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The northern spotted owl lives in the Pacific Northwest, and the Mexican spotted owl lives in Mexico and the southwestern portion of the United States. The two owls are similar in appearance but with slightly different coloration.
The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of incipient speciation. (credit “northern spotted owl”: modification of work by John and Karen Hollingsworth, USFWS; credit “Mexican spotted owl”: modification of work by Bill Radke, USFWS)

Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely for speciation to take place. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls; in the north, the climate is cooler than in the south; the other types of organisms in each ecosystem differ, as do their behaviors and habits; also, the hunting habits and prey choices of the owls in the south vary from the northern ones. These variances can lead to evolved differences in the owls, and over time speciation will likely occur unless gene flow between the populations is restored.

In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species, which is called adaptive radiation    . From one point of origin, many adaptations evolve causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island, which leads to geographical isolation for many organisms ( [link] ). The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved, including the eight shown in [link] .

The illustration shows a wheel, with the founder species that gave rise to various honeycreeper birds at the hub. Between spokes of the wheel are six modern honeycreeper species. Five of these, the ‘Apapane, Liwi, ‘Amakihi, ‘Akiapola’au and Maui Parrotbill, eat insects and/or nectar and have long, think beaks. The sixth bird, the Nihoa Finch, eats insects, seeds, and bird eggs, and has a short, fat beak.
The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics.

Notice the differences in the species’ beaks in [link] . Change in the genetic variation for beaks in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The fruit and seed-eating birds have thicker, stronger beaks which are suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach their nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another well-studied example of adaptive radiation in an archipelago.

Concept in action

Click through this interactive site to see how island birds evolved; click to see images of each species in evolutionary increments from five million years ago to today.

Speciation without geographic separation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? A number of mechanisms for sympatric speciation have been proposed and studied.

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Source:  OpenStax, Natural history supplemental. OpenStax CNX. Aug 19, 2014 Download for free at http://legacy.cnx.org/content/col11695/1.1
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