Macroevolution

Macroevolution is evolution on a scale at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes of allele frequencies within a species or population. Macroevolution and microevolution describe fundamentally identical processes on different scales.

The process of speciation may fall within the purview of either, depending on the forces thought to drive it. Paleontology, evolutionary developmental biology, comparative genomics and genomic phylostratigraphy contribute most of the evidence for macroevolution‘s patterns and processes.

Origin of the term

Russian entomologist Yuri Filipchenko first coined the terms “macroevolution” and “microevolution” in 1927 in his German language work, “Variabilität und Variation”. Since the inception of the two terms, their meanings have been revised several times. The term macroevolution fell into a certain disfavour when it was taken over by writers such as the paleontologist Otto Schindewolf to describe their theories of orthogenesis. This was the vitalist belief that organisms evolve in a definite direction due to an internal “driving force”.

Macroevolution includes changes occurring on geological time scales, in contrast to microevolution, which occurs on any time scale.

Early cetaceans like Ambulocetus natans possessed hindlimbs, derived from their walking ancestors, but no longer useful in their marine environment.

Macroevolution and the modern synthesis

Within the modern synthesis of the early 20th century, macroevolution is thought of as the compounded effects of microevolution. Thus, the distinction between micro- and macroevolution is not a fundamental one – the only difference between them is of time and scale. As Ernst W. Mayr observes, “transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species…it is misleading to make a distinction between the causes of micro- and macroevolution”. However, time is not a necessary distinguishing factor – macroevolution can happen without gradual compounding of small changes; whole-genome duplication can result in speciation occurring over a single generation – this is especially common in plants.

Changes in the genes regulating development have also been proposed as being important in producing speciation through large and relatively sudden changes in animals’ morphology.

Types of macroevolution

There are many ways to view macroevolution, for example, by observing changes in the genetics, morphology, taxonomy, ecology, and behavior of organisms, though these are interrelated. Sahney et al. stated the connection as “As taxonomic diversity has increased, there have been incentives for tetrapods to move into new modes of life, where initially resources may seem unlimited, there are few competitors and possible refuge from danger. And as ecological diversity increases, taxa diversify from their ancestors at a much greater rate among faunas with more superior, innovative or more flexible adaptations.”

Molecular evolution occurs through small changes in the molecular or cellular level. Over a long period of time, this can cause big effects on the genetics of organisms. Taxonomic evolution occurs through small changes between populations and then species. Over a long period of time, this can cause big effects on the taxonomy of organisms, with the growth of whole new clades above the species level. Morphological evolution occurs through small changes in the morphology of an organism. Over a long period of time, this can cause big effects on the morphology of major clades. This can be clearly seen in the Cetacea, where throughout the group’s early evolution, hindlimbs were still present. However over millions of years the hindlimbs regressed and became internal.

Abrupt transformations from one biologic system to another, for example the passing of life from water into land or the transition from invertebrates to vertebrates, are rare. Few major biological types have emerged during the evolutionary history of life. When lifeforms take such giant leaps, they meet little to no competition and are able to exploit many available niches, following an adaptive radiation. This can lead to convergent evolution as the empty niches are filled by whichever lifeform encounters them.

Research topics

Subjects studied within macroevolution include:

• Adaptive radiations such as the Cambrian Explosion.
• Changes in biodiversity through time.
• 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.

Adapted from Wikipedia, the free encyclopedia