Note: These materials are intended as supplements for students in Ant. 301. These pages are in development and will contain errors.

EVOLUTION AS A PROCESS

 

EVOLUTION AS A PROCESS

The frequency of the HbA allele (p) plus the frequency of the HbS allele (q) is 1.0 (or 100%). That is, p + q = 1.

Thus: 1 - q = p The frequency of HbA is 1 - .11 = .89

Square both sides of the equation

(p + q)2 = (1)2 or p2 + 2pq + q2 = 1

where

p2 = .892 = .7921 - the frequency of HbAHbA homozygotes

2pq = 2(.89*.11 )= .1958 - the frequency of HbA HbS heterozygotes

q2 = .112 = .0121 - frequency of HbSHbS homozygotes

More than 20% of the West African population (0.1958 + 0.012) possesses the allele HbS, believed to be responsible for more than 100,000 deaths annually throughout the world. The obvious question is why such a seemingly deleterious trait would reach such high frequencies in West Africa. One possibility is that the heterozygote phenotype, HbA HbS, is not deleterious. In this situation, removal of all homozygote HbSHbS phenotypes each generation only removes a small proportion of the HbS alleles. Since the frequency of the recessive HbS in the next generation will be those contributed by heterozygous parents, the rate of homozygote removal can be calculated:

where:

Complete removal of HbSHbS homozygotes reduces the frequency of HbS to .099 in one generation. Thus, continuing the process for 10 more generations reduces the frequency of HbS to .052. This means that about 10% of the eleventh generation would be bearers of HbS ( p2 + 2pq + q2 = 1). If this process continued indefinitely, HbS would be expected to be reduced to a low level, the same level that reflects the mutation rate at which HbS is reintroduced into the population. Although this expectation approximates the situation in North America, the sickle-cell trait does not behave in this way in West Africa. Rather, in West Africa, it persists at a greater rate. Therefore some other element must be involved.

To search for that other element, other processes must be considered. The process of differential reproduction of genotypes is called natural selection, and since it violates the Castle-Hardy-Weinberg assumption of random gamete exchange, it is an important evolutionary force. The net reproductive rate of a genotype, called reproductive success or fitness, is usually expressed as relative fitness (w) and computed by dividing the reproductive rate of a specified genotype by the genotype with the greatest fitness. The difference between fitness of one genotype and another is called the selection coefficient. For HbSHbS genotypes in the example above, s, the selection coefficient is (1.0 - 0) = 1. If however, 1 out of every 99 homozygote HbSHbS could survive and reproduce, then s would be (1.0-.01) = 0.99. If s is less than 1.0, the result of selection on successive generations is calculated using the formula:

High rates for the sickle-cell character would be understandable if the selection coefficients for homozygote HbAHbA and heterozygote HbA HbS gave an advantage to the heterozygote that compensated for the loss of HbSHbS genotypes in each generation. A sample of West African phenotypes provides counts among adults (From F.B. Livingston, Abnormal Hemoglobins in Human Populations, Aldine, 1967)

The observed proportion of HbS in the population of gametes represented by these adults is:

However, if we use expectations from the Castle-Hardy-Weinburg assumption to form phenotypes of 12,387 people with a frequency of HbS at 0.123, our estimates vary from what was observed (see table below). If we assume that the heterozygote has a theoretical fitness of 1.0, we can estimate the relative fitness (w).

 

Genotype  Observed Phenotype Frequency Expected phenotype Frequency   Ratio of Observed/Expected  w (Relative Fitness)  S (Selection coefficient)
 HbSHbS   29  187.4  0.155  0.155/1.12 = 0.14  1 -.14=.86
 HbA HbS 2,993  2,672.4  1.12  1.12/1.12 = 1.0  1-1=0
 HbAHbA  9,365  9,527.2  0.983  0.983/1.12 = 0.88  1-.88=.12
 Total  12,387  13,387    
           


We can compute the value or frequency of HbS at which equilibrium should occur; that is, the proportion of HbS alleles at which the disadvantage of HbSHbS balances the disadvantage of HbAHbA:

qA = sA /( sA + sS ) = .12/(.12 + .86) = 0.122

The observed value (0.123) agrees closely with the model of selection coefficients that indicates a heterozygote advantage for the sickle-cell character (0.122).

What, then, is the advantage of the HbAHbS phenotype? Worldwide distribution of the HbS allele corresponds to the distribution of malaria, a disease that kills more than 1,000,000 people annually. Experiments and clinical observations verify that heterozygous HbA HbS individuals are much less vulnerable to malaria than are homozygous HbAHbA individuals. The malaria parasite multiplies in red cells, possibly deriving nutrition from hemoglobin. Some hemoglobin variants, including HbS, are less susceptible to this parasite. The HbAHbA phenotype is more vulnerable to malaria, especially a virulent variety known as falciparum malaria ( produced by the microorganism, Plasmodium falciparum). Though homozygous HbSHbS individuals only occasionally reach reproductive age, heterozygous HbSHbA individuals do have better health, and it is their reproductive success that maintains the high rate of HbS alleles in the population.

A polymorphism such as HbAHbS, that is maintained by selection in favor of the heterozygote phenotype, is called a balanced polymorphism. Selection coefficients cause the relative frequencies of alleles to rise or fall until they reach equilibrium with both HbA and HbS genes represented. If the heterozygote were less fit than either homozygote, the allele with an initially higher frequency would continue to increase until the other allele(s) was eliminated. If individuals possessing HbA had an unconditional advantage, the population would in time become virtually all HbAHbA. Only selection favoring a heterozygote will produce a balanced polymorphism. A transient polymorphism refers to a polymorphism in which one allele is in the process of displacing another. It is theoretically possible for neutral polymorphisms to exist, if two or more alleles are equal in fitness, but it is more likely that apparently neutral polymorphisms reflect minor differences in fitness responding to a variable environment.

Details of a malarial parasite's life cycle make a difference in selection coefficients. Malaria became more prevalent with the spread of agriculture, presumably because slash-and-burn techniques in tropical forests expanded the number of small bodies of standing water, the preferred habitat for mosquitoes reproduction, especially of Anopheles gambae. Introduction of a complex Malaysian agricultural tradition into East Africa by sailors from the Pacific during the first century AD. included cultivation of taro, yams, bananas, and coconuts. This "root and tree" crop tradition has the highest correlation with HbS, as Malaysian cultivars dispersed through equatorial Africa. Though probably originating in West Africa, the HbS allele appears to follow agricultural practices that promote mosquito habitats.

The rate at which allele frequencies change can be estimated. In an environment characterized by malaria, a single mutation or a single immigrant could introduce the HbS allele to the local breeding population. The selection coefficients used in the example above would reach equilibrium in about 100 generations.

Most human malaria varieties (Plasmodium malariae, P. ovale, and P. vivax) are similar to malaria species found in the African apes and are not very lethal, suggesting a long co-evolutionary relationship between parasite and host. Indeed, some human genotypes are resistant, the most extreme being one of the alleles of the Duffy system (Fy4), a red cell coat protein, which provides immunity to Plasmodium vivax. The allele Fy4 approaches 100% in some African populations, producing immunity to P. vivax. A variety of hemoglobin polymorphisms in human populations may be the result of selection pressure from malaria. For example, people with glucose-6-phosphate dehydrogenase deficiency (G6PD) lack an enzyme that metabolizes glucose in their red blood cells. G6PD is an X-linked trait with several alleles that are known by their varying activity at producing an enzyme vital to the production of glutathione (GSH) GSH is required to maintain the membrane around red blood cells. Some alleles that produce G6PD deficiency are advantageous in a malarial environment, although they produce hemolytic anemia in response to eating fava beans a dietary staple in the Mediterranean area) and to the antimalarial drug primaquine. If a person with reduced G6PD actively eats fava beans or is exposed to pollen by walking through a field when plants are in flower, their GSH is rapidly oxidized, resulting in hemolysis, the rupture of red blood cells. This reaction can be mild or severe, depending on the G6PD genotype. Individuals have had fatal hemolytic reactions from the smell of cooking fava beans.

Plasmodium falciparum malaria, a more virulent disease that accounts for more than 95% of all human fatalities, is phylogenetically more related to the avian malarias than to ape and human Plasmodium species. Perhaps the transfer of P. falciparum to humans correlates with increased exposure to infected birds after the domestication of fowl (McCutchan, 1991). Falciparum malaria, being a relatively new human parasite, has not had time for coevolutionary processes to produce a less lethal parasite-host relationship, although Hbc may be replacing Hbs in some West African populations, and Hbc homozygotes are less disadvantaged than Hbs homozygotes (return to outline)..

Stabilizing Selection

Stabilizing selection, exemplified by the hemoglobin system previously discussed, also can be seen in polygenic characters, such as those affecting birth weight. A plot of birth weight against mortality illustrates the stabilizing influence of increasing mortality when neonatal birth weight departs from an optimal value of 7.5 lbs. Lower birth weights produce higher infant mortality and higher birth rates increase obstetrical difficulties with late pregnancy and birth. It is also probable that some behavioral characteristics such as intelligence, boldness, or playfulness of mammals also reflect stabilizing selection.

A clear understanding of selection is vital to modern medicine and agriculture. When a physician prescribes an antibiotic for an infection, it is dispensed with the instructions, "Take all of these. Do not stop the treatment when symptoms are relieved." These instructions are based upon an evolutionary concept of selection. Genetic variations in bacteria arise spontaneously from mutation. Sometimes these variants are resistant to a particular antibiotic, but under normal conditions such variants may be unimportant. Effective antibiotic treatment reduces infection to the point that a combination of body defenses and treatment produces a cure. However, inadequate antibiotic therapy (either not powerful enough or taken for only a short time) destroys only those bacterial variants that are most susceptible to treatment, thereby selecting for antibiotic-resistant strains. Indiscriminate use of antibiotics in medicine or as supplements in livestock feed is also likely to select for antibiotic-resistant microorganisms. The physician's instructions are intended to protect the patient from a recurrence of the illness by a more resistant bacterial phenotype.

Resistance to Disease

Natural changes in resistance to disease illustrate selection and interaction between biological agents. Use by modern medicine and agriculture of more biological agents to substitute for antibiotics and pesticides promotes more experience with these kinds of systems. In 1950, Myxomatosis, a viral disease naturally infecting Brazilian wild rabbits, was introduced into Australia as a biological agent to control the introduced European rabbits (Oryctolagus cuniculus). Transmitted by mosquito, this nonlethal virus of the Brazilian rabbit was quite virulent in the European rabbits, producing death in 98% of exposes. After a few years, the rabbit mortality rate at infection declined to about 25% as a result of changes in both rabbit and virus. Those rabbits whose phenotypes varied in some way that allowed them to survive the viral infection had a strong selective advantage in this context. Mutations in the virus which reduced virulence, also increased the average life span of infected rabbits, allowing a sick animal to spread the attenuated (less virulent) microorganism to many more potential hosts. Thus rabbits became more resistant and the virus became less virulent. These joint changes in the genotypes of interacting species are called coevolution. Many chronic human diseases, for example leprosy, smallpox, syphilis, and tuberculosis, may be products of similar coevolution. One promise of the developing science of molecular biology is that organisms can be designed and modified, thus engineering our own coevolution.

Mutation

Just as selection is responsible for order in biological systems, mutation is the source of new alleles. In sexual reproduction, the link between parent and child is through the gamete, where genetic recombination plays an important role in increasing the number of genotype combinations in a population. A mutation is a change due to an alteration in DNA. Agents, such as X-rays, that break a DNA strand and allow a segment to be lost or incorporated are mutagens. Mutations are the only source of new DNA codes outside of biological agents (such as viruses) or genetic engineering. DNA molecules are relatively stable and the genetic code is usually accurately duplicated in each of the homologous chains that form a DNA strand. Damage to one chain is repaired by using the remaining intact chain as a template. There are effective DNA repair mechanisms, based upon enzymes, that check for damage, excise defective segments, and replace them.

In reality then, a mutation is an alteration in the code that escapes this repair process. Even if a mutation occurs, the genetic code from the matching analogous chromosome should provide the codes needed for protein synthesis. The diploid nature of human cells protects against the consequences of many mutations. Since disturbances of the code associated with the X chromosome are not protected by this redundancy in males, X-linked mutations are easier to detect in males.

Extensive rearrangements, likely to be lethal to the gamete or zygote that acquires them, do not enter the gene pool. DNA errors can be due to a single base modification (alteration, substitution, insertion, deletion) or due to mispairing of sequences between chromosomes during meiosis, which results in duplications in one of a pair of chromosomes and deficiencies in the other. If two chromosomes differ slightly in DNA sequence, the repair mechanism acting upon the unpaired nucleotides, using one strand as a template, can convert one genotype into another. Errors in chromosome pairing may be made more frequent by DNA sequences that do not appear to be required for normal function, but which are duplicated in large numbers on many chromosomes, providing transposable elements. The particular structure of DNA thus can produce hot spots, places where errors are more likely to occur.

The more times DNA is replicated, the greater the likelihood of an error in replication. The apparent increase in mutation defects in progeny of older males, but not in older females, is probably due to the fact that all germ cell DNA replication occurs in the embryonic and fetal tissues of females, while DNA replication in spermatogonial cells of males continues for many years.

Our knowledge of mutations is limited. Most known mutations are recognized because they attract medical attention. A partial list of estimates of human mutation rates is in the following table:

 

Estimates of Mutation rates in Humans

 

 Trait  Mutants per Million Gametes
 Autosomal dominant Traits  
  von Hippel-Lindau syndrome   0.2
 Aniridia   2.6 - 5.0
 Acrocephalosyndactyly   3.0 - 4.0
 Neurofibromatosis   5.0 - 12.3
 Polycystic kidney disease   6.0 - 13.0
   
 X-linked Recessive Traits  
 Hemophilia B   2.0 - 3.0
 Hemophilia A   32.0 - 57.0
   
 Duchenne muscular dystrophy   43.0 - 105.0
   
   

{From Sutton 1988 page 271 with data from F. Vogel, R. Rathenberg, 1975, Adv. Hum. Genet. 5:223]

 

Mutations that occur in somatic cells produce clones of cells that differ from surrounding cells. Though somatic mutations do not contribute directly to the gene pool, they may seriously compromise the individual that possesses them since such mutations play a role in carcinogenesis .

Cancer, Mutagens, and the Immune System.

A common mutagen is ultraviolet radiation from sunlight. Overexposure causes damage to skin tissues (sunburn) and numerous mutations in exposed cells. Fortunately, repair mechanisms protect most of the damaged cells and those which escape repair are usually controlled by the immune system. However, extensive exposure overwhelms both the repair and defensive mechanisms, producing skin cancer. Evidence of a weakened immune response to deviant cells comes from experiments in which skin tumors from one laboratory mouse are transplanted to another. A skin tumor transplanted to a healthy mouse will be rejected; one transplanted to a mouse with a history of sunburn will grow.

This example illustrates much of current theory about cancer. Cancer appears to be triggered by specific mutagens called carcinogens, which damage certain regulatory genes in the affected cell. Common mutagens include caffeine, benzo(a)pyrene (a component of cigarette smoke), nitrous acid, ozone, and the active chemicals in most solvents. Certain viruses appear to behave as carcinogens that target growth-regulatory genes in specific tissues. The body's immune system destroys cancer cells when they are detected. However, it is possible that some types of cancer avoid immune responses. The immune system itself can be weakened by stress, age, malnutrition, and perhaps by exposure to large numbers of cancer cells (return to outline).

Genetic Drift

Genetic drift, also called the Sewall Wright effect, is an accidental change in gene allele frequencies (sampling error), and is more likely to occur in small populations. Indeed, the probability of such chance fluctuations increases as population size decreases. This process is particularly evident when a new population is formed by a small number of individuals leaving a larger group. Imagine a small number of people in canoes traveling from a continent to an island, where they begin a new population. The extensive genetic diversity of the continental population is not represented among the small number of travelers. Or imagine a scientist who captures a breeding pair of mice from a field and uses them as breeding stock in a laboratory. The laboratory stock does not possess the full range of genetic variation in alleles that one might see in the wild mouse population. These two examples represent special cases of genetic drift, the founder effect. Small populations were the rule in much of human evolutionary history rather than the exception. Fluctuations in population size that reduce the effective breeding population briefly to small numbers are called bottlenecks. The most important effect of small groups is rapid reduction of heterozygosity, resulting in high frequencies of surviving alleles in future progeny (return to outline).

Isolation

An isolating mechanism is any thing or process that inhibits random pairing of adults in a breeding population. It can be a barrier that prevents gamete exchange, such as geographic distance, or it can be something that promotes the pairing of certain individuals, such as kinship rules for preferred marriages. An isolating barrier produces divergence in gene allele frequencies between the separated groups. Each isolate experiences its own events and mutations. A lowering of the barrier that allows gene flow can produce changes in either or both populations.

Migration across an isolating barrier from one population to another produces a shift in allele frequencies in the direction of the immigrants. If allele frequencies for a locus are known for both populations, the consequences of gene flow can be estimated from the equation:


Pt = (1 - m)t ( P0 - P) + P

where

Pt = frequency of allele A1 in host population after t generations

m = proportion of migrants in a host population

P0 = initial frequency of allele A1 in the host population

P = frequency of allele A1 among migrants

Nonrandom mating is one of the most common departures from the Castle-Hardy-Weinberg equilibrium. A population may be stratified into castes, tribes, or ethnic groups, with people tending to marry within their own group. Mate selection based on resemblance or differences in a preferred phenotype is called assortative mating, a process that changes the distribution of genotypes and phenotypes in a population without changing allele frequencies. Choosing a contrasting phenotype in a mate is called negative assortative mating or heterogamy. This strategy increases the probability of heterozygous genotypes. Selecting a similar phenotype, positive assortative mating, increases the probability of homozygous genotypes. Inbreeding, mating between closely related individuals, can also have a similar effect. A shift from heterogamy to homogamy or inbreeding increases the probability of a recessive allele being expressed in a phenotype and thus exposed to natural selection. This can be unfortunate if the character is deleterious, but it is useful in animal husbandry and agriculture where selective breeding is used to change the characteristics of domesticated species (return to outline).

Evolution: The Current Synthesis

 

The current theory of evolution integrates the concepts of selection, mutation, drift, and isolation into a single perspective of evolutionary change. The concept of evolution is widely accepted because it alone makes sense out of the homologies and adaptations of the organic world. Though a simple idea, it has been difficult to use because we traditionally think in terms of types, not breeding populations. It is easy to pay lip service to modern evolutionary theory, to use the clichés that describe evolution, yet continue to think in terms of types and typologies. In terms of this current synthesis, there never was a "first human" or a "first" anything after the first DNA molecule. Evolutionary change is a shifting of gene allele frequencies in response to evolutionary processes. Every new individual was preceded by parents with different genotypes and who were members of a population with different gene allele frequencies.

Absolute stability among sexually reproducing populations is a virtual impossibility. Apparent uniformity in a body form or locomotor system can be maintained by stabilizing selection in the absence of environmental fluctuations, but organisms possess so many gene loci and alleles that it is unthinkable that unchanging conditions, indefinitely retaining existing frequencies for all loci, could be sustained. Evolutionary changes in biological systems of one sort or another should be expected as long as life exists.

Rates of evolutionary change have been the focus of much discussion and research. It is possible (but not probable) that new species sometimes appeared as a result of sudden and catastrophic rearrangements of genetic materials. These "hopeful monsters" may occur in asexual species, and they may, in fact, be frequent among plants, but they should be exceptionally rare in mammals. Selection itself does not create new genotypes. On the other hand, evolutionary rates may reflect the strength of processes like mutation, selection, and isolation. Subpopulations of the same species, isolated from each other and subject to dramatic drift and selection pressures, may diverge quickly. The more Mendelian a character, the more sensitive it is to drift. At the other extreme, both Mendelian and continuous variables that reflect numerous loci are easily altered by selection pressure. Indeed, continuous variables may be more immediately responsive since their inherent heterozygosity may provide a great range of alleles for selection without waiting for advantageous mutations.

If a population is genetically homogeneous, natural selection can not discriminate between genotypes. Mild selection pressures are most effective in altering gene allele frequencies in moderate-sized populations. Selection effects are somewhat negated by the tendency for the Castle-Hardy-Weinberg equilibrium to be sustained in large populations, or by genetic drift in small populations. Consequently, the observation that widely distributed species are divided by clines into subspecies and local breeding populations has important implications with respect to natural selection. The nature, amount, and rate of evolutionary changes reflect the particular combination of effective population size, mutation, selection, drift, and isolation pressures that a species experiences.

N. Eldredge and Stephen Jay Gould (1972) proposed that new species emerge suddenly and remain mostly unchanged until they become extinct, a theory known as punctuated equilibrium. In their view, instead of being a gradual series of subtle changes, speciation is dramatic and rapid. Some scientists have used the punctuated evolution model to argue that speciation occurs so rapidly that natural selection can not be viewed as a major element in evolutionary processes, going so far to write, " that theory [neo-Darwinian evolutionary theory] as a general proposition is effectively dead, despite its persistence as a text-book orthodoxy" (Gould, 1980).

Part of the problem between an extreme gradualistic view and a punctuated view of evolution, is confusion about the nature of species. In the Darwinian view, a species is an evolving reproductive community, recognizably discrete from other such communities in time and space. A species is not a "fixed' entity since by genetic changes, it can evolve into something different. The strongest advocates of punctuated equilibrium argue that this view of species and of evolution is profoundly wrong since it presumes that

"...all evolution reduces to nothing more than change in gene content and frequency within species. The process is adaptive, as natural selection guides change in response to changing environments. It is at once a simple view and a sophisticated version of Darwin's original conception. And it is, in a profound sense, probably wrong. It is one of the greatest myths of twentieth-century biology" (Eldredge and Tattersall, 1982; page 43)

Those advocating punctuated equilibrium as a model for evolution view species as single organic entities, separated in time and space from all other entities. Under normal conditions they do not evolve, but by some mechanism that is not known, species sometimes are transformed, very suddenly, into something different. As long as a species exists, it is a single unit. Progressive adaptive change does not contribute to or account for speciation.

The concept of punctuated equilibrium is cited by creation scientists who argue that punctuated evolution demonstrates the presence of supernatural intervention, sometimes evoking an 18th century model of the great chain of being where supernatural forces keep all creatures in their place and on a path toward perfection, a process called guided design.

Mainstream thinking in evolutionary biology does not sympathize with the advocates of punctuated equilibrium. The synthetic theory of evolution always included the idea that evolution proceeded at varying rates. Natural selection is one of the central organizing concepts of all modern biology. It has not been falsified by contradictory data nor even seriously challenged by a proposed alternative explanation. Speciation via neo-Darwinian mechanisms has been observed to occur. For example, rabbits released on St. Helena when Napoleon was exiled are no longer cross-fertile with their European ancestral stock. The synthetic theory can account for abrupt change in small populations or in altered environments. Consider what happens for example, if an environmental change makes a dominant trait lethal. Rates of evolutionary change observed in the geological record are in fact less than rates estimated on living populations of a variety of animal species (Williams, 1987). Observed rates of morphological change, however irregular in tempo, are more surprising in their conservativeness. For an anatomical feature or a system to persist for periods long enough to be meaningful to a geologist, the system must experience consistent and long-term stabilizing selection.

Debate has arisen over the importance and frequency of neutral variation. Especially in small populations, random drift could establish alleles that are neutral to selective pressure. Among such characters, there could be cumulative random changes constantly occurring through time. There may be many such characters in our genotype, and as they are identified, they may serve as useful "clocks" that measure the time depth between speciation events (return to outline).

ADAPTATION

 

Adaptation, in evolutionary terms, is the functional modification of structure, physiology, and behavior of an organism that promotes survival in a particular ecological niche. Adaptation is literally the conformity between an organism and its environment. A niche can be viewed as the total range of conditions in which an organism thrives, or conversely, a niche is the sum of the adaptations of an organism to its particular environment. A specialized organism (a specialist) has a narrow set of tolerance limits to one or more environmental factors, whereas a generalized organism (a generalist) has broader tolerance to variations in the niche. The abundance of a species (in biomass per unit area) reflects these adaptations. A specialist may not be abundant unless its particular habitat is widespread. In contrast, generalists occupy more habitats, have a greater geographic distribution, and are usually more common (greater biomass).

Fisher (1930) proposed that minor random changes (mutations) in an organism have a greater chance of improving functional adaptations than major changes. More specifically, the probability of a change in an organism or environment improving conformity is inversely related to the magnitude of the change. These situations are analogous to focusing a microscope that is already near focus. Small random changes are as likely as not to improve focus, but gross changes invariably take the microscope out of focus. In this analogy, generalists have greater depth of field and can tolerate greater organismal and environmental variation. This analogy however, does not apply to directed changes such as selected breeding and husbandry directed by humans toward domesticated animals.

Sometimes adaptation is facilitated by opportunistic evolution. A morphological or physiological character that has a functional relationship to one environment or activity may coincidentally be useful in a new environment or situation. For example, a manipulative, prehensile hand readily moved from a function in climbing and feeding to tool use among primates once appropriate cognitive abilities evolved. In contrast, even though sea mammals developed many cognitive skills, their environment and anatomy channeled them in different directions.

An organism possesses a complex of related behavioral, morphological, and physiological traits that complement and enhance each other in terms of adaptive conformity between itself and its niche. Such mutually adaptive traits are called an adaptive suite. Pianka (1978; page 93) illustrates the concept of the adaptive suite by describing interrelated features of the desert horned lizard, Phrynosoma platyrhinos. Horned lizards are specialists who eat little except ants, a small prey that must be consumed in large numbers, and thus requires long periods of feeding. Since ants contain much indigestible chitin, horned lizards have relatively large stomachs (about 13% of body weight), a tank-like body, and relatively slow locomotion relying on a spiny body and cryptic behavior to avoid predation. Since they are reluctant to move, they experience greater fluctuations in body temperature than other species of lizards living in the same area and which move more freely into or out of sun and shade. The tank-like body and low activity rate allow Phrynosoma to carry heavier eggs and to commit a higher reproductive investment in their eggs. All of these interrelated features contribute to the horned lizard's adaptive suite. The expression of the adaptive suite in the organism's anatomy is called the total morphological pattern, an idea used in paleontology since the geology of Georges Cuvier and emphasized in modern physical anthropology by the British anatomist, W.E. Le Gros Clark (return to outline).

Speciation and Adaptive Radiation

The process of transformation of a parent species into one or several progeny species is called speciation. Speciation may occur when geographic barriers isolate populations of the parent species from each other. Indeed, the only way to avoid genetic divergence is to re-establish gene flow. This is called allopatric speciation. Allopatric, meaning in another land, contrasts with sympatric, which means sharing the same area. Sympatric speciation is relatively common in plants where rates of euploidy (changes in chromosome number) is high and new species are sometimes produced by hybridization, but it is presumed to be a rare process in mammals.

The importance of the concept of natural selection (Darwinism) is that it provided a hypothetical organizing principle that explained how adaptation and speciation could occur. As a hypothesis, natural selection has become stronger and more credible after more than a century of challenge and testing. The reality that evolution occurs is directly observable in both fossil and living species. The synthetic theory of evolution provides an explanation of process, and natural selection remains a primary element of that theory. Evolutionary theory remains the central organizing concept in biological science.

When an organism, through the process of speciation, enters a new niche or a new environment, it tends to disperse and adapt to the various regions of that new environment. This divergence of new group and consequent speciation from an ancestral organism into new habitats and new niches is called adaptive radiation. A major factor that promotes adaptive radiation is absence of competition in the new niches. Another factor that may advance adaptive radiation is the concept of competitive exclusion, an idea formulated by the Russian biologist G.F. Gause in 1934. Competitive exclusion is the principle that if two or more species occupy the same niche in the same locality, competition tends to eliminate all but one of them. A situation in which multiple species occupy identical niches in the same area is viewed by biologists as being a highly unstable or temporary condition. Direct competition however, can produce niche diversification, a frequently observed situation in which actual or potential competitors specialize to use slightly different components of the local environment.

Major adaptive radiations of the vertebrates include the evolution of the bony fishes, amphibians, reptiles, birds, mammals, and Miocene apes. Potentially, any species could be the parent species of an adaptive radiation. This is one reason biologists view all existing species as successful. A rare or unremarkable species can give rise to an adaptive radiation that populates the planet's niches with its daughter species (return to outline).

 

SPECIES

The Reverend John Ray's seventeenth century concept of a species has been a cornerstone of biology. It represents the indivisible unit of an organism, isolated from other species by a barrier of infertility, interbreeding among themselves, and sharing a common gene pool. For all its appeal however, the concept has never been fully realistic since individuals are part of populations, and in the real world one can find many degrees of variation, divergence, and isolation between populations. Species only appear to be discontinuous when viewed as isolates in space and time. Life is a continuum and every population is connected through its parent population to all living things. As each new individual is conceived, life is passed (in the form of DNA or RNA) to the next generation. Thus, reproduction is not an act of creation, but simply a way of passing life to the future (with slight genetic modifications (return to outline).

Classification

For accuracy of communication, it is necessary to devise ways of identifying and naming organisms. This process, known as taxonomy, follows the eighteenth century tradition of Carolus Linnaeus. A list of traits is used to differentiate organisms into morphologically distinct groups. Groups must be morphologically or biochemically discontinuous or they cannot be easily recognized. Most modern taxonomists consider a species to be a group of interbreeding natural populations that is reproductively isolated from other such groups. Realizing that in some instances, this is sometimes an impractical criterion, taxonomists make compromises and approximations to facilitate communication.

Each organism is given a two-term binomial name called a Latin binomial that consists of its generic name (genus) and a specific name (species), followed by the name of the taxonomist who first described the species and the date of publication of that description. Valid binomial species names are written in italics with the genus capitalized. Humans are designated Homo sapiens Linnaeus, 1758. However, in practice the taxonomist's name and the date of publication are omitted in most popular or nonscientific contexts.

Systematics is the science of arranging groups of organisms on the basis of their evolutionary relatedness. A genus is group of related species, genera are grouped to form families, which are further grouped into orders, and orders into classes. Related classes become phyla and groups of phyla become kingdoms. The process of creating a name or taxon is an exercise in taxonomy. Integrating Systematics and taxonomy to place an organism into the nomenclature of biology is called classification. Humans are classified as follows:

KINGDOM - Animalia

PHYLUM - Chordata

SUBPHYLUM - Vertebrata

CLASS - Mammalia

ORDER - Primates

SUBORDER - Anthropoidea

FAMILY - Hominidae

GENUS - Homo

SPECIES - Homo sapiens Linnaeus, 1758

The importance of taxonomy is organization and the importance of taxonomic nomenclature is communication; that is, it allows all scientists to understand readily which organism is being discussed, by its taxonomic name. A set of rules and an international nomenclature commission help minimize variation between investigators' use of nomenclature. The 10th edition of Linneaus's Systema Naturae, published in 1758, is considered the starting point. Only scientific names published in that volume or after that date are considered valid. Conflicts between names are resolved according to the law of priority: "The first name proposed for a taxonomic unit that is published and properly described supersedes all subsequent proposed names." There can be no duplication. Once used, a name with the same spelling cannot be used again for a different species in the same genus in order to prevent homonyms. Tautonyms, the use the same name for both genus and species are permissible, for example, Gorilla gorilla Savage and Wyman, 1847. The use of Latin and Greek language in scientific names facilitates international understanding of the meaning of the terms now, as it did in the time of Linnaeus. A race or subspecies can be designated by a trinomial name, but the group on which the definition of species is based must repeat the species term. Thus Gorilla gorilla gorilla Savage and Wyman, 1847 could designate the lowland gorilla subspecies and differentiate it from Gorilla gorilla beringei Matschie, 1903, the mountain gorilla.

Systematics serves a different purpose and, when done properly, a classification attempts to portray the evolutionary relationships between organisms, something quite outside Linnaeus' frame of reference. Systematics assumes that closely related taxa will be similar morphologically and biochemically, but the degree of similarity should decrease with time. Systematists try to distinguish between traits that are homologous (due to common ancestry) and those that are analogous (due to common function), a distinction formulated in 1843 by Richard Owen (1804-1892), a superintendent of the N atural History Department of the British Museum. Homologies are identified on the basis of careful comparative anatomy and sometimes are verifiable by demonstration of a common embryonic origin. Homology is the fundamental concept underlying comparative anatomy and Systematics.

Systematics bases its taxonomy on evolutionary theory of patterns of evolutionary descent. The results are often graphed as phylogenetic trees. Ernst Haeckel, who formulated the first zoological classification that incorporated systematics in the modern sense, drew the first phylogenetic tree in 1864. Modern versions of Haeckel's tree illustrate the anatomical relationships of the primates to other animals.

Gould (1989) argues that superficial consideration of such trees are misleading since evolutionary trees usually illustrate increasing diversity of species with time. There is an important difference between diversity (the number of species in a particular lineage) and disparity (difference in body plans). Life on earth was characterized by an initial disparity in anatomical plans that has been successively decimated by extinction events. The relatively few lineages that survive speciate but in Gould's view, the history of life is one of progressive loss of disparity. Although the disparity of life may be reduced with the passage of time, diversity generally increases in surviving lineages. Thus ecosystems that have great stability over long periods of time (such as tropical rain forests) are characterized by a great diversity of species.

Since proteins are chains of amino acids, one can compare amino acid sequences of homologous proteins and construct phylogenetic trees from evidence from molecular biology just as one would from fossils. A widely cited example is cytochrome c, a small enzyme composed of 104 amino acid units. A modern phylogenetic tree based upon the cytochrome c molecule differs from Haeckel's original proposal.

Accumulation of small changes in a species that transforms its phenotype is called anagenesis, a process whereby a parent species is replaced by a daughter species later in time, and is represented by a continuous line. Splitting of the phylogenetic lineage, with a parent species giving rise to more than one daughter species, is called cladogenesis.

In phenetics, or numerical taxonomy, systematists select as many characters as possible and group organisms into clusters by the calculated similarity in characters. Cladistics, in contrast, uses a different approach. A distantly related organism is selected as an outgroup and certain characteristics are compared between each of the organisms under study and those of the outgroup. Those characteristics shared with the outgroup are considered primitive; that is, they are inherited from the common ancestor of both the study group and the outgroup. Those characteristics shared by the study groups, but not the outgroup, are termed derived because they have a common ancestral population. Such shared derived characters are called synapomorphies. The more synapomorphies that two organisms share, the closer their common ancestry. A diagram of relationships based upon an analysis of synapomorphies is called a cladogram, and common descent groups are called clades. An important assumption in systematics is Dollo's law: "Complex characters once lost in evolution tend not to be regained in their original form." Thus, analogous characters that serve the same function may reappear, but a close examination will reveal differences in detail. It seems that once genetic information is gone, replacement information at a later time usually differs enough for the resultant trait to be recognized as distinct. Common exceptions to Dollo's law would reversals in body size or the loss of characters to revert to an ancestral state.

Parallel evolution is the independent acquisition of similar characters among closely related species. Convergence is the development of superficially similar characters among unrelated or distantly related species (return to outline).

Subspecies

If different populations within a species occur, they vary as a consequence of evolutionary processes occurring within those populations. Consequently, it is sometimes possible to subdivide a species into identifiable subspecies. A subspecies is simply a recognizably distinct subpopulation within a species that occupies a particular geographic area. Subspecies designations are conventionally considered valid if as many as three-fourths of the individuals are recognizable as belonging to that subgroup on the basis of some morphological or biochemical characteristic. Subspecies may represent intermediate stages in the process of speciation, the formation of new species. A small amount of gene flow, interbreeding across a population boundary, slows or prevents genetic divergence between subspecies.

Two computations assess genetic similarity between populations. Genetic identity (I) is an estimate of the extent to which two populations share the same alleles for loci where alleles are known to vary in that organism.

Genetic distance (D) estimates the degree to which they differ consistently.

D = -loge I

Genetic identity (I) equals 1.0 if two populations (X and Y) have the same alleles in the same proportions. I is zero when they do not share any alleles in common. Genetic distance (D) is an estimate of the number of allelic substitutions required to account for the degree of difference between the populations. Distance can range from 0 (identical) to values greater than 1, since a single gene may be substituted more than once. Generally, research on the genetics of South American fruit flies (Drosophila) suggests the following:

- Samples representing the same deme (breeding population) have I values near 1.0 and D values slightly above 0.

- I values as low as 0.8 and D scores greater than 0.2 reflect different subspecies.

- Sibling species have I values near 0.5 and D values above 0.6.

- Clearly distinct (but related) species exhibit I and D values of 0.35 and 1.05 respectively.

- Generally, I and D estimates are proportional to the degree of reproductive isolation in the recent history of the two species.

One way to handle subspecies variation is to identify clines, measurable gradients in some phenotypic characteristic that reflect genetic gradients among populations of a particular organism. A geocline is a gradient across geographic distance; a chronocline is a gradient through time; and an ecocline is a change associated with changing ecological conditions. Use of clines to describe subspecific variation is consistent with the concept of organisms as populations with varying allele frequencies and it is a little less typological than an imposition of subspecies designations upon segments of a breeding population (return to outline).

Bergman's rule and Allen's rule are examples of clines frequently seen in mammals. Bergman's rule states that within a species, populations in colder climates have larger body sizes than populations in warm climates. Allen's rule predicts that extremities tend to be smaller in cooler climates, producing a more spherical body shape. Both conditions, presumed to be climatic adaptations, produce a lower ratio of surface area (skin) to body weight in colder habitats, making conservation of body heat more efficient. (The more linear the departure from a sphere, the geometric form with the least surface area relative to volume, the greater the relative surface area will be.) Warm climates present animals with the converse problem, the need to lose body heat generated by metabolism rather than conserve it. The higher surface area-to-weight ratio in hot dry climates promotes heat loss through the skin and cooling of the body.

Subspecies need not occur in clines, and it is important not to confuse the two concepts. Clines are gradients in populations, whereas subspecies are incipient species (return to outline).

The Mendelian Population

A Mendelian population is an interbreeding group of organisms sharing a common gene pool that is recognizable as a phenotypically distinct group from other populations within a species. Distributed over a substantial area, a Mendelian population is divisible into demes, local populations from which mates are usually chosen.

The primary problem with the concept of a Mendelian population is that it is a population phenomenon and our culture trains us to think in terms of types, not populations. Many social problems of today; ethnic cleansing, racial hatred, etc. are due to our ignoring important within-group variation and between-group continuity in order to focus only on differences between groups or on the groups themselves. Unfortunately misconceptions about population differences produce great harm. It may help to understand the nature of the differences if we consider what conditions would be required for such variation to disappear:

1. all of humanity would have to be a single breeding population in a single environment, and

2. each individual would have to be equally likely to breed with any other person on earth.

The closer we approach that condition, the less populational differentiation we will exhibit.

Genetic diversity, the mechanisms of heredity, and evolutionary processes combine to produce extreme genetic individuality. Except for identical twins, there are no two people on earth who are exactly alike. The hereditary and evolutionary processes that produce this marvelous individual diversity also promote deme similarity. Alleles are distributed by independent assortment and recombination so that even though there is a replicator effect (individuals, chromosomes, regions of chromosomes), genetic segregation tends to disperse characteristics independently and randomly. This genetic diversity provides protection against a pathogen that might select against all members of a population unless some are different enough t o survive the event.

Replicators

Level Replicator Effective unit for selection

 Level  Replicator  Effective unit for selection
 1  nucleotide  
 2  codon  
 3  small segment of chromosome  
 4  gene (cistron)  +
 5  larger segment of chromosome  
 6  chromosome  +
 7  cell (asexual)  +
 8  cell (sexual)  +
 9  individuals  +
 10  group  
 11  population (deme)  
 12  race  
 13  species  
 14  community  
 1  all planetary live  


 

Different demes may experience different histories and selection pressures. A mutation may occur in one deme and not in another. Selection pressure from a particular disease microorganism may exist on one deme and not another. Without sufficient gene flow between demes, genetic population differences accumulate. Such phenotypic differences, produced by ongoing evolutionary processes (mutation, selection, migration, isolation) in contemporary populations, constitute the differences that scientists sometimes call microevolution. Since identical events do not occur in all human populations at the same time, different populations diverge in their genetic characters. Variability among loci known to have multiple alleles is greater within populations (91.2%) than between them (8.8%) in spite of the processes that promote differentiation (Avers, 1989 page 282).

Distribution maps of single traits tend not to coincide with each other and the concept of cline is not an alternative to the population concept. A population comparison looks at a constellation of traits to identify a deme, whereas clines require the examination of one trait at a time. Because it is impossible to see a Mendelian population, scientists use contrasting phenotypes between populations as guides to their boundaries.

Further Reading

Dawkins, Richard 1986. The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design. New York: W. W. Norton & Co.

Huxley, Julian 1942. Evolution: The Modern Synthesis. Reprinted in 1964. New York: John Wiley & Sons.

Mayr, Ernst and William b. Provine 1980. The Evolutionary Synthesis: Perspectives on the Unification of Biology. Cambridge. Harvard University Press.

Simpson, George Gaylord 1964. This View of Life: The World of an Evolutionist. New York: Harcourt, Brace & World, Inc.

Huxley, T. H. 1959 Man's Place in Nature. Ann Arbor: University of Michigan Press.

Gould, Stephen Jay. 1989. Wonderful Life: The Burgess Shale and the Nature of History. New York: W W Norton and Company.

 

 

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15 Aug 2004
Department of Department of Anthropology, College of Liberal Arts , UT Austin
Comments to cbramblett@mail.utexas.edu