Proc Natl Acad Sci U S A. 1960 Apr; 46(4): 494-506.
THE BALANCE BETWEEN COHERENCE AND VARIATION IN EVOLUTION
By JENS CLAUSEN AND WM. M. HIESEY
DEPARTMENT OF PLANT BIOLOGY, CARNEGIE INSTITUTION OF WASHINGTON, STANFORD, CALIFORNIA
Read before the Academy, November 18, 1959
What are the mechanisms that cause living things to differentiate into recognizable species and subspecific entities rather than to evolve into a flow of continuous variation? This age-old question has puzzled evolutionists, and no adequate answer has been demonstrated. One of the reasons for our highly fragmentary understanding of evolutionary processes is that only scant information has been available about mechanisms governing variation in truly wild organisms.
A recently concluded analysis of 20-year experiments on the hereditary mechanisms that separate ecological races of wild plants of western North America shows that coherence is as much a part of evolution as is variation. Even characters of evolutionary entities below the level of the species are held together by coherence mechanisms that are balanced against others producing variation. The fact that the forces of coherence are as integral a part of evolutionary mechanisms as are the forces of variation makes the process of speciation comprehensible.
Studies on the mechanisms of inheritance in wild organisms have hitherto largely been avoided because their inheritance has been known to be relatively complex. For experimental purposes, organisms that have evolved within the protective environment created by human civilization have been selected because they frequently differ in rather spectacular characters whose modes of inheritance are simpler to study. Emphasis has also been placed on the effects of individual genes as they relate to the inheritance of one or a few traits, and on the effects of mutant genes having conspicuous effects, rather than on their wild-type alleles having many small but cumulative effects.
Natural selection tends to discard the average mutant and does not directly operate on single genes, but rather upon combinations of characters regulated by constellations of genes. Races of wild plants are products of natural selection acting through geologic periods of time. They are adjusted to living in arrays of distinct environments and differ by many traits, both morphological and physiological.
GENETIC STRUCTURE OF WILD RACES
In an attempt to fill some of the gaps in our knowledge concerning mechanisms of natural evolution, our group has undertaken to analyze the hereditary structure of ecological races of selected species of wild plants. Some of these are on record,9-11,13-16,20-22 and others are still unpublished.
These investigations have shown that nearly every character distinguishing one ecological race from another is controlled by a small system of genes. The combined effects of all the genes of such a system can be easily noticed, but each component gene induces much less effect than genes commonly studied in the laboratory as mutants. The physiological effects of genes in wild races tend to counterbalance each other so that their expression is buffered. About 35 years ago Erwin Baur2,3 demonstrated that in crossings between wild and cultivated snapdragons (Antirrhinum), segregation for many small differences occurred, but no actual genetic analysis was then attempted.
The Coherence Mechanism.—The basic mechanism by which coherence is achieved is provided through the chromosomes. The several genes that regulate the phenotypic expression of a character, such as petal shape or anthocyanin coloration of stems and leaves, are usually located on separate chromosomes, but each chromosome carries genes that contribute to the control of many other characters as well. Some of the genes that regulate two distinct characters may thus be linked, whereas other genes governing these characters recombine freely. The hereditary mechanism that regulates the phenotypic expression of the characters operates, therefore, like a loose, partly disengaging network. If natural selection favors a particular trait of an organism, its effect is transmitted to the progeny through the chromosomes that carry the genes controlling that trait, and through these it will also be partially linked with several other distinct characters regulated by genes carried by the same chromosomes.
Potentials of Variation.—Natural races carry much greater potentials of variability than has previously been assumed. Crossings between wild races greatly increase the expression of variability. Epistatic (covering) effects of certain genes are frequent; likewise, genes of separate races may complement each other so that much hitherto unexpressed variability becomes released and subject to selection. Genes having additive, subtractive, and complementary effects on a character are frequently carried by separate races of the species and cause transgressive segregations in interracial crosses. In the case of physiological differences, such segregations may far exceed the limits found in the parental races.
Number of Genes per Character.—From evidence thus far obtained, each morphological difference has been found to be regulated by a system composed of a relatively small number of genes. In Potentilla and the Madiinae of western North America, the parental expressions of segregating individual characters have been approached, attained, or exceeded in F, populations consisting of 500 to 1,000 individuals under circumstances suggesting that systems on the order of three to six pairs of genes govern such characters.11-15 These findings are supported by other data previously obtained in diallel intercrossings between forms of the two wild pansy species of northwestern Europe, Viola tricolor and V. arvensis.6-8,15 The highly variable floral colors of wild pansies can be accounted for by assuming interactions between 11 pairs of genes, three of which also concern the development of anthocyanin in stems and leaves. The occurrence of an inconspicuous honey spot on the front of the pistil is regulated by the complementary and oppositional interactions of four pairs of genes.
The genetic regulation of certain physiological characters, such as date of flowering in Potentilla, is more complex, however; the F2 segregation with regard to this trait greatly exceeded the limits of the parents. Considering the dates of flowering of the cloned F2 individuals in three contrasting environments, it was estimated that a minimum of 20 pairs of genes regulate this character in the interracial crosses. In contrast, other physiological characters, such as winter-dormancy and frost resistance, appeared to be governed by three or four pairs of genes.
Combining data from crosses between several agricultural races of barley (Hordeum), Wexelsen32,33 found that 6 pairs of genes, having somewhat unequal effects, control the length of the internodes between spikelets of the ear. His conclusions were based on detailed genetic analyses of segregating F2's, F3's, and F4's resulting from selfing, and were supported by the fact that two of the multiple genes were linked with two groups of genes governing other morphological characters located on separate chromosomes.
The estimates above refer to the minimal number of gene differences in organisms where it has been possible to conduct a careful and fairly representative analysis of interracial crosses. These estimates are far below the 100 to 200 genes per character that were suggested by J. Rasmusson28 without substantiating data, but have been repeatedly quoted.1,26 The high estimates were based on statistical deductions rather than directly on observed segregations, and they assumed small, linearly additive differences. A moderate number of gene differences can, however, produce great diversity in segregation when the system contains complementary and oppositional genes, or when both parents carry a different set of genes having additive or subtractive effects.
Correlation in Inheritance.—Only few studies on correlations between segregating characters are on record, although there are many on linkages between individual genes. The calculation of correlation becomes a major operation when many segregating characters within a species are studied. The extensive study of correlations in progenies of interracial Potentilla hybrids15 were facilitated through the use of punched card techniques.
Supporting evidence on correlations between characters is available from interracial hybrids within Layia and Madia,9,14,15 and from subspecific hybrids of the Solidago sempervirens complex,4 and Gilia capitata.19 It was also reported in the subspecific hybrid of the lowland Antirrhinum majus x alpine A. molle.3
Strong correlations were observed between 8 segregating characters in a cross between two agricultural races of barley, one race developed in California and the other in Sweden.32,33 The segregations of 5 of these characters were widely transgressive.
At the interspecific level data are also available on correlations between segregating characters in two parallel hybrids of Nicotiana.1,29,30 Many potential recombinations were presumably eliminated, because in the F1 only 6 of the 9 parental chromosomes were fully homologous, about half of the pollen was sterile, and the number of seeds per capsule was reduced to one-third of that in the parents. The initial F2 populations were small, and the parental expressions of the characters were not attained among the F2's. The data suggest that strong limitations in recombination follow interspecific crosses in Nicotiana, contrasting sharply with the widely transgressive character of segregations following intraspecific, interracial crossings in Potentilla and in barley.
A recent paper on the cross Nicotiana langsdorffii x N. sanderae31 strengthens the case for initial limitations in recombination and provides additional significant evidence that the tight coherences can be occasionally broken to release variation in later generations. Three selection programs were conducted among successive progenies of this hybrid until the F10, aiming, respectively, for strains having short corolla tubes as in langsdorffii, long tubes as in sanderae, and intermediate tubes as in the F1. Uniformity comparable to that of the parents was achieved by the F6. A tube as short as that of the langsdorffii parent was never attained. The long corolla of sanderae was approached, but actually not attained, when the long-tube selection became stabilized in the F6. In the F9 and F10 generations, however, the long-tube selection suddenly surpassed the sanderae length in a slight, but statistically significant, transgression. Such a shift presumably could occur following a rare crossover between two genes having opposite effects on tube length and being tightly linked on one chromosome. A greater transgression in length of corolla tube was attained by backcrossing the F5 of the plus selection to the sanderae parent and selecting until the F4 following the backcross. It is therefore obvious that even interspecific crosses may, in due time, release tightly stored variability and produce transgressive variation.
EVOLUTIONARY BACKGROUND OF POTENTILLA GLANDULOSA
Potentilla glandulosa Lindl. is a semi-woody perennial species of the rose family. It belongs to an evolutionally old circumboreal complex of closely related species that was once named a genus, Drymocallis, but now is considered to be a section of the genus Potentilla and somewhat related to the strawberry. Seven species of the Drymocallis complex all have 7 pairs of chromosomes. The chromosome pairing is normal in interracial hybrids of P. glandulosa,12 and an interspecific hybrid was fertile.27
Forms of Potentilla arguta Pursh are considered to be the immediate ancestors of P. glandulosa. On the North American continent, arguta is probably the oldest species of the Drymocallis complex. It occupies by far the largest area, extending from the Atlantic Ocean to the Pacific, and northward diagonally from about the 38th latitude in the east to the 60th latitude in the west.12
Potentilla glandulosa is more variable than arguta, but occupies a more limited area in the western United States essentially south of the general region of arguta,12 an area that has been the scene of great mountain-building processes during the 60 million years of the Cenozoic era. During the first period of that era, the Eocene, the California part of the present glandulosa region is known to have had an essentially tropical vegetation; the mountains were low hills, and the Central Valley was a huge bay of the Pacific Ocean. Like the tree species of the western United States, it is assumed that the ancestors of glandulosa migrated from the north, and gradually occupied the new niches that became available as the climates changed from tropical to warm temperate, and as the mountains rose.
Potentilla glandulosa is the most highly diversified species of the Drymocallis complex, and has evolved a series of ecological subspecies and races fitting the highly diversified niches in its topographically complex area.12,15 There is reason to believe that the ancestors of its outer Coast Range and high-mountain races have been in position during the last 20 to 30 million years, from the beginning of the Miocene period. and that since that time the forms inhabiting the mountains have undergone gradual selection, adjusting them to survival in the climates of the rising mountains. The climatically youngest environment in the glandulosa area is the warm and dry inner foothill region. It has existed only since the California Coast Ranges started their uplift and blocked the moist and cooling influence of the Pacific Ocean—an event that presumably began about a million years ago.
The differentiation into climatic races within this species has affected its breeding system. The lower altitude races are self-compatible and have small, relatively inconspicuous petals. The races of the montane to alpine altitudes of the Sierra Nevada are highly self-incompatible and have large, showy petals. Comparable changes have also taken place in the life forms of the races. The low altitude ones are woody cushion plants that have heavy tap roots and branch from the crown (chamaephytes), whereas those of high altitudes spread by slender, subterranean rhizomes and die to the ground level in the autumn (hemicryptophytes).
SEGREGATION AND COHERENCE IN POTENTILLA HYBRIDS
Two hybrids from four contrasting races of Potentilla glandulosa were analyzed with regard to the gene systems that regulate their differences, and the degrees of correlation between parental characters were determined among their segregating progeny. One cross was followed through the second generation in one environment. The progenies of the other cross were studied through the selfed third generation, and second generation plants of this hybrid were cloned and transplanted to three contrasting environments where their performance over many years was carefully studied.
Coast Range x Alpine Hybrids.—A cross between a plant of a coastal race of Potentilla glandulosa from near sea level at Santa Barbara, California, and another of an alpine race from Upper Monarch Lake, at 3,300 meters altitude in the Sierra Nevada, yielded F1 progeny that in some characters was intermediate between the parents, in others favored one or the other parent, and in vigor exceeded both. The coastal parent was self-fertile, the alpine self-sterile, and dominance for self-fertility in the F1 was so strong that a single selfed plant yielded nearly 30,000 good seeds. The Coast Range race is unable to survive in the alpine climate, and the alpine in the Coast Range, but the F1 hybrids surpassed both by surviving in Coast Range, mid-altitude, and alpine gardens.
The two races differed in many morphological and physiological traits. A sample of 1,000 F2 hybrid progeny were grown and showed a striking amount of segregation in all characters, and transgressive segregation in many, 10 per cent of the segregants even exceeding the vigorous F1 in size. Fourteen character differences distinguishing the parents were studied with respect to their degree of correlation, or frequency of association together, in the F2 progeny.
Figure 1 summarizes the data in the form of a diagram. It is evident that most characters are not inherited independently of each other, but are partially linked to others in varying degrees. Among the 91 possible combinations of the 14 characters studied, 67 were associated together in some degree, and 24 showed no significant correlation. The majority of parental characters were therefore directly or indirectly correlated, although also a great deal of genetic recombination occurred among all of them. Probably all of these characters are either directly or indirectly related to the success of the races in their respective environments.
|Fig. 1.—Coast Range x alpine races of Potentilla. Degrees of correlations between 14 segregating characters in an F, progeny of approximately 1,000 individuals. Correlation coefficients, r, above the 1 per cent level (0.08) considered significant.|
Foothill x Subalpine Hybrids.—A second hybrid was a cross between a plant of the foothill race from Oak Grove in the Sierra Nevada foothills at 760 m and another of the subalpine race from a slope above our Timberline station situated in a hanging valley on the east side of the crest of the Sierra Nevada at 3,050 meters altitude. The contrasts in characters between the parents of this hybrid were fully as great as between those of the former, and there were certain significant differences between the two crosses, especially in the expression of hybrid vigor and in the inheritance of self-fertility.
The F1's of the foothill x subalpine cross exceeded only slightly in size the intermediate position between the parents, and almost all of the F2 plants varied between the ranges of the parents. Six of the selfed F3 progenies, however, showed luxuriance of growth to the extent that 45 per cent of their 2,228 plants surpassed the tall foothill parent; three other F3 progenies showed minor degrees of hybrid vigor. The genetic potentials for hybrid vigor therefore existed in this cross, but the proper gene combinations had not been achieved until the third generation, whereas in the Coast Range x alpine cross the F1 generation already exceeded the largest parent.
The inheritance of self-fertility showed similar differences. The F1 of the foothill x subalpine cross was only about 40 per cent self-fertile, in contrast with the dominance of self-fertility in the first cross. Two selfed F1 plants yielded 1,070 highly variable F2 plants despite the less than 50 per cent self-fertility, and 575 F2 plants were grown to maturity. Twenty-two F2 plants, representing morphologically diverse types, were selfed for progeny testing. Two of these were self-sterile—and 10 others were nearly self-sterile, yielding from 16 to 57 offspring per plant (0.2 to 1 seedling per fruit receptacle). The other 10 F2 plants were so self-fertile that only small fractions of the harvested seeds could be sown. These 10 F2 plants varied in their degree of self-fertility and produced from 1,200 to 28,000 good seeds per selfed plant, or from 4 to 25 per receptacle. Full self-fertility was therefore recovered within a sample of 22 selected F2 plants. The frequency of more or less self-sterile F2 plants suggests that, in addition to genes that promote self-fertility, others that inhibit it were also present, in contrast with their absence in the first cross.
|FIG. 2.—Foothill x subalpine races of Potentilla. Degrees of correlations between 12 segregating characters in an F2 progeny of approximately 575 individuals. Correlation coefficients above the 5 per cent level (0.09) considered significant.|
Although the two crosses differed in these and other details, they both demonstrated the principle that the parental characters are held together by coherence mechanisms that are balanced against great potentials for recombination. In the foothill x subalpine cross, the correlations between 12 segregating characters were tested, and the findings are depicted in the diagram, Figure 2.
The 12 characters permit 66 possible combinations of pairs of characters, and 38 of these were significantly correlated, whereas the other 28 appeared to be uncorrelated. The correlation coefficients varied between 0.10 and 0.64. These are moderate to fairly strong correlations that permit various degrees of recombination.
Almost all of the characters of the foothill x subalpine cross appear to have significance in relation to the success of the parental races in their native habitats, although at least one obviously appears not to have any such significance. The character notched petals has so far been found in only one deviant subalpine plant in the wild. For this reason, it was thought that this character represented a recent spontaneous mutation. The genotypic analysis indicated, however, that notched petals is regulated by at least three pairs of genes: a dominant but hypostatic gene for notch, and two multiple, epistatic genes that inhibit the phenotypic expression of the notch gene. Although the notch character is so rare, it was found to be partially linked with six other characters (Fig. 2).
No deep significance should be attached to the apparent differences between the degrees of coherence among the foothill x subalpine characters, and the coherences among those of the Coast Range x alpine cross. The observed correlations are resultants of individual linkages between the genes that govern two segregating characters. The correlations shift according to the interactions between oppositional, complementary, and hypostatic genes that regulate them. Linkage between genes of positive effect can, for example, neutralize the effect of linkage between negative genes governing the same two characters. The degree of correlation may therefore vary widely depending upon the particular genotype of the parent individuals used. The exact percentage is therefore less significant than the fact that the characters of races cohere and also recombine.
Selective Effects.—Moderate coherence, such as that observed in the interracial foothill x subalpine cross, has real significance for survival of the segregating progeny under field conditions. This was shown by cloning each of the 575 F2 plants of this cross and planting a part of each at our three altitudinal transplant stations where they were exposed to the effects of contrasting environments, and the responses were studied over a period of years. The stations are: at Stanford, in central California in the oak savanna region near sea level; at Mather, in the yellow pine forest of the Sierra Nevada at 1,400 m; and at Timberline, at the upper reaches of the lodgepole pine forest at 3,000 m near the summit of the Sierra Nevada. The climates of these stations range between the mild Coast Range climate at Stanford having an all-year growing season, to the inhospitable Timberline climate where the growing season is compressed within 2 1/2 late summer months with relatively short days and cold nights. Detailed notes were taken on each plant over a period of 5 years at Stanford, and of 9 years at each of the mountain stations.
SURVIVAL PERCENTAGES OF 509 F2 CLONES OF THE FOOTHILL x SUBALPINE CROSS OF Potentilla
IN RELATION TO INDEX VALUES
|Classes of Index Values|
|Survival percentages||At Timberline
|Number of clones||90||330||89|
|Index values of parents:|
In order to provide a numerical scale for the degree of resemblance of each hybrid plant to its parents, each character of the parental plants, the F1, and all the F2's, was rated in such a way that a low value was assigned to the expression of that character in the subalpine parent, and a high value to its expression in the foothill parent, making allowance for the F2 segregation concerning that character. An index of value of each plant was obtained by adding the values of all 12 characters that were studied. As seen in Table 1, the index value of the subalpine parent was 19, that of the foothill parent was 54, and for the F1 it was 34. The scale made allowance for values from 12 to 64, and those F2 plants of which all characters could be scored varied between 22 and 50.
For the sake of simplification, the index values of the F2 plants were arbitrarily grouped into three classes, each ranging roughly over 10 values. These were: low, 20-29; intermediate, 30-39; and high, 40-50. The index values of the plants of the low class were closest to the value of the subalpine parent, those of the high approached the foothill race, and the intermediate group clustered more or less around the value of the F1 type. These classes are purely numerical, and do not necessarily imply superficial resemblance to a particular parent. It would have been impossible to classify the segregants by sight, because the characters were quite thoroughly shuffled.
Each cloned plant was also classified according to its period of survival at each station. The subalpine parent died repeatedly at the lowland station, but survived during all 9 years at Mather and Timberline. In contrast, the foothill parent survived for the entire experimental period at the lowland and mid-altitude stations, but consistently died promptly at high altitude. The F1 individuals combined the tolerances of both parents and survived indefinitely at all three stations. An F2 plant was considered to be a survivor at a particular station if it survived during the entire. period of the experiment. It was listed as a nonsurvivor if it died once or twice. The percentages of only the survivors are listed in Table 1. The percentages of the nonsurvivors are 100 minus the survival percentage.
Ninety of the 509 fully classified F2 plants are in the subalpine-like 20-29 class, and 75 per cent of the 90 survived all 9 years at the Timberline station, the other 25 per cent being nonsurvivors (Table 1). Sufficient recombinations had occurred among the subalpine-like class so that 18 per cent of this group were able to survive the full 5 years at the lowland station.
At the other extreme, a group of 89 F2 plants had high index values between 40 and 50, statistically approaching the value of the foothill parent. Seventy per cent or more of this group survived at the lowland and mid-altitude stations, but more than 13 per cent of the group had, through recombination, acquired the capacity to survive all 9 years in the extreme climate at Timberline.
The 330 F2 plants having intermediate index values between 30 and 39 tended to follow the foothill parent at the two lower stations, but a fairly large number of individuals, 34 per cent, were able to survive at 3,000 meters.
It is especially significant from the evolutionary point of view that 148 F2 plants (29.7 per cent) were able to survive at all three stations during the entire period. These plants represented a wide range of index values, although the majority belonged to the large intermediate group. None of the parents, nor any of the previously sampled plants from the wild," had demonstrated such a wide range of tolerance.
These data clearly indicate that the physiological traits that enable the subalpine race to survive at high altitudes, and the foothill one at low, cohere strongly with the kind, of characters that can be scored to produce index values. The coherence is nevertheless flexible enough to permit some recombined subalpine-like plants to survive at low altitudes, and some of the foothill-like segregants to live at high altitude. Likewise, the interracial crossings resulted in many recombinations that showed new growth patterns and presumably would be able to fit new niches. Many of these innovations had a widely increased range of tolerance. There is reason to believe that these findings apply to ecological races of many kinds of organisms.
IMPLICATIONS OF COHERENCE
Coherence mechanisms have not been prominent in discussions of evolution, although their existence should have been predictable. For example, at the chemical level, the significance of coherence through chemical bonds has long been recognized, as well as variations represented by the composition and arrangement of side chains. At the highly complex level of the living cell, the structures of DNA and RNA molecules exemplify coherence within molecular chains and variation as different sequences of nucleotides.
Early experimenters, such as Linnaeus,21 Koelreuter,24 and Gärtner,18 knew of the strong coherences that counteract interspecific hybridization. Darwin17 cited the experiments of the latter two authors but doubted the general significance of their findings, because he knew that orchid hybrids and willow hybrids were fertile. Darwin, in general, de-emphasized the distinction between species and varieties and emphasized that natural variation provides the materials for natural selection. Variation is, indeed, the important element in change, but distinct kinds of organisms could not have remained distinct and recognizable throughout geologic epochs without some kind of inherent restraining influence.
Alexis Jordan in 184623 had already shown through many experiments that even local populations of certain plant genera, for example the violets, could be morphologically distinct and developed true from seed. Darwin does not mention Jordan's discovery, but he may have missed its significance because Jordan, following the trend of the time, named such distinct populations species. What Jordan observed was undoubtedly a combination of genetic drift34 and genetic coherence. The coherence systems that exist at the level of the ecological race are carried by the individuals of the race, and as such are also a feature of the local populations of which races are composed.
The finely balanced genetic equilibria that make it possible for an organism to live and to adjust to its environment are so complex that many generations are required to reassemble them after they have been broken down through hybridization. The coherence mechanism protects the continuancy of the balances.
Coherence, combined with natural selection, makes it possible for two ecological races to exist in adjacent but ecologically distinct habitats without losing their identities. Races of Potentilla in moist, cold meadows are contiguous with others on adjacent dry, warm slopes, but they keep distinct although they are interfertile.12,15 So do the adjacent and interfertile annual inland, and perennial maritime sand dune races of Viola tricolor in west Jutland, Denmark.5
The arrangement whereby genes which govern a character are located on separate chromosomes provides a fair degree of resilience in the inheritance of the character, and in the adjustment of the race to minor differences in environment. From the opposite point of view, if the genic controls of characters were closely assembled on single chromosomes, recombination would involve large steps and the coherence-variation equilibrium would become impaired. It should be remembered, however, that genes located remote from each other on a chromosome can function as if they were independent of each other.
Conclusions.—The evidence available from experiments on the inheritance of ecological races indicates that the survivors throughout geologic periods have been those kinds of organisms that possess a fair degree of inherited coherence balanced against potentials of variation that can be made available through interracial crossing. It is the device of coherence that enables ecological races to function as reservoirs for potential variability.
As a result of their dynamic coherence-variation balances, natural entities remain unchanged so long as the environments remain essentially the same. Major changes in the environments cause migrations of races, interracial crossings, release of variability, and changed selective pressures. These events can lead to establishment of new races adjusted to new environments. Certain species have nevertheless been able to retain their basic coherence mechanism relatively unchanged throughout geologic periods, as has been demonstrated, for example, in the chromosomal homology of hybrids between the Old World and New World sycamores of the genus Platanus.15
Once an equilibrium between coherence and variation has become established, it adjusts to all levels of speciation, ranging from the local population to the ecological races, the species, and beyond. A species may age during its evolutionary processes, but as long as it is able to maintain enough local populations, enough ecological races, and maintain an effective balance between coherence and variation within its entities, it is still, in a biological sense, a youthful species:
The restraints of coherence become stronger as evolution proceeds toward entities of higher order. In organisms like Drosophila, Oenothera, and Holocarpha, inversions and other transpositions of segments of chromosomes isolate groups of genes into super-genes. These rearrangements within chromosomes increase coherence and decrease flexibility. At the level of distinct, although closely related, species the genomes have become sufficiently distinct so that interchange of their genes results in unfitness of a major percentage of the hybrid progeny, thereby increasing coherence and decreasing effective recombination, as was discussed under Nicotiana. A further step toward evolutionary separation is the differentiation of whole sets of chromosomes that cohere and can be added but not recombined, a step that often distinguishes subgenera or sections of a genus.
Distinct genera have lost their ability to intercross and may carry vast cores of heredity held together by unbroken coherences. Their component species and ecological races, however, represent many coherence-variation equilibria, and through them they carry vast possibilities for evolution and adjustment to new conditions.