What is it called when one species splits into two or more groups that can no longer interbreed?

It's a bit like a character in a movie going off and having adventures that change him so drastically that when he returns, the folks in his hometown no longer recognize the way he looks and behaves. The biological equivalent is "allopatric speciation," an evolutionary process in which one species divides into two because the original homogenous population has become separated and both groups diverge from each other.

In their separate niches, the two groups go their own evolutionary ways, accumulating different gene mutations, being subjected to different selective pressures, experiencing different historical events, finally becoming incapable of interbreeding should they ever come together again. For many years this has been regarded as the main process by which new species arise.

Often this type of speciation occurs in three steps. First, the populations become physically separated, often by a long, slow geological process like an uplift of land, the movement of a glacier, or formation of a body of water. Next, the separated populations diverge, through changes in mating tactics or use of their habitat. Third, they become reproductively separated such that they cannot interbreed and exchange genes.

Under normal conditions, genes in a given population are exchanged through breeding, so that even if some variation occurs, it is limited by this "gene flow." But gene flow is interrupted if the population becomes divided into two groups. One way this happens is by "vicariance," geographical change that can be slow or rapid.

An example of vicariance is the separation of marine creatures on either side of Central America when the Isthmus of Panama closed about 3 million years ago, creating a land bridge between North and South America. Nancy Knowlton of the Smithsonian Tropical Research Institute in Panama has been studying this geological event and its effects on populations of snapping shrimp. She and her colleagues found that shrimp on one side of the isthmus appeared almost identical to those on the other side -- having once been members of the same population.

But when she put males and females from different sides of the isthmus together, they snapped aggressively instead of courting. They had become separate species, just as the theory would predict.

11 April 2005

Lecture 36

Reading, Chapter 11

C. Speciation

Speciation is the process by which new species arise from existing species. Two patterns for the process of speciation have been proposed: phyletic speciation and divergent speciation.

1. Phyletic speciation

Phyletic speciation is a process of gradual change in a single population. The modern form of the organism differs from the original form so much that the two can be considered separate species. Phyletic speciation could be drawn as a line. Species A becomes species B, which becomes species C, etc. In the past, phyletic speciation has been proposed for human evolution and the evolution of the horse. The problem with phyletic speciation is that it would only occur if there were a gradual change in the selective regime that progressively favored the modern form. This seems an unlikely occurrence in nature and the fossil record does not support phyletic speciation for either human or horse evolution. For these reasons, natural phyletic speciation is believed to be rare. Artificial selection in domestic animals and plants approximates phyletic speciation, however. The familiarity of this sort of evolution is probably the only reason that phyletic speciation was ever considered as a hypothesis of natural speciation.

2. Divergent speciation

If phyletic speciation is drawn as a line, divergent speciation has the form of a branching tree. Species A splits into species A and B. Species B may subsequently branch into species C, and so on. Species A, B and C may exist all at the same time and any of them may be ended by extinction at some point in the process. Divergent speciation is consistent with fossil evidence of biological evolution and with the known mechanisms of biological evolution

Divergent speciation, the branch points in the tree described above, results from reproductive isolation of two parts of a population. Reproductive isolation means that interbreeding between the two groups is prevented by some barrier. Once interbreeding ends, two processes cause the isolated group to become different from the parent population:

1) Genetic variation occurs independently in the two groups. Lack of interbreeding prevents sharing of these independent genetic variations. Thus, the genetic variation on which natural selection acts is different in the two groups.

2) Selection may be different for the two groups, especially if they live in different places. If selection differs, different variants will be favored in the two groups.

Over time, the two populations become sufficiently different that they can no longer interbreed even if barriers to interbreeding are removed. Speciation has occurred.

 a. Geographic isolation

Divergent speciation requires reproductive isolation, which can occur in two general ways

Allopatric speciation is the term applied to divergent speciation when the two subpopulations are isolated by geographical barriers. For example, two isolated subpopulations would occur if a few individuals colonized an island that was far enough from the parent population that new colonists were rare. Charles Darwin's ideas about natural selection were stimulated when he observed finches on the Galapagos Islands. The different islands all had finches that were obviously related but that had beaks specialized for eating the locally available foods. These different finches likely resulted from allopatric speciation. Other geographical barriers that may lead to allopatric speciation include mountain ranges raised by geological uplift, separation of continents by continental drift and sea level changes, and changes in the courses of rivers caused by uplift or earthquakes.

b. Breeding barriers

Sometimes interbreeding between two subpopulations is prevented even though they occupy the same geographic area. Breeding barriers can result from changes in a few individuals that prevent normal breeding behavior or from genetic events that prevent fertility between a few individuals and the rest of the population. Speciation occurring from such reproductive isolation is called sympatric speciation.

Sympatric speciation in animals may be very rare. A possible example is that of the cichlid fishes of two lakes in Africa. A number of related but distinct species of cichlids occupy the same habitat in these lakes but gene sequences indicate that the species in each lake are more closely related to species in their own lake than to species in the other lake. This situation implies that migration of fishes between lakes has not occurred recently and that each lake's complement of species arose by sympatric speciation within the lake.

The mechanism by which this speciation occurred is unknown. One possibility is that mutations causing color differences in a few individuals may have prevented breeding behavior between the normal fish and the color mutants. Body colors can identify fish to members of their own species. A mutation changing body color could make the mutants sexual outcasts, except to each other. Such reproductive isolation may have contributed to the observed speciation.

Unlike animals, plants show evidence of frequent sympatric speciation. This is because plants, unlike animals, can experience an increase in their number of chromosomes and still live. An increase in chromosome number in an individual plant is a condition called "polyploidy". More than half of flowering plant species appear to be polyploid and to have arisen by sympatric speciation.

Polyploidy in plants arises in two ways: autopolyploidy (self-polyploidy) and allopolyplody (also known as hybridization). Autopolyploidy occurs when eggs and sperm form that have two sets of homologous chromosomes instead of the usual one. If the plant has both male and female flower parts and can self-fertilize, as many plants can, these diploid eggs and sperm can fuse to form offspring having four sets of chromosomes instead of the usual two. The new individual may also be self fertile but its different chromosome number prevents breeding with members of its parent population. Thus a new, reproductively isolated species may be born in a single generation.

Allopolyploidy occurs when two related species having different chromosome numbers interbreed, leading to offspring with a unique chromosome number that cannot breed with either parent species. If the hybrid is or can become capable of fertilizing itself, a new species is born.

Natural polyploidy events such as those described above are rare on a human timescale but appear to have occurred frequently in the evolution of flowering plants. In agriculture, breeding practices can accelerate polyploidy events and lead to new food plant species in a few generations. The evolution of modern wheat cultivars by successive polyploidy events is an example of this that is presented on pages 240 and 241 of your textbook.

D. Extinction

The majority of species that have lived on Earth are extinct. Fossil evidence suggests that the average lifetime of a species is roughly 10 million years. Extinction occurs when selection overwhelms the genetic diversity in a population, which may occur when something changes in the selective regime, e.g. a new predator, a new disease, climate change, etc.

Extinctions are sometimes clustered into "mass extinctions". The disappearance of nearly all dinosaur species 65 million years ago is an example of a mass extinction. The presently accepted hypothesis for how this mass extinction occurred was that a comet struck the earth. The impact is proposed to have thrown large amounts of dust into the upper atmosphere, blocking sunlight for a number of years and killing plants on which dinosaurs and other animals fed.

E. Punctuated equilibrium

In addition to mass extinctions, the fossil record suggests periods of rapid speciation during the earth's history separated by longer periods of little change. This observation led to the "punctuated equilibrium" hypothesis of speciation proposed by S. J. Gould and Miles Eldredge. Punctuated equilibrium proposes that new species can arise in a few hundred thousand years and then persist for millions of years relatively unchanged. It contradicts the prior view of "Neo-Darwinian gradualism", which states that speciation is a gradual process that proceeds at a constant speed without starts and stops.

Punctuated equilibrium is not the "truth". It is a revision of the prior hypothesis that is more consistent with fossil evidence and an improved view of the complex history of life on earth. It is also a demonstration that, within the constraints of human nature, biological evolution is the product of science rather than faith. Hypotheses of evolution are subject to testing and revision, just like any other scientific hypotheses.

What is it called when a species splits into two?

Is when a population is separated from another population and is no longer able to interbreed with the original population?

What are the two types of speciation?

How could 1 species separate into 2 species?