In a broad sense, biological co-evolution is "the change of a biological object triggered by the change of a related object". Coevolution can occur at multiple levels of biology: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as covarying traits between different species in an environment. Each party in a co-evolutionary relationship exerts selective pressures on the other, thereby affecting each others' evolution. Species-level co-evolution includes the evolution of a host species and its parasites, and examples of mutualism evolving through time. Evolution in response to abiotic factors, such as climate change, is not coevolution (since climate is not alive and does not undergo biological evolution). Evolution in a one-on-one interaction, such as that between predator and prey, host-symbiont or host-parasitic pair, is coevolution. But many cases are less clearcut: a species may evolve in response to a number of other species, each of which is also evolving in response to a set of species. This situation has been referred to as "diffuse coevolution". And, certainly, for many organisms, the biotic (living) environment is the most prominent selective pressure, resulting in evolutionary change.
Examples of co-evolution include pollination of Angraecoid orchids by African moths. These species co-evolve because the moths are dependent on the flowers for nectar and the flowers are dependent on the moths to spread their pollen so they can reproduce. The evolutionary process has led to deep flowers and moths with long probosci.
Co-evolution also occurs between predator and prey species as in the case of the Rough-skinned Newt (Taricha granulosa) and the common garter snake (Thamnophis sirtalis). In this case, the newts produce a potent nerve toxin that concentrates in their skin. Garter snakes have evolved resistance to this toxin through a set of genetic mutations, and prey upon the newts. The relationship between these animals has resulted in an evolutionary arms race that has driven toxin levels in the newt to extreme levels. Coevolution processes were modeled by Leigh Van Valen as the theory of the Red Queen. Emphasizing the importance of the sexual conflict, Thierry Lodé privileged the role of antagonist interactions (notably sexual) in evolution leading to an antagonist coevolution.
Co-evolution does not imply mutual dependence. The host of a parasite, or prey of a predator, does not depend on its enemy for survival.
The existence of mitochondria within eukaryote cells is an example of co-evolution as the mitochondria has a different DNA sequence than that of the nucleus in the host cell. This concept is described further by the endosymbiotic theory.
Co-evolutionary algorithms are also a class of algorithms used for generating artificial life as well as for optimization, game learning and machine learning. Pioneering results in the use of co-evolutionary methods were by Daniel Hillis (who co-evolved sorting networks) and Karl Sims (who co-evolved virtual creatures).
Flowers have converged to take advantage of similar birds. Flowers compete for pollinators and adaptations reduce deleterious effects of this competition. Bird-pollinated flowers usually show higher nectar volumes and sugar production. This reflects high energy requirements of the birds. Energetic criteria are the most important determinants of flower choice by birds. Following their respective breeding seasons, several species of hummingbirds co-occur in North America, and several hummingbird flowers bloom simultaneously in these habitats. These flowers seem to have converged to a common morphology and color. Different lengths and curvatures of the corolla tubes can affect the efficiency of extraction in hummingbird species in relation to differences in bill morphology. Tubular flowers force a bird to orient its bill in a particular way when probing the flower, especially when the bill and corolla are both curved; this also allows the plant to place pollen on a certain part of the bird’s body. This opens the door for a variety of morphological co-adaptations.
An important requisite for attraction is conspicuousness to birds, which reflects the properties of avian vision and habitat features. Birds have their greatest spectral sensitivity and finest hue discrimination at the long wavelength end of the visual spectrum. This is why red is so conspicuous to birds. Hummingbirds may also be able to see ultraviolet “colors” (Stiles 1981). The prevalence of ultraviolet patterns and nectar guides in nectar-poor entomophilous flowers allows the bird to avoid these flowers on sight. Two subfamilies in the family Trochilidae are Phaethorninae and Trochlinae. Each of these groups has evolved in conjunction with a particular set of flowers. Most Phaethorninae species are associated with large monocotyledonous herbs, and members of the subfamily Trochilinae are associated with dicotyledonous plant species.
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