Natural Selection Research Paper

Due to the excess in fecundity of organisms living in an environment with inadequate resources to support all the young, organisms compete over such resources. Competition can be direct, by aggressive interactions or by defending territories, or indirect, by eating food that is no longer available to other individuals. Organisms compete especially with conspecifics, because conspecifics are most similar in their needs and use of resources. Such inter and intraspecific competition is an important although not the only factor promoting evolution.

Individuals that are better able to cope with such competition because, for example, they can more efficiently digest food than other individuals in the same population, use more energy for reproduction, and may leave more offspring surviving into the next generation. If the ability to digest food is heritable, it will be passed on to the offspring and also improve the progeny’s reproductive efficiency. Because only successful competitors survive and reproduce, any character that improves such rivalry will spread in a population. This is in brief how natural selection works, which is a basic mechanism of evolution (recent textbooks on evolution by natural selection are Ridley 1996, Futuyma 1998, Stearns and Hoekstra 2000).

1. The Concept Of Natural Selection

Charles Darwin (1809–82) was among the first to formulate a scientific theory how evolution works. As an explanation for how the diversity of life with so many species and large variations in characters came to exist, he formulated the concept of natural selection (Darwin 1859). Natural selection was his central principle of how evolutionary change occurs, and it is still considered to be of paramount importance in evolutionary biology. Other factors definitely also play an important role in evolution, as, for example, genetic drift or chance events, but natural selection of hereditary variations usually is raised as the first and main hypothesis to explain even complex morphological, physiological, or behavioral characteristics of organisms.

Darwin based his theory on three general observations:

 (a) Variation: Members of a species typically do not look alike but differ in some characters.

(b) Heredity: Offspring are more similar to their parents than to other conspecifics, suggesting that parents pass on characters to the progeny.

(c) Differential reproduction: Some parents leave more progeny than others, due to some inherited characters.

Darwin had developed his theory without knowing the process of how exactly offspring inherit their parents’ characters. Gregor Mendel published his discovery of genes and of the principles of genetics as a contemporary of Darwin. However, it was not before the 1930s until the ‘Modern Synthesis’ combined the contributions of genetics, systematics, and paleontology into a new neo-Darwinian Theory (Young 1994). Due to the growth of genetics, we nowadays have detailed knowledge about how genes encode the information that is given from one generation to the next. Genes are copied and transmitted to the offspring when organisms reproduce. The variation that Darwin observed, thus, is based on genetic differences among individuals. Within populations, genes typically occur in two or more alternative forms, so-called alleles. Different alleles produce different forms of the same protein and result in different morphological, physiological, biochemical, behavioral, or other properties of an organism that develop through action of genes and environment. In other words, differences in genotype (the genetic constitution of an individual) result in different phenotypes.

As already emphasized by Darwin, given the three conditions as described before, natural selection always operates. If different genes or differences in hereditary characters have an influence on the survival or reproduction of an individual, such characters also promote with a high probability the individual’s offspring survival or reproduction. As a consequence, an allele responsible for improved reproduction becomes more common in the population and will increase in frequency in the next generation.

Natural selection operates whenever genotypes differ in their relative contribution of offspring to the next generation. Generally speaking, natural selection acts on differences in individual lifetime reproductive success, and thus results in the maximization of an individual’s biological fitness. Selection on individuals will favor alleles that help to build an organism with a relatively high probability to survive and reproduce successfully in its environment. Traits that benefit an organism to maximize its biological fitness function as adaptations.

2. The Importance Of The Environment

When natural selection works, a modified trait that confers higher fitness replaces another version that now yields lower fitness. This process of individual selection takes place in a situation with numerous selective factors, and most traits of organisms, actually are adaptations to the prevailing environmental conditions. The ecological environment includes both abiotic features, as climate, salinity, soil type, or availability of water, and biotic factors, as food supply, prey, predators, parasites, or conspecifics. The features of the environment that are important differ from species to species because of their different evolutionary histories.

Given that the relationship between genetic differences and reproductive success in the natural environment remains constant, alternative alleles at a specific gene locus are completely replaced. Environmental conditions, however, can change unpredictably and over long periods of time. Furthermore, catastrophic geologic events as mass extinctions, even if rare, impose strong forces and have a major influence on the course of evolution. This is why selection pressure is not constant over many generations and why scientists typically study the adaptive value of a trait by focusing strictly on its current fitness effects.

3. Genetic Variation Arises Through Mutations

According to the concept of individual selection populations should differ in traits. An organism is a complex system, the development of which is controlled by a genetic program embodied in its DNA, the genetic material. When the DNA is replicated within the cell, errors in copying occur from time to time. Each copying error is a mutation that results in an altered genotype. If such an alteration changes the bodily structure or the phenotype of an organism, natural selection begins to operate on such trait.

4. Natural Selection Explains Evolution And Adaptation

The process of natural selection explains both evolution and adaptation. It is ‘a process by which the forms of organisms in a population that are best adapted to the environment increase in frequency relative to less well-adapted forms over a number of generations’ (Ridley 1996). This fact is illustrated beautifully in long-term field studies of a group of finches living on the Galapagos Archipelago. This group of birds became known as Darwin’s finches because Charles Darwin had been the first to study them when he visited the islands during his voyage on the H. M. vs. Beagle (1831–36). He had been impressed by the way how different species of finches differed in the size and shape of the beak. Already in the 1940s, David Lack identified 14 separate species of finches that differ in the beak, body size, and male and female plumage. Otherwise, the birds are so similar in appearance and in behavior that he was convinced that they were all descended from a single ancestral species that colonized the Galapagos Islands.

Most of these species can be found on several islands. The island forms of the same species often also differ in the same characters that distinguish different species: in beak, body size, and plumage. Lack concluded that ‘new species have arisen when well-differentiated island forms have later met in the same region and kept distinct’ (Lack 1947). Members of the ancestral species have colonized one island after the other. After having arrived on an island they reached species level due to geographic isolation from other members of the original founder population. After they reached species level they recolonized the islands from which they had come.

Lack also provided evidence that the main beak differences between the species are adaptations to differences in diet based on the observation of clear correlations between a species’ beak size and its method of feeding. Seed-eating species had heavy finch-like beaks, the flower-probing species had rather long beaks, and so on.

The correlation with diet was evident even in closely related species where the beaks differ only in size, as in the three species of seed-eating ground-finches of the genus Geospiza. These species eat many of the same seeds; however, they also show a clear preference for different sized seeds in proportion to the size of their beak. Detailed studies by Peter Grant and his coworkers have not only supported David Lack’s main conclusion but also shown that variable environmental conditions result in changes in mean beak size within populations even over short periods of time as few generations (Grant 1986, Grant and Grant 1989). During longer periods of droughts populations of the large ground-finch Geospiza fortis declined fairly dramatically on the island Daphne. This was because the birds feed on seeds produced by the island’s plants, and seed production generally is reduced during dry periods. During the next year, however, the scientists observed an increase in the average size of the birds, indicating that survival was not random but that large ground finches survived better than small ones. The supply of smaller and softer seeds, which the finches can feed on easily, soon had been exhausted and the birds were forced to handle larger and harder seeds. Larger birds with larger beaks are better able to cope with the large seeds. As a consequence of strong natural selection in favor of the larger birds, mean body size and beak size in the population of Geospiza fortis on Daphne had increased from one season to the next. During wet periods as during El Nino years, however, with an excess production of small plant seeds, small sized birds with relatively small beaks had an advantage. As a consequence, natural selection on body size and beak size fluctuates in ground finches, dependent on the climatic condition and food availability during a given season.

5. Modes Of Selection

The Galapagos ground finches are an example of directional natural selection. During dry periods natural selection produces an increase in body size because larger finches have a higher fitness. If, however, intermediate phenotypes have the highest fitness, this mode of selection is called stabilizing or balancing selection. Here, natural selection acts against changes of a specific character and keeps the population constant over time. Birth weight in humans is an example of stabilizing selection that is generally a common mode of selection in nature. Babies with a lower or higher than average body weight at birth do not survive as well as babies of an average birth weight. A third possibility is for natural selection to be disruptive when both extremes of a phenotype are favored over the intermediate types. Such disruptive selection may work in sexually dimorphic species, with males being much larger than females (see Ridley 1996).

Furthermore, there are cases where the fitness of a phenotype depends on its frequency in the population, so that the fitness of an individual with a specific character is not constant in a given environment but is frequency dependent. In many frogs, males aggregate during the breeding season near or at ponds and call for mates, usually during the night. Calling males attract females that come to the pond to lay their eggs. Some males, however, are silent and simply remain near others that are calling and try to grasp and mate with a bypassing female that approaches a caller. Calling is energetically expensive, especially when producing low-frequency calls that are most attractive to females. Furthermore, calling males sometimes engage in aggressive fights over access to females. Males that are silent do not have to pay these costs. However, the highest benefit is expected for a silent male in a large chorus of calling conspecifics that attract a lot of females. If too many males are silent, on the other hand, no or only few females are attracted to the site. As a consequence, the reproductive success of a silent male is dependent on the frequency of calling males in its environment.

Frequency-dependent selection, thus, can result in populations containing two—or even more—alternative characters or traits. A form of natural selection that is called heterozygous advantage (or over- dominance for fitness), however, can also maintain such a polymorphism. It is the case if individuals carrying two different alleles at the specific gene locus (heterozygote individuals) have a higher fitness than individuals being homozygoes for either of the alleles.

As a consequence, both alleles are maintained in the population by the superior survival or reproduction of the heterozygote. A well-documented example of heterozygous advantage at a single gene locus is sickle-cell hemoglobin in humans. It was one of the first polymorphisms that were understood in terms of its selection process. Hemoglobin occurs in different variants with HbA being the normal type. Persons homozygous for HbS, however, suffer severe anemia and usually die before they reach adulthood. Heterozygotes have slight anemia, but in parts of Africa where falciparum malaria is common, they have a higher survival probability than homozygotes for normal hemoglobin HbA. Other examples of heterozygous advantage are rather rare, and this mode of selection does not seem to be a major cause of polymorphisms in nature.

6. Adaptation As A Compromise Between Different Selection Pressures

Natural selection has produced many adaptations that benefit an individual organism. Many evolutionary and behavioral scientists all over the world have collected a wealth of information about how individual selection has shaped the life of such diverse organisms as elephants, toads, sticklebacks, dung flies, or mites (recent text books summarizing such examples are Krebs and Davies 1993, Alcock 1998). Nevertheless, when studying such adaptations, however, it is important to realize that selection from different environmental factors simultaneously affects an individual’s behavior. Searching for food in an environment with predators certainly requires different behavioral rules than feeding in a safe place. Adaptations, therefore, typically have to be understood as a compromise between different needs of an organism.

Michael Ryan and his co-workers studied the mating behavior of the Tungara frog (Physalaemus pustulosus) on Barro Colorado Island, Panama. Males of this species are active during the night and produce rather complex calls, consisting of ‘whining’ components that are followed by another component that can be described as ‘chucks.’ Females prefer males that call and especially those that produce chucks. Nevertheless, some males do not produce chucks especially when they are alone or in small groups. Given that a male’s attractiveness for females depends on chucks, why don’t they always produce complete calls? The reason is that Tungara frogs have a special enemy, a frog-eating bat. Fringe-lipped bats (Trachops cirrhosus) track their prey, especially males, by their calls during the night, and they can better localize males that produce whine–chuck combinations in comparison to males with incomplete calls. The risk of being eaten by a predator is lower for frogs in large choruses because a bat has more individuals to choose from. Males calling alone or in small groups, however, suffer much higher costs in terms of predation. Thus, by producing only whining calls, which are less attractive to females but also harder for bats to locate, they can increase the probability to survive. During the next night, they might end up in a larger chorus and have a better chance to reproduce successfully (Ryan 1990, Ryan et al. 1990).

7. Sexual Selection As A Special Case Of Natural Selection

Given that individuals ultimately compete over reproduction, selection is typically strong on any characters that influence an organism’s access to mating partners. Charles Darwin introduced the term ‘sexual selection’ to describe ‘the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction’ (Darwin 1871).

From a biological point of view, males and females in sexually reproducing species differ in the size and number of their gametes. Males typically produce a relatively large number of tiny, mobile sperm cells that contain mainly DNA and some mechanism to provide energy so that sperms can move towards an egg within the female reproductive tract. Females, on the other hand, typically produce much larger gametes. Such egg cells are immobile, and besides genetic material they are also equipped with proteins and carbohydrates as energy stores for the embryo. As a consequence, the difference between the sexes in the size of the gametes can be expressed as a difference in parental investment, the time, energy, and risk that a parent expends in each progeny. This difference starts at the level of the gametes, and often is amplified by nourishment of offspring during pregnancy and maternal care even after the birth of the offspring. The energy available for reproduction, however, is generally limited for each organism. Therefore, investment in one offspring reduces the parent’s ability to invest in other or future progeny (Trivers 1972).

A male’s reproductive success is not so much limited by its sperm production but rather by its access to females. Thus, males are expected to increase their reproductive success by mating with as many females as possible. The fitness of a female, however, that invests relatively more in each offspring, is likely to be more reduced by unproductive matings. Females, therefore, are expected to be more selective in mating with males. As a consequence, sexual selection is typically stronger among males, which compete against each other for access to females. This pattern is reversed in some species such as seahorses (Syngnathidae) or phalaropes (Phalaropodidae) where males care for the progeny.

Sexual selection occurs in two types, intrasexual and intersexual selection. Intrasexual selection refers to males competing among each other for access to females. In red deer (Cer us elaphus) males aggregate in autumn during the rut—the breeding season—and engage in roaring contests and parallel marches. Males gain information about the size and fighting capacity of a competitor during such contests. Only males in very good condition can sustain roaring at a high rate for several minutes, and parallel marches allow judging each other’s body size. Smaller males usually leave the area after such contests and avoid energy consuming fights that they are likely to lose. Males that are similar in size and condition, however, engage in vicious fights with antler-clashing and shoving matches until the less competitive one flees the ground. The winning male acquires the group of females the deer were fighting over, and copulates with them as soon as they are in oestrous (Clutton-Brock et al. 1982).

Sometimes competition among males over reproduction can be more subtle. In species where fertile females mate with several males as in many insects, males have evolved extraordinary devices so that their sperm has an advantage over those of others in fertilizing the female’s eggs. Male damselflies have a penis with lateral horns that enable a male to scrub out a female’s genital tract. This behavior allows removing sperm from other competitors already stored in the female’s sperm storage organ before releasing its own sperm into the female (Waage 1979).

Characters as large size sperm-removing devices or large horns or antlers confer an advantage over access to receptive females and, thus, are adaptive. There are other elaborate male characters, however, as the peacock’s train that cannot be explained by that mechanism. In peacocks or in some African widow birds, males have extraordinarily long tail feathers that even seem to lower survival because they reduce maneuverability, the power of flight, and they make the bird more conspicuous for predators. Further-more, these male characters do not contribute an advantage in brood care because males do not provide resources or other care for the progeny.

It is difficult to explain the existence of such characters by natural selection. Darwin was well aware of that problem and suggested that the reduced survival of males with such handicaps is more than compensated by the bearers increased advantage in reproduction because of female choice. Males with exaggerated traits exist because females prefer to selectively mate with males with longer or brighter or more colorful feathers or other characters.

Darwin introduced the term intersexual for this second type of sexual selection to emphasize that such characters exist because males compete over being chosen or preferred by females. The idea that female choice of males can explain extraordinary characters like the peacock’s train is intriguing, nevertheless the question arises why females should do so.

In species where males provide parental care, male exaggerated traits might enable females to identify large and competitive males that successfully protect and care for the progeny. In pied flycatcher (Fidecula hypoleuca), females prefer males with black and white plumage over those with a duller, browner appearance. To test whether such preference evolved because black-and-white males are better able to care for the progeny, Sætre and co-workers removed the females and measured offspring growth reared exclusively by either black-and-white or dull males. Indeed blackand-white fathers could feed their young better which consequently had a higher weight than offspring cared for by dull males only (Sætre et al. 1994).

In species where males do not offer resources or parental care, the following three main hypotheses currently are discussed why females choose males with exaggerated traits.

First, females prefer the trait because it is an indicator of male health. Hamilton and Zuk (1982) have suggested that in birds bright or colorful plumage is a sign of health, because sick or parasitized individuals cannot produce such a trait. A similar argument can be applied for other extravagant traits or behaviors in other animals. Male appearance, thus, informs a female about the potential sexual partner’s health or parasite load. If resistance to disease or parasitic infection is heritable, females pass on the males’ genes for resistance to their offspring when picking a male with characters that indicate that they are parasite-free. This evolutionary mechanism results in the selection of ‘good genes’ because sons and daughters may inherit their father’s viability advantages. Experimental evidence from many species supports the hypothesis of ‘good genes selection’ by female preference of exaggerated traits, as in sticklebacks or barn swallows (Milinski and Bakker 1990, Møller 1994).

Second, females prefer males with an exaggerated trait because, again, it is an indicator of male health. Females use this information to mate with males that are less likely to transmit lice, mites, fleas, or bacterial pathogens which typically are harmful to the females themselves, or to their progeny. According to this ‘healthy male hypothesis,’ choosy females benefit because they may avoid contagious diseases and parasites.

Third, in the 1930s the evolutionary biologist R. A. Fisher suggested the idea of ‘runaway selection’ to explain genetic benefits for females with preferences for exaggerated traits in males (Fisher 1930). A choosy female benefits because her sons inherit a trait that makes them very attractive to females, and because her daughters will prefer males with such an attractive trait. The idea that female mate choice genes and genes for the preferred male trait are inherited together is the basis for the concept of a runaway process: as new mutations occur, ever more extreme female preferences and male characteristics spread together in a population. According to this theory, the adaptive value to choosy females holds even if the male exaggerated trait actually reduces the survival probability of the bearer of the trait. The runaway process stops only if the trait or behavior gets too costly or risky. In other words, if the bearer of the trait has such a low survival probability that he dies before he even reproduced, natural selection will counterbalance sexual selection for exaggerated, attractive characters (for a detailed review of the underlying arguments based on mathematical models see Andersson 1994).

Natural selection and sexual selection operate fundamentally the same way. Both forms of selection require that individuals that differ in heritable characters differ in the number of surviving offspring. Although sexual selection is just a special case of natural selection, Darwin already intended to emphasize the selective consequences of sexual interactions among conspecifics by introducing the distinction between the two processes. Sexual selection explains characters that cannot be understood as adaptations in dealing with other aspects of an individual’s environment, such as access to food or avoiding predators or parasites.

8. Altruism As A Challenge To The Concept Of Individual Selection

Conspecifics are a major environmental factor for many organisms because they are not only partners to mate with—in sexual species—but also partners in competitive and cooperative interactions. It is the special focus of scientists in the field of sociobiology and behavioral ecology to study such interactions among organisms (Wilson 2000). The interesting aspect here is to understand how individuals in social species influence each other’s reproduction. In this context special emphasis is on any behavior where individuals help others to raise young. Biologists term a behavior ‘altruistic’ if it increases the number of offspring produced by the recipient and includes fitness costs—a decreased number of young produced—for the acting individual. In many animal species individuals invest time and energy in the care of nonoffspring. Such behavior is common in social insects, where many or most members of a society may even spend their entire life in caring for and rearing the young of another female (Holldobler and Wilson 1990). Helping behavior or all-parental care also occurs in cooperative breeding birds and mammals (Brown 1987, Stacey and Koenig 1990, Konig 1997, Solomon and French 1997). The provision of care to nonoffspring in animal societies has attracted substantial scientific attention because of its apparent contradiction to the concept of individual selection. Can natural selection ever favor altruistic behavior that decreases the reproduction of the actor?

In the 1960s, Hamilton (1964) raised the point that relatedness can be of paramount importance for the evolution of such altruistic behavior. Hamilton realized that an individual can maximize its fitness not only by direct reproduction but also indirectly by helping kin to produce and/or to rear additional offspring. With a certain probability—dependent on the degree of relatedness between the donor and the recipient of help—such offspring will carry copies of alleles identical by descent to the helping individual. John Maynard Smith named this genetical mechanism kin selection (Maynard Smith 1964). According to Hamilton’s idea, altruistic behavior is more common in social groups of related individuals than in groups consisting of unrelated conspecifics.

Besides this mechanism of kin selection, altruistic behavior can also evolve via mutualism (each partner gains fitness benefits by performing the altruistic act) or reciprocity (as long as the altruistic act is reciprocated at some later date, both partners will gain fitness benefits; Dugatkin 1998). Both mechanisms allow for cooperation among unrelated individuals, however, require specific conditions so that the altruistic act is stabilized within groups.

Natural selection has not only resulted in many adaptations that benefit individuals in competitive interactions. It has also produced highly cooperative behaviors as the phenotypically altruistic behavior of nonoffspring care, which nevertheless is genetically ‘selfish’ and, thus, adaptive.

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