Some mutations do not affect the functioning of the proteins encoded by the mutated genes. An allele that does not affect the fitness of an organism is called a neutral allele. Such alleles, untouched by natural selection, may be lost, or their frequencies may increase with time, purely by genetic drift. Therefore, neutral alleles often accumulate in a population over time, providing it with considerable genetic variation. Much of the variation in those characters we can observe with our unaided senses is not neutral, but much molecular variation apparently is. Modern molecular techniques enable us to measure variation in neutral alleles and provide the means by which to distinguish adaptive from neutral variation.
Frequency-dependent selection maintains genetic variation within populations
Natural selection often preserves variation as a polymorphism: the coexistence within a population, at frequencies greater than mutations can produce, of two or more alleles at a locus. A polymorphism may be maintained when the fitness of a genotype (or phenotype) varies with its frequency relative to that of other genotypes (or phenotypes) in a population. This phenomenon is known as frequency-dependent selection. Asmall fish that lives in Lake Tanganyika, in East Africa, provides an example of frequency-dependent selection. The mouth of this scale-eating fish, Perissodus microlepis, opens either to the right or to the left as a result of an asymmetrical jaw joint; the direction of opening is genetically determined. P. microlepis approaches its prey (another fish) from behind and dashes in to bite off several scales from its flank. “Right-mouthed” individuals always attack from the victim’s left; “left-mouthed” individuals always attack from the victim’s right. The distorted mouth enlarges the area of teeth in contact with the prey’s flank, but only if the scaleeater attacks from the appropriate side. Prey fish are alert to approaching scale-eaters, so attacks are more likely to be successful if the prey must watch both flanks. Vigilance by the prey favors equal numbers of rightmouthed and left-mouthed scale-eaters, because if one form were more common than the other, prey fish would pay more attention to potential attacks from the corresponding flank. Over an 11-year period in which the scale-eaters in Lake Tanganyika were studied, the polymorphism was found to be stable: The two forms of P. microlepis remained at about equal frequencies.
Genetic variation is maintained in geographically distinct subpopulations
Much of the genetic variation in large populations is preserved as differences among members in different places (subpopulations). Subpopulations often vary genetically because they are subjected to different selective pressures in different environments. Plant species, for example, may vary geographically in the chemicals they synthesize to defend themselves against herbivores. Some individuals of the clover Trifolium repens produce the poisonous chemical cyanide. Poisonous individuals are less appealing to herbivores - particularly mice and slugs - than are non-poisonous individuals. However, clover plants that produce cyanide are more likely to be killed by frost because freezing damages cell membranes and releases the toxic cyanide into the plant’s own tissues. In populations of Trifolium repens, the frequency of cyanide-producing individuals increases gradually from north to south and from east to west across Europe. Poisonous plants make up a large proportion of clover populations only in areas where winters are mild. Cyanideproducing individuals are rare where winters are cold, even though herbivores graze clovers heavily in those areas.
2.3.2. Light and time effects as aspects of evolution’ phenomena and “natural” classification
Long-day Plants and Short-day plants
The botanists went on testing to confirm this discovery with many species of plants. Following this single lead, they were able to answer a host of questions that had long troubled both professional botanists and gardeners.
The investigators found that plants are of three general types, which they called day-neutral, short-day, and long day. Day–neutral plants flower without regard to day length. Short-day plants flower in early spring or fall; they must have a light period shorter than a critical length (Table 2.2). Long-day plants, which flower chiefly in the summer, will flower only if the light periods are longer than a critical length (Table 2.3).
Day neutral plants are plants in which flowering is not dependant upon photoperiod. Examples include celery, geranium and tomato.
In 1938, two investigators, Karl C. Humner and James Bonner, began to study Photoperiodism, using the cocklebur as their experimental tool. The cocklebur is a short-day plant requiring 15.5 hours or less of light per 24-hour cycle to flower. It is particularly useful for experimental purposes because a single exposure under laboratory conditions to a short-day cycle will introduce flowering two weeks later, even if the plant is immediately returned to long-day- conditions. The cocklebur can withstand a great deal of rough treatment, surviving even if its leaves are removed. Hamner and Bonner showed that it is the leaf blade of the cocklebur that responds to the photoperiod. A plant stripped of all its leaves cannot be induced to flower. But, if as little as one-eighth of a fully expanded leaf is left on the stem, the single short-day exposure induces flowering.
|Species of short day (long night) plant||Critical duration of darkness (hours)|
|Species of long day (short night) plant||Critical duration of light (hours)|
Latitude and season
Photoperiodism affects the geographic distribution of plant species since daylight varies with latitude and season. At the equator, the 12-hour day length alternate with 12-hour periods of darkness throughout the whole year. However, in other parts of the world, seasonal variations occur in the length of the daylight period. The further away from the equator a geographical location is situated, the longer its day length in summer and the shorter its day length in winter.
Short day plants (which need a critical duration of darkness) tend to live near the equator where they can flower all year or live in temperature regions where they flower from late autumn to early spring. They cannot reproduce in arctic environments because the temperature is too low for growth when the nights are long in winter.
Long-day plants (which need a critical duration of light) tend to inhabit extreme northern latitudes of temperature regions where they can flower from late spring to early autumn when the day length is long.
By responding to a photoperiod of particular length, all members of a species produce their flowers at the same time of year. This allows cross-pollination to occur, often on a large scale.
Effect of light on timing of breeding in animals
Behavior in animals is described as rhythmicalwhen it is repeated at definite intervals. Although such behavior isendogenous(under internal control), the time, at which it occurs, is influenced by an external factor.
Many birds and mammals areseasonalbreeders. Their gonads (tastes and ovaries) become active only at the certain time of the year. These changes are triggered by the arrival of daily photoperiods of a certain critical length.
In birds, the reproductive activity oflongdaybreeders is stimulated by the increasing day length that occurs in spring. Hormones are secreted which promote the enlargement and activity of gonadal tissues and the production of sex cells.
In larger mammals (e.g. sheep and deer), which require a longer period of gestation, ashortdaybreedingcycle occurs. Gonadal activity and reproductive behavior are triggered by the decreasing photoperiods, which occur in autumn.
In each case, seasonal breeding period is timed so that the offspring will be born in spring during favorable environmental conditions. Several months of plentiful food and mild temperatures during summer will allow the young animals to grow and become strong before winter and unfavorable conditions return.
The onset of shorter day lengths also triggers a series of events that lead to the migration of certain birds (e.g. swallow) and the hibernation of certain mammals (e.g. hedgehog) in autumn. These forms of behavior are further examples of photoperiodism. They are of biological significance since they enable the animal to survive the winter when conditions would be severe and the food would be scarce.
Circadian rhythms – regular rhythms of growth and activity that occur approximately on 24-hour basis.
Some plants have flowers that open in the morning and close at dusk or they spread their leaves in the sunlight and fold them toward the stem at night. More recent studies have shown that less evident activities, such as photosynthesis, auxin production, and the rate of cell division, also have regular daily rhythms, which continue even when all environmental conditions are kept constant. These regular day-night cycles have come to be called circadian rhythms, from the Latin words circa, meaning “about”, and dies, “day”. Circadian rhythms now have been found throughout the plant and animal kingdoms.
For a number of years, biologists debated whether it might not be some environmental force, such as cosmic rays, the magnetic field of the Earth, or the Earth’s rotation, that was setting the rhythms. Attempts to settle this recurrent controversy have led to countless experiments under an extraordinary variety of conditions. Organisms were taken down to salt mines, shipped to the South Pole, flown halfway around the world in airplanes, and, most recently, orbited in satellites. Although there is still a vocal minority that believes that circadian rhythms are under the influence of a subtle geophysical factor, most workers now agree that the rhythms are endogenous – that is, they originate within the organism. The strongest evidence in support of this belief is that the rhythms are not exact. Different species and different individuals of the same species often have slightly different, but consistent, rhythms, often as much as an hour or two longer or shorter than 24 hours. Nothing, however, is known about the physical or chemical nature of this internal timing device, which is often referred to as a biological clock.
Biological clocks are believed to play an essential role in many aspects of plant and animal physiology. For instance, insects are more active in the early evening hours. Bats that feed on insects begin to fly each evening just when the insects are most available.
Changing the Rhythms
Although circadian rhythms probably originate within the organisms themselves, they can be modified by external conditions – the fact that is, of course, important to the survival of both individuals and species. For instance, a plant whose natural daily rhythm shows a peak every 26 hours, when grown under continuous dim light can adjust its rhythm to 14 hours of light and 10 of darkness. It can also adjust to 11 hours of light and 11 hours of dark (or 22 hours). Such adjustment to an externally imposed rhythm is known as entrainment. If the new rhythm is too far removed from the original one, however, the organism will ‘escape” the entrained rhythm and revert to its natural one. A plant that kept on an artificial or forced rhythm, even for a long period of time, will revert to its normal internal period when returned to continuous dim light.
Adaptations to Climate Change
The angiosperms evolved during a relatively mild period in the Earth’s history. As the climate became colder and, as a consequence, water became locked in snow and ice for part of the year, the angiosperms, which already possessed some adaptations to drought (perhaps because of highland origins), were placed under new environmental stress. Some did not survive, and some were pushed southward. Those that did survive in colder, drier areas did so because of selection for characteristics that offered advantages in these relatively unfavorable environments. Chief among such characteristics is the capacity to remain dormant during periods when water is in short supply and when climatic conditions are unfavorable for delicate growing buds, shoots, new leaves, and root tips. Modern plants are classified as annuals, biennials, and perennials, depending on their characteristic patterns of active growth and dormancy.
Annuals, Biennials, and Perennials
Among annual plants, which include many of our weeds, wild flowers, garden flowers, vegetables, and grasses and most other monocots, the entire cycle from seed to vegetative plant to flower to seed again takes place within a single growing season. All vegetative organs (roots, stems, and leaves) die, and only the dormant seed bridges the gap between one generation and the new one. Plants with no woody stems, such as most annuals, are known as herbs.