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(s) of a single species. In other words, microevolution is the scale of evolution that is limited to intraspecific (within-species) variation, while macroevolution extends to interspecific (between-species) variation. The evolution of new species (speciation) is an example of macroevolution. This is the common definition for 'macroevolution' used by contemporary scientists.Template:EfnTemplate:EfnTemplate:EfnTemplate:EfnTemplate:EfnTemplate:EfnTemplate:EfnTemplate:EfnTemplate:Efn However, the exact usage of the term has varied throughout history.

Macroevolution addresses the evolution of species and higher taxonomic groups (genera, families, orders, etc) 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 (dubbed the ‘Eclipse of Darwinism’) 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 (1927). 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 believed that the origin of families must require the sudden appearance of new traits which are different in greater magnitude compared to the characters required for the origin of a genus or species. However, this view is no longer consistent with contemporary understanding of evolution. Furthermore, the Linnaean ranks of ‘genus’ (and higher) are not real entities but arbitrary concepts.

The term macroevolution was adopted by Filipchenko's protégé Theodosius Dobzhansky in his book ‘Genetics und the Origin of Species’ (1937) and in The Material Basis of Evolution (1940) 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 (or evo-devo). 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 (see below).

Microevolution vs Macroevolution

There has been considerable debate regarding the connection between microevolution and macroevolution.

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.

Within microevolution, the evolutionary process of changing heritable characteristics (e.g. changes in allele frequencies) is described by population genetics, with mechanisms such as mutation, natural selection, and genetic drift, and speciation (e.g. sympatric and allopatric speciation), phyletic gradualism and punctuated equilibrium. Macroevolution asks how higher taxonomic groups (genera, families, orders, etc) have evolved across geography and vast spans of geological time. Important questions and topics include:

• How different species are related to each other is addressed by phylogenetics.
• The rates of evolutionary change and across time in the fossil record. Why do some groups experience a lot of change while others remain morphologically stable, as in living fossils?
• Mass extinctions and evolutionary diversifications, e.g. the Permian-Triassic and Cretaceous-Paleogene events, the Cambrian Explosion and Cretaceous Terrestrial Revolution.
• Why different taxonomic groups (even in spite of having similar ages) exhibit different survival/extinction rates, species diversity, and/or morphological disparity.
• Long-term trends in evolution, e.g. trends towards complexity or simplicity.
• How species and higher taxa have evolved, e.g. via gene duplication, heterochrony, novelty in evo-devo, facilitated variation, and constructive neutral evolution.

Macroevolutionary processes

Speciation

According to the modern definition, the evolutionary transition from the ancestral to the daughter species is microevolutionary, because it results from selection (or, more generally, sorting) among varying organisms. However, speciation has also a macroevolutionary aspect, because it produces the interspecific variation species selection operates on. Another macroevolutionary aspect of speciation is the rate at which it successfully occurs, analogous to reproductive success in microevolution.

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.

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 (modified limbs), feathers (modified reptile scales), lungs (modified swim bladders, e.g. found in fish), or even the heart (a muscularized segment of a vein).

The same concept applies to the evolution of "novel" tissues. Even fundamental tissues such as bone can evolve from combining existing proteins (collagen) with calcium phosphate (specifically, hydroxy-apatite). This probably happened when certain cells that make collagen also accumulated calcium phosphate to get a proto-bone cell.

Research topics

Subjects studied within macroevolution include:

• Adaptive radiations such as the Cambrian Explosion.
• Changes in biodiversity through time.
• Evo-devo (the connection between evolution and developmental biology)
• Genome evolution, like horizontal gene transfer, genome fusions in endosymbioses, and adaptive changes in genome size.
• Mass extinctions.
• Estimating diversification rates, including rates of speciation and extinction.
• The debate between punctuated equilibrium and gradualism.
• The role of development in shaping evolution, particularly such topics as heterochrony and phenotypic plasticity.

See also

• Extinction event
• Interspecific competition
• Microevolution
• Molecular evolution
• Punctuated equilibrium
• Red Queen hypothesis
• Speciation
• Transitional fossil
• Unit of selection

Further reading

• What is marcroevolution? (pdf) https://onlinelibrary.wiley.com/doi/full/10.1111/pala.12465

External links

• Introduction to macroevolution
• Macroevolution as the common descent of all life
• Macroevolution in the 21st century Macroevolution as an independent discipline.
• Macroevolution FAQ