Adaptive radiation

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Four of the 14 finch species found in the Galapagos Archipelago, which are thought to have evolved via an adaptive radiation that diversified their beak shapes, enabling them to exploit different food sources. Darwin's finches.jpeg
Four of the 14 finch species found in the Galápagos Archipelago, which are thought to have evolved via an adaptive radiation that diversified their beak shapes, enabling them to exploit different food sources.

In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic interactions or opens new environmental niches. [1] [2] Starting with a single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits. The prototypical example of adaptive radiation is finch speciation on the Galapagos ("Darwin's finches"), but examples are known from around the world.

Contents

Characteristics

Four features can be used to identify an adaptive radiation: [2]

  1. A common ancestry of component species: specifically a recent ancestry. Note that this is not the same as a monophyly in which all descendants of a common ancestor are included.
  2. A phenotype-environment correlation: a significant association between environments and the morphological and physiological traits used to exploit those environments.
  3. Trait utility: the performance or fitness advantages of trait values in their corresponding environments.
  4. Rapid speciation: presence of one or more bursts in the emergence of new species around the time that ecological and phenotypic divergence is underway.

Conditions

Adaptive radiations are thought to be triggered by an ecological opportunity [3] or a new adaptive zone. [4] Sources of ecological opportunity can be the loss of antagonists (competitors or predators), the evolution of a key innovation or dispersal to a new environment. Any one of these ecological opportunities has the potential to result in an increase in population size and relaxed stabilizing (constraining) selection. As genetic diversity is positively correlated with population size [5] the expanded population will have more genetic diversity compared to the ancestral population. With reduced stabilizing selection phenotypic diversity can also increase. In addition, intraspecific competition will increase, promoting divergent selection to use a wider range of resources. This ecological release provides the potential for ecological speciation and thus adaptive radiation. [3]

Occupying a new environment might take place under the following conditions: [6]

  1. A new habitat has opened up: a volcano, for example, can create new ground in the middle of the ocean. This is the case in places like Hawaii and the Galapagos. For aquatic species, the formation of a large new lake habitat could serve the same purpose; the tectonic movement that formed the East African Rift, ultimately leading to the creation of the Rift Valley Lakes, is an example of this. An extinction event could effectively achieve this same result, opening up niches that were previously occupied by species that no longer exist.
  2. This new habitat is relatively isolated. When a volcano erupts on the mainland and destroys an adjacent forest, it is likely that the terrestrial plant and animal species that used to live in the destroyed region will recolonize without evolving greatly. However, if a newly formed habitat is isolated, the species that colonize it will likely be somewhat random and uncommon arrivals.
  3. The new habitat has a wide availability of niche space. The rare colonist can only adaptively radiate into as many forms as there are niches. [4]

Relationship between mass-extinctions and mass adaptive radiations

Phanerozoic biodiversity blank 01.png
All genera
"Well-defined" genera
Trend line
"Big Five" mass extinctions
Other mass extinctions
Million years ago
Thousands of genera
Phanerozoic biodiversity blank 01.png
Phanerozoic biodiversity as shown by the fossil record

A 2020 study found there to be no direct causal relationship between the proportionally most comparable mass radiations and extinctions in terms of "co-occurrence of species", substantially challenging the hypothesis of "creative mass extinctions". [7] [8] [9]


Examples

Darwin's finches

Darwin's finches are an often-used textbook example of adaptive radiation. Today represented by approximately 15 species, Darwin's finches are Galapagos endemics famously adapted for a specialized feeding behavior (although one species, the Cocos finch ( Pinaroloxias inornata ), is not found in the Galapagos but on the island of Cocos south of Costa Rica). [10] Darwin's finches are not actually finches in the true sense, but are members of the tanager family Thraupidae, and are derived from a single ancestor that arrived in the Galapagos from mainland South America perhaps just 3 million years ago. [11] Excluding the Cocos finch, each species of Darwin's finch is generally widely distributed in the Galapagos and fills the same niche on each island. For the ground finches, this niche is a diet of seeds, and they have thick bills to facilitate the consumption of these hard materials. [10] The ground finches are further specialized to eat seeds of a particular size: the large ground finch ( Geospiza magnirostris ) is the largest species of Darwin's finch and has the thickest beak for breaking open the toughest seeds, the small ground finch ( Geospiza fuliginosa ) has a smaller beak for eating smaller seeds, and the medium ground finch ( Geospiza fortis ) has a beak of intermediate size for optimal consumption of intermediately sized seeds (relative to G. magnirostris and G. fuliginosa). [10] There is some overlap: for example, the most robust medium ground finches could have beaks larger than those of the smallest large ground finches. [10] Because of this overlap, it can be difficult to tell the species apart by eye, though their songs differ. [10] These three species often occur sympatrically, and during the rainy season in the Galapagos when food is plentiful, they specialize little and eat the same, easily accessible foods. [10] It was not well-understood why their beaks were so adapted until Peter and Rosemary Grant studied their feeding behavior in the long dry season, and discovered that when food is scarce, the ground finches use their specialized beaks to eat the seeds that they are best suited to eat and thus avoid starvation. [10]

The other finches in the Galapagos are similarly uniquely adapted for their particular niche. The cactus finches ( Geospiza sp.) have somewhat longer beaks than the ground finches that serve the dual purpose of allowing them to feed on Opuntia cactus nectar and pollen while these plants are flowering, but on seeds during the rest of the year. [10] The warbler-finches ( Certhidea sp.) have short, pointed beaks for eating insects. [10] The woodpecker finch ( Camarhynchus pallidus ) has a slender beak which it uses to pick at wood in search of insects; it also uses small sticks to reach insect prey inside the wood, making it one of the few animals that use tools. [10]

The mechanism by which the finches initially diversified is still an area of active research. One proposition is that the finches were able to have a non-adaptive, allopatric speciation event on separate islands in the archipelago, such that when they reconverged on some islands, they were able to maintain reproductive isolation. [11] Once they occurred in sympatry, niche specialization was favored so that the different species competed less directly for resources. [11] This second, sympatric event was adaptive radiation. [11]

Cichlids of the African Great Lakes

The haplochromine cichlid fishes in the Great Lakes of the East African Rift (particularly in Lake Tanganyika, Lake Malawi, and Lake Victoria) form the most speciose modern example of adaptive radiation. [12] [13] [14] These lakes are believed to be home to about 2,000 different species of cichlid, spanning a wide range of ecological roles and morphological characteristics. [15] Cichlids in these lakes fill nearly all of the roles typically filled by many fish families, including those of predators, scavengers, and herbivores, with varying dentitions and head shapes to match their dietary habits. [14] In each case, the radiation events are only a few million years old, making the high level of speciation particularly remarkable. [14] [13] [12] Several factors could be responsible for this diversity: the availability of a multitude of niches probably favored specialization, as few other fish taxa are present in the lakes (meaning that sympatric speciation was the most probable mechanism for initial specialization). [12] Also, continual changes in the water level of the lakes during the Pleistocene (which often turned the largest lakes into several smaller ones) could have created the conditions for secondary allopatric speciation. [14] [12]

Tanganyika cichlids

Lake Tanganyika is the site from which nearly all the cichlid lineages of East Africa (including both riverine and lake species) originated. [16] Thus, the species in the lake constitute a single adaptive radiation event but do not form a single monophyletic clade. [16] Lake Tanganyika is also the least speciose of the three largest African Great Lakes, with only around 200 species of cichlid; [13] however, these cichlids are more morphologically divergent and ecologically distinct than their counterparts in lakes Malawi and Victoria, an artifact of Lake Tanganyika's older cichlid fauna. Lake Tanganyika itself is believed to have formed 9–12 million years ago, putting a recent cap on the age of the lake's cichlid fauna. [13] Many of Tanganyika's cichlids live very specialized lifestyles. The giant or emperor cichlid ( Boulengerochromis microlepis ) is a piscivore often ranked the largest of all cichlids (though it competes for this title with South America's Cichla temensis , the speckled peacock bass). [13] It is thought that giant cichlids spawn only a single time, breeding in their third year and defending their young until they reach a large size, before dying of starvation some time thereafter. [13] The three species of Altolamprologus are also piscivores, but with laterally compressed bodies and thick scales enabling them to chase prey into thin cracks in rocks without damaging their skin. [13] Plecodus straeleni has evolved large, strangely curved teeth that are designed to scrape scales off of the sides of other fish, scales being its main source of food. [13] Gnathochromis permaxillaris possesses a large mouth with a protruding upper lip, and feeds by opening this mouth downward onto the sandy lake bottom, sucking in small invertebrates. [13] A number of Tanganyika's cichlids are shell-brooders, meaning that mating pairs lay and fertilize their eggs inside of empty shells on the lake bottom. [13] Lamprologus callipterus is a unique egg-brooding species, with 15 cm-long males amassing collections of shells and guarding them in the hopes of attracting females (about 6 cm in length) to lay eggs in these shells. [13] These dominant males must defend their territories from three types of rival: (1) other dominant males looking to steal shells; (2) younger, "sneaker" males looking to fertilize eggs in a dominant male's territory; and (3) tiny, 2–4 cm "parasitic dwarf" males that also attempt to rush in and fertilize eggs in the dominant male's territory. [13] These parasitic dwarf males never grow to the size of dominant males, and the male offspring of dominant and parasitic dwarf males grow with 100% fidelity into the form of their fathers. [13] A number of other highly specialized Tanganyika cichlids exist aside from these examples, including those adapted for life in open lake water up to 200m deep. [13]

Malawi cichlids

The cichlids of Lake Malawi constitute a "species flock" of up to 1000 endemic species. [14] Only seven cichlid species in Lake Malawi are not a part of the species flock: the Eastern happy ( Astatotilapia calliptera ), the sungwa ( Serranochromis robustus ), and five tilapia species (genera Oreochromis and Coptodon ). [14] All of the other cichlid species in the lake are descendants of a single original colonist species, which itself was descended from Tanganyikan ancestors. [16] The common ancestor of Malawi's species flock is believed to have reached the lake 3.4 million years ago at the earliest, making Malawi cichlids' diversification into their present numbers particularly rapid. [14] Malawi's cichlids span a similarly range of feeding behaviors to those of Tanganyika, but also show signs of a much more recent origin. For example, all members of the Malawi species flock are mouth-brooders, meaning the female keeps her eggs in her mouth until they hatch; in almost all species, the eggs are also fertilized in the female's mouth, and in a few species, the females continue to guard their fry in their mouth after they hatch. [14] Males of most species display predominantly blue coloration when mating. However, a number of particularly divergent species are known from Malawi, including the piscivorous Nimbochromis livingtonii , which lies on its side in the substrate until small cichlids, perhaps drawn to its broken white patterning, come to inspect the predator - at which point they are swiftly eaten. [14]

Victoria's cichlids

Lake Victoria's cichlids are also a species flock, once composed of some 500 or more species. [12] The deliberate introduction of the Nile Perch ( Lates niloticus ) in the 1950s proved disastrous for Victoria cichlids, and the collective biomass of the Victoria cichlid species flock has decreased substantially and an unknown number of species have become extinct. [17] However, the original range of morphological and behavioral diversity seen in the lake's cichlid fauna is still mostly present today, if endangered. [12] These again include cichlids specialized for niches across the trophic spectrum, as in Tanganyika and Malawi, but again, there are standouts. Victoria is famously home to many piscivorous cichlid species, some of which feed by sucking the contents out of mouthbrooding females' mouths. [17] Victoria's cichlids constitute a far younger radiation than even that of Lake Malawi, with estimates of the age of the flock ranging from 200,000 years to as little as 14,000. [12]

Adaptive radiation in Hawaii

An `i`iwi (Drepanis coccinea). Note the long, curved beak for sipping nectar from tubular flowers. Iiwi on native mint - Hakalau Forest NWR.jpg
An ʻiʻiwi (Drepanis coccinea). Note the long, curved beak for sipping nectar from tubular flowers.

Hawaii has served as the site of a number of adaptive radiation events, owing to its isolation, recent origin, and large land area. The three most famous examples of these radiations are presented below, though insects like the Hawaiian drosophilid flies and Hyposmocoma moths have also undergone adaptive radiation. [18] [19]

Hawaiian honeycreepers

The Hawaiian honeycreepers form a large, highly morphologically diverse species group of birds that began radiating in the early days of the Hawaiian archipelago. While today only 17 species are known to persist in Hawaii (3 more may or may not be extinct), there were more than 50 species prior to Polynesian colonization of the archipelago (between 18 and 21 species have gone extinct since the discovery of the islands by westerners). The Hawaiian honeycreepers are known for their beaks, which are specialized to satisfy a wide range of dietary needs: for example, the beak of the ʻakiapōlāʻau ( Hemignathus wilsoni ) is characterized by a short, sharp lower mandible for scraping bark off of trees, and the much longer, curved upper mandible is used to probe the wood underneath for insects. [10] Meanwhile, the ʻiʻiwi ( Drepanis coccinea ) has a very long curved beak for reaching nectar deep in Lobelia flowers. [18] An entire clade of Hawaiian honeycreepers, the tribe Psittirostrini, is composed of thick-billed, mostly seed-eating birds, like the Laysan finch ( Telespiza cantans ). [18] In at least some cases, similar morphologies and behaviors appear to have evolved convergently among the Hawaiian honeycreepers; for example, the short, pointed beaks of Loxops and Oreomystis evolved separately despite once forming the justification for lumping the two genera together. [20] The Hawaiian honeycreepers are believed to have descended from a single common ancestor some 15 to 20 million years ago, though estimates range as low as 3.5 million years. [21]

Hawaiian silverswords

A mixture of blooming and non-blooming Haleakala silverswords (Argyroxiphium sandwicense macrocephalum). Young silverswords, Haleakala.jpg
A mixture of blooming and non-blooming Haleakalā silverswords (Argyroxiphium sandwicense macrocephalum).

Adaptive radiation is not a strictly vertebrate phenomenon, and examples are also known from among plants. The most famous example of adaptive radiation in plants is quite possibly the Hawaiian silverswords, named for alpine desert-dwelling Argyroxiphium species with long, silvery leaves that live for up to 20 years before growing a single flowering stalk and then dying. [18] The Hawaiian silversword alliance consists of twenty-eight species of Hawaiian plants which, aside from the namesake silverswords, includes trees, shrubs, vines, cushion plants, and more. [21] The silversword alliance is believed to have originated in Hawaii no more than 6 million years ago, making this one of Hawaii's youngest adaptive radiation events. [21] This means that the silverswords evolved on Hawaii's modern high islands, and descended from a single common ancestor that arrived on Kauai from western North America. [21] The closest modern relatives of the silverswords today are California tarweeds of the family Asteraceae. [21]

Hawaiian lobelioids

Hawaii is also the site of a separate major floral adaptive radiation event: the Hawaiian lobelioids. The Hawaiian lobelioids are significantly more speciose than the silverswords, perhaps because they have been present in Hawaii for so much longer: they descended from a single common ancestor who arrived in the archipelago up to 15 million years ago. [21] Today the Hawaiian lobelioids form a clade of over 125 species, including succulents, trees, shrubs, epiphytes, etc. [22] Many species have been lost to extinction and many of the surviving species endangered.

Caribbean anoles

Anole lizards are distributed broadly in the New World, from the Southeastern US to South America. With over 400 species currently recognized, often placed in a single genus ( Anolis ), they constitute one of the largest radiation events among all lizards. [23] Anole radiation on the mainland has largely been a process of speciation, and is not adaptive to any great degree, but anoles on each of the Greater Antilles (Cuba, Hispaniola, Puerto Rico, and Jamaica) have adaptively radiated in separate, convergent ways. [24] On each of these islands, anoles have evolved with such a consistent set of morphological adaptations that each species can be assigned to one of six "ecomorphs": trunk–ground, trunk–crown, grass–bush, crown–giant, twig, and trunk. [24] Take, for example, crown–giants from each of these islands: the Cuban Anolis luteogularis , Hispaniola's Anolis ricordii , Puerto Rico's Anolis cuvieri , and Jamaica's Anolis garmani (Cuba and Hispaniola are both home to more than one species of crown–giant). [23] These anoles are all large, canopy-dwelling species with large heads and large lamellae (scales on the undersides of the fingers and toes that are important for traction in climbing), and yet none of these species are particularly closely related and appear to have evolved these similar traits independently. [23] The same can be said of the other five ecomorphs across the Caribbean's four largest islands. Much like in the case of the cichlids of the three largest African Great Lakes, each of these islands is home to its own convergent Anolis adaptive radiation event.

Other examples

Presented above are the most well-documented examples of modern adaptive radiation, but other examples are known. Populations of three-spined sticklebacks have repeatedly diverged and evolved into distinct ecotypes. [25] On Madagascar, birds of the family Vangidae are marked by very distinct beak shapes to suit their ecological roles. [26] Madagascan mantellid frogs have radiated into forms that mirror other tropical frog faunas, with the brightly colored mantellas ( Mantella ) having evolved convergently with the Neotropical poison dart frogs of Dendrobatidae, while the arboreal Boophis species are the Madagascan equivalent of tree frogs and glass frogs. The pseudoxyrhophiine snakes of Madagascar have evolved into fossorial, arboreal, terrestrial, and semi-aquatic forms that converge with the colubroid faunas in the rest of the world. These Madagascan examples are significantly older than most of the other examples presented here: Madagascar's fauna has been evolving in isolation since the island split from India some 88 million years ago, and the Mantellidae originated around 50 mya. [27] [28] Older examples are known: the K-Pg extinction event, which caused the disappearance of the dinosaurs and most other reptilian megafauna 65 million years ago, is seen as having triggered a global adaptive radiation event that created the mammal diversity that exists today. [6] Also the Cambrian Explosion, where vacant niches left by the extinction of Ediacaran biota during End-Ediacaran mass extinction were filled up by the emergence of new phyla. [29]

See also

Related Research Articles

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species. He also identified sexual selection as a likely mechanism, but found it problematic.

<span class="mw-page-title-main">Cichlid</span> Family of fishes

Cichlids are fish from the family Cichlidae in the order Cichliformes. Traditionally Cichlids were classed in a suborder, the Labroidei, along with the wrasses (Labridae), in the order Perciformes, but molecular studies have contradicted this grouping. On the basis of fossil evidence, it first appeared in Tanzania during the Eocene epoch, about 46–45 million years ago. The closest living relative of cichlids is probably the convict blenny, and both families are classified in the 5th edition of Fishes of the World as the two families in the Cichliformes, part of the subseries Ovalentaria. This family is large, diverse, and widely dispersed. At least 1,650 species have been scientifically described, making it one of the largest vertebrate families. New species are discovered annually, and many species remain undescribed. The actual number of species is therefore unknown, with estimates varying between 2,000 and 3,000.

<span class="mw-page-title-main">Lake Tanganyika</span> Rift lake in east-central Africa

Lake Tanganyika is an African Great Lake. It is the second-oldest freshwater lake in the world, the second-largest by volume, and the second-deepest, in all cases after Lake Baikal in Siberia. It is the world's longest freshwater lake. The lake is shared among four countries—Tanzania, the Democratic Republic of the Congo (DRC), Burundi, and Zambia, with Tanzania (46%) and DRC (40%) possessing the majority of the lake. It drains into the Congo River system and ultimately into the Atlantic Ocean.

<i>Anolis</i> Genus of lizards

Anolis is a genus of anoles, iguanian lizards in the family Dactyloidae, native to the Americas. With more than 425 species, it represents the world's most species-rich amniote tetrapod genus, although many of these have been proposed to be moved to other genera, in which case only about 45 Anolis species remain. Previously, it was classified under the family Polychrotidae that contained all the anoles, as well as Polychrus, but recent studies place it in the Dactyloidae.

Allopatric speciation – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow.

<span class="mw-page-title-main">Sympatric speciation</span> Evolution of a new species from an ancestor in the same location

In evolutionary biology, sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap so that they occur together at least in some places. If these organisms are closely related, such a distribution may be the result of sympatric speciation. Etymologically, sympatry is derived from Greek συν (sun-) 'together', and πατρίς (patrís) 'fatherland'. The term was coined by Edward Bagnall Poulton in 1904, who explains the derivation.

<span class="mw-page-title-main">Directional selection</span> Type of genetic selection favoring one extreme phenotype

In population genetics, directional selection is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes. The increases are independent of the dominance of the allele, and if the allele is recessive, it will eventually become fixed.

<span class="mw-page-title-main">Peripatric speciation</span> Speciation in which a new species is formed from an isolated smaller peripheral population

Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from exchanging genes, it can often be difficult to distinguish between them. Nevertheless, the primary characteristic of peripatric speciation proposes that one of the populations is much smaller than the other. The terms peripatric and peripatry are often used in biogeography, referring to organisms whose ranges are closely adjacent but do not overlap, being separated where these organisms do not occur—for example on an oceanic island compared to the mainland. Such organisms are usually closely related ; their distribution being the result of peripatric speciation.

<span class="mw-page-title-main">Evolutionary ecology</span> Interaction of biology and evolution

Evolutionary ecology lies at the intersection of ecology and evolutionary biology. It approaches the study of ecology in a way that explicitly considers the evolutionary histories of species and the interactions between them. Conversely, it can be seen as an approach to the study of evolution that incorporates an understanding of the interactions between the species under consideration. The main subfields of evolutionary ecology are life history evolution, sociobiology, the evolution of interspecific interactions and the evolution of biodiversity and of ecological communities.

<span class="mw-page-title-main">Disruptive selection</span> Natural selection for extreme trait values over intermediate ones

In evolutionary biology, disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups. In this more individuals acquire peripheral character value at both ends of the distribution curve.

An evolutionary radiation is an increase in taxonomic diversity that is caused by elevated rates of speciation, that may or may not be associated with an increase in morphological disparity. A significantly large and diverse radiation within a relatively short geologic time scale is often referred to as an explosion. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment, they are termed adaptive radiations.

<span class="mw-page-title-main">Character displacement</span>

Character displacement is the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species' distributions do not overlap. This pattern results from evolutionary change driven by biological competition among species for a limited resource. The rationale for character displacement stems from the competitive exclusion principle, also called Gause's Law, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.

<span class="mw-page-title-main">Lake Malawi National Park</span> National park in Malawi

Lake Malawi National Park is a national park at the southern end of Lake Malawi in Malawi, Southeast Africa. It is the only national park in Malawi that was created with the purpose of protecting fish and aquatic habitats. Despite this being its main purpose, Lake Malawi National Park includes a fair amount of land, including a headland, the foreshore and several small rocky islands in Lake Malawi.

<span class="mw-page-title-main">Silversword alliance</span> Group of Hawaiian plants that show remarkable diversity

The silversword alliance, also known as the tarweeds, refers to an adaptive radiation of around 30 species in the composite or sunflower family, Asteraceae. The group is endemic to Hawaii, and is derived from a single immigrant to the islands. For radiating from a common ancestor at an estimated 5.2±0.8 Ma, the clade is extremely diverse, composed of trees, shrubs, subshrubs, mat-plants, cushion plants, rosette plants, and lianas.

<span class="mw-page-title-main">Hybrid speciation</span> Form of speciation involving hybridization between two different species

Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was thought to be particularly difficult to achieve, and thus hybrid species were thought to be very rare. With DNA analysis becoming more accessible in the 1990s, hybrid speciation has been shown to be a somewhat common phenomenon, particularly in plants. In botanical nomenclature, a hybrid species is also called a nothospecies. Hybrid species are by their nature polyphyletic.

<span class="mw-page-title-main">Lepidophagy</span> Fish eating scales of other fish

Lepidophagy is a specialised feeding behaviour in fish that involves eating the scales of other fish. Lepidophagy is widespread, having evolved independently in at least five freshwater families and seven marine families. A related feeding behavior among fish is pterygophagy: feeding on the fins of other fish.

<span class="mw-page-title-main">Hawaiian honeycreeper</span> Subfamily of birds

Hawaiian honeycreepers are a group of small birds endemic to Hawaiʻi. They are members of the finch family Fringillidae, closely related to the rosefinches (Carpodacus), but many species have evolved features unlike those present in any other finch. Their great morphological diversity is the result of adaptive radiation in an insular environment. Many have been driven to extinction since the first humans arrived in Hawaii, with extinctions increasing over the last 2 centuries following European discovery of the islands, with habitat destruction and especially invasive species being the main causes.

Island ecology is the study of island organisms and their interactions with each other and the environment. Islands account for nearly 1/6 of earth’s total land area, yet the ecology of island ecosystems is vastly different from that of mainland communities. Their isolation and high availability of empty niches lead to increased speciation. As a result, island ecosystems comprise 30% of the world’s biodiversity hotspots, 50% of marine tropical diversity, and some of the most unusual and rare species. Many species still remain unknown.

Eukaryote hybrid genomes result from interspecific hybridization, where closely related species mate and produce offspring with admixed genomes. The advent of large-scale genomic sequencing has shown that hybridization is common, and that it may represent an important source of novel variation. Although most interspecific hybrids are sterile or less fit than their parents, some may survive and reproduce, enabling the transfer of adaptive variants across the species boundary, and even result in the formation of novel evolutionary lineages. There are two main variants of hybrid species genomes: allopolyploid, which have one full chromosome set from each parent species, and homoploid, which are a mosaic of the parent species genomes with no increase in chromosome number.

Nonadaptive radiations are a subset of evolutionary radiations that are characterized by diversification that is not driven by resource partitioning. The species that are a part of a nonadaptive radiation will tend to have very similar niches, and in many cases will be morphologically similar. Nonadaptive radiations are driven by nonecological speciation. In many cases, this nonecological speciation is allopatric, and the organisms are dispersal-limited such that populations can be geographically isolated within a landscape with relatively similar ecological conditions. For example, Albinaria land snails on islands in the Mediterranean and Batrachoseps salamanders from California each include relatively dispersal-limited, and closely related, ecologically similar species often have minimal range overlap, a pattern consistent with allopatric, nonecological speciation. In other cases, such as certain damselflies and crickets from Hawaii, there can be range overlap in closely related species, and it is likely that sexual selection plays a role in maintaining species boundaries.

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Further reading