Kenneth J McNamara. Encyclopedia of Life Sciences: Supplementary Set. Volume 22, Wiley, 2007.
An evolutionary trend can be defined as a persistent, directional change in a character state, or set of character states, resulting in a significant change through time.
What Are Evolutionary Trends?
Evolutionary trends can occur within species (as microevolutionary trends) and supraspecifically (as macroevolutionary trends). The biological, and particularly the palaeontological, literature contains numerous examples. The most celebrated example of a microevolutionary trend is a change in one character state—colour—in the peppered moth, Biston betularia. This shows that there have been directional changes, arising from natural selection, over a 100-year period from the original pale, speckled grey colour, to the black, so-called carbonaria form. The reasons for this are, however, not as clear cut as often portrayed. Originally considered to have occurred in response to increases in pollution in industrial areas, more recently it has been shown that this is an oversimplification and other factors may be involved.
Typical examples of macroevolutionary trends include general trends towards increasing body size in horses throughout the Cenozoic, combined with reduction in digit number; increases in body size and cranial capacity during hominin evolution; trends in changing complexity of suture lines in ammonites; trends of increased body size and reduced appendages in whales; trends in digit loss in ceratopsian dinosaurs; integration of eukaryotic cells to form the first multicellular life, and subsequent increase in size and complexity (McNamara, 1990).
The periods of time over which trends can occur vary from the historical (days to years), in the case of microevolutionary trends, to many millions of years in macroevolutionary trends. For instance, the very large-scale trend of the evolution of organisms from the marine biota, then their adapting to terrestrial habitats, before evolving the ability to fly, occurred over hundreds of millions of years. The longer the period over which the trend occurs, the higher the taxonomic level at which it operates. Thus the shortest-term trends are intraspecific, whereas the longest may be interphylum, or even interkingdom.
Traditionally, two basic patterns of evolutionary trends have been recognized: anagenetic and cladogenetic (McKinney, 1990). Although it has been argued that many so-called anagenetic trends may be little more than accumulated cladogenesis, filtered through a higher-level process of species sorting, most researchers accept the concept of the two patterns. Anagenetic trends are defined as unidirectional changes in a single, nonbranching lineage, involving only one species at a single point in time. Cladogenetic trends consist of directional branching (speciation) events, involving a number of species that are evolving simultaneously. Anagenetic trends are often associated with microevolutionary trends, and cladogenetic ones with macroevolutionary trends.
Most of the evolutionary trends that have been recognized in the fossil record are large-scale. These may be either ‘passive’ or ‘driven’ (McShea, 1994). In passive trends, for example, mean size will be expected to increase if groups originate at a small body size, as there is a natural lower boundary beneath which the organism is physiologically constrained. The extent of variation will therefore be skewed towardslargerbodysize. Such trends will also arise as a natural result of increase in variance (Gould, 1988), resulting in a directional shift in one or a number of traits. Similarly, trends towards increased complexity (which are a likely outcome of increase in size) may be passive. If the first organisms were simple, subsequent evolution could only be towards the more complex.
In a driven system, however, the bias that causes a trend may occur as a result of natural selection and species selection. Two of the more important examples are selectivity in extinctions and predation pressure. In the former, extinction rates may, for example, over a protracted period be higher in species of larger body size.
The resultant overall trend will therefore be towards decrease in body size. Similarly, it has been shown that changing levels of predation pressure (which themselves will influence selectivity of extinction) can affect the directionality of evolution, at both micro-and macroevolutionary levels.
Evolutionary trends are influenced not only by intrinsic changes within the organisms themselves but also by extrinsic factors, most notably the inherent polarities that abound in the physical world in which the species are evolving. Examples of these physical polarities in the marine realm, which are represented by environmental gradients along which evolutionary trends evolve, include the bathymetric gradient, from deep to shallow or vice versa, and the corresponding gradients of low to high levels of hydrodynamic activity and low to high temperatures. Even intrinsically trends exist on very short timescales in the form of ontogenetic gradients. During ontogeny, organisms change in a unidirectional manner in morphology, size and complexity, and sometimes physiologically and behaviourally, all traits that are subject to evolutionary trends on much longer timescales. This reinforces the notion of a close relationship between ontogeny and phylogeny and will be discussed in terms of the generation of evolutionary trends below.
Distinguishing Macroevolutionary from Microevolutionary Trends
Microevolutionary trends are the directional changes that occur within species, whereas macroevolutionary trends are directional evolution above the species level. Microevolution occurs by changes over time within a species due to interactions between natural selection, environment and variations inherent in the gene pool of the species. These can occur by genetic drift, gene flow, mutation, nonrandom mating and natural selection.
Genetic drift can be defined as chance random changes in gene frequency in small populations. This will result in the preservation or extinction of particular genes. Such events, even in small populations, can have profound effects, significantly altering the gene pool of the following generation. Gene flow can also play a role in microevolution. This is the passage of genes typical of one breeding population into the gene pool of another, and occurs when two or more different populations begin to interbreed. Fertile individuals move into or out of a population, transferring gametes between populations. If movement is unidirectional, this can result in a microevolutionary trend.
Another agent of microevolution is mutation, that is, a change in the DNA of an organism, creating a new allele. Although not common, mutations are important because they generate new alleles. As mutations are random in nature, their role in promoting microevolutionary trends will be minimal. One agent of microevolution that is inherently nonrandom, however, is nonrandom mating. An example, is the mating of males and females with a certain phenotypic trait. Unlike gene flow, in nonrandom mating organisms tend to mate with others in close proximity, including closely related partners. A similar effect is assortive mating, where individuals will mate with partners that closely resemble themselves in certain characteristics.
Probably the most significant agent inducing microevolutionary trends is the extrinsic factor of natural selection. This will result in the survival and, hence, reproductive success of individuals or groups best suited to their environment, so leading to the persistence of genetic qualities most appropriate to that particular environment. If the outcome of natural selection is to shift the mean value of variable traits unidirectionally over time, then micro-evolutionary trends will ensue.
A classic example of a microevolutionary trend arising from natural selection has been demonstrated in the Gala´pagos Island finch, Geospiza fortis. This is one of the largest of the finches on the islands, and possesses a large and strong bill. The presence of El Niño conditions in the Pacific Ocean, when ocean temperatures in the equatorial eastern Pacific increase substantially, has had a strong influence on the type of bill that is selected for in this species (Gibbs and Grant, 1987). The cyclical waxing and waning of oceanic temperatures causes a corresponding cyclicity in rainfall. During drought conditions, forms of Geospiza fortis with larger adult body size are preferentially selected. Accompanying larger body size is the possession of a relatively larger bill, arising from the allometric nature of the growth of this structure. These birds are able to feed from large, hard seeds that otherwise would be ignored when smaller, more readily obtainable seeds are present under more amenable conditions. During these periods of low rainfall, the amount of seed set will be reduced, causing increased pressure for the limited resources. As part of the natural range of variation within the species, some individual adult birds will be larger than others and have correspondingly larger bills. It is these individuals that will thrive more during these drought periods, resulting in microevolutionary trends in both body and bill size. Conversely, during periods of higher rainfall, seed biomass will be high and selection will favour smaller birds with smaller bills that can feed on the more abundant, softer seeds. The evolutionary trends will be reversed. Microevolutionary trends have been documented in many other groups from the fossil record, such as foraminifers, radiolarians, mammals and echinoids.
Stanley (1979) has proposed that macroevolutionary trends are generated by analogous mechanisms to those that cause microevolutionary trends. ‘Phylogenetic drift’ is equivalent to genetic drift, ‘directed speciation’ to mutation pressure, and ‘species selection’ to natural selection. While the direction of speciation is inherently random, stochastic fluctuations result in significant net change. Species selection equates to natural selection, but rather than acting on the individual it is concerned with those factors acting on species, such as shifting rates of speciation (rather than reproduction) or variations in longevity of species, based on survival against extinction (rather than survival against death). Whereas the source of variability within populations will be due to mutation/recombination in microevolution, in macroevolutionary trends it is directed speciation.
Macroevolutionary trends have been documented in all groups in the fossil record (McNamara, 1990). These range from interspecific trends in clades, to major trends at the level of families, orders and classes. For example, Jackson and McKinney (1990) investigated trends in bryozoans, corals and coralline algae and found that many extended for tens of millions of years and some even up to 200 million years. They regard many as being ‘progressive trends’, both in terms of morphological and also ecological changes, such as clades moving from near to offshore facies. These trends can be explained as the consequence of species selection, involving escalation in defences employed in biological interactions, or progressive adaptation to routine physical processes, such as wave action. Similar explanations have been proposed for macroevolutionary trends at the species level in brachiopods and echinoids (McNamara, 1997). The major factor influencing trends at this level in echinoids is predation pressure. The same factor explains major trends in the class at the ordinal level.
Evolutionary Trends in Size
Probably the most commonly observed evolutionary trends in the fossil record are those involving body size, in particular the apparent frequency of body size increases. This has been codified as ‘Cope’s rule’ and can be defined as the widespread tendency of animal groups to evolve towards large size. This is most clearly seen in long-term body size trends arising, in part, from Gould’s concept of increase in variance. Such cladogenetic diffusion is asymmetrical to larger size, because it starts at smaller body sizes, so there is ‘nowhere to go but up’.
More debatable is whether single clades show a similar dominance of trends to increased body size. Studies of fossil molluscs have found just as many trends towards smaller size as towards large size, suggesting that these organisms do not show such dominance. However, comparable studies on fossil mammals from throughout the Cenozoic have revealed that, within lineages, descendent species are on average about 9% larger than ancestral species, indicating a possible taxonomic bias.
More detailed studies on equine evolution also show a cladogenetic pattern of an overall evolutionary trend towards increasing size, when viewed over the 55 million year history of the group. However, this was not constant. For the first 30 million years there was virtually no trend to increased body size. Between 25 and 10 million years ago, however there was a rapid, eightfold increase.
When a wide range of groups are analysed for cladogenetic body size trends, they show as many trends for decrease in body size as for increase. Studies focusing on anagenetic trends, though, reveal a preponderance of size increase. However, there is an important scaling phenomenon that affects these results. For time spans of less than one million years, size decrease often occurs with a similar frequency to size increase. Studies spanning more than one million years show a dominance of trends of increase in size (McKinney, 1990).
A number of different processes are thought to generate trends in body size evolution. ‘Diffusion with drift’ is one term used to describe the general cladogenetic ‘diffusion’ away from a small-sized ancestor to larger descendants. Selection pressure promotes size increase. One important factor is predator-prey relationships ‘driving’ evolution. This can be prey species evolving either larger or smaller body size in order to escape from the optimum foraging range of predators. Energy expenditure by the predator becomes a factor when the prey species is large. If more energy has to be expended than is gained by consuming the prey, then the potential prey species has extended beyond the optimum foraging range of the predator. However, if the predator evolves a larger body size, then there is selection pressure on even larger body size in the prey, so driving a trend of increased body size.
Anagenetic trends towards smaller body size relate to ‘crypsis’—the ability to hide from potential predators. Many cladogenetic patterns of reduced size arise from selective extinctions, the preferential extinction of large-bodied mammals during the Pleistocene being a classic example. Trends of reduced size can also arise from selection for certain life history strategies, in particular more ‘r’ selected forms that, among their other features, have smaller body sizes than forms at the ‘K’ end of the ‘r-K continuum’.
In addition to predation pressure, many other biotic and abiotic factors select for body size. Environmental factors promoting larger body size include regularly abundant and low-nutrient food supply, ambient temperatures, seasonality, sex selection and female fecundity (McKinney, 1990). Selection on developmental timing can also affect size trends. Thus in hominin evolution there has been a trend towards delaying the onset of maturity, resulting in the evolution of larger body size. Conversely, examples of selection for earlier maturation in descendent species have been documented in a number of groups, such as trilobites and ammonoids (McNamara, 1990). Directional changes to rates of growth will similarly affect trends in body size. In marine invertebrates, for example, larger body size is often a consequence of faster growth rates.
Large-Scale Trends—Complexity and Progress in Evolution
Any discussion of ‘progress’ in evolution inevitably runs into the concept of ‘orthogenesis’. There was a school of thought prevalent in the late nineteenth and early twentieth centuries that firmly believed that there was an inherent directionality in evolution, with a progression from more simple to more complex patterns of morphology and behaviour. This was called orthogenesis. The implication was that there was some form of inbuilt mechanism driving evolution to bigger and better things. It was even argued that the power of orthogenesis was so strong and so inevitable that evolutionary trends could drag lineages down dead ends, producing characters that led to their extinction. The oft-quoted example of this was the extinct Irish ‘elk’, Megaloceros giganteus. Actually a deer, this cervid possessed huge antlers, up to 3.5 m across, that were considered to have become maladaptive because they became tangled in the branches of trees, resulting in the species’ extinction. More recent research suggests that these structures were in fact eminently adapted to their environment. Although the notion of orthogenesis has been completely discredited, its effect has been to make the concept of evolutionary trends noticeably unpopular, particularly when it comes to discussing trends of increased complexity.
The concept of complexity has frequently been discussed in evolution, but it has rarely been resolved. Evolutionary trends have often been thought to involve an increase in morphological and physiological complexity of the sensory systems in particular. Larger size and greater morphological complexity of the brain, such as in hominin evolution, are therefore viewed as reflecting evolutionary ‘success’. Such a simplistic view that larger means more complex and that morphological complexity equates with evolutionary complexity, and therefore greater evolutionary success, is not necessarily the case. The smallest, morphologically most simple, species can be highly specialized and ecologically complex. Evolutionary trends can also proceed towards the smaller and simpler, as well as towards the larger and more complex.
Defining complexity poses many problems. Is a bat more complex than a bacterium? It has more cells, and more different types of cells, which could be an appropriate criterion to use. More cell types means more specialized functions and increased behavioural complexity. However, when the fossil record has been examined to assess whether or not organisms become more complex through time, the results have been equivocal. For example, the morphological complexity of the vertebral columns of a range of mammals—camels, whales, squirrels, pangolins and chevrotains—over the past 30 million years shows no evidence of trends towards increased complexity (McShea, 1994). Consequently, the scale at which trends are examined can influence the patterns that they show.
The fossil record would appear to show that, over billions of years, evolution proceeded from the simple to the more complex. This occurred firstly by increases in cell complexity, from bacterial prokaryotic cells to eukaryotic cells. This was followed by increases in genome size, then the evolution of multicellularity, and an increase in the number of cells. This was accompanied by an increase in the diversity of cell types, reflected in an increase in anatomical, physiological and behavioural complexity, through simple, jawless fishes to amphibians, reptiles, birds and mammals.
From a broader biological perspective, it has been argued that the major trend in the evolution of life has been towards increased complexity. Maynard Smith and Szathmaŕy (1999), for example, have identified eight major transitions to support this view, presented in chronological order:
- Early replicating molecules to populations of molecules in compartments
- Unlinked, independent replicators to replicator linkage in chromosomes
- The transition from an ‘RNA world’ to a ‘DNA world’ with the origin of the genetic code
- Prokaryotesto eukaryotes
- Asexual clones to sexual populations
- Single-celled protists to multicellularity
- Solitary individuals to colonial organisms
- From primitive societies to the emergence of human societies and the origin of language
Most of these transitions involve trends of increasing nesting of parts in wholes, or of hierarchical complexity, with trends to the emergence of new levels of organization. Each is a series of chance events, or preadapted situations favoured by short-term, selective advantage.
Mechanisms Generating Evolutionary Trends
Over the last two hundred years a wide range of proposals have been made to explain the underlying mechanisms that have driven evolutionary trends, such as that complexity increased owing to the activity of invisible fluids within organisms; that selection favoured increases in the ability of organisms to modify the environment; or that organisms have become more ‘energy intensive’ through time. Factors that have been thought to have contributed to large-scale evolutionary trends include complexity, size, adaptiveness, entropy, energy intensiveness, evolutionary versatility and developmental factors, in particular heterochrony. The first two, complexity and size, have been discussed above in their respective sections.
One of the more enigmatic trends at higher taxonomic levels is increased adaptiveness, or fitness. If an environment deteriorates directionally, then individuals’ levels of adaptiveness within a species should increase. Such correspondence between environmental gradients and morphological or adaptive gradients commonly results in evolutionary trends (McNamara, 1990). However, if environmental changes are too complex, then adaptations may not be cumulative, or directional. One problem in formulating such adaptive trends is how to assess changing levels of adaptiveness over time. Features that could be used include decreasing chances of becoming extinct, or replacement of one species by another.
Change in a closed system not in equilibrium will tend towards increasing the amount of disorder, or entropy, of the system. It has been argued that evolution can be viewed as a form of entropically driven production, resulting in splitting and branching of taxa (cladogenesis) and their diffusion into multidimensional descriptive species (McShea, 1998). Thus, it would be expected that a group of repeated taxa (a clade) would grow and diffuse throughout its genotypic space. The component species will diverge from each other, increasing the entropy of the clade. In following the second law of thermodynamics, such a trend would be intrinsically driven, and could eventuate in the absence of selection.
It has been suggested (Vermeij, 1987) that the extent of organisms’ growth and reproduction is limited only by their ability to locate, consume and defend resources (to eat or be eaten). Limiting factors in their ability to do this are competitors, predators and dangerous prey. In such a system, evolutionary changes that are favoured will be those that improve the organisms’ ability to attack, by the evolution of more sophisticated weapons or locomotory ability, or to escape, using more effective defence techniques. These involve crypsis, toxicity, information gathering and processing systems, growth and metabolic rate. To enable these changes to occur, an increase in energy flow into the organism must occur, or an increase in its energy intensiveness. Vermeij has termed this ‘escalation’.
Increases in energy intensiveness will probably occur where resources are greater and there has been a relaxation of constraints. Vermeij (1987) has identified two periods, one in the early Palaeozoic, the other in the late Mesozoic, when nutrient and energy levels in the biosphere were increased by submarine volcanism. This, he argues, was the driving force behind increases in evolutionary innovation and diversity.
This has been defined as ‘a function of the number of degrees of interdependence in development, or number of independent dimensions along which variation can occur in evolution’ (McShea, 1998). Evolutionary versatility is likely to increase in evolution, as it increases the extent of possible adaptive strategies, so improving functional effciency. It will be more likely to occur when energy resources are high, adaptive constraints are reduced and selection for energy intensiveness is greatest.
Major evolutionary changes in a trend are often constrained by morphological and functional trade-offs, with one structure improving at the expense of another. Many such trade-offs have a developmental basis (McNamara, 1997). Examples include increase in body and limb size in ratite birds at the expense of wings, and increase in body size and brain size in hominins at the expense of jaw and gut size and complexity. These have all arisen from heterochrony.
Despite the spurious and misleading claim made elsewhere in this encyclopedia (see the article ‘Heterochrony’) that ‘heterochrony may have outlived its usefulness’, there is a vast biological and palaeontological literature that supports the notion that heterochrony is a fundamental aspect of evolution (McNamara, 1997). It has been argued that heterochrony plays an important role in evolutionary trends (McNamara, 1990), including both anagenetic and cladogenetic trends, and both micro-and macroevolutionary trends. This relationship between evolutionary trends and heterochrony arises because evolutionary trends are, like ontogenetic trajectories, unidirectional. However, for trends to develop, in addition to the intrinsic factor of heterochrony, extrinsic factors are also critical. Selection of either progressively more paedomorphic or more peramorphic traits must take place along an environmental gradient, such as in the aquatic environment from deep to shallow water, or from coarse to fine-grained sediments.
An evolutionary trend from ancestors to descendants that show increasingly more paedomorphic characters is called a ‘paedomorphocline’. If the trend shows increasing peramorphic descendants, it is called a ‘peramorphocline’ (McNamara, 1990). Collectively these are called hetero-chronoclines. The driving force behind heterochronoclines is often competition or predation pressure. With the induction of a heterochronocline by competition, the persistence of the ancestral form constrains selection to one direction, along an environmental gradient away from the ancestral species. The resultant phylogenetic pattern will be one of cladogenesis. Selection caused by predation pressure will induce an anagenetic heterochronocline. Such trends have been described in taxa such as spatangoid echinoids, where predation pressure has resulted in heterochronoclines evolving along an environmental gradient from coarse to fine-grained sediments, reflecting, perhaps, shallow to deep water (McNamara, 1997). An example of a cladogenetic paedomorphocline is the Cenozoic brachiopod lineage of Tegulorhynchia–Notosaria (McNamara, 1990). This developed along an environmental gradient of deep to shallow water. Many examples of heterochronoclines have also been described in ammonoids (McNamara, 1990).