/or breeding to further pass on "average" genes. In contrast, population functions in stabilizing selection mode when the intermediate individuals are the most populous. Disruptive selection occurs in times of change, such as habitat change or change in resources availability.

Disruptive Selection and Speciation

The bell curve is not typical in shape when exhibiting disruptive selection. In fact, it looks almost like two separate bell curves. There are peaks at both extremes and a very deep valley in the middle, where the average individuals are represented. Disruptive selection can lead to speciation, with two or more different species forming and the middle-of-the-road individuals being wiped out. Because of this, it's also called "diversifying selection," and it drives evolution.

Disruptive selection happens in large populations with lots of pressure for the individuals to find advantages or niches as they compete with each other for food to survive and/or partners to pass on their lineage.

Like directional selection, disruptive selection can be influenced by human interaction. Environmental pollution can drive disruptive selection to choose different colorings in animals for survival.

Disruptive Selection Examples: Color

Color, in regards to camouflage, serves as a useful example in many different kinds of species, because those individuals that can hide from predators the most effectively will live the longest. If an environment has extremes, those who don't blend into either will be eaten the most quickly, whether they're moths, oysters, toads, birds or another animal.

Peppered moths: One of the most studied examples of disruptive selection is the case of ​London's peppered moths. In rural areas, the peppered moths were almost all a very light color. However, these same moths were very dark in color in industrial areas. Very few medium-colored moths were seen in either location. The darker-colored moths survived predators in the industrial areas by blending in with the polluted surroundings. The lighter moths were seen easily by predators in industrial areas and were eaten. The opposite happened in rural areas. The medium-colored moths were easily seen in both locations and were therefore very few of them left after disruptive selection.​​

Oysters: Light- and dark-colored oysters could also have a camouflage advantage as opposed to their medium-colored relatives. Light-colored oysters would blend into the rocks in the shallows, and the darkest would blend better into the shadows. The ones in the intermediate range would show up against either backdrop, offering those oysters no advantage and make them easier prey. Thus, with fewer of the medium individuals surviving to reproduce, the population eventually has more oysters colored to either extreme of the spectrum.

Disruptive Selection Examples: Feeding Ability

Evolution and speciation isn't all a straight line. Often there are multiple pressures on a group of individuals, or a drought pressure, for example, that is just temporary, so the intermediate individuals don't completely disappear or don't disappear right away. Timeframes in evolution are long. All types of diverging species can coexist if there are enough resources for them all. Specialization in food sources among a population might occur in fits and starts, only when there is some pressure on supply.

Mexican spadefoot toad tadpoles: Spadefoot tadpoles have higher populations in the extremes of their shape, with each type having a more dominant eating pattern. The more omnivorous individuals are round-bodied, and the more carnivorous are narrow-bodied. The intermediate types are smaller (less well-fed) than those at either extreme of body shape and eating habit. A study found that those at the extremes had additional, alternate food resources that the intermediates didn't. The more omnivorous ones fed more effectively on pond detritus, and the more carnivorous ones were better at feeding on shrimps. Intermediate types competed with each other for food, resulting in individuals with ability on the extremes to eat more and grow faster and better.

Darwin's finches on the Galapagos: Fifteen different species developed from a common ancestor, which existed 2 million years ago. They differ in beak style, body size, feeding behavior, and song. Multiple types of beaks have adapted to different food resources, over time. In the case of three species on Santa Cruz Island, ground finches eat more seeds and some arthropods, tree finches eat more fruits and arthropods, vegetarian finches feed on leaves and fruit, and warblers typically eat more arthropods. When food is abundant, what they eat overlaps. When it's not, this specialization, the ability to eat a certain type of food better than other species, helps them survive.

Sexual Selection is another type of Natural Selection. In sexual selection, the female of the species tends to choose mates based on a group traits they show that are more attractive. 

The fitness of the males is judged based on their attractiveness and those who are found more attractive will reproduce more and more of the offspring will also have those traits.  

Sexual dimorphism is the difference in morphology between male and female members of the same species. Sexual dimorphism includes differences in size, coloration, or body structure between the sexes. For example, the male northern cardinal has a bright red plumage while the female has a duller plumage. Male lions have a mane, female lions do not.

Examples of Sexual Dimorphism

• Male elk (Cervus canadensis) grow antlers, while female elk do not have antlers.

• Male elephant seals (Mirounga sp.) develop an elongated snout and fleshy nose that they inflate as a sign of aggression when competing with other males during the mating season.

• Male birds of paradise (Paradisaeidae) are noted for their elaborate plumage and complex mating dances. Females are far less ornate.

In most cases, when size differences exist between the male and female of a species, it is the male that is the larger of the two sexes. But in a few species, such as birds of prey and owls, the female is the larger of the sexes and such a size difference is referred to as reverse sexual dimorphism.