Speciation can take place with or without geographic separation

Genetic Changes/Reproductive Barriers (Allopatric Review)

Many different processes can produce such genetic changes, including natural selection under different environmental conditions, genetic drift, and sexual selection. Once formed, reproductive barriers that arise in allopatric populations can prevent interbreeding with the parent population even if the populations come back into contact.

Example of Polyploids

Many important agricultural crops--such as oats, cotton, potatoes, tobacco, and wheat--are polyploids. For example, the wheat used to for bread is an allohexaploid (six sets of chromosomes, two sets from each of three different species).

Did the species on different sides of the isthmus originate by allopatric speciation? (example)

Morphological and genetic data group these shrimp into 15 pairs of sister species, parts whose member species are each other's closest relative. In each of these 15 pairs, one of the sister species lives on the Atlantic side of the isthmus, while the other lives on the Pacific side. This fact strongly suggests that the two species arose as a consequence of geographic speciation.

Evolution in Mosquitofish populations (example)

On Andros Island, in the Bahamas, populations of the mosquitofish colonized a series of ponds that later became isolated from one another. Genetic analyses indicate that little or no gene flow currently occurs between the ponds. The environments of these ponds are very similar except that some contain predatory fishes, while others do not.

Separate Gene Pools Diverge After Geographic Isolation

Once geographic isolation has occurred, the separate gene pools may diverge. Different mutations arise, and natural selection and genetic drift may alter allele frequencies in different ways in the separated populations. Reproductive isolation may then evolve as a by-product of the genetic divergence that results from selection or drift.

How did so many species--more than double the number of freshwater fish species known in all of Europe--originate within a single lake? (example)

One hypothesis is that subgroups of the original cichlid populations adapted to different food sources and the resulting genetic divergence contributed to speciation in Lake Victoria. But sexual selection, in which (typically) females select males bases on their appearance, may also have been a factor.

Polyploidy is Mostly Found in Plants

Polyploidy speciation occasionally occurs in animals. However, polyploidy is far more common in plants. In fact, botanists estimate that more than 80% of plant species alive today are descended from ancestors that formed by polyploidy speciation.

Polyploidy

A species may originate from an accident during cell division that results in extra sets of chromosomes, a condition called polyploidy.

Allopatric Speciation

(from the Greek allos, other, and para, homeland) Gene flow is interrupted when a population is divided into geographically isolated subpopulations.

First pre-zygotic barrier is habitat isolation (example)

Apple maggot flies usually mate on or near their host plant. This results in a prezygotic barrier (habitat isolation) between populations that feed on apples and populations that feed on hawthorns. Furthermore, because apples mature more quickly than hawthorn fruit, natural selection has favored apple-feeding flies with rapid development.

Evidence of Allopatric Speciation (snapping shrimp example)

Consider the 30 species of snapping shrimp in the genus Alpheus that live off of the Isthmus of Panama, the land bridge that connects South and North America. Fifteen of these species live on the Atlantic side of the isthmus, while the other 15 live on the Pacific side. Before the isthmus formed, gene flow could occur between the Atlantic and Pacific populations of snapping shrimp.

Geographic Distance Between Two Populations Increases Reproductive Isolation

Field studies also show that reproductive isolation between two populations generally increases as the geographic distance between them increases, a finding consistent with allopatric speciation.

Allopolyploid

However, an infertile hybrid may be able to propagate itself asexually (as many plants can do). In subsequent generations, various mechanisms can change a sterile hybrid into a fertile polyploid called an allopolyploid. The allopolyploids are fertile when mating with each other but cannot interbreed with either parent species; thus, they represent a new biological species.

Allopatric Speciation Review

In allopatric speciation, a new species forms in geographic isolation from its parent population. Geographic isolation severely restricts gene flow. Intrinsic barriers to reproduction with members of the parent population may then arise as a by-product of genetic changes that occur within the isolated population.

First type of Polyploidy

In plants, a failure of cell division could double a cell's chromosome number from the original number of (2n) to a tetraploid number (4n).

Selection Differences Between Ponds (example)

In ponds with predatory fishes, selection has favored the evolution of a mosquitofish body shape that enables rapid bursts of speed. In ponds without predatory fishes, selection has favored a different body shape, one that improves the ability to swim for long periods of time.

Sympatric Speciation

In sympatric speciation (from the Greek syn, together), separation occurs in populations that live in the same geographic area.

Evidence of Allopatric Speciation (Laboratory Studies)

Laboratory studies show that reproductive barriers can develop when populations are isolated experimentally and subjected to different environmental conditions.

Second Type of Polyploidy

A second form of polyploidy can occur when two different species interbreed and produce hybrid offspring. Most such hybrids are sterile because the set of chromosomes from one species cannot pair during meiosis with the set of chromosomes from the other species. However, an infertile hybrid may be able to propagate itself asexually (as many plants can do).

Post-zygotic barrier on Host Plant (example)

Researchers also have identified alleles that benefit the flies that use one host plant but harm the flies that use the other host plant. Natural selection operating on these alleles has provided a post zygotic barrier to reproduction, further limiting gene flow. Altogether, although the two populations are still classified as subspecies rather than separate species, sympatric speciation appears to be well under way.

How have these different selective pressures affected the evolution of reproductive barriers? (example)

Researchers found that female mosquitofish prefer to mate with males whose body shape is similar to their own. This preference establishes a behavioral barrier to reproduction between mosquitofish from ponds with predators and those from ponds without predators. Thus, as a by-product of selection for avoiding predators, reproductive barriers have formed in these allopatric populations.

Cichlids with Differing Coloration (example)

Researchers have studied two closely related sympatric species of cichlids that differ mainly in the coloration of breeding males: Breeding Pundamilia pundamilia males have a blue-tinged back, whereas breeding Pundamilia nyererei males have a red-tinged back.

Sexual Selection and Habitat Differentiation (Sympatric Review)

Sympatric speciation also can result from sexual selection. Finally, sympatric speciation can occur when a subset of a population becomes reproductively isolated because of natural selection that results from a switch to a habitat or food source not used by the parent population.

Sympatric Speciation: Habitat Differentiation

Sympatric speciation can also occur when a subpopulation exploits a habitat or resource not used by the parent population. Consider the North American apple maggot fly, a pest of apples. The fly's original habitat was the native hawthorn tree, but for about 200 years ago, some populations colonized apple trees that had been introduced by European settlers.

Sympatric Speciation Review

Sympatric speciation, in contrast, requires the emergence of a reproductive barrier that isolates a subset of a population from the remainder of the population in the same area. Though rarer than allopatric speciation, sympatric speciation can occur when gene flow to and from the isolated subpopulation is blocked. This can occur as a result of polyploidy, a condition win which an organism has extra sets of chromosomes.

How formidable must a geographic barrier be to promote allopatric speciation?

The answer depends on the ability of the organisms to move about. Birds, mountain lions, and coyotes can cross rivers and canyons--as can the windblown pollen of pine trees and the seeds of some flowering plants. In contrast, some rodents may find a wide river or deep canyon a formidable barrier.

First Polyploidy Events

The first polyploidy events that eventually led to modern wheat probably occurred about 8,000 years ago in the Middle East as a spontaneous hybrid of an early cultivated wheat species and a wild grass.

Allopatric Speciation is Exhibited In Isolated Regions

The importance of allopatric speciation is also suggested by the fact that regions that are isolated or highly subdivided by barriers typically have more species than do otherwise similar regions that lack such features. For example, many unique plants and animals are found on geographically isolated Hawaiian Islands.

Examples of Allopatric Speciation

The water level in a lake may subside, resulting in two or more smaller lakes that are now home to separated populations. Or a river may change course and divide a population of animals that cannot cross it.

Coloration Acts as a Reproductive Barrier (example)

Their results suggest that mate choice based on male breeding coloration can act as a reproductive barrier that keeps the gene pools of these two species separate.

Sympatric Speciation: Sexual Selection

There is evidence that sympatric speciation can also be driven by sexual selection. Clues to how this can occur have been found in cichlid fish from one of Earth's hot spots of animal speciation, East Africa's Lake Victoria. This lake was once home to as many as 600 species of cichlids. Genetic data indicate that these species originated within the last 100,000 years from a small number of colonizing species that arrived from other lakes and rivers.

Second pre-zygotic barrier is gene flow (example)

These apple-feeding populations now show temporal isolation from the hawthorn-feeding flies, providing a second prezygotic barrier to gene flow between the two populations.

Polyploids are Produced in Labs

Today, plant geneticists generate new polyploids in the laboratory by using chemicals that induce meiotic and mitotic errors. By harnessing the evolutionary process, researchers can produce new hybrid species with desired qualities, such as a hybrid that combines the high yield of wheat with the hardiness of rye.

Autopolyploid

Two distinct forms of polyploidy have been observed in plant (and a few animal) populations. An autopolyploid (from the Greek autos, self) is an individual that has more than two chromosome sets that are all derived from a single species.

Physical Separation is Not a Biological Barrier

While geographic isolation prevents interbreeding between members of allopatric populations, physical separation is not a biological barrier to reproduction. Biological barriers are intrinsic to the organisms themselves. Hence, it is biological barriers that can prevent interbreeding when members of different populations come into contact with one another.

Tetraploid

A tetraploid can produce fertile tetraploid offspring by self-pollinating or by mating with other tetraploids. In addition, the tetraploids are reproductively isolated from 2n plants of the original population, because the triploid (3n) offspring of such unions have reduced fertility. Thus, in just one generation, autopolyploidy can generate reproductive isolation without any geographic separation.

Allopatric Speciation Can Occur Without Geologic Remodeling

Allopatric speciation can also occur without geologic remodeling, such as when individuals colonize a remote area and their descendants become geographically isolated from the parent population.

How can reproductive barriers form between sympatric populations while their members remain in contact with each other?

Although such contact (and the ongoing gene flow that results) makes sympatric speciation less common than allopatric speciation, sympatric speciation can occur if gene flow is reduced by such factors as polyploidy, sexual selection, and habitat differentiation.

Divergence Times Consistent with Geologic Changes (example)

Furthermore, genetic analyses indicate that the Alpheus species originated from 9 to 3 million years ago, with the sister species that live in the deepest water diverging first. These divergence times are consistent with geologic evidence that the isthmus formed gradually, starting 10 million years ago, and closing completely about 3 million years ago.

Allopatric vs Sympatric Speciation

In allopatric speciation the populations are geographically isolated, while in sympatric speciation they are not.