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The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially interbreeding individuals. In fact, the presence in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be completed.
Given the extraordinary diversity of life on the planet there must be mechanisms for speciation: the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration found in On the Origin of Species (Figure 1a). Compare this illustration to the diagram of elephant evolution (Figure 1b), which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct.
For speciation to occur, two new populations must be formed from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation (allo– = “other”; –patric = “homeland”) involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym– = “same”; –patric = “homeland”) involves speciation occurring within a parent species remaining in one location.
Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why there might not be more than two species formed at one time except that it is less likely and multiple events can be conceptualized as single splits occurring close in time.
A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across the range of the species, is relatively free because individuals can move and then mate with individuals in their new location. Thus, the frequency of an allele at one end of a distribution will be similar to the frequency of the allele at the other end. When populations become geographically discontinuous, that free-flow of alleles is prevented. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population. 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.
Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion forming a new valley, a group of organisms traveling to a new location without the ability to return, or 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, 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: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms.
Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate sub-species 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).
Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely it is that speciation will occur. 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 types of organisms in each ecosystem differ, as do their behaviors and habits; also, the hunting habits and prey choices of the southern owls vary from the northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur.
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. This is called adaptive radiation because many adaptations evolve from a single point of origin; thus, 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. The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved, including the six shown in Figure 3.
Notice the differences in the species’ beaks in Figure 3. Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another example of adaptive radiation in an archipelago.
Click through this interactive site to see how island birds evolved in evolutionary increments from 5 million years ago to today.
Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. The process of speciation within the same space is called sympatric speciation; the prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” A number of mechanisms for sympatric speciation have been proposed and studied.
One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition called aneuploidy (Figure 4).
In Figure 4, which is most likely to survive, offspring with 2n+1 chromosomes or offspring with 2n-1 chromosomes?
[reveal-answer q=”462162″]Show Answer[/reveal-answer]
[hidden-answer a=”462162″]Loss of genetic material is almost always lethal, so offspring with 2n+1 chromosomes are more likely to survive.[/hidden-answer]
Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy (Figure 5). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.
For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes produced by this plant species. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species.
The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid. The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. Figure 6 illustrates one possible way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.
The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations described here are unlikely to survive and produce normal offspring.) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.
Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction: if individuals of the two populations were to be brought together, mating would be less likely, but if mating occurred, offspring would be non-viable or infertile. Many types of diverging characters may affect the reproductive isolation, the ability to interbreed, of the two populations.
Reproductive isolation can take place in a variety of ways. Scientists organize them into two groups: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of the development of an organism that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place; this includes barriers that prevent fertilization when organisms attempt reproduction. A postzygotic barrier occurs after zygote formation; this includes organisms that don’t survive the embryonic stage and those that are born sterile.
Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, called temporal isolation, can act as a form of reproductive isolation. For example, two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May (Figure 7).
In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the other populations of the same species. This situation is called habitat isolation. Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. For example, a cricket population that was divided after a flood could no longer interact with each other. Over time, the forces of natural selection, mutation, and genetic drift will likely result in the divergence of the two groups (Figure 8).
Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction from taking place. For example, male fireflies use specific light patterns to attract females. Various species of fireflies display their lights differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern and would not mate with the male.
Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place; this is called a gametic barrier. Similarly, in some cases closely related organisms try to mate, but their reproductive structures simply do not fit together. For example, damselfly males of different species have differently shaped reproductive organs. If one species tries to mate with the female of another, their body parts simply do not fit together. (Figure 9).
In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and diameter, which prevents the plant from being cross-pollinated with a different species (Figure 10).
When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages. This is called hybrid inviability because the hybrid organisms simply are not viable. In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own; this is called hybrid sterility.
Habitat Influence on Speciation
Sympatric speciation may also take place in ways other than polyploidy. For example, consider a species of fish that lives in a lake. As the population grows, competition for food also grows. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new food source was found at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish; therefore, they would breed together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area and keeping separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them.
This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 11 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location but have come to have different morphologies that allow them to eat various food sources.
Speciation: 4 Types of Speciation | Evolution
Speciation is the formation of one or more new species from an existing species.
A species is a collection of demes. The deme is a group of populations with common gene pool.
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Types of Speciation:
Speciation is of following types.
1. Allopatric Speciation (alios— other, patria— native land):
In this type of species formation, a part of the population becomes geographically isolated from the main population. The population becomes entirely separated and finally constitutes a new species. Thus geographic isolation brings about allopatric speciation. An important example of this type of speciation is formation of Darwin’s finches that formed separate species in the Galapagos Islands (Fig. 7.55A).
2. Sympatric Speciation (sym— together, patria— native land):
In this type of species formation, a small segment of the original population becomes isolated reproductively. As the isolating mechanism comes into force, a new subspecies emerges. In due course of time a new species is formed. Thus sympatric speciation is the formation of species within a single population without geographical isolation. The reproductive isolation brings about sympatric speciation (Fig. 7.55B).
3. Parapatric Speciation:
It separates adjacent population. Parapatric speciation takes place when a population of a species enters a new niche or habitat. It occurs only at the edge of the parent species range. Although there is no physical barrier between these populations, yet the occupancy of a new niche results as a barrier to gene flow between the population of new niche. Two species are produced due to reproductive isolation from single one. Such type of speciation is found in flightless grasshoppers, smails and annual plants.
4. Quantum Speciation:
It is the rapid and abrupt mode of species formation. Grant (1971) defined quantum speciation “the budding off a new and very different daughter species from a semi-isolated peripheral population of the ancestral species”. This type of speciation is based on the observation of H.L. Carson on Drosophila inhabiting Hawaii Island.
The quantum speciation is a sudden and rapid speciation. It does not produce subspecies or intermediate stage. Genetic drift or chance plays a major role in quantum speciation.
SPECIES AND SPECIATION
Taxonomists classify organisms according to their similarities and differences. The fundamental unit of classification is the species. Unfortunately, it is difficult to define species. According to a biological definition, a species is a group of populations in which genes are actually, or potentially, exchanged through interbreeding.
This definition has certain problems.
- Taxonomists often work with. morphological characteristics. The morphological and ecological information provide reproductive criterions.
- Some organisms do not reproduce sexually. Therefore, other criteria are applied in these cases.
- There are also problems of fossil material. Paleontologists describe species of extinct organisms. But they cannot test their reproductive criterion.
- Populations of similar organisms are isolated from each other. Therefore,
gene exchange is geographically impossible. The biologists can transplant individuals to test a reproductive criterion. Mating may take place in transplanted individuals. But it does not prove that mating can also occur in natural setting.
The biologists are trying to understand the problems associated with the biological definition. The taxonomists use morphological, physiological, mbryological, behavioral, molecular, and ecological criteria for describing species. They realized that all of these have a genetic basis.
rhe formation of new species is called ,speciation. The subpopulations are prevented from breeding. This is called reproductive isolation Natural selection Ind genetic drift act on reproductively isolated populations. Therefore, the revolution takes a different course in these subpopulations.
Reproductive isolation can occur in different ways.
Premating isolation: It prevents mating to take place. For example:
- Impenetrable barriers like rivers or mountain ranges separate sub populations.
- Sometimes, courtship behavior patterns of two animals are not mutually appropriate. Therefore, mating does not occur.
- Some individuals have different breeding periods. Some individual occupy different habitats. Therefore, they are unable to breed with each other.Postmating isolation: In this case, mating occurs but successful fertilization and development is prevented. For example,
- There are different conditions in the reproductive tract of a female. It may not support the sperm of another individual. So fertilization does not take. place successfully.
- Sometimes, hybrids are sterile. Therefore, postmating isolation occurs. For example, mule is produced by the mating of a male donkey and a mare. Mule is a sterile hybrid.
- Mismatched chromosomes cannot synapse properly during meiosis. Thus any gametes produced are not viable.
- Sometimes, the development of fertilized egg or embryo is failed. It is also a post mating isolation.
The speciation occurs when sub populations become geographically isolated from one another is called allopatric speciation. For example, a mountain range or river permanently separate members of a population. A aptations for different environments or neutral selection take place in these s parate populations. The members of this population cannot reproduce successfully with each other. Many biologists believe that allopatric speciation is the most common kind of speciation.
Allopatric speciation in Darwinian finches
The Darwinian finches on the Galapagos Islands are example of allopatric speciation and adaptive radiation. A number of new forms diverge from an ancestral form during adaptive radiation. These adaptations occur in response to the opening of major new habitats.
Fourteen species of finches evolved from the original finches in the Galapagos Islands. Ancestral finches emigrated from the mainland. They were distributed among a few of the islands of the Galapagos. Their populations became isolated o various islands overtime. The original population displayed some genetic on variation. More variation arises in these ancestral populations. The original finches were seed eaten. They arrived in islands of Galapagos. They filled their habitat rapidly. Variations occur within original finch population. So some birds started exploiting new islands and habitats. Mutation changed the genetic composition of the isolated finch populations. It introduced further variations. Natural selection favored some variations. These variations promoted successful reproduction.
The combined forces of isolation, mutation, and natural selection developed m my adaptations in these finches. These adaptations allowed the finches to diverge into a_number of species. These species have specialized feeding habits. Si of the 14 species of finches have beaks specialized for crushing of seeds of different sizes. Others finches feed on flowers of the prickly cactus. Some other feed on forests on insects and fruit.
The speciation occurs in small local population called demes is called parapatric speciation. Deme is a small and local population. For example, all of the frogs in a particular pond or all of the sea urchins in a particular tide pool make up a deme. Individuals of on deme are more likely to breed with one another than with other individuals in the larger population. They experience the same environment. So they face similar selection pressures. The individuals developmental stages, or gametes can move among the demes of a population Therefore, the demes are not completely isolated from each other. But these demes face relative isolation. Its members experience different selection pressures than u•her members of the population. So speciation can occur in them. Most evolutic lists theoretically agree that parapatric speciation is possible. But no certain cast.3 are known. Therefore, parapatric speciation has less importance in the evolution of animal .groups than allopatric speciation.
Speciation – Definition, Causes, Process, and Types of Speciation
Speciation is the procedure of development of a new genetically independent group of organisms, called species, through the lapse of evolution.
The procedure of splitting of genetically homogenous population into two or more populations that undergo genetic differentiation and eventual reproductive isolation is called speciation. The entire passage of evolution relies on the origin of new populations (types) that have higher adaptive efficiency than their forefathers.
Speciation occurs in two ways:
- Change of old species into new species over time.
- Dividing of a single species into many, that is the multiplication of species.
Causes of Speciation
In the fruit fly example, some fruit fly larvae were washed up on an island, and speciation began since populations were prevented from interbreeding by geographical isolation. Researchers believe that geographic isolation is a common way for the process of speciation to start: rivers change course, mountains rise, continents drift, organisms migrate, and when a constant population is divided into two or smaller sized populations.
It doesn’t even need to be a physical barrier like a river that separates two or more groups of organisms– it might just be undesirable habitat between the two populations that prevents them from mating with one another.
As discussed by Charles Darwin, different individuals in a species may establish particular distinct qualities which are useful and affect the genetic makeup of the individual. Under such conditions, these attributes will be conserved, and over time, new species might be formed.
However, in this case, the necessary element of this factor is that speciation happens only when a single species splits into a number of species leading to the multiplication of species.
Chromosomal mutations have the potential to serve as separating mechanisms, and the locking up and defense of an especially beneficial gene enhance through a chromosomal mutation. These mutations, when maintained from one generation to another, may lead to the formation of new species.
4.Reduction of gene flow
Nevertheless, speciation may likewise occur in a population without any particular extrinsic barrier to gene flow. Think of a circumstance in which a population crosses a broad geographic range, and breeding throughout the population is not random. Individuals in the far west would have no opportunity of mating with individuals in the far eastern end of the range. So, we have actually lowered gene flow, but not total isolation.
This may or may not sufficient to cause speciation. Speciation would also need different selective pressures at opposite ends of the variety, which would alter gene frequencies in groups at different ends of the variety so much that they would not have the ability to mate if they were reunited.
Process of Speciation
Traditionally, speciation has been observed as a three-stage procedure:
- Isolation of populations
- Divergence in characteristics of separated populations (e.g. mating system or habitat use).
- Reproductive isolation of populations that keeps isolation when populations enter into contact again (secondary contact).
The procedure of speciation begins with the isolation of the subpopulation of a species which might either take place through physical isolation (allotropic speciation) or hereditary isolation (sympatric speciation). When the population is separated, a gradual accumulation of little genetic modifications leads to a subpopulation of a type that eventually builds up many changes that the subpopulations become different species.
Over time, the subpopulation now ends up being genetically independent and will continue to diverge by mutation, selection, and genetic drift. Eventually, the genetic differentiation between the subpopulation ends up being so high that the formation of hybrids in between them would be physiologically, developmentally, or behaviourally impossible even if the modes of the separation were abolished.
Types of Speciation
The category of the modes or types of speciation is based on how much the geographical separation of the initial population contributes to the reduced gene flow and eventually, the formation of new species.
Allopatric speciation is the mode of speciation in which the initial population is divided into two by a barrier resulting in reproductive seclusion.
The model for allopatric speciation was presented by Mayr. It is based on the principle that new species occur when some physical geographical barrier divides the big population of a species into two or more little populations. The individuals of these isolated populations can not interbreed because of their physical isolation.
Physical isolation may take place either due to physical barriers like vast stretches of ocean, high mountains, glaciers, deep river valleys, large rivers or deserts, or a substantial range due to a bigger geographical variety.
Each separated population begins to adjust to their apart environments while collecting differences and developing independently into new species. Allopatric speciation can take place even in cases in which the barrier allows some individuals to cross the barrier to mate with the members of the other groups. For speciation even to be considered “allopatric,” gene circulation between the soon-to-be species needs to be significantly minimized– however it does not have to be totally lowered to absolutely zero.
Example of Allopatric speciation
The timeless example of allopatric speciation is that of Darwin’s finches. The divergent populations of finches living in the Galapagos Islands were observed to have differences in functions such as body size, color, and beak length or shape. The distinctions resulted because of the various kinds of food available in various Islands.
Sympatric speciation is the process of the development of new species from an original population that are not geographically separated.
It is based upon the establishment of new populations of a species in different environmental niches and the reproductive isolations of founders of the new population from the individuals of the source population.
Gene flow in between daughter and adult population throughout sympatric speciation is postulated to be inhibited by intrinsic aspects, such as chromosomal changes and non-random mating. Making use of a new niche might instantly reduce gene flow with individuals making use of a different niche. This mode of speciation prevails in herbivore insects when they begin feeding and mating on a new plant or when a new plant is presented within the geographical range of the species.
The gene flow is then reduced between the species that concentrate on a specific plant which may ultimately lead to the development of new species. The choice leading to expertise requires to be actually strong for the population to diverge. Thus, sympatric speciation is a sporadic event in multicellular organisms or arbitrarily mating populations.
Example of Sympatric speciation
Sympatric speciation is observed in apple maggot flies which 200 years ago laid eggs and reproduced just on hawthorns and now lays eggs on both hawthorns and domestic apples. As a result, gene flow between parts of the population that mate on various kinds of fruit is reduced, and in fewer than 200 years, some hereditary differences between these two groups of flies have actually progressed.
Parapatric speciation is a highly uncommon case of speciation that occurs when a population is continually dispersed within a geographic area with no specific barriers to gene flow. Nonetheless, the population does not mate randomly within the population, however rather individuals’ mate more commonly with their closest geographical adjacent neighbors, resulting in irregular gene flow.
Non-random breeding might increase the rate of dimorphism within populations, in which differed morphological forms of the same species are displayed. The result of parapatric speciation is one or more distinct sub-populations (called ‘sister species’), which have little, continuous overlaps in their biogeographic range and are genotypically dimorphic.
Example of Parapatric speciation
The grass specieAnthoxanthum odoratum where some species living near the mine have ended up being tolerant to heavy metals nevertheless, other plants that do not live around the mines are not tolerant. But due to the fact that the plants are close together, they might fertilize each other and lead to new types.
Peripatric speciation is a type of allopatric speciation that takes place when populations that have ended up being isolated have very few individuals. Through this process, the population goes through a genetic bottleneck. Within the little sub-population, organisms which have the ability to make it through within the new environment might bring genes that were unusual within the main population but that triggers a small variation to behavior or morphology.
Through repeated mating, the frequency of these, when uncommon, genes boost within the small population. This is known as the ‘founder effect’. In time, the characteristic that was determined by the gene ends up being fixed within the population, resulting in a separated species that is evolutionarily distinct from the main population.
Example of Peripatric speciation
The Australian bird Petroica multicolor and London Underground mosquito, a variation of the mosquito Culex pipiens, which entered the London Underground in the 19th century are examples of Peripatric speciation.
The process of formation of new species is called speciation. It occurs when some populations of the same species split down and then undergo reproductive isolation. It occurs through the lapse of evolution. Geographical isolation, natural selection, chromosomal changes, reduction in gene flow are some factors responsible for speciation.
The types of speciation depend on how much the geographical separation of the initial population contributed to gene flow. The population divided by the barrier and result in reproductive seclusion is allopatric speciation. When some members of the same species undergo genetic changes but are not separated geographically is sympatric speciation.
When a population is continually dispersed within a geographic area with no specific barriers to gene flow is parapatric speciation. Peripatric is a type of allopatric speciation when there are fewer individuals after reproductive isolation.
Speciation is how a new kind of plant or animal species is created. Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics.
Biology, Geography, Physical Geography
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Speciation is how a new kind of plant or animal species is created. Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics. The demands of a different environment or the characteristics of the members of the new group will differentiate the new species from their ancestors.
An example of speciation is the Galápagos finch. Different species of these birds live on different islands in the Galápagos archipelago, located in the Pacific Ocean off South America. The finches are isolated from one another by the ocean. Over millions of years, each species of finch developed a unique beak that is especially adapted to the kinds of food it eats. Some finches have large, blunt beaks that can crack the hard shells of nuts and seeds. Other finches have long, thin beaks that can probe into cactus flowers without the bird being poked by the cactus spines. Still other finches have medium-size beaks that can catch and grasp insects. Because they are isolated, the birds don&rsquot breed with one another and have therefore developed into unique species with unique characteristics. This is called allopatric speciation.
There are five types of speciation: allopatric, peripatric, parapatric, and sympatric and artificial.
Allopatric speciation (1) occurs when a species separates into two separate groups which are isolated from one another. A physical barrier, such as a mountain range or a waterway, makes it impossible for them to breed with one another. Each species develops differently based on the demands of their unique habitat or the genetic characteristics of the group that are passed on to offspring.
When Arizona's Grand Canyon formed, squirrels and other small mammals that had once been part of a single population could no longer contact and reproduce with each other across this new geographic barrier. They could no longer interbreed. The squirrel population underwent allopatric speciation. Today, two separate squirrel species inhabit the north and south rims of the canyon. On the other hand, birds and other species that could easily cross this barrier continued to interbreed and were not divided into separate populations.
When small groups of individuals break off from the larger group and form a new species, this is called peripatric speciation (2). As in allopatric speciation, physical barriers make it impossible for members of the groups to interbreed with one another. The main difference between allopatric speciation and peripatric speciation is that in peripatric speciation, one group is much smaller than the other. Unique characteristics of the smaller groups are passed on to future generations of the group, making those traits more common among that group and distinguishing it from the others.
In parapatric speciation (3), a species is spread out over a large geographic area. Although it is possible for any member of the species to mate with another member, individuals only mate with those in their own geographic region. Like allopatric and peripatric speciation, different habitats influence the development of different species in parapatric speciation. Instead of being separated by a physical barrier, the species are separated by differences in the same environment.
Parapatric speciation sometimes happens when part of an environment has been polluted. Mining activities leave waste with high amounts of metals like lead and zinc. These metals are absorbed into the soil, preventing most plants from growing. Some grasses, such as buffalo grass, can tolerate the metals. Buffalo grass, also known as vanilla grass, is native to Europe and Asia, but is now found throughout North and South America, too. Buffalo grass has become a unique species from the grasses that grow in areas not polluted by metals. Long distances can make it impractical to travel to reproduce with other members of the species. Buffalo grass seeds pass on the characteristics of the members in that region to offspring. Sometimes a species that is formed by parapatric speciation is especially suited to survive in a different kind of environment than the original species.
Sympatric speciation (4) is controversial. Some scientists don&rsquot believe it exists. Sympatric speciation occurs when there are no physical barriers preventing any members of a species from mating with another, and all members are in close proximity to one another. A new species, perhaps based on a different food source or characteristic, seems to develop spontaneously. The theory is that some individuals become dependent on certain aspects of an environment&mdashsuch as shelter or food sources&mdashwhile others do not.
A possible example of sympatric speciation is the apple maggot, an insect that lays its eggs inside the fruit of an apple, causing it to rot. As the apple falls from the tree, the maggots dig in the ground before emerging as flies several months later. The apple maggot originally laid its eggs in the fruit of a relative of the apple&mdasha fruit called a hawthorn. After apples were introduced to North America in the 19th century, a type of maggot developed that only lays its eggs in apples. The original hawthorn species still only lays its eggs in hawthorns. The two types of maggots are not different species yet, but many scientists believe they are undergoing the process of sympatric speciation.
Artificial speciation (5) is the creation of new species by people. This is achieved through lab experiments, where scientists mostly research insects like fruit flies.
There are four major variants of speciation: allopatric, peripatric, parapatric, and sympatric.
Illustration by Ilmari Karonen, courtesy Wikimedia. CC-BY-SA-3.0
There are nearly 150 species of the small anolis lizard on the islands of the Caribbean Sea, all of which descended from as few as two initial species.
The Hawaiian islands are home to some of the most stunning examples of speciation. Over 1000 species of fruit fly have developed there and are found nowhere else on Earth!
Watch the video: ΒΙΟΛΟΓΙΑ Β ΛΥΚΕΙΟΥ, 1ο Κεφάλαιο - Ενότητα: Πρωτεΐνες, Αμινοξέα u0026 Επίπεδα οργάνωσης των πρωτεϊνών (May 2022).