Have you ever wondered how polar bears came to be white? It makes perfect sense; the white bears blend into the white arctic terrain easily. That makes it difficult for their prey to see them approach, and this has allowed them to survive for millennia in the polar region. Polar bears evolved to adapt to their environment. Every species adapts through the generations to survive in its habitat, or else it faces extinction.
But the whiteness of the polar bear becomes somewhat curious when you consider that, at one time, they are believed to have branched off of the brown bear family tree [source: Polar Bears International]. So how exactly did some brown bears become polar bears?
Like every other species that diverges -- splits into two or more distinct species -- brown bears and polar bears underwent a speciation event. This is an occurrence where members of the same species separate until, for a number of different possible reasons, they are no longer able to reproduce with one another. This means that there is no longer a genetic flow between the species.
Genetic flow is the term for the potential of genetic differences in a single species to be passed around within a species. Two members of a species that can mate may pass genes between them. Think about it this way: Your mother's genes differ from your father's. Each brought his or her own lineage to your conception, and you are the result of these lineages, this genetic flow. When there is no longer a genetic flow between members of a formerly single species, reproductive isolation has been fulfilled. They no longer share the same gene pool. When this occurs, speciation is complete, and the one species has now become two (or more).
But how does speciation take place? Evolutionists still don't have a definitive answer. There are a number of different ideas of how reproductive isolation could take place. On the next page, we'll take a look at the conditions under which scientists suspect speciation takes place.
Types of Speciation
Speciation events are believed to happen under a number of conditions. Scientists generally believe it takes place because members of the same species have become isolated from one another. There are four types of speciation that evolutionists have identified as being the likeliest explanations for speciation.
In the case of allopatric speciation, an actual geographic boundary physically separates the species. A river or a mountain range, for example, may cause a species to diverge.
With parapatric speciation, a species is spread out across large areas with diverse environments. They adapt to these new areas and gradually become separate species. There's no defining geographical feature that separates these animals -- they may become separate species simply because of the distance that separates the groups.
Peripatric speciation occurs when a small group becomes isolated from the main body of the species. Since this cluster is just a small portion of the whole population of the species, all of the genetic differences which make the species robust may not be present. This presents what evolutionary biologists call a bottleneck. Some of the genes that flow within a species have been clipped off and separated from the gene pool.
In an evolutionary bottleneck, a small population is successful at producing subsequent generations. This has happened at various times in human history. Most Europeans, for example, are all descendents of only a few hundred ancestors, who lived in the midst of a bottleneck [source: The Polymath].
While a small group may be successful in repopulating, there are repercussions that may arise from a bottleneck. The genes present in this small population will become robust, since the genetic flow is much bulkier than it would be in a larger population, where genetic differences -- including abnormalities -- are spread out. This is called the founder effect. The genes of founding members of what eventually becomes a large population become much more frequent than they are in similar, larger populations. Several members of an Amish sect in Pennsylvania suffer microcephaly, a generally fatal disease where the brain does not attain its full size. These Amish are all descended from a single couple who arrived in Pennsylvania in the 18th century.
In the case of sympatric speciation, members of a species continue to live side by side, but still separate into different species. For example, some insects feed and reproduce on a single type of fruit. If some members of this species try another type of fruit, their offspring may be raised to visit that fruit as well. If this happens, the members of a species may diverge into two species based solely on the fruit they feed on and lay eggs in.
In each of these types of speciation, the species must undergo the process of reproductive isolation. For example, the damselfly evolved to form differing genitalia, and members became physically incapable of mating with one another. Reproductive isolation needn't be so drastic, however. Members of a single species can evolve to create different mating rituals, ones that won't attract members of the other species. They may also mate at different times of the day, month, season or year. Or they can evolve to mate in different places, for example on two different types of fruit, as in the example given above for sympatric speciation.
So when a population finds itself isolated in one way or another from the rest of a species, speciation occurs, right? Not necessarily. Find out more about the disagreement over speciation on the next page.
Gradual vs. Punctual
It seems that when members of a species end up isolated from other members of their species a speciation event is likely to occur. Scientists can compare the genes of similar species to show that the common DNA they share strongly suggests the two species were previously one. But how does this process occur? Unfortunately, evolutionists haven't been able to pin down exactly how speciation takes place. Instead, two main camps have evolved, so to speak, with very different opinions on how the process of speciation takes place.
There are those who believe in phyletic gradualism, the thought that the process of speciation takes place constantly and over long periods of time. Others believe in punctuated equilibrium. These people believe that speciation takes places in quick jumps and starts, with long periods of time between events. Both groups have the fossil record to contend with.
The fossil record is the sum total of all of the fossils that have been discovered thus far. Placed together, they form a relatively clear picture of the evolution of life on Earth. But there are, however, gaps in the fossil record -- periods when new species suddenly emerge without any apparent transition from one species to a radically different, related species.
Those who subscribe to punctuated equilibrium say the fossil record is proof their theory concerning speciation is the correct one. If speciation takes place in sudden jumps and starts, there should be no record of transition from one species to a sister species. Instead, members of a species find themselves reproductively isolated from other members and suddenly, speciation occurs.
For the part of the gradualists -- who generally claim among their ranks Charles Darwin, the father of evolutionary theory -- the fossil record is simply incomplete. The process of speciation includes a long transition from one species to a related species. In the gradualists' eyes, these transition members simply haven't been culled from the layers of silt and rock in the Earth yet. In other words, the fossil record, to the gradualists, is simply incomplete.
But the dispute over how speciation happens may be put to rest. A perfect natural experiment has literally washed ashore. And the setting couldn't be more perfect. To find about it, read the next page.
A Bit About Island Life
In mid-September 1995, Hurricane Marilyn struck the Caribbean. The storm killed at least 13 people and caused more than $2 billion in damages. Many people lost their homes to the force of the hurricane. But not everyone who was affected by Marilyn was human. A group of 15 green iguanas is believed to have drifted from the island of Guadeloupe to Anguilla after clinging to storm debris for three weeks. When the green iguanas set foot on Anguilla, they became potential founding members of a new species.
If the process of speciation does take place, it could take a while -- if the gradualists are correct. Or it could happen rather quickly, as the punctuated equilibrium adherents believe. Of course, the iguanas may not live long enough to thrive in their new environment, if they are unable to adapt to the differences of life in Anguilla quickly enough.
But the setting of the iguana's new environment speaks volumes about evolutionary isolation. On the island, the iguanas are physically isolated and could evolve to become reproductively isolated from the other members of their species. Islands represent the ultimate in isolation. As such, island living has produced some radical swings in phenotypes -- outward characteristics -- among species. Both gigantism and dwarfism have been discovered as a result of speciation.
Perhaps no recent example of radical development toward the smallish has been as documented as the case of Homo floresiensis. On Flores, an island in Indonesia, a small skull was discovered in 2005. It was small, about the size of a chimp's skull, and that's probably what the skull's discoverers would have believed it to be -- had it not also been surrounded by several primitive tools. These tools suggested that the skull belonged to a human, or at least a human relative. But for a skull that small, it would have been a very tiny human. Testing of the skull found that the person it belonged to had lived 18,000 years ago on Flores. Calculating from the size of the skull, the researchers who found it believed that the person stood just more than 3 feet tall. News of the find rocked the scientific world, and even captured the imagination of some of the general public.
Whether or not the skull represents a new type of human is being challenged. Researchers at Penn State University believe that the skull found represents a Homo sapiens who suffered from a developmental disease -- microcephaly, just like the Amish group in Pennsylvania. But the notion that a group of small humans, distinct from Homo sapiens evolved on an island is an example of what is called the island rule -- a generalization about speciation where animals on islands often grow much smaller or larger than their mainland cousins.
Biologists are still trying to explain why this happens. The most logical explanation is that the environmental pressures on an island are much different than those on the mainland. The smaller sizes of islands offer fewer food sources. This would seem to explain dwarfism found in many species. But what of those that grow to gigantic proportions, like the Komodo dragon? It's possible that these species have developed to be larger than their cousins because islands offer fewer predators and less competition for the food sources available.
The prevailing theory concerning the island rule is that a variety of local environmental factors cause animals in island isolation to evolve to be large or small. It's possible that there is no rule of thumb that accounts for all island evolution across the board. Perhaps Anguilla's newest arrivals -- the green iguanas -- will shed light on the island rule, as well as speciation.
For more information on evolution and other related topics, visit the next page.
Related HowStuffWorks Articles
More Great Links
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