Biological species concept: This concept states that "a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups. It suggested a critical test of species-hood: two individuals belong to the same species if their gametes can unite with each other under natural conditions to produce fertile offspring.
This concept also emphasized that a species is an evolutionary unit. Members share genes with other members of their species, and not with members of other species. Although this definition clearly is attractive, it has problems.
Can you test it on museum specimens or fossil data? Can it explain the existence of species in a line of descent, such as the well-known lineage of fossil horses? Obviously not. In fact, one cannot apply this definition easily, or at all, with many living organisms.
What if species do not live in the same place? What about the hybrids that we know occur in zoos? These problems are serious enough that some biologists recently argued for a return to the morphological species concept. So what is the best way to define a species? Most scientists feel that the biological species concept should be kept, but with some qualifications. It can only be used with living species, and cannot always be applied to species that do not live in the same place.
The real test applies to species that have the potential to interbreed. Most importantly, the biological species concept helps us ask how species are formed, because it focuses our attention on the question of how reproductive isolation comes about.
Let us first examine types of reproductive isolation, because there are quite a few. Types of Reproductive Isolation There are many barriers to reproduction. Each species may have its own courtship displays, or breeding season, so that members of the two species do not have the opportunity to interbreed. Or, the two species may be unable to interbreed successfully because of failure of the egg to become fertilized or to develop.
This suggests a simple and useful dichotomy, between pre-mating or prezygotic i. Remember that a zygote is the cell formed by the union of two gametes and is the basis of a developing individual. Prezygotic isolating mechanisms Ecological isolation: Species occupy different habitats. The lion and tiger overlapped in India until years ago, but the lion lived in open grassland and the tiger in forest. Consequently, the two species did not hybridize in nature although they sometimes do in zoos.
Temporal isolation: Species breed at different times. In North America, five frog species of the genus Rana differ in the time of their peak breeding activity. Behavioral isolation: Species engage in distinct courtship and mating rituals see Figure 1.
In some cases, these pre-zygotic isolating mechanisms fail to prevent inter-breeding among individuals from separate populations. In these cases, viable hybrids may form, or the consequences of a successful mating attempt may end in failure, either due to the production of inviable zygotes or sterile, non-reproductive offspring. These diverse pre- and post-zygotic barriers are of great importance to speciation biologists because they determine how reproductively-isolated populations are from one another, which indicates how far along the often continuous process of speciation that populations are.
For example, reproductive isolation is weak in the early stages of speciation, but changes to strong or complete in later stages of speciation Figure 2. One or more of the many types of isolating mechanisms may play a role in the evolution of species along a continuum Figure 2. But how and why might reproductive barriers to genetic exchange evolve? Figure 2: Schematic illustration of the continuous nature of divergence during speciation, with three arbitrary points along the speciation continuum depicted.
Numerous types of differentiation can vary quantitatively, with the magnitude of differentiation representing a measure of how far speciation has proceeded. Two headed arrows represent mating between individuals. All rights reserved. A major area of debate among speciation biologists is the geographic context in which it occurs Figure 3.
Ernst Mayr emphatically defended his view that speciation was most likely when populations became geographically isolated from one another, such that evolution within isolated populations would lead to enough differences among them that speciation would be an eventual outcome.
The central idea here is that when populations are geographically separated, they will diverge from one another, both in the way they look and genetically. These changes might occur by natural selection or by random chance i. This view of speciation of geographically isolated populations — termed allopatric speciation — is still widely held among speciation biologists as playing a major role in the evolution of biodiversity e.
However, speciation might also occur in overlapping populations that are not geographically isolated i. The problem here is how do populations that are living in the same area, and exchanging genes, diverge from one another? This could occur, for example, if insects adapted to living on different plants within the same geographic region Feder et al. It will be interesting to see how many new examples emerge now that the idea of sympatric speciation is becoming less controversial.
Parapatric speciation refers to populations that are situated in geographic proximity to one another, usually with abutting but non-overlapping ranges. Here, a small proportion of each population are in actual contact with one another, and thus considered in sympatry, whereas the majority of individuals reside far enough apart that frequent encounters with one another are rare Figure 3.
There are putative examples of parapatric speciation in salamanders Niemiller et al. Reprinted from Mallet et al. The s saw a reclassification of modes of speciation away from schemes that focus solely on the geographic mode of divergence and towards a focus on the evolutionary process driving genetic divergence i. This reclassification was motivated — at least in part — by renewed interest in the extent to which the evolutionary processes which cause adaptation within species also tend to create new species.
Further, although the geographic mode of divergence has important implications for speciation via patterns of gene flow and sources of selection, speciation research has reached the point where we can directly test the role of different evolutionary process in driving speciation Butlin et al.
We outline several processes that can drive speciation. Recent years have seen renewed efforts to address these questions. For example, populations living in different ecological environments e. These same evolutionary changes can also result in the populations evolving into separate species.
For example, adaptation to different environments might cause differences between populations in the way individuals tend to look, smell, and behave. In turn, these differences might cause individuals from different populations to avoid mating with one another, or hybrids exhibit reduced fitness if mating occurs. Thus, the populations cease exchanging genes, thereby diverging into separate species because of the adaptive changes that occurred via natural selection.
More specifically, ecological speciation is defined as the process by which barriers to gene flow evolve between populations as a result of ecologically-based divergent selection between environments. This process makes some simple predictions. For example, ecologically-divergent pairs of populations should exhibit greater reproductive isolation than ecologically-similar pairs of populations of similar age Funk Figure 4 illustrates an example that supports this prediction.
Other predictions are that traits involved in divergent adaptation will also cause reproductive isolation, and that levels of gene flow in nature will decrease as ecological differences between populations increase.
Figure 4 Ecological speciation in host-plant associated populations of Timema cristinae walking-stick insects individual populations feed on either the Ceanothus spinosus host plant or on Adenostoma fasciculatum.
Pairs of populations feeding on the same host plant species, but in different geographic localities, are ecologically similar and assumed to not be subject to divergent selection. In contrast, pairs of populations feeding on different host plant species are ecologically divergent and subject to divergent selection. This pattern is independent from neutral genetic divergence, a proxy for time since divergence.
A current debate is whether sexual selection can lead to speciation in the absence of ecological divergence van Doorn et al. Indeed, compelling examples that implicate an important role of sexual selection leading to new species sometimes also involve the evolution of different signals used in mate-selection among populations in different ecological contexts, such as light environment Seehausen et al.
Here, signals used in mate-selection become adapted to new ecological environments where the transmission of these traits is more perceptible or audible in a new habitat. Another mechanism of speciation that involves chance events is speciation by polyploidization. Polyploidy, or the presence of three or more complete sets of chromosomes, has been documented in a wide variety of taxa.
Because polyploidy can lead to hybrid infertility, it is viewed as a mechanism that can rapidly lead to the formation of new species, potentially without selection for the divergence of other characters. Recent advances in genomics now allow such studies to be taken to the genome-wide level, where biologists can examine hundreds of thousands of gene regions, rather than just a handful.
A genomic island is any gene region, be it a single nucleotide or an entire chromosome, which exhibits significantly greater differentiation than expected under neutrality i. The metaphor thus draws parallels between genetic differentiation observed along a chromosome and the topography of oceanic islands and the contiguous sea floor through which they are connected. Following this metaphor, sea level represents the threshold above which observed differentiation is significantly greater than expected by neutral evolution alone.
Thus, an island is composed of both directly selected and tightly linked loci. Major remaining questions concern the size, number and distribution i. Clear answers to these questions will likely require experimental studies that measure selection at the genomic level to directly quantify how selection acts on the genome.
Nevertheless, the integration of geographic, ecological, and new genomic approaches is likely to yield new insight into speciation over the coming decades. See text for details. Divergent natural selection : Selection that acts in contrasting directions between two populations, usually with reference to ecological differences between their environments e. Ecological speciation : A speciation process in which divergent natural selection drives the evolution of reproductive incompatibility i.
Mutation-order speciation : A speciation process in which different and incompatible mutations alleles fix in separate populations that are experiencing similar selective regimes. Dobzhansky-Muller Incompatibility : Hybrid dysfunction arising from negative interactions epistasis between alleles at two or more loci: an allelic substitution at a locus causes no reduction in fitness on its own genetic background, but leads to reduced fitness when placed on the alternative background.
Genomic Island : A region of the genome where differentiation between populations is stronger than expected in the absence of divergent selection stronger than occurs via purely neutral processes such as genetic drift alone.
Natural selection : Differential survival of classes of entities such as alleles which differ in some characteristic s. Sexual selection : Differential reproductive success of classes of entities such as alleles which differ in some characteristic s. Reproductive Isolation : Genetically-based differences between populations which reduce or prevent genetic exchange between them i. Darwin, C. London, UK: John Murray, Feder, J. Genetic differentiation between sympatric host races of Rhagoletis pomonella.
Nature , 61—64 Funk, D. Isolating a role for natural selection in speciation: Host adaptation and sexual isolation in Neochlamisus bebbianae leaf beetles. Evolution 52 , — Maan, M.
Mechanisms of species divergence through visual adaptation and sexual selection: Perspectives from a cichlid model system. Current Zoology 56 , — 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. 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—such as shelter or food sources—while 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—a 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. Illustration by Ilmari Karonen, courtesy Wikimedia. Holy Anolis! There are nearly species of the small anolis lizard on the islands of the Caribbean Sea, all of which descended from as few as two initial species.
Pretty Fly The Hawaiian islands are home to some of the most stunning examples of speciation.
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