Macroevolution


Macroevolution comprises the evolutionary processes and patterns which occur at and above the species level. In contrast, microevolution is evolution occurring within the population of a single species. In other words, microevolution is the scale of evolution that is limited to intraspecific variation, while macroevolution extends to interspecific variation. The evolution of new species is an example of macroevolution. This is the common definition for 'macroevolution' used by contemporary scientists. However, the exact usage of the term has varied throughout history.
Macroevolution addresses the evolution of species and higher taxonomic groups and uses evidence from phylogenetics, the fossil record, and molecular biology to answer how different taxonomic groups exhibit different species diversity and/or morphological disparity.

Origin and changing meaning of the term

After Charles Darwin published his book On the Origin of Species in 1859, evolution was widely accepted to be real phenomenon. However, many scientists still disagreed with Darwin that natural selection was the primary mechanism to explain evolution. Prior to the modern synthesis, during the period between the 1880s to the 1930s many scientists argued in favor of alternative explanations. These included 'orthogenesis', and among its proponents was the Russian entomologist Yuri A. Filipchenko.
Filipchenko appears to have been the one who coined the term 'macroevolution' in his book Variabilität und Variation. While introducing the concept, he claimed that the field of genetics is insufficient to explain "the origin of higher systematic units" above the species level.
Filipchenko's also claimed that a new taxon cannot evolve from an older one with a lower rank; e.g. a species cannot evolve into a family. It must originate from a preceding family. Furthermore, the evolution of a new family must require the sudden appearance of new traits which are different in greater magnitude compared to the new traits required for the evolution of a genus or species.
However, Filipchenko's views are not consistent with contemporary understanding of evolution. Furthermore, the Linnaean ranks of 'genus' are not real entities but arbitrary concepts. These traditional taxonomic concepts break down when they are applied to common ancestry.
Nevertheless, Filipchenko’s distinction between microevolution and macroevolution had a major influence on evolutionary biology. The term macroevolution was adopted by Filipchenko's protégé Theodosius Dobzhansky in his book 'Genetics und the Origin of Species', a seminal piece that contributed to the development of the Modern Synthesis. The term was also used by critics of the Modern Synthesis. A good example of this is the book The Material Basis of Evolution by the geneticist Richard Goldschmidt, a close friend of Filipchenko. Goldschmidt suggested saltational evolutionary changes which found a moderate revival in the 'hopeful monster' concept of evolutionary developmental biology. Occasionally such dramatic changes can lead to novel features that survive.
As an alternative to saltational evolution, Dobzhansky suggested that the difference between macroevolution and microevolution reflects essentially a difference in time-scales, and that macroevolutionary changes were simply the sum of microevolutionary changes over geologic time. This view became broadly accepted in the middle of the last century but it has been challenged by a number of scientists who claim that microevolution is necessary but not sufficient to explain macroevolution. This is the decoupled view.

Microevolution vs Macroevolution

Micro- and macroevolution are both supported by overwhelming evidence. This fact remains uncontroversial within the scientific community. However, there has been considerable debate regarding the connection between microevolution and macroevolution.
Broadly speaking, there are two views regarding this issue. The 'Extrapolation' view holds that macroevolution is merely cumulative microevolution. The 'Decoupled' view holds that there are separate macroevolutionary processes that cannot be sufficiently explained by microevolutionary processes alone. Most scientists who adopt the second viewpoint are not claiming that macroevolution is incompatible with microevolution. Rather, they see macroevolution as an autonomous field of study regarding the deep history of life. For this reason, a full understanding of macroevolution requires insights that are not limited to microevolution. An example of this argument has been made by Francisco J. Ayala.
Microevolution is characterized by the evolutionary process of changing heritable characteristics and changes in allele frequencies within populations. This involves mechanisms such as mutation, natural selection, and genetic drift as studied in the field of population genetics. In contrast, macroevolution concerns how species and
and higher taxonomic groups have evolved across geography and vast spans of geological time. For example, whether speciation is sympatric or allopatric; and whether the common mode of macroevolution is better described in terms of phyletic gradualism or punctuated equilibrium. These and other important questions and topics are researched within various scientific fields, which makes the study of macroevolution highly interdisciplinary. Examples of these include:

Speciation

According to Hautmann, speciation has both micro- and macroevolutionary aspects. Specifically, speciation also involves the classic process of descent with modification, i.e. morphological transformation observed across many generations. This is microevolutionary. In contrast, the species variation produced by speciation, and the rate at which it successfully occurs, is macroevolutionary. Stephen J. Gould also saw species as the basic unit of macroevolution.
Speciation is the process in which populations within one species change to an extent at which they become reproductively isolated, that is, they cannot interbreed anymore. However, this classical concept has been challenged and more recently, a phylogenetic or evolutionary species concept has been adopted. Their main criteria for new species is to be diagnosable and monophyletic, that is, they form a clearly defined lineage.
Charles Darwin first discovered that speciation can be extrapolated so that species not only evolve into new species, but also into new genera, families and other groups of animals. In other words, macroevolution is reducible to microevolution through selection of traits over long periods of time. In addition, some scholars have argued that selection at the species level is important as well. The advent of genome sequencing enabled the discovery of gradual genetic changes both during speciation but also across higher taxa. For instance, the evolution of humans from ancestral primates or other mammals can be traced to numerous but individual mutations.
According to the Resource-use hypothesis, the diversification of terrestrial species is closely related to global climatic changes, particularly the Cenozoic alternation of warming and cooling episodes. Global analysis of terrestrial mammals supports the view that these physical environmental changes have shaped macroevolutionary patterns by promoting biome specialisation. This specialization leads to significantly higher rates of vicariance and speciation in biome specialist lineages compared to generalist lineages.

Evolution of new organs and tissues

One of the main questions in evolutionary biology is how new structures evolve, such as new organs. Macroevolution is often thought to require the evolution of structures that are 'completely new'. However, fundamentally novel structures are not necessary for dramatic evolutionary change. As can be seen in vertebrate evolution, most "new" organs are actually not new—they are simply modifications of previously existing organs. For instance, the evolution of mammal diversity in the past 100 million years has not required any major innovation. All of this diversity can be explained by modification of existing organs, such as the evolution of elephant tusks from incisors. Other examples include wings, feathers, lungs, or even the heart.
The same concept applies to the evolution of "novel" tissues. Even fundamental tissues such as bone can evolve from combining existing proteins with calcium phosphate. This probably happened when certain cells that make collagen also accumulated calcium phosphate to get a proto-bone cell.

Examples

Evolutionary faunas

A macroevolutionary benchmark study is Sepkoski's work on marine animal diversity through the Phanerozoic. His iconic diagram of the numbers of marine families from the Cambrian to the Recent illustrates the successive expansion and dwindling of three "evolutionary faunas" that were characterized by differences in origination rates and carrying capacities. Long-term ecological changes and major geological events are postulated to have played crucial roles in shaping these evolutionary faunas.