How can speciation occur

Note: speciation can also occur unnaturally in the laboratory or due to selective breeding and so forth. Allopatric speciation is when populations become separated geographically and diverge over time due to natural selection , mutations , and genetic drift within each population. For example, if a river changes its course over time and two populations of a species that used to interbreed and exchange genes no loner exchange genes because they can't or don't want to cross the river , those two populations will evolve separately and become distinct species.

Parapatric speciation occurs when two populations are not completely separated. Two populations are largely separated from one another but occasional contact still occurs. However, interbreeding between the two populations is disadvantageous and results in offspring with lower fitness. Sympatric speciation occurs when there is no geographical barrier but populations diverge regardless.

What happened is that a long time ago, the continents of North and South America were separated, and the oceans were connected.

When the two land masses merged, populations of species were isolated on either side. Over time, these fish have diverged enough to be separate species. Species can split without such clear boundaries, too. When species diverge like the apple maggot flies - without a complete, physical barrier - it's called Sympatric Speciation. Sympatric speciation can occur for all kinds of reasons. All it takes is something that makes one group have less sex with another. For one species of Monarch flycatchers Monarcha castaneiventris , it was all about looks.

At some point, a small group of them developed a single amino acid mutation in the gene for a protein called melanin, which dictates the bird's color pattern. Some flycatchers are all black, while others have chestnut colored bellies. Even though the two groups are perfectly capable of producing viable offspring, they don't mix in the wild.

Researchers found that the birds already see the other group as a different species. The males, which are fiercely territorial, don't react when a differently colored male enters their turf. Like the apple maggot flies, the flycatchers are no longer interbreeding, and have thus taken the first step towards becoming two different species. These might seem like little changes, but remember, as we learned with dogs, little changes can add up.

Because they're not interbreeding, these different groups will accumulate even more differences over time. As they do, they will start to look less and less alike. The resultant animals will be like the species we clearly see today. Perhaps some will adapt to a lifestyle entirely different from their sister species - the orcas, for example, may diverge dramatically as small changes allow them to be better suited to their unique prey types.

Others may stay fairly similar, even hard to tell apart, like various species of squirrels are today. The point is that all kinds of creatures, from the smallest insects to the largest mammals, are undergoing speciation right now. We have watched species split, and we continue to see them diverge. Speciation is occurring all around us. Evolution didn't just happen in the past; it's happening right now, and will continue on long after we stop looking for it. Soltis, D. McPheron, B.

Uy, J. Geographic isolation In the fruit fly example, some fruit fly larvae were washed up on an island, and speciation started because populations were prevented from interbreeding by geographic isolation. Scientists think that geographic isolation is a common way for the process of speciation to begin: rivers change course, mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations.

Reduction of gene flow However, speciation might also happen in a population with no specific extrinsic barrier to gene flow. Imagine a situation in which a population extends over a broad geographic range, and mating throughout the population is not random.

Individuals in the far west would have zero chance of mating with individuals in the far eastern end of the range. So we have reduced gene flow, but not total isolation. Typically, environmental conditions, such as climate, resources, predators, and competitors, for the two populations will differ causing natural selection to favor divergent adaptations in each group.

Different histories of genetic drift, enhanced because the populations are smaller than the parent population, will also lead to divergence. Given enough time, the genetic and phenotypic divergence between populations will likely affect characters that influence reproduction enough that were individuals of the two populations brought together, mating would be less likely, or if a mating occurred, offspring would be non-viable or infertile.

Many types of diverging characters may affect the reproductive isolation inability to interbreed of the two populations. These mechanisms of reproductive isolation can be divided into prezygotic mechanisms those that operate before fertilization and postzygotic mechanisms those that operate after fertilization.

Prezygotic mechanisms include traits that allow the individuals to find each other, such as the timing of mating, sensitivity to pheromones, or choice of mating sites. If individuals are able to encounter each other, character divergence may prevent courtship rituals from leading to a mating either because female preferences have changed or male behaviors have changed.

Physiological changes may interfere with successful fertilization if mating is able to occur. Postzygotic mechanisms include genetic incompatibilities that prevent proper development of the offspring, or if the offspring live, they may be unable to produce viable gametes themselves as in the example of the mule, the infertile offspring of a female horse and a male donkey.

If the two isolated populations are brought back together and the hybrid offspring that formed from matings between individuals of the two populations have lower survivorship or reduced fertility, then selection will favor individuals that are able to discriminate between potential mates of their own population and the other population.

This selection will enhance the reproductive isolation. Isolation of populations leading to allopatric speciation can occur in a variety of ways: from a river forming a new branch, erosion forming a new valley, or a group of organisms traveling to a new location without the ability to return, such as seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the biology of the organism and its potential for dispersal.

If two flying insect populations took up residence in separate nearby valleys, chances are that individuals from each population would fly back and forth, continuing gene flow. However, if two rodent populations became divided by the formation of a new lake, continued gene flow would be unlikely; therefore, speciation would be more likely. Biologists group allopatric processes into two categories. If a few members of a species move to a new geographical area, this is called dispersal.

If a natural situation arises to physically divide organisms, this is called vicariance. Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate subspecies of spotted owls exist.

The northern spotted owl has genetic and phenotypic differences from its close relative, the Mexican spotted owl, which lives in the south [Figure 2]. The cause of their initial separation is not clear, but it may have been caused by the glaciers of the ice age dividing an initial population into two. 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 [Figure 3].