Macroevolution

Speciation

According to Hautmann^[4], 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.^[2] Stephen J. Gould also saw species as the basic unit of macroevolution.^[e]

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.^[29][30]

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.^[31] In addition, some scholars have argued that selection at the species level is important as well.^[32] 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.^[33]

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 (stenobiomic) lineages compared to generalist lineages.^[34]

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.^[35] 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),^[36] lungs (modified swim bladders, e.g. found in fish),^[37][38] or even the heart (a muscularized segment of a vein).^[39]

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.^[40]

Evolutionary faunas

A macroevolutionary benchmark study is Sepkoski's^[41][42] 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.^[43]

Stanley's rule

Macroevolution is driven by differences between species in origination and extinction rates. Remarkably, these two factors are generally positively correlated: taxa that have typically high diversification rates also have high extinction rates. This observation has been described first by Steven Stanley, who attributed it to a variety of ecological factors.^[44] Yet, a positive correlation of origination and extinction rates is also a prediction of the Red Queen hypothesis, which postulates that evolutionary progress (increase in fitness) of any given species causes a decrease in fitness of other species, ultimately driving to extinction those species that do not adapt rapidly enough.^[45] High rates of origination must therefore correlate with high rates of extinction.^[4] Stanley's rule, which applies to almost all taxa and geologic ages, is therefore an indication for a dominant role of biotic interactions in macroevolution.

Evolution of multicellularity

The evolution of multicellular organisms is one of the major breakthroughs in evolution. The first step of converting a unicellular organism into a metazoan (a multicellular organism) is to allow cells to attach to each other. This can be achieved by one or a few mutations. In fact, many bacteria form multicellular assemblies, e.g. cyanobacteria or myxobacteria. Another species of bacteria, Jeongeupia sacculi, form well-ordered sheets of cells, which ultimately develop into a bulbous structure.^[46][47] Similarly, unicellular yeast cells can become multicellular by a single mutation in the ACE2 gene, which causes the cells to form a branched multicellular form.^[48]

Evolution of bat wings

The wings of bats have the same structural elements (bones) as any other five-fingered mammal (see periodicity in limb development). However, the finger bones in bats are dramatically elongated, so the question is how these bones became so long. It has been shown that certain growth factors such as bone morphogenetic proteins (specifically Bmp2) is over expressed so that it stimulates an elongation of certain bones. Genetic changes in the bat genome identified the changes that lead to this phenotype and it has been recapitulated in mice: when specific bat DNA is inserted in the mouse genome, recapitulating these mutations, the bones of mice grow longer.^[49]

Limb loss in lizards and snakes

Snakes evolved from lizards. Phylogenetic analysis shows that snakes are actually nested within the phylogenetic tree of lizards, demonstrating that they have a common ancestor.^[50] This split happened about 180 million years ago and several intermediary fossils are known to document the origin. In fact, limbs have been lost in numerous clades of reptiles, and there are cases of recent limb loss. For instance, the skink genus Lerista has lost limbs in multiple cases, with all possible intermediary steps, that is, there are species which have fully developed limbs, shorter limbs with 5, 4, 3, 2, 1 or no toes at all.^[51]

Human evolution

While human evolution from their primate ancestors did not require massive morphological changes, our brain has sufficiently changed to allow human consciousness and intelligence. While the latter involves relatively minor morphological changes it did result in dramatic changes to brain function.^[52] Thus, macroevolution does not have to be morphological, it can also be functional.

The study of human (brain) evolution benefits from the fact that human and ape genomes are available so that the genomes of our common ancestor can be reconstructed.^[53] Even though the precise genetic mechanisms that shaped the human brain are not known, the mutations involved in human brain evolution are largely known, given that the genes expressed in the brain are relatively well understood.^[54]

Evolution of viviparity in lizards

Most lizards are egg-laying and thus need an environment that is warm enough to incubate their eggs. However, some species have evolved viviparity, that is, they give birth to live young, as almost all mammals do. In several clades of lizards, egg-laying (oviparous) species have evolved into live-bearing ones, apparently with very little genetic change. For instance, a European common lizard, Zootoca vivipara, is viviparous throughout most of its range, but oviparous in the extreme southwest portion.^[55][56] That is, within a single species, a radical change in reproductive behavior has happened. Similar cases are known from South American lizards of the genus Liolaemus which have egg-laying species at lower altitudes, but closely related viviparous species at higher altitudes, suggesting that the switch from oviparous to viviparous reproduction does not require many genetic changes.^[57]