ella[191] and protein sorting machinery[192] evolved by the recruitment of several pre-existing proteins that previously had different functions.[144] Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.[193][194]

An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations.[195] This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features.[196] These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals.[197] It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles.[198] It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.[199]

Co-evolution


Common Garter Snake (Thamnophis sirtalis sirtalis) which has evolved resistance to tetrodotoxin in its amphibian prey.
Further information: Co-evolution
Interactions between organisms can produce both conflict and co-operation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called co-evolution.[200] An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.[201]

Co-operation
Further information: Co-operation (evolution)
Not all co-evolved interactions between species involve conflict.[202] Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil.[203] This is a reciprocal relationship as the plants provide

losophical approaches: interbreeding, ecological and phylogenetic.[211] The biological species concept (BSC) is a classic example of the interbreeding approach. Defined by Ernst Mayr in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups".[212] Despite its wide and long-term use, the BSC like others is not without controversy, for example because these concepts cannot be applied to prokaryotes,[213] and this is called the species problem.[210] Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.[210][211] "

Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules.[214] Such hybrids are generally infertile. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype.[215] The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many types of animals,[216] with the gray tree frog being a particularly well-studied example.[217]

Speciation has been observed multiple times under both controlled laboratory conditions and in nature.[218] In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four mechanisms for speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.[219][220] As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.[221]

The second mechanism of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effec

their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time.[156][157]

Adaptation
For more details on this topic, see Adaptation.


Homologous bones in the limbs of tetrapods. The bones of these animals have the same basic structure, but have been adapted for specific uses.
Adaptation is the process that makes organisms better suited to their habitat.[158][159] Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection.[160] The following definitions are due to Theodosius Dobzhansky.

Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.[161]
Adaptedness is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats.[162]
An adaptive trait is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.[163]
Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell.[164] Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment,[165] Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing,[166][167] and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol.[168][169] An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing or

roversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability).[170][171][172][173][174]



A baleen whale skeleton, a and b label flipper bones, which were adapted from front leg bones: while c indicates vestigial leg bones, suggesting an adaptation from land to sea.[175]
Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all these structures from a common mammalian ancestor.[176] However, since all living organisms are related to some extent,[177] even organs that appear to have little or no structural similarity, such as arthropod, squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called deep homology.[178][179]

During evolution, some structures may lose their original function and become vestigial structures.[180] Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include pseudogenes,[181] the non-functional remains of eyes in blind cave-dwelling fish,[182] wings in flightless birds,[183] and the presence of hip bones in whales and snakes.[175] Examples of vestigial structures in humans include wisdom teeth,[184] the coccyx,[180] the vermiform appendix,[180] and other behavioural vestiges such as goose bumps[185][186] and primitive reflexes.[187][188][189]

However, many traits that appear to be simple adaptations are in fact exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process.[190] One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation.[190] Within cells, molecular machines such as the bacterial flagella[191] and protein sorting machinery[192] evolved by the recruitment of several pre-existing proteins that previously had different functions.[144] Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structur