Chapter 25 Chapter Thirteen Development After Synthesis-1

The history of evolutionary biology can be divided into several fairly well-defined phases.From 1859 to around 1895, what evolutionists were concerned about was how to prove biological evolution and determine some common ancestor sequences.The study of phylogeny was the main work that evolutionists concentrated on at that time.From around 1895 until the beginning of evolutionary synthesis (1936), debates within the field of evolutionary biology also dominated research and writing.The important question at this stage is: Is evolution gradual or abrupt?Is genetics soft or hard?Are genetic changes due to mutational pressure or to selection pressure?

The period from 1936 to the 1960s was dominated by evolutionary synthesis and in-depth study of the details of new discoveries.The population perspective dominates all research, and diversity, especially at the population and species levels, has received renewed attention; the adaptive significance of variation has been analyzed to be due to selection, but all genetic explanations are governed by the concept of gene frequency. Later developments in evolutionary biology were scattered.These developments include a keen interest in the random components of variation and recognition of the diversity of genetic material (in the form of various forms of DNA).It has established extensive links with ecology and behavioral biology, and the study of the evolution of biopolymers and their role in evolution has become a very important branch of evolutionary biology.As a result of all these developments, the study of evolution has become a highly differentiated science.And it's much more than that!

The extension of evolutionary thinking to all branches of biology has torn down the curtain between evolutionary biology and other fields of biology, so that it is now impossible to say whether some fields such as evolutionary ecology, evolutionary behavior and molecular evolution should include in evolutionary biology or in adjacent fields in which they have been merged.Perhaps most importantly, biologists were finally able to respectfully ask why questions without being suspected of being teleologists. A unified explanation of the evolutionary process has a very favorable impact on improving the status of evolutionary biology in the entire field of biology.By rejecting all theories or principles (such as those of vitalism, teleology) that contradict physicochemical explanations, evolutionary biology is more subject to criticism than it was at a previous stage when it was ridiculed as "speculative" by experimental biologists. respect.The new insight revealed by the elucidation of the structure of DNA in 1953 that living things are composed of two fundamentally different parts, historical parts (genetic programs) and functional parts (translated proteins), Immediately demand that the analysis of the causes of all biological phenomena be extended to the historical part.This makes people realize that any reasonable comprehensive biological analysis should not only but also must include the study of the evolutionary history of all components of organisms.This expansion of evolutionary thinking has affected every branch of biology.

Evolutionary biology is indeed a striking example of shifting interests and changing research programs in a scientific field.My simplified statement, however, belies the fact that hardly any line of research inquiry is completely closed, even if new lines or ideas that are more promising and promising are revealed.Nor does my statement mention that the roots of every new line of research or line of thought usually go back decades before it yields much fruit.Every new technology and a researcher's transition from a field he is familiar with to another field are likely to initiate new routes or new ideas.

It is clearly impossible to faithfully characterize the full complexity of progress in evolutionary biology, and for that matter any field of science. In 1946, the United States established a special society to support the research on evolutionary issues. In 1947, Mayer founded the journal "Evolution" (Evolution), which specializes in the study of evolutionary biology. "American Naturalist" once transformed into a journal of experimental biology in the 1930s, and then reverted to a special journal of evolutionary biology after evolutionary synthesis.Periodicals dedicated to evolution also appeared in the United States and other countries.The number of new textbooks on evolution is also increasing, as are courses in evolutionary biology being taught at colleges and universities.The relevant literature has grown to such an extent that regular publication of review articles is now necessary.

This influx of activity poses a serious problem for historians, and it is now entirely impossible to analyze recent developments satisfactorily.The best I can do is to outline some of the main features of recent research and to raise at least some of the unresolved questions that have particularly troubled this generation of evolutionists.I can only list some modern magazines and some newly published textbooks.Next, I will start with the evolutionary issues that population genetics and molecular biology have paid attention to in recent years. Population genetics has been testing the conclusions of mathematical population genetics in the field and on experimental populations in the laboratory as its main task since the early 1900s.This work is governed by the idea that evolution is defined as "changes in gene frequencies in a population".The finest in this research tradition is the series Genetics of Natural Populations (1938-1976) by Dubzhansky and his collaborators, which focuses on the fruit fly (DrosoPhila Pseudoobscusa) and its sister species (relatives) ( Lewontin et al, 1981).Dubzhansky tried to determine the following values:

Selection pressure, gene flow, effective population size (number), lethal and other recessive frequencies that are concealed, and potentially other factors of evolutionary significance.The advantage of this study is that this species, like most other species in the genus Drosophila, has a large number of intraarm inversions (as detected by the banding pattern of the giant salivary glands), each of which There is some sort of different geographic scope.Dubzhansky found that the relative frequency of an inversion varied not only with geography but also with season (in some cases over several years).Many regularities suggest that this frequency is controlled by selection and are confirmed by experiments. Mayr (1945) tried to explain the arrangement of genes as an adaptation of ecotypes, that is, carriers of different inversions can take advantage of different local niches (niches).This explanation was later proved by Coluzzi et al. (1977) from the gene sequence of mosquitoes (Anopheles).Most striking is that carriers of different gene arrangements not only have different fitness in different niches, but also have different behavioral abilities to find suitable niches.

An important technological advance in the study of fruit fly populations is the invention of the "population cage" by Teissier and IHeritier, a group of fruit flies of different sizes and genetically heterogeneous Can persist for many generations in population cages without introgression of new heterologous genes; placing such population cages under different temperature and food conditions allows the relative fitness of different genes or combinations of genes to be examined and selection calculated pressure.Immediately adopted by Dubzhansky et al., this technique is now in use with various modifications in many genetics laboratories.It provides a very efficient method for the experimental study of natural selection in populations.

13.1 Molecular Biology Since the branch of biology has been called biochemistry, many of its discoveries have been important to evolutionary biology, although this was not recognized at first.Mention can be made here of the discovery of nuclides by Michel in 1869, the research of Nuttall in immunology, the research of Garrod on inborn errors of metabolism, the research of Landsteiner on blood types, and the later work of Beadle and Tatum.Yet molecular biology didn't really take off until the discovery of the structure of DNA in 1953.At first this had little effect on previously established concepts of evolution.The most important immediate impact was the later discovery that translation from nucleic acids into peptides and proteins is a one-way street (the "central dogma").This finding provides the most convincing evidence for outright denying the inheritance of acquired traits.

The exceptionally precise and reliable replication of germplasm at each nuclear fission was not a conceptual problem until recently.Essentialists take this for granted, while those who believe in soft inheritance think it is wrong.But biophysicists are amazed at how nearly error-free this complex replication process works.Of course, occasional errors have been found, which geneticists call mutations.To an evolutionist, the magnitude of this error is not particularly disturbing, since he knows that there will be substantial loss of gametes and zygotes either before or during development.What was surprising was the discovery of a repair mechanism that could subsequently "fix" errors in the replication process.The existence of such a mechanism calls into question the definition of "mutation rate," but it helps explain the observed rare occurrence of replication errors.

The discovery that the genetic code is largely the same in all living things, including prokaryotes, is an important additional piece of evidence that all extant life on Earth can be traced back to a single origin.This discovery in molecular biology, along with some others, went a long way toward simplifying and unifying biology, yet there were also others that required some revision of the existing theory of genetics or at least some changes to our understanding of genetic processes. some fixes. Most of the early research in molecular biology was carried out in viruses and bacteria, and it was assumed by Occams razor that discoveries made in prokaryotes could be applied unchanged to eukaryotes.However, research in recent years has shown that this assumption is not necessarily reasonable.In particular, it is now clear that the structure of eukaryotic chromosomes is very complex, which is fundamentally different from the simple tandem DNA double helix structure of prokaryotic organisms. Its DNA is tightly combined with certain proteins, especially histones, to form a size Different nucleosomes seem to have different functions.At present this type of research is mainly related to physiological genetics, but there is no doubt that knowledge of the structure of DNA in eukaryotic chromosomes will eventually provide answers to some hitherto unsolved evolutionary questions, such as the control of evolutionary tendencies, many evolutionary routes Stability of phenotypes in the middle, rapid transfer to new evolutionary grades during genetic revolutions, and so on.We may well be on the threshold of a major discovery. When Nirenberg and Matthaei succeeded in deciphering the genetic code in 1961, it was generally considered that the last piece of the puzzle in molecular biology had been solved.Unexpectedly, since then, completely unexpected discoveries have followed one after another, and the pace is even faster.The main impact of these discoveries until now has been on the physiology of genes, but there is no doubt that all of this also has important evolutionary implications, which will inevitably emerge when the molecular processes are better understood. Genetic processes are governed by submicroscopic structures, and molecular biologists have developed new techniques with uncommon ingenuity to infer molecular structures, processes, and how they change.In fact in this respect molecular evolution can be understood more deeply due to the use of new technologies than by reference to new concepts.Starch gel electrophoresis, first employed by Clem Markert, is the most important of these techniques.Soluble proteins can be separated from each other by moving different distances in the gel placed in the electric field according to their molecular size and electrical properties. Different staining techniques can be used to see the separated proteins in the gel.Using this method, the genotype of an individual can be determined directly without any selection analysis.20, 30 or even more than 70 loci (loci) can be analyzed simultaneously for the determination of alleles.This method can determine the degree of heterozygosity of individuals and populations, which is.It cannot be done by the previous methods.It also compares the geographic population of a species with related species to determine how much of the allelic profile is the same or different.The biggest disadvantage of this method is that it can only show the variation of structural (enzyme) genes.Another disadvantage is that it cannot separate alleles with equal charge, thus underestimating the number of alleles.More alleles can often be found using complementary approaches (thermolysis, changing pH). Since only a small number of enzymes have been analyzed in detail, it is debated how much genetic variability is missed in conventional ice methods. Because this technique is so delicate and can be used even by non-biochemists, it has been used since Hubby and Lewontin (for Drosophila) and Harris (for humans) in 1966 to study heterozygosity in individuals and populations. There was an upsurge in the study of enzyme mutations.The number of new discoveries made possible thanks to this technique is enormous: new relatives (sister species), quantification of degrees of difference between closely or distantly related species, correlations between resistance to change and speciation, Correlation of geographic variation of enzymes with climate or other environmental factors, etc. One of the conclusions drawn from these studies (confirmed somewhat by the behavior of other macromolecules) is that there is a certain regularity in the rate of molecular change over geological time, that is, the rate at which amino acids are replaced in evolution. regularity.Therefore, some scholars (firstly Pauling and Zuckerkandl, and later especially Savich and Wilson) advocated that this regularity can be used to formulate a "molecular clock" (molecular clock), and the degree of difference between homologous molecules can be used to infer the relationship between two evolutionary routes. The age of the bifurcation point (Wilson et al, 1977). Current bifurcation ages calculated by molecular clocks differ considerably from those calculated by paleontologists based on the (admittedly inadequate) fossil record.There is also other evidence that caution must be exercised in applying the molecular clock concept. For example the same molecule may change more rapidly in some germline sequences than in others over the same geological time interval. It also seems to be the case that the rate of change occasionally drops sharply in some germline sequences.For example, the molecular distance between humans and orangutans is smaller than that between some species in the genus Drosophila. Another difficulty is that the concept of a molecular clock implies an inherent law of change, or spontaneity, so to speak. Molecular clocks are sometimes described in terms of mutations occurring every two million years.Such a statement is of course entirely misleading; mutations at the same locus occur frequently, but are consistently eliminated by sampling error or natural selection until the molecular background has changed enough to favor a change in the three-dimensional structure of the molecule.In other words, the molecular clock is pseudo-controlled by natural selection rather than the rate of mutation.This has been confirmed by many examples of polymers, but the most convincing example is hemoglobin.Substitution of just one of the more than 300 amino acids in hemoglobin can be very harmful. Sickle cell anemia, for example, is caused by the substitution of a glutamic acid on the beta chain of hemoglobin for a valine.More than 200 hemoglobin mutations have now been found in humans (discovered as "cryptic" blood types), and although they are not the cause of serious blood disorders in many cases, none of these mutations have been successfully detected in humans Ancestors are fixed or manifested in polymorphic forms.The selection of these mutations by natural selection has been shown by the fact that the hemoglobin of the orangutan, a distant relative of man, is almost identical to that of man, although literature documents a high mutation rate for hemoglobin. A possible explanation for the molecular clock phenomenon is that each macromolecule also typically interacts with 10–25 other macromolecules in the cell.However, when some of these other macromolecules evolve in response to specific selective forces, sooner or later these changes will put selective pressure on the original molecule to replace an amino acid for the best possible fit with its genetic background and restore stability. state. Since all genes contain DNA, it has been assumed since 1953 that all genes are basically identical in function and evolutionary characteristics.Research over the past 20 years has shown that this is not the case.There are many different types of genes, such as enzyme genes, structural (insoluble) protein genes, regulatory genes, etc., and there may be many more genes that we don't know yet.The nucleus of a higher organism contains enough DNA for five million or so genes.And genetic studies have found evidence for only about 10,000 or at most 50,000 traditional (enzyme) genes.They (among other kinds?) belong to the so-called special order, but there are also several types of "repeated DNA" and a lot of apparently "inactive" DNA (their function has been a mystery).Most of what is not enzymatic DNA apparently has a regulatory function.The study of differences in the evolutionary behavior of different types of genes has only just begun (Davidson and Britten, 1973; 1979). Since the end of the 1960s, especially since 1975, new discoveries in molecular genetics have been made one after another at such an astonishing speed that a non-professional cannot keep up.In addition, some of the findings were quite unexpected, and their explanations are controversial.These findings concern the genomes of eukaryotes.For example, certain genes (transposons) have been found to change positions on chromosomes.Stranger still is the discovery that many genes contain sequences that cannot be transcribed into the message RNA (mRNA) but are excised during transcription ("introns", introns), the remainder of the gene ("exons", excons) Later, they were "spliced" together to become functional mRNA.This begs two questions; how did such a peculiar system evolve?Is it contained in a mere inactive stable factor or has a function not yet known?The teleological answer that this apparently nonfunctional DNA is being stored "in case it is needed" is utterly unsatisfactory.A fairly popular explanation is that this extra DNA is parasitic (let's say it) and that organisms cannot prevent its replication and accumulation (Orgel and Crick, 198O). While there are sound arguments in favor of this hypothesis, it is intuitively unpalatable to Darwinists, since natural selection must have produced defense mechanisms against such wasteful parasitism.Given how little is currently known about how gene regulation works, it would be premature to write off whether introns are genetically introverted.All we know now is that it may be important to separate certain segments of genes (exons) from each other before transselection.There is now real evidence that introns help regulate the splicing of genes. Equally puzzling is how closely related species or genera differ from each other in their repetitive DNA and other components of their genomes without much visible change in morphology, sometimes without even losing the ability to interbreed.How this might affect evolutionary potential remains entirely unclear.Since the pioneering studies of Mirsky and RIS (1951), it has been known that different types of organisms contain different amounts of DNA in their cells (nuclei).The least content is prokaryotes and fungi, and the most content is tail animals, lungfish and some plants.It is also known that there are certain regularities (with exceptions to almost all of them), such as that annual plants generally have less DNA than related perennials or trees.Slow-growing (long-speaking) species had more DNA than their related species.Large differences in the DNA content of different taxa seem to support the idea that the excess DNA is mostly not of high selection value.But as long as our knowledge of gene control in eukaryotes remains as shallow as it is now, further evolutionary inferences are premature. Evolutionists since Lamarck have been familiar with the principle of "mosaieevolution," the idea that different components of a phenotype may evolve at very different rates.This inconsistency in the rate of evolution has now been found to hold true for molecular evolution as well.For example, Wilson et al. (1974) believed that the enzyme genes of mammals and anurans (such as frogs) evolved at roughly the same speed, while the regulatory genes controlling morphological evolution of mammals changed much faster than those of babies.Genes controlling color patterns in South American mimic butterflies show strong geographic variation with little individual variation, whereas enzyme genes in these species show strong individual variation with little geographic variation (Turner, Johnson, and Eames, 1979 ).Researchers have also recently discovered a large difference in variability between genes for enzymes and genes for other proteins.Finally, changes in genes that control speciation appear to be independent of enzyme genes.This is a new field of evolutionary biochemistry, and I foresee significant results in the near future.This is already evident: Different classes of genes appear to respond to different selection pressures and follow their own evolutionary pathways.The results of studying one class of genes (such as enzyme genes) cannot be generalized to all classes of genes.This seems to be true for responses to selection pressure, for variability (degree of heterozygosity), and for molecular clocks.Chromosomal changes in different organisms also have very different evolutionary speeds.Karyotypes (karyotypes) appear to be very stable in some classes of organisms, while in others (such as some mammals) they change extremely rapidly. Each set of genes may have played a different role in evolution.Differences in enzyme genes apparently accumulate gradually at a fairly normal rate and are thus ideally scaled for molecular clocks.Speciation appears to be mostly independent of enzyme genes.Why there are different classes of genes may be due to their different functions, but our understanding of these functions is extremely limited. Chetvinikov's concept of genetic background began to take on new meaning.It has been recognized that the study of the role of genes must be supplemented by the study of gene interactions. Lerner's Genetic Homeostasis (1954), a pioneering discussion of the function of genotypes, cites extensive evidence for the significance of gene interactions.This idea was reinforced by Dubzhansky's work on "synthetic lethals," in which he pointed out that certain combinations of genes or chromosomes lead to higher fitness and when combined with other chromosomes lethal.This is a blow to the idea that genes have fixed and unchanging fitness values, although these findings, lacking an analysis of the reasons for such relativity, represent only the beginning of a new field of research. (Mayr, 1963, Chapter 10; see also Mayr, 1974; Carson, 1977). The study of molecular evolution has revealed the surprising fact that most of the macromolecules of higher organisms can be traced back to protists, but protists may only have a small fraction (one ten thousandth) of the nucleic acid content of higher organisms, which other Where did all the genes for come from? The geneticists who first considered this question were apparently members of Morgan's group (Metz, 1916; Bridgs, 1918).Intricate and detailed studies by Sturtevant, Briges, Muller show that new genes are created when a segment of a chromosome is inserted into an existing chromosome.This can be accomplished either by unequal crossovers or by major chromosomal mutations (especially translocations).Chromosomal analysis of the Drosophila salivary glands provides a good opportunity to confirm duplications that are entirely inferred from genetic evidence.In other cases entire chromosomes may be added to the genome (due to non-disjunction) or the genome as a whole may be duplicated (through the process of polyploidy).The study of gene duplications by early geneticists has evolved considerably in recent years (eg, Ohno, 1970).The evolutionary advantage of small duplications is that they interfere with the normal functional activity of the genome much less than sometimes major translocations or additions to entire chromosomes (eg, Downs syndrome) or genomes.Therefore, small-scale duplications are easier to incorporate into the gene pool, and the duplicated genes can only display new functions and be more divergent from their sister genes through divergent mutations.It has been doubted whether such repetitions could produce entirely new proteins, but the history of evolution has been clarified for too few macromolecules to draw exhaustive conclusions.It is quite possible, but not necessarily, however, that some of the most important types of macromolecules arose very early in the history of life. When Darwin proposed the theory of common ancestry in 1859, he realized that at first there must have been "prime life," which he expressed in a more or less biblical phrase: life "was first impregnated into a few bodies or into a single body" (: 490).This is a very bold statement, because the differences between the countless species of organisms are so great that there cannot be only one at least.Even if scholars who study phylogeny succeed in tracing the ancestors of animals and plants back to algae and flagellates, it seems completely impossible that prokaryotes (bacteria, etc.) and eukaryotes (higher organisms) have the same origin.However, this has been proven to be certain by molecular biological research.Not only are the chemical compositions of all life forms generally similar, especially the genetic codes are also exactly the same (including prokaryotes), which clearly shows that the life that exists on the earth now has only originated once.There is now a correct theory about the origin of eukaryotes (Margulis, 1981).All organisms currently living on the earth have undoubtedly descended from a single ancestral species.If life had several independent sources, all the others have been overwhelmed or annihilated in competition by the one which now dominates the world. The origin of life from inanimate matter may be through spontaneous generation.As it happens, the theory of spontaneous generation came under fire when Pasteur and others experimentally rejected the possibility of common ancestry at the same time as Darwin proposed it (Farlery, 1974).This put evolutionists in a dilemma, and Darwin had to resign himself to saying: "It is absurd to mention the origin of life at this time. If this is the case, we can also consider the origin of matter." In 1871 he was meditating again: "It is often said that all the conditions for the first generation of living things are present, and will be in the past. However, if (good fellow! What a great if!) We can think of some mild little pond, and have ammonia, phosphate, light, heat, electricity, all that, and imagine forming some protein-like compound, ready to undergo more complex changes; uptake, which cannot occur before biogenesis" (LLD, III: 18). The reason why the question of the origin of life was so difficult to study in the decades after 1859 is because the whole question had to be formulated anew.People often consider the sudden emergence of living species from inanimate matter in terms of model thinking, and regard the earth as if its atmosphere and other environmental conditions have remained fixed throughout geological epochs.These assumptions must be radically revised.The botanist Schneidan (1863) was apparently the first to mention the origin of life, "the first cell" It was possible to form under the completely different atmospheric conditions of the young Earth.This is now fully confirmed.The young Earth is now thought to have a reducing atmosphere, composed mostly of water vapor, methane and ammonia.Free oxygen (which can oxidize other substances thereby destroying the precursors of any possible life) was practically non-existent at the time of the origin of life on Earth (approximately 3.5-3.8 billion years ago).Oxygen began to accumulate about 1.9 billion years ago when it was produced by photosynthetic organisms that evolved at that time. The second amendment concerns life.The essentialist conception of the sudden origin of life must be replaced by the gradualist conception of evolution in this matter.We now recognize that the origin of life was as gradual as the origin of man.Just as Homo sapiens is related to lower primates through a series of intermediate anthropomorphic animals, so life has a series of precursors.These intermediate molecular stages between inanimate matter and well-structured organisms are now absent in nature.They cannot survive in an oxidizing atmosphere and under the action of various microorganisms that feed on organic molecules.In the reducing atmosphere, under the action of ultraviolet radiation and lightning, organic compounds such as pyrimidine, purine and amino acid can indeed be produced as building materials for life.This has been experimentally confirmed by Miller, with Urey's advice and guidance.Haldane (1929) and O'Paring (1924) had previously proposed scenarios to explain how inanimate matter transitions to animate. Fox et al. (1979) have also made imaginative contributions to solving this problem. Surprisingly, rather than simplifying the task of interpretation, the discoveries of molecular biology have complicated it. Even in the simplest organisms, polypeptide chains (proteins) are assembled from amino acids under the direction of a genetic program (nucleic acids).In fact, there is a very perfect "symbiotic" relationship between nucleic acid and protein, and it is impossible to imagine how the other will function without one.How was the first primitive protein assembled and replicated without nucleic acid?If nucleic acids have no other function than controlling protein assembly, how did they originate and survive in the original "organic soup"? (See Chapter 19 for further introduction on this issue). The problem of the origin of life, that is, to imagine and prove each step from simple molecules to the first organisms showing function, is the most serious challenge faced by scholars of molecular evolution.The near impossibility of fully realizing the origin of life makes it extremely rare to see this state of affairs.This is why so many biologists consider the origin of life to be a unique event.The chance (probability) that this rare phenomenon can happen several times is very small, even though there are millions of planets in the universe. The above brief introduction to the recent advances in molecular biology reveals the close relationship between molecular biology research and evolutionary biology research.Molecular biologists' keen interest in evolution is manifested in the founding of the Journal of Molecular Evolution and in the publication of a series of proceedings and reviews of recently held symposiums (eg, Ayala, 1976).As evolutionists say, the study of molecular evolution has become an important branch of evolutionary biology. It is often said that besides Darwin's theory of evolution, there is now a "molecular theory" of evolution.The veracity of this claim is questionable.Two of the more important phenomena of evolution occurring at the molecular level, hard inheritance (advocated from Weismann 1883 to the Morgan school) and mutation (De Vry, 1901; Morgan, 1910a), at least in principle as early as molecular Genetics was accepted decades before its rise.It is not yet certain whether certain recent discoveries in molecular genetics (repeated DNA, splicing of genes, wandering genes) will at all require a revision of the theory of synthetic evolution.It is likely that the new findings simply amplify the magnitude of genetic variation required for natural selection to work and impose some control or constraint on the action of natural selection. I use molecular biology as an example to illustrate the growing affinity between evolutionary biology and other branches of biology.Positive interactions have also been shown between evolutionary biology and many other biological disciplines.Evolutionary issues now appear to be dominating the field of ecology and are of great importance in behavioral biology; this can be seen in recent textbooks on ecology and ethology. While evolutionary synthesis hasn't solved every problem in evolutionary biology, it has at least created a united front. A search of the recent literature on evolutionary issues reveals that differences of opinion still exist on some particular evolutionary issues. The contrary opinion, however, is not against any of the fundamental arguments of the synthetic theory; it is simply a difference of opinion about some evolutionary pathways.I try below to shed light on the nature of these divergences by presenting some of the unresolved problems in three main areas of evolutionary biology: the theory of natural selection, the problem of speciation, and evolution above the level of species. process (macroevolution). 13.2 Natural Selection Most of the powerful resistance to natural selection characterized by the post-Darwinian period and Mendelianism was overthrown by evolutionary synthesis.Resistance, or resistance, is so powerful because all anti-Darwinists agree on this point, and neo-Lamarckians and mutationists are equally vehemently opposed to natural selection.The first 30 years of this century are best known for Johannsen's selection experiments.He had been working in chemistry labs early on, so he took a completely non-biological approach to selection experiments.In order to obtain suitable experimental materials, he initially tried to establish "purelines".Not surprisingly, the results of many generations of inbreeding in a sample of this class of genetically identical individuals do not respond to selection.Johnson thus concluded (1915: 609, 613) that selection in self-crossing species cannot produce out-of-the-ordinary results, "even the most precise experiments with hybrid animals and plants confirm convincingly that our看法，即选择不能取得超出将原来就已在体质上不同的生物分离或单纯加以隔离的结果：对不同的个体进行选择不能产生任何新东西。按选择的指令改变生物类型的说法从来没有证实过！”他最后断言“非常明显遗传学完全剥夺了达尔文选择学说的基础…进化问题仍然是一个完全没有解决的问题”（659页）。他的这个结论在实验生物学家中被普遍接受，甚至T． H．摩根（1932）也说“自然选择学说所暗示的、在某个种群中选择最不词一般的个体，下一代就将更不一般，现在已经知道这是错误的。”迟至1936年，两位着名的英国动物学家，G． C． Robson和O． W． Richards还说，“我们并不认为可以不考虑自然选择作为进化的一种可能因素。然而迄今支持它的肯定证据还是如此之少…以致我们没有理由把它看作是进化的主要动因。”在20年代和30年代的这种知识文化背景下无怪乎达尔文主义者要花费极大的力量来驳斥反选择主义者的各种论点。 反达尔文主义者的怀疑态度也并不是完全没有道理。无论是在自然界还是在实验室中自然选择的直接征据几乎一直到20世纪中叶还非常少。由Bummus（1896）证实的作为冰暴结果的麻雀差别死亡率是几十年来的唯一证据，因而被选择主义者一直反复引用。 更糟的是进化综合以前在达尔文主义者内部对选择的看法也有分歧。其中大多数人跟着达尔文也承认某种软式遗传，如用进废退。华莱士是最坚定的早期选择主义者并且首先支持魏斯曼否定软式遗传的论点，因而是一个“自然选择万能”论者（Allmacht derNaurzuchtung）。实际上华莱士甚至将隔离机制的起源也完全归之于选择，而达尔文则无从想象这种同域过程，因而在这一点上两人的看法相左。现代研究物种形成的学者倾向于同意达尔文的观点。魏斯曼和华莱土在他们无条件地支持自然选择上很孤立，绝大多数进化主义者对之都有不同程度的保留（反对自然选择效力的意见见第十一章；更详细的介绍见Kellogg，1907；Mayr and Provine，1980多以及大量的反对达尔文主义的文献。）促使对自然选择的意见发生转变的因素很多，其中最重要的可能有以下几种： （1）实验室的选择实验以及动植物育种家的大量工作真正证明了选择有效。在自然界中进行的实验，例如Kettlewell的工业黑化现象（Industdd melanism，Ford，1964），特别有说服力。3Q年代台西尔等创用的种群笼方法（见前）很快就被杜布赞斯基以及研究果蝇的其他工作者采用，并5！起了对不同遗传品种在不同温度，温度，食物，群聚，竞争等条件下自然选择一实验的高潮。（2）遗传学家否定了软式遗传，这就实际上除了经由自然选择的渐进进化而外没有任何其他可供选择的余地。（3）驳斥了生物伪绝大多数特征没有选择价值的说法。甚至Haldane（1932：116）也曾经说过，“毫无疑问（动植物）的无数性状并不显示具有选择价值，而且这些性状正是那些使分类学家得以区别物种的性状。”后来通过一些学者（例如主席，尤其是E．B．Ford的牛津大学研究组）的研究终于证实以前的许多所谓“中性”性状经过深入细致研究也都具有选择价值。（4）Norton，霍尔丹，菲舍以及其他人的计算指陈即使极小的选择优势如果连续许多代就能显示其重要意义。（5）种群思想的传播，特别是新系统学者论证了物种和高级分类单位中的不连续性可以通过地理成种作用和灭绝产生，因而不需要骤变。 杜布赞斯基在他的《遗传学与物种起源》（1937）一书中有一整章（共43页）讨论自然选择。他的阐述之所以特别吸引人是因为他不仅仅把自然选择看作是一种学说而且是一种可以由实验证实的过程。另外他还指出选择和渐进的适应性地理变异（例如壬席的气候规律就是这种变异的反映）并不矛盾。这就不再需要用拉马克的解释作掩护，而在以前由于突变论者的论点博物学家都只能被迫如此。迈尔（1963：182—203）详细分析了前十几年选择主义所引起的一些问题，其中有下面五个问题要单独提出来进一步讨论。 区分自然选择有几种方法。其中有一种是根据选择压力施加于变异曲线的段落来区分。稳定化选择（Stabilizing Selection）指的是指向变异曲线两个尾部的选择；这相当于本质论者的“淘汰”，也就是说一切偏离“正常”的都被排斥。定向选择（directive selection）是在曲线的一个尾部被自然选择选中，另一尾部被排斥，结果是曲线的均值稳定增高。多样化（歧化）选择或分裂选择（diversifying selection或disruptive selection）是曲线的两个尾部都被选中，如在具有拟态或其他多形性的物种中所发现的双峰曲线。 自然选择的概率（几率）性质本质论者很难理解自然选择是一种统计现象而不是全或无现象。哲学家C． S． Peirce比他的同时代人或许对这一点了解更清楚并指出自然选择虽然在个别情况下可能不起作用，但“变异和自然选择…在长时间里将…使动物适应它们的环境。”Mayr（1963：184）也同样强调选择的概率性质。尽管哲学家可能仍然谈论“最适者生存”，但是生物学家已不再使用这样的决定论语言。 由于采用了“进化是由突变和选择引起的”这个公式，某些遗传学家为一个广泛流传的错误概念倒出了不少力。这个公式被他们解释为突变了的基因是选择的真正目标。 与此相对映的是，自从达尔文以来的博物学家和具有洞察力的遗传学家则一直强调不是基因而是整体生物（能繁殖的个体）才是选择的单位。这就意味着重组的效应和基因调节效应以及发育中的表现型对环境的反应能力对选择来说和突变同样重要，但在数量上比突变的重要性要高出好几个数量级。然而当菲舍（1930）和其他数学遗传学家选用基因作为选择单位并对每个基因赋予一定的适合值时就发生了困难。适合度被重新定义为某个基因对下一代基因库所作的贡献（另见霍尔丹，1957）。这转过来又引出了一个非常成问题的进化定义（“种群中基因频率的变化”）并引起了颇有道理的批评，即单个基因的频率变化使很多（事实上是绝大多数）进化现象无法解释。目前对选择学说的责难有一些（如果不是大多数）就包括对基因是选择单位这种非达尔文主义的假定的抨击。 这一点必须强调，因为它表明某些学者新近提出的“内部选择”概念是多么容易引起误解和混淆视听。根本无法将选择分为两部分，一部分是由外部环境引起，另一部分由生理和发育等内部因素造成。这样的划分之所以不可能是因为选择的结果决定于外部环境与作为整体的生物的生理过程之间的相互作用。没有内部选择。所有发育过程和调节过程对某个个体的适合度所起的作用不是有利就是不利，担是这只有当某一个体置身于（暴露于）外部环境（包括同一物种或另一物种中个体的竞争）时才能估价。达尔文早已充分觉察到这些内部因素的重要性，例如从他在讨论相关（correlation）肘（：143－15o）就可明显看出。当一个现代学者仍旧将已过时的公式“突变与选择”看作是达尔文主义者的主张肘，就无怪乎他会认为这公式不足以解释特定的进化反应（evolutionary response）。凡是继续还用这公式的人决不可能了解进化演变的真正原因。着名的进化主义者放弃把突变当作选择的目标已有40多年。 由于表现型的整体是选择的目标，所以不可能同时使表现型的所有组成部分按相同的程度改善。选择不能达到尽善尽美，因为种群中的成员在为繁殖成功的竞争中只要较优而不必一定是完美无缺就足以取胜。更何况每个遗传型是多种选择压力的调和折衷结果，其中某些选择压力可能彼此措抗，例如性选择和避免被捕食的隐蔽色（Endler，1978）。由于遗传型的内聚性（cohesion），往往不可能只改善表现型的某一组成部分而不有损于另外部分。当转移到新的适应区后，在原先的适应区中的某些适应就不再有利。水生哺乳类必须尽可能减少和淘汰陆生生活方式的一切特殊适应。两足的类人猿仍然为他们过去的四足历史所苦（付出代价）。 进化主义者长期称之为进化的折衷（妥协）被现代生态学家称作进化的最优化处理。 每一种进化性进展（例如跳得更快，拥有更多的后代，利用新的食物资源）都有其代价，自然选择决定这进展所增加的利益与所付出的代价究竟是否相称。这样一来结果便是表现型往往是为特定功能（或应答某一特殊的选择压力）所特别选择的性状的拚凑产物，而其他性状则是作为整体的遗传型的副产物并且是选择真正容许的。自从达尔文以来，博物学家就一直自问物种之间的差异应当分在两类中的哪一类中。例如白氏（Borchells）斑马和格氏（Grevys）斑马斑纹上的差别究竟是由于非洲不同地区（这两种斑马的来源地）的选择压力不同的结果还是由于这两种斑马的遗传型对斑纹选择的应答不同？ 只要某些遗传学家相信每个基因有一个独立的适合度，各有最适合的适合值，这样就可以认为表现型的每个方面都是对特殊选择（ad hoc selection）的专门应答。然而作为整体的个体才是选择目标，而且多数（如果不是全部）基因是彼此互相作用的事实便对表现型就选择作出应答反应施加了严格限制。这就是为什么人还有阑尾、易受伤的能骼关节以及一些结构不完善的窦的原因。Gregory（1913；1936）将特殊适应的总体称作“习性”（habitus），对容许保留的过去残留物称为“遗产”（heritage）。 并不是表现型的每一个细节都是由特殊选择塑造的这一结论由于Book（1959）称之为“多重途径”（multiple pathways）的现象而更加使人倍服。例如远洋无脊椎动物具有多种多样的机制使之浮在水面：气泡，油滴、体表增大。在每种情况下自然选择（它总是见机行事）都是利用最容易取得所需要的适应的那一部分可以利用的变异。 将某一生物尽可能地分解成非常多的部分并论证其中每一部分的选择值的原子还原诊者（atomistic－reductionist）的做法使整个适应概念陷入争议之中。争议竟然达到这样的程度，即反对选择主义的人所提出的对自然选择的反对意见（例如Grasse，1977a）是完全站得住脚的。总之，选择是概率性的，在小种群中采样误差必然产生随机效应，生物集成为整体总是严格限制了个别性状的反应。的确，就总体来说生物能很好地适应它们的环境，因为那些不能适应的繁殖成功率极低从而无法生存。但是这并不意味生物表现型在结构和功能效率各个方面都是最理想的。 就本质论者看来，选择完全是一种消极因素，是将有害的脱离正常的东西加以淘汰的力量。因此达尔文的反对者坚持本质论的教条认为选择不可能创造任何新事物。他们这样说显示了他们既不懂选择的两步过程又不明白它的种群实质。选择的头一步是产生无限数量的新变异，即新的遗传型和表现型，特别是通过基因重组而不是经由突变。第二步是第一步的产物要经受自然选择的检验（考验）。只有能够通过这种严格考验的个体才能成为下一代基因库的参与者。切特维尼可夫和杜布赞斯基等曾经正确指出这种在基因重组和选择所取得的极其有限的下一代“祖先”之间的循环往复确实是一种创造过程。它为每一代提供了一个新的起点，并且提供了新的机会来利用新环境和新的基因群体（genetic constellations）。 13．3自然选择还没有解决的问题 对上面所提到的五个有关选择问题的解释基本上没有什么争议。但是还有一些其他问题在进化生物学家中仍有分歧。下面就对其中的几个问题进行讨论。 过去50年中关于自然种群中遗传变异性的水平问题有两个相持不下的学派。就穆勒和大多数经典遗传学家看来，每个等位基因都有不同的选择值，其中有一个（一般是“野生型”）是“最佳的”，因而是种群中占优势的基因。他认为自然选择的功能就是淘汰另一个较劣的等位基因，它的贮量不断地由突变来补充。根据这种推理这个学派断言种群中的大多数个体在多数座位（位点）上应当是纯合型，否则有害的隐性基因的负担（“遗传负荷”）就会变得太大。穆勒、Crow等是这种传统观点的最积极的拥护者。 另一个学派的领导人是杜布赞斯基（Mather，Lerner，Mayr，B．Wallace及他们的学生也都属于这一学派）。这个学派认为遗传型是一个许多基因的和谐平衡系统，其中任何等位基因的杂合子往往优于纯合子。此外，这个学派还否认基因的绝对适合值，所以几个等位基因都可以是“最佳的”，在每一种情况下要根据它们的遗传背景和当时的外部选择压力而定。平衡学派的观点来源于切特维尼可夫的遗传背景概念，这个概念在将遗传型看作是一个平衡系统的学说中得到了发展（Dobzhansky，1951；Mather，1943）。 借助于遗传分析的古典技术来确定种群中隐蔽隐性的频率是办不到的，因为每一次只能有一个座位可以成为纯合型。因此这就不可能澄清“古典”学派与“平衡”学派之间的争论。1966年采用了酶电泳方法后从Hubby和Lewontin对果蝇及Harris对人的研究中终于发现了等位基因的多形性具有令人吃惊的高水平。他们发现（并被以后研究者普遍证实）即使单个个体其基因座位可能约有百分之十或更多是杂合型，物种则可达30％～50％。因而这问题看来似乎已得到澄清并对杜布赞斯基的平衡学说有利。这似乎也表明达尔文深信遗传变异实际上是无穷的是对的。 然而正像大多数新的研究路线一样，酶变异性研究所引出的新问题比它已解答的问题还要多。为什么某些物种比其他物种的变异性水平高得多？物种的变异性水平和它的生态有什么关系？种群的变异性有哪些部分是由选择维持的、其他哪些部分是由于机率（实际是中性等位基因的突变）？酶基因的变异性和遗传型的其他DNA的变异性有什么关系？在努力回答这些问题的研究中运用电泳方法研究酶基因变异性现在已经成为进化遗传学的最活跃的领域之一（Lewontin，1974；Ayala，1976；Ayala etal.，1974b）。 在这种高度遗传变异性中最有争议的问题是它的来源。采样误差以及对低劣纳合子的选择压力可能会大大降低等位基因变异性的水平。在一个基因座位上4个、5个、甚至10个以上的等位基团是怎样能够在一个种群中同肘保持下来的？ 自从60年代在自然种群中发现了大量的遗传变异性后，认为这种变异大多数在选择上是中性的论点再度被提了出来。这一学说的拥护者——King及Jukes（1969），Crow及木材（Kimura）（1970）——将由于随机过程（基本上是中性突变）引起的遗传变化称为“非达尔文进化”。这一名称很容易引起误解，因为拉马克主义，直生论以及突变主义也都是非达尔文进化的形式。另外有些人将之称为“随机游动进化”或许更合适。 自此之后关于在自然种群中观察到的遗传变异性有多少是出于选择、有多少是源于机遇引起了热烈争论。奇怪的是，在这场争论中意识形态似乎也起了某种作用，因为从总体来说马克思主义者比非马克思主义者更强调随机游动进化。我本人的看法是选择比非达尔文进化的支持者所承认的要重要得多，但是在某些基因座位上的大部分变异也确实具有随机成分。 很有可能单是杂合子的选择优越性并不能保持这样的高水平的遗传多样性。然而也有其他因素有利于遗传多样性的保持（Mayr，1963：234-258）。就多态的蜗牛和昆虫而言，某一稀有的表现型在一定程度上不致于被捕食，因为捕食者的“搜索形象” （search image）已习惯于更常见的表现型（Clark，1962）。也曾经有人揭示（首先是Petit与Ehrman，1969）某些物种的雌性动物首先选择与稀有遗传型的雄性动物交配；这也有利于防止稀有遗传型从种群中消失。也曾发现选择值发生改变的情况，看来依赖于频率的选择（frequency－dependent selection）是保持种群的遗传变异性的一种十分重要的机制。 目前已有不少证据表明不同的基因型不仅在物种生境的各种不同亚生境中显示优越性而且宁愿选择这样的亚生境并有能力找到它们。这和复杂栖息地的遗传多样性一般高于简单栖息地的发现是一致的（Nevo，1978；Powell and Taylor，1979）。保持遗传变异性的另一机制是霍尔丹早已指出的（1949）防止寄生物和病原体。提供免疫性的基因（抗体形成等等）的高度遗传变异性能够保护种群免于毁灭性的损失，，因为病原体无法对付希有的免疫基因。最后，如果上位相互作用正像我们认为的那样很重要，则低频率的基因也可以保持住，因为它们在某种组合下具有高选择值。考虑到目前发现了这样多的由选择控制的机制，所有这些机制都能使二倍体基因库贮存遗传变异性，这就促使人们作出这样的结论，即大部分观察到的种群遗传变异性完全可能是自然选择伯结果。 霍尔丹（1957）与Kimura（木村，1960）曾经作过一些计算表明在一个大种群中用一个在选择上处于优势的等位基因取代一个等位基因所付出的代价是多么“昂贵”。他们由之作出了进化必定是非常慢地在进行的结论，也就是说在相当少的基因座位上同时进行，否则总死亡率将会高得惊人。这一结论和已被普遍接受的进化演变的高速度（例如淡水鱼）以及大多数自然种群中高度的杂合现象直接相矛盾。霍尔丹显然作了一些不切实际的假定。迈尔（1963：262）和后来的一些其他学者（Lewontin，1974）指出了霍尔丹所作出的一些简化假定的种类。例如在某一物种中由于密度依赖性竞争（density－dependent competition）的缘故在所有的后代中只有一小部分进行繁殖，在每一代中死亡率是如此之高。因而压低这具有有害的纯合子的“可放弃的多余” （expendable surplus）无论如何也不是严重的负担。更重要的是霍尔丹的计算适用于大种群，而速度快的进化演变最常见于小种群中（见下）。就密度高的物种来说霍尔丹可能是正确的，这已通过化石记录所显示的这一类物种的进化惰性表明，但是他的计算对小种群是无效的，尤其是创始者种群（founderpopulations），大多数极其重要的进化事态正是发生在这一类种群中。 作为整体现象的自然选择当自然选择学说遭到严重非难时就不会认真考虑自然选择还可以进一步细分的问题。 现在成于自然选择已经确定无疑，新问题便又出现，例如，是不是有一种可以称为群体选择（groupselection）的过程？像达尔文那样，将性选择从自然选择中划分出去是否合理？这两个问题已经引起广泛争论，下面有必要简单介绍一下争论的实质。 个体是选择的主要单位，这一论点遭到了某些主张群体选择的进化主义者的诘难（Wynne－Edwards，1962）。支持群体选择的人声称有一些现象可能并不是个体选择的结果。他们特别指出了整个种群的某些特征，例如异常的性比（sex ratios）、突变速度、扩散距离、性二形性的程度以及促进自然种群中近交（in-vreeding）或远交（out-breeding）的某些机制。支持群体选择的学者认为种群之间的这一类差别只有当整个种群（小区种群，deme）比其他小区种群更占优势时才能显出，因为这是由于上述因素的遗传结构有所不同的结果。这类群体选择究竟是否发生以及达到什么程度，在目前仍然还有激烈争论，但一般的看法是，这类情况的绝大多数可以按个体选择来解释，也许只有社群动物除外（Lack，1968；Williams，1966）。 关于群体选择的争论表明，在选择的某些方面的确还有含糊不清的地方。进化主义者已经意识到过去往往把很多十分不同的现象搅合在一起，只有将它们分腾不同的组成部分才能充分了解选择的作用。 早在18世纪末期，某些动物育种家就曾提到，雌性动物偏爱强健的雄性并认为这种现象解释了性二形性。某一个体对异性个体具有更大的吸引力从而取得繁殖优势的现象被达尔文称为性选择。达尔文将之和自然选择（准确的涵义）加以严格区分，后者是在全面的适合度（对环境的耐受力，资源利用，对捕食者的阻挠能力，对疾病的抵抗力等等）上发挥作用（表现优势）。达尔文对性选择的重视从他早期的笔记（1840年左右）就可看出，然而在中他只花了不到三页的篇幅讨论这个问题（1859：87－g0）。在《人类由来》（1871）一书中他却用了比人类进化更多的篇幅来讨论性选择。 尽管如此，更能说明达尔文对这问题具有浓厚兴趣的莫过于他和华莱士讨论性二形性产生原因时的长期通信（Kottler，1980）。达尔文一华莱士的通信，是有关性选择重要意义这一迄未结束的长期争论的开端（关于这一争论的早期情况见Kellogg，1907：106－128）。达尔文将性选择与自然选择严格区分开的努力遭遇到强有力的反对。到了1876年甚至华莱士也放弃了性选择，在随后的年代中大多数实验生物学家也是如此，因为他们（如T．H．摩根）只注意近期原因（例如有哪些激素或基因与性二形性有关）。 数学种群遗传学家彻底否定性选择，因为他们认为进化是基因频率的变化并将适合度定义为为下一代基因库提供基因。由于这个定义实际上对自然选择和性选择都同样适用，所以这两类选择之间的区别也就湮灭了。 近年来生物个体又被看作是选择的主要目标，恢复达尔文的性选择概念也就是名正言顺的了（Campbell，1972）。不可否认，达尔文曾把性二形性的某些方面包括在性选择内，这些（例如雄性动物的攻击性某些方面）如果列在自然选择中将更合适。然而这样一来所剩下的就全是雄性装饰物（及鸣叫或歌唱）方面，达尔文将这些解释为是由于“雌性挑选”（female choice）的结果。虽然雌性挑选原理在过去100多年中得到大多数博物学家的支持，但大部分生物学家以及几乎所有的非生物学家都反对它，因为这赋予雌性动物以某种鉴别能力，而这种能力是“它们（雌性动物）所不可能拥有的”。然而近来行为学者以及其他领域的博物学家的研究却肯定地证明了不仅是雌性脊椎动物而且昆虫和其他无脊椎动物的雌性一般都很“害羞”（coy），从不和它们所遇到的第一个雄性动物交配。事实上，对最后与之交配的雄性动物的挑选往往是一种时间拖得很长的过程。在这类情况下雌性挑选已是确凿无疑的事实，虽然雌性挑选所根据的标准究竟是什么还并不清楚。 这和雄性动物形成了强烈的对比，雄性动物一般倾向于和任何雌性交配，甚至往往不分是否同一物种的雌性动物。雌雄动物在这方面相差悬殊的原因已由Bateman（1948）根据投资原理（prinCiple of investment）加以阐明，后来Trivers（1972）进一步加以充实。雄性动物有足够的精子使非常多的雌性动物受精，因而它在每次交配中的投资很少。相反，雌性产卵很少（至少在雌性挑选的物种中是如此），而且还要投入大量的时间和物资来孵化或用于胚胎发育，在孵化后还要照顾抚育幼仔。如果在挑选配偶时发生失误（例如产生低劣的或不育的杂种）雌性动物就会损失其全部生育潜力。雌性挑选原理还解释了很多过去一直无法解释的现象，例如，为什么具有贝氏拟态的蝴蝶的多形性通常只限于雌蝴蝶，因为雌蝴蝶能分辨其配偶具有物种特异性的彩斑，如果彩斑相差太远就不会与之交配（触发机制，releasingmechanism）。 目前有一种合理的倾向是把性选择概括地解释为能促进繁殖优势的任何形态或行为特征（性状）。迈尔（1963：199－201）指出某些类别的自然选择潜在的“自私”的一面，特别是能提高个体繁殖成功的机会而并没有提高物种的一般适应能力。Hamilton（1964），Trivers（1972）和Dawkins（197z）都曾指陈这一类自然选择非常广泛而且对动物行为和进化趋向有多么深刻的影响。Wilson（1975）曾对有关文献作过评论。繁殖自私似乎是生存竞争的一种温和表现方式，不像社会达尔文主义者所描述的生存竞争是血淋淋的残酷斗争。 在19世纪80年代和90年代当社会达尔文主义与真正达尔文主义混淆在一起时，合作与利它现象（altruism）常被引用来作为人类道德趋向的进化证据，这种趋向似乎不可能是自然选择的结果。这种看法忽视了合作（尤其是在社群生物中）可能具有选择上的好处。达尔文在谈到“我是在广泛的、隐喻的意义上使用生存竞争这个词的，包括生物之间彼此依存”（1859：62）。 利它现象及其进化是霍尔丹在1932年提出的，现在又成为注意力的焦点。利它通常被定义为一种有利于另一个体（“受益者”）的活动而对利它者似乎无利的现象。霍尔丹指出如果受益者（与利它者）的亲缘关系很接近，则利它性状将被自然选择选中，这样一来受益者的生存就有利于它和利它者所共有的基因。例如某个利它活动有十分之一的机会使利它者付出生命作为代价，而受益者却是它（利它者）的子女、同胞兄妹、孙子孙女，它们全都和它共同占有十分之一以上的基因，自然选择就会促使利它现象发展。 这