SPECIATION

SPECIATION 

Speciation is the process by which new species of organisms arise. Earth is inhabited by millions of different organisms, all of which likely arose from one early life-form that came into existence about 3.5 billion years ago. It is the task of taxonomists to decide which out of the multitude of different types of organisms should be considered species. The wide range in the characteristics of individuals within groups makes defining a species more difficult. Indeed, the definition of species itself is open to debate.

Concepts of Species

In the broadest sense, a species can be defined as a group of individuals that is "distinct" from another group of individuals. Several different views have been put forward about what constitutes an appropriate level of difference. Principal among these views are the biological-species concept and the morphological-species concept.

The biological-species concept delimits species based on breeding. Members of a single species are those that interbreed to produce fertile offspring or have the potential to do so. The morphological-species concept (from the ancient Greek root "morphos," meaning form) is based on classifying species by a difference in their form or function. According to this concept, members of the same species share similar characteristics. Species that are designated by this criterion are known as a morphological species.

Organisms within a species do not necessarily look identical. For example, the domestic dog is considered to be one species, even though there is a huge range in size and appearance among the different breeds. For naturally occurring populations of organisms that we are much less familiar with, it is much more difficult to recognize the significance of any character differences observed. Therefore deciding what characteristics should be used, as criteria to designate a species can be difficult.

Speciation : Natural Selection and Genetic Drift

Before the development of the modern theory of evolution, a widely held idea regarding the diversity of life was the "typological" or "essentialist" view. This view held that a species at its core had an unchanging perfect "type" and that any variations on this perfect type were imperfections due to environmental conditions. Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) independently developed the theory of evolution by natural selection, now commonly known as Darwinian evolution.

The theory of Darwinian evolution is based on two main ideas. The first is that heritable traits that confer an advantage to the individual that carries them will become more widespread in a population through natural selection because organisms with these favorable traits will produce more offspring. Since different environments favor different traits, Darwin saw that the process of natural selection would, over time, make two originally similar groups become different from one another, ultimately creating two species from one. This led to the second major idea, which is that all species arise from earlier species, therefore sharing a common ancestor.

When so much change occurs between different groups that they are morphologically distinct or no longer able to interbreed, they may be considered different species; this process is known as speciation. A species as a whole can transform over time into a new species (vertical evolution) or split into more separate populations, each of which may develop into new species (adaptive radiation).

Modern population geneticists recognize that natural selection is not the only factor causing genetic change in a population over time. Genetic drift is the random change in the genetic composition of a small population over time, due to an unequal genetic contribution by individuals to succeeding generations. It is thought that genetic drift can result in new species, especially in small isolated populations.

Isolating Mechanisms

Whether natural selection and genetic drift lead to new species depends on whether there is restricted gene flow between different groups. Gene flow is the movement of genes between separate populations by migration of individuals. If two populations remain in contact, gene flow will prevent them from becoming separate species (though they may both develop into a new species through vertical evolution).

Gene flow is restricted through geographic effects such as mountain ranges and oceans, leading to geographic isolation. Gene flow can also be prevented by biological factors known as isolating mechanisms. Biological isolating mechanisms include differences in behavior (especially mating behavior), and differences in habitat use, both of which lead to a decrease in mating between individuals from different groups.

When geographic separation plays a role in speciation, this is known as allopatric speciation, from the Greek roots allo, meaning separate, and "patric," meaning country. In allopatric speciation, natural selection and genetic drift can act together.

For example, imagine a mudslide that causes a river to back up into a valley, separating a population of rodents into two, one restricted to the shady side of the river, the other to the sunny side. Because coat thickness is a genetically inherited trait, eventually, through natural selection, the population of animals on the cooler side may develop thicker coats. After many generations of separation, the two groups may look quite different and may have evolved different behaviors as well, to allow them to survive better in their respective habitats. Genetic drift may occur especially if either or both populations remain small. Eventually these two populations may be so different as to warrant designation as different species.

It is also possible for new species to form from a single population without any geographic separation. This is known as "ecological" or "sympatric" (from the Greek root sym, meaning same) speciation, and it results in ecological differences between morphologically similar species inhabiting the same area. Sympatric speciation can occur in flowering plants in a single generation, due to the formation of a polyploid. Polyploidy is the complete duplication of an organism's genome, for example from n chromosomes to 4n. Even higher multiples of n are possible. This increase in a plant's DNA content makes it reproductively incompatible with other individuals of its former species.

Formation of new and distinct species, whereby a single evolutionary line splits into two or more genetically independent ones. One of the fundamental processes of evolution, speciation may occur in many ways. Investigators formerly found evidence for speciation in the fossil record by tracing sequential changes in the structure and form of organisms. Genetic studies now show that such changes do not always accompany speciation, since many apparently identical groups are in fact reproductively isolated (i.e., they can no longer produce viable offspring through interbreeding). Polyploidy is a means by which the beginnings of new species are created in just two or three generations.

Speciation is the evolutionary process by which new biological species arise. There are four modes of natural speciation, based on the extent to which speciating populations are geographically isolated from one another: allopatric, peripatric, parapatric and sympatric. Speciation may also be induced artificially, through animal husbandry or laboratory experiments. Observed examples of each kind of speciation are provided throughout. 

Natural speciation

All forms of natural speciation have taken place over the course of evolution, though it still remains a

subject of debate as to the relative importance of each mechanism in driving biodiversity.

There is debate as to the rate at which speciation events occur over geologic time. While some

evolutionary biologists claim that speciation events have remained relatively constant over time,

some paleontologists such as Niles Eldredge and Stephen Jay Gould have argued that species usually

remain unchanged over long stretches of time, and that speciation occurs only over relatively brief

intervals, a view known as punctuated equilibrium.

Allopatric (geographic)

During allopatric speciation, a population splits into two geographically isolated allopatric

populations (for example, by habitat fragmentation due to geographical change such as mountain

building or social change such as emigration). 

The isolated populations then undergo genotypic

and/or phenotypic divergence as they (a) become subjected to dissimilar selective pressures or (b)

they independently undergo genetic drift. 

When the populations come back into contact, they have

evolved such that they are reproductively isolated and are no longer capable of exchanging genes.

Observed instances : Island genetics, the tendency of small, isolated genetic pools to produce unusual

traits, has been observed in many circumstances, including insular dwarfism and the radical changes

among certain famous island chains, like Komodo and Galapagos, the latter having given rise to the

modern expression of evolutionary theory, after being observed by Charles Darwin. Perhaps the most

famous example of allopatric speciation is Darwin's Galápagos Finches.

Peripatric (Mostly geographic)

In peripatric speciation, new species are formed in isolated, small peripheral populations, which are

prevented from exchanging genes with the main population. It is related to the concept of a founder

effect, since small populations often undergo bottlenecks. Genetic drift is often proposed to play a

significant role in peripatric speciation. E.g.

·         Mayr bird fauna

·         The Australian bird Petroica multicolor

·         Reproductive isolation occurs in populations of Drosophila subject to population bottlenecking

Parapatric (Somewhat geographic)

In parapatric speciation, the zones of two diverging populations are separate but do overlap. There is

only partial separation afforded by geography, so individuals of each species may come in contact or

cross the barrier from time to time, but reduced fitness of the heterzygote leads to selection for

behaviours or mechanisms which prevent breeding between the two species.

Ecologists refer to parapatric and peripatric speciation in terms of ecological niches. A niche must be

available in order for a new species to be successful.

Observed instances

·         Ring species

·         The Larus gulls form a ring species around the North Pole.

·         The Ensatina salamanders, which form a ring round the Central Valley in California.

·         The Greenish Warbler (Phylloscopus trochiloides), around the Himalayas.

·         the grass Anthoxanthum has been known to undergo parapatric speciation in such cases as mine

contamination of an area.

Sympatric (Non-geographic)

In sympatric speciation, species diverge while inhabiting the same place. Examples of sympatric

speciation are found in insects which become dependent on different host plants in the same area.

Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which

self-fertilize to produce a tetraploid zygote.

Polyploidy is a mechanism often attributed to causing some speciation events in symparty. Not all

polyploids are reproductively isolated from their parental plants, so an increase in chromosome

number may not result in the complete cessation of gene flow between the incipient polyploids and

their parental diploids.

Polyploidy is observed in many species of both plant and animal like wheat, Salsify or      goatsbeard,

Cichlids of Lake Victoria, Lake Tanganyika and Lake Malawi, Xenopus    laevis, and an African frog.

Hybridization between two different species sometimes leads to a distinct phenotype. This phenotype

can also be fitter than the parental lineage and as such natural selection may then favor these

individuals. Eventually, if reproductive isolation is achieved, it may lead to a separate species.

However, reproductive isolation between hybrids and their parents is particularly difficult to achieve

and thus hybrid speciation is considered an extremely rare event.

Reinforcement

Reinforcement is the process by which natural selection increases reproductive isolation. It may occur after two populations of the same species are separated and then come back into contact. If their reproductive isolation was complete, then they will have already developed into two separate incompatible species. If their reproductive isolation is incomplete, then further mating between the populations will produce hybrids, which may or may not be fertile. If the hybrids are infertile, or fertile but less fit than their ancestors, then there will be no further reproductive isolation and speciation has essentially occurred (e.g., as in horses and donkeys.) If the hybrid offspring are more fit than their ancestors, then the populations will merge back into the same species within the area they are in contact.

Reinforcement is required for both parapatric and sympatric speciation. Without reinforcement, the geographic area of contact between different forms of the same species, called their "hybrid zone," will not develop into a boundary between the different species. And also without reinforcement they will have uncontrollable interbreeding. Reinforcement may be induced in artificial selection experiments as described below.

Artificial speciation

New species have been created by domesticated animal husbandry, but the initial dates and methods of the initiation of such species are not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with Ovis orientalis, one species from which they are descended. Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, gaur, yak, etc., as they readily produce fertile offspring with them.

The best-documented creations of new species in the laboratory were performed in the late 1980s. William Rice and G.W. Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies which came out of two of the eight exits were set apart to breed with each other in their respective groups. After thirty-five generations, the two groups and their offspring would not breed with each other even when doing so was their only opportunity to reproduce.

Diane Dodd was also able to show allopatric speciation by reproductive isolation in Drosophila pseudoobscura fruit flies after only eight generations using different food types, starch and maltose. Dodd's experiment has been easy for many others to replicate, including with other kinds of fruit flies and foods (Fig 3.3). The history of such attempts is described in Rice and Hostert (1993).

Genetics Drift

Hybrid speciation

Hybridization between two different species sometimes leads to a distinct phenotype. This phenotype can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if reproductive isolation is achieved, it may lead to a separate species. However, reproductive isolation between hybrids and their parents is particularly difficult to achieve and thus hybrid speciation is considered an extremely rare event. The Mariana Mallard arose from hybrid speciation.

Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in Heloconius butterflies and sunflowers. Polyploid speciation, which involves changes in chromosome number, is a more common phenomenon, especially in plant species.

Gene transposition as a cause

Theodosius Dobzhansky, who studied fruit flies in the early days of genetic research in 1930s, speculated that parts of chromosomes that switch from one location to another might cause a species to split into two different species. He mapped out how it might be possible for sections of chromosomes to relocate themselves in a genome. Those mobile sections can cause sterility in inter-species hybrids, which can act as a speciation pressure. In theory, his idea was sound, but scientists long debated whether it actually happened in nature. Eventually a competing theory involving the gradual accumulation of mutations was shown to occur in nature so often that geneticists largely dismissed the moving gene hypothesis.

However, recent research shows that jumping of a gene from one chromosome to another can contribute to the birth of new species. This validates the reproductive isolation mechanism, a key component of speciation.

Interspersed repeats

Interspersed repetitive DNA sequences function as isolating mechanisms. These repeats protect newly evolving gene sequences from being overwritten by gene conversion, due to the creation of non-homologies between otherwise homologous DNA sequences. The non-homologies create barriers to gene conversion. This barrier allows nascent novel genes to evolve without being overwritten by the progenitors of these genes. This uncoupling allows the evolution of new genes, both within gene families and also allelic forms of a gene. The importance is that this allows the splitting of a gene pool without requiring physical isolation of the organisms harboring those gene sequences.

Human speciation

Humans have genetic similarities with chimpanzees and gorillas, suggesting common ancestors. Analysis of genetic drift and recombination suggests humans and chimpanzees speciated apart 4.1 million years ago.