Evolution influences every aspect of the form and behavior of organisms. Most prominent are the specific behavioral and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by co-operating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.
These outcomes of evolution are sometimes divided into macroevolution, which is evolution that occurs at or above the level of species, such as extinction and speciation, and microevolution, which is smaller evolutionary changes, such as adaptations, within a species or population.[126] In general, macroevolution is regarded as the outcome of long periods of microevolution.[127] Thus, the distinction between micro- and macroevolution is not a fundamental one - the difference is simply the time involved.[128] However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels - with microevolution acting on genes and organisms, versus macroevolutionary processes acting on entire species and affecting the rate of speciation and extinction.[129][130][131]
A common misconception is that evolution has goals or long-term plans, but in reality, evolution has no long-term goal and does not necessarily produce greater complexity.[132][133] Although complex species have evolved, this occurs as a side effect of the overall number of organisms increasing, and simple forms of life remain more common.[134] For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size,[135] and constitute the vast majority of Earth's biodiversity.[136] Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is more noticeable.[137] Indeed, the evolution of microorganisms is particularly important to modern evolutionary research, since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time.[138][139]
Adaptation
Adaptation is one of the basic phenomena of biology,[140] and is the process whereby an organism becomes better suited to its habitat.[141][142] 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, or the ability of horses to run fast and escape predators. 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.[143] The following definitions are due to Theodosius Dobzhansky.
- 1. Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.[144]
- 2. 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.[145]
- 3. 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.[146]
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.[147] Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment,[148] Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing,[149][150] and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol.[151][152] 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 organisms' evolvability).[153][154]
Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organization 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.[156] However, since all living organisms are related to some extent,[157] 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.[158][159]
During adaptation, some structures may lose their original function and become vestigial structures.[160] 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,[161] the non-functional remains of eyes in blind cave-dwelling fish,[162] wings in flightless birds,[163] and the presence of hip bones in whales and snakes.[155] Examples of vestigial structures in humans include wisdom teeth,[164] the coccyx,[160] and the vermiform appendix.[160]
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.[165] 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.[165] Within cells, molecular machines such as the bacterial flagella[166] and protein sorting machinery[167] evolved by the recruitment of several pre-existing proteins that previously had different functions.[126] 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.[168][169]
A critical principle of ecology is that of competitive exclusion: no two species can occupy the same niche in the same environment for a long time.[170] Consequently, natural selection will tend to force species to adapt to different ecological niches. This may mean that, for example, two species of cichlid fish adapt to live in different habitats, which will minimize the competition between them for food.[171]
An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations.[172] This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features.[173] These studies have shown that evolution can alter development to create new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals.[174] 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.[175] It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.
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