Changes in habitat Before Darwin, adaptation was seen as a fixed relationship between an organism and its habitat. It was not appreciated that as the
climate changed, so did the habitat; and as the habitat changed, so did the
biota. Also, habitats are subject to changes in their biota: for example,
invasions of species from other areas. The relative numbers of species in a given habitat are always changing. Change is the rule, though much depends on the speed and degree of the change. When the habitat changes, three main things may happen to a resident population: habitat tracking, genetic change or extinction. In fact, all three things may occur in sequence. Of these three effects only genetic change brings about adaptation. When a habitat changes, the resident population typically moves to more suitable places; this is the typical response of flying insects or oceanic organisms, which have wide (though not unlimited) opportunity for movement. This common response is called
habitat tracking. It is one explanation put forward for the periods of apparent stasis in the
fossil record (the
punctuated equilibrium theory).
Genetic change Without
mutation, the ultimate source of all
genetic variation, there would be no genetic changes and no subsequent adaptation through evolution by natural selection. Genetic change occurs in a population when mutation increases or decreases in its initial frequency followed by random genetic drift, migration, recombination or natural selection act on this genetic variation. One example is that the first pathways of enzyme-based metabolism at the very origin of life on Earth may have been co-opted components of the already-existing
purine nucleotide metabolism, a metabolic pathway that evolved in an ancient
RNA world. The co-option requires new mutations and through natural selection, the population then adapts genetically to its present circumstances. The coat color of different wild mouse species matches their environments, whether black lava or light sand, owing to adaptive mutations in the
melanocortin 1 receptor and other
melanin pathway genes. Physiological resistance to the heart poisons (
cardiac glycosides) that
monarch butterflies store in their bodies to protect themselves from predators are driven by adaptive mutations in the target of the poison, the
sodium pump, resulting in target site insensitivity. These same adaptive mutations and similar changes at the same amino acid sites were found to evolve in a parallel manner in distantly related insects that feed on the same plants, and even in a bird that feeds on monarchs through
convergent evolution, a hallmark of adaptation. Convergence at the gene-level across distantly related species can arise because of evolutionary constraint. Habitats and biota do frequently change over time and space. Therefore, it follows that the process of adaptation is never fully complete. Over time, it may happen that the environment changes little, and the species comes to fit its surroundings better and better, resulting in stabilizing selection. On the other hand, it may happen that changes in the environment occur suddenly, and then the species becomes less and less well adapted. The only way for it to climb back up that fitness peak is via the introduction of new genetic variation for natural selection to act upon. Seen like this, adaptation is a genetic
tracking process, which goes on all the time to some extent, but especially when the population cannot or does not move to another, less hostile area. Given enough genetic change, as well as specific demographic conditions, an adaptation may be enough to bring a population back from the brink of extinction in a process called
evolutionary rescue. Adaptation does affect, to some extent, every species in a particular
ecosystem.
Leigh Van Valen thought that even in a stable environment, because of antagonistic species interactions and limited resources, a species must constantly had to adapt to maintain its relative standing. This became known as the
Red Queen hypothesis, as seen in host-
parasite interactions. Existing genetic variation and mutation were the traditional sources of material on which natural selection could act. In addition,
horizontal gene transfer is possible between organisms in different species, using mechanisms as varied as
gene cassettes,
plasmids,
transposons and viruses such as
bacteriophages.
Co-adaptation with flowering plants. In
coevolution, where the existence of one species is tightly bound up with the life of another species, new or 'improved' adaptations which occur in one species are often followed by the appearance and spread of corresponding features in the other species. In other words, each species triggers reciprocal natural selection in the other. These
co-adaptational relationships are intrinsically dynamic, and may continue on a trajectory for millions of years, as has occurred in the relationship between
flowering plants and
pollinating insects.
Mimicry ; the others show
Batesian mimics: three
hoverflies and one
beetle. Bates' work on Amazonian
butterflies led him to develop the first scientific account of
mimicry, especially the kind of mimicry which bears his name:
Batesian mimicry. This is the mimicry by a palatable species of an unpalatable or noxious species (the model), gaining a selective advantage as
predators avoid the model and therefore also the mimic. Mimicry is thus an
anti-predator adaptation. A common example seen in temperate gardens is the
hoverfly (Syrphidae), many of which—though bearing no sting—mimic the
warning coloration of aculeate
Hymenoptera (
wasps and
bees). Such mimicry does not need to be perfect to improve the survival of the palatable species. Bates, Wallace and
Fritz Müller believed that Batesian and
Müllerian mimicry provided
evidence for the action of natural selection, a view which is now standard amongst biologists.
Trade-offs All adaptations have a downside: horse legs are great for running on grass, but they cannot scratch their backs;
mammals' hair helps temperature, but offers a niche for
ectoparasites; the only flying penguins do is under water. Adaptations serving different functions may be mutually destructive. Compromise and makeshift occur widely, not perfection. Selection pressures pull in different directions, and the adaptation that results is some kind of compromise.
Examples Consider the antlers of the
Irish elk, (often supposed to be far too large; in
deer antler size has an
allometric relationship to body size). Antlers serve positively for defence against
predators, and to score victories in the annual
rut. But they are costly in terms of resources. Their size during the
last glacial period presumably depended on the relative gain and loss of reproductive capacity in the population of elks during that time. As another example,
camouflage to avoid detection is destroyed when vivid
coloration is displayed at mating time. Here the risk to life is counterbalanced by the necessity for reproduction. Stream-dwelling salamanders, such as
Caucasian salamander or
Gold-striped salamander have very slender, long bodies, perfectly adapted to life at the banks of fast small rivers and mountain
brooks. Elongated body protects their
larvae from being washed out by current. However, elongated body increases risk of desiccation and decreases dispersal ability of the salamanders; it also negatively affects their
fecundity. As a result,
fire salamander, less perfectly adapted to the mountain brook habitats, is in general more successful, have a higher fecundity and broader geographic range. 's trainin full display The
peacock's ornamental train (grown anew in time for each mating season) is a famous adaptation. It must reduce his maneuverability and flight, and is hugely conspicuous; also, its growth costs food resources. Darwin's explanation of its advantage was in terms of
sexual selection: "This depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction." The kind of sexual selection represented by the peacock is called '
mate choice,' with an implication that the process selects the more fit over the less fit, and so has survival value. The recognition of sexual selection was for a long time in abeyance, but has been rehabilitated. The conflict between the size of the human
foetal brain at birth, (which cannot be larger than about 400 cm3, else it will not get through the mother's
pelvis) and the size needed for an adult brain (about 1400 cm3), means the brain of a newborn child is quite immature. The most vital things in human life (locomotion, speech) just have to wait while the brain grows and matures. That is the result of the birth compromise. Much of the problem comes from our upright
bipedal stance, without which our pelvis could be shaped more suitably for birth.
Neanderthals had a similar problem. As another example, the long neck of a giraffe brings benefits but at a cost. The neck of a giraffe can be up to in length. The benefits are that it can be used for inter-species competition or for foraging on tall trees where shorter herbivores cannot reach. The cost is that a long neck is heavy and adds to the animal's body mass, requiring additional energy to build the neck and to carry its weight around. ==Shifts in function ==