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Punctuated Gradualism

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Introduction

Working to understand how species evolve over time is an important area of study within evolutionary biology. This area can be complex considering how much species differ and the difficulty in comparing evolutionary changes. Much of the changes being observed have occurred over millions of years. This means that unique methods have to be used to identify the changes over time. This also means that the understanding of one species can be applied to other species to increase the overall understanding within evolutionary biology.

One hypothesis is that the punctuated equilibrium model most accurately describes species evolution. This model can be described as a species undergoing multiple periods of rapid evolution split around periods of little evolution. A second hypothesis is one that differs from the previous one and can be explained by species that undergo gradual change over long periods of time. This model is known as the phyletic gradualism model. The final hypothesis is a mixture of both hypotheses. This is described as both phyletic gradualism and punctuated equilibrium being present amongst or within species. 

Phyletic Gradualism

Phyletic gradualism is supported by studies done on many different groups of organisms. One study that supports this hypothesis analyzes phylogenetic trees. The study examines the relationships between sister groups and the neighboring ancestors.[1] This study found that the distance between the branches was more significant than the number of branches.[1] This found that the length of the branches could be maintained due to the gradualism of the evolutionary changes.[1] Another way to study this hypothesis is through the use of marine planktonic microfossils. This is due to the abundance of them still present today containing evidence dating back millions of years. One study based on this group focuses on the Eocene time period.[2] The study presented evidence of minor evolutionary change through time.[2] An example of these changes could be small fluctuations in body size rather than big changes rapidly. A second similar study was done on macroperforate planktonic foraminifera. The study showed that the species has remained stable in terms of evolutionary change over millions of years.[3] The study was able to make these conclusions by observing the fossils and the extent of the changes over time.[3] It found that the species had minute changes through time and no big clade events.[3] These are a couple of approaches that were used to support the phyletic gradualism hypothesis.

Dental fossils can also be used to prove the accuracy of a hypothesis. A study looked into the dental records within a group of arvicolid rodents.[4] The study observed the differences between generations over time.[4] They were able to conclude that there was a high level of consistency over time.[4] Another study done on several species of Belemnitella studied several characteristics, some unique to a species and others common amongst the multiple species.[5] The study showed parallels between multiple groups.[5] It found that there were minimal unique evolutionary changes amongst the groups.[5] The studies looked at body changes and found that changes in body shape resulted more from genetics than evolutionary pressures.[5] These are several studies done to support the hypothesis of phyletic gradualism.

Punctuated Equilibrium

The next hypothesis to be considered is that of species evolving under the punctuated equilibrium model. Instead of the gradual changes, there are more drastic, rapid changes. Genomic sequencing is another method that has allowed for an alternate way to test these hypotheses. This is a technological advancement that has enabled the study of a species over time. A study using this method explored Anole lizards.[6] The study showed periods of strong selection.[6] Strong selection during defined times is consistent with the ideas of punctuated equilibrium as strong selection occurring leads to more rapid evolutionary changes. The study also found the importance of structure and behavior in the ways that the organisms interacted and reproduced.[6] This shows the reasoning behind how the species were able to evolve. The genes selected for were passed down more frequently resulting in accelerated evolutionary change. Expanding on the way that the changes in a species have been observed, a study was done on echolocating whales. They were able to create visuals of the whale’s heads and compare the evolution of the head and apply it to likelihood of survival.[7] The study found isolated changes within whale telescoping over time.[7] It found that there were times of great evolution followed by extended periods of a lack of it.[7] This shows that there were likely pressures encouraging evolution.

Other studies used different methods of analyzing data and declaring it significant. A study worked in addressing the difference between marine and terrestrial organisms.  This study used p-values and regression lines to contradict conclusions found in the previous studies of marine plankton.[8] The study identifies that the species developed by the production of individual clades.[8] This study also held importance in highlighting the differences that evidence can present when considering very different species.[8] It states how marine animals can evolve more gradually due to a predicted more stable environment.[8] The contradictions between this study and the other studies previously mentioned shows potentially differences in how data can be analyzed. Another different, but unique approach to understanding punctuated gradualism was done in a study of landscapes and environments. This could hold distinct significance, as this is a big factor upon which selection acts. The study found that the landscapes succumbed to defined changes that effected different species.[9] Identifying changes in environments can lead to reinforcement about how a species may have evolved. 

A study looks at changes in body size. This study concludes that the punctuated equilibrium model is the most likely mechanism for which organisms change.[10] This study looked at both micro-evolutionary patterns and fossil records.[10] The study places importance in considering the alternate gradual hypothesis, but uses quantitative data to make their conclusion about rapid, isolated changes in body type.[10]

Another avenue in which evolutionary characteristics are studied is within cancer research. There are many types of cancer where many similarities and differences can be identified. As with all other studies, consistencies can be found and applied to the many other areas within the greater study of how organisms evolve. One study looked at pancreatic cancer. This study can be directly compared to the progression of other cancers.[11] Pancreatic cancer has been known as a rapidly progressing cancer.  This study examines the punctuated genomic change that results in the rapid progression of this cancer.[11]

Lastly, a study looks at yeasts and uses DNA to support evidence that can be seen on phylogenic trees.[12] The study shows how phylogenetic trees can tell how yeasts evolve and the study concludes that punctuated equilibrium is at play.[12] These studies are evidence for the depth in which the punctuated equilibrium model is supported.

Punctuated Gradualism

Considering these hypotheses, it can be concluded that there is strong evidence supporting both phyletic gradualism and punctuated equilibrium. It can also be concluded that both ideas can be applied within different species. Organisms have a great propensity to adapt and evolve depending on the circumstances, so it can be hard to predict the environments species were exposed to during that time. The result is trying to apply evidence to predict how organisms evolved in the past and comparing it to the present. Not only can phyletic gradualism and punctuated equilibrium apply to different species, both ideas can also apply within species. This can be shown in a study done on the body size evolution in the radiolarian Pseudocubus vema.[13] This study concluded that this group went through periods of alternations between phyletic gradualism and punctuated equilibrium.[13]

One study focuses on how efforts to apply only one hypothesis to a phenomenon can be inaccurate.[14] It supports how difficult it can be to show that only one hypothesis is at play at any given time.[14] Another study also displays the importance of considering both hypotheses. The study supports that there can always be both hypotheses at play at any time.[15] Another related study focuses on the extent of undefined area when trying to compare the two modes of evolution.[16]

Another study expresses the importance of defining what is meant to be identified before research is done. The study directly challenges phyletic gradualism versus punctuated equilibrium and shows how many factors can come into play when comparing the two modes of evolution.[17]

These studies show how there is evidence that supports both punctuated equilibrium and phyletic gradualism. It can be difficult to apply one hypothesis to the majority when there will always be variance in environments. Some environments present challenges that require quick adaptation for survival, while others are relatively stable. In addition, organisms differ in the amount of traits upon which selection can act. This along with replication time can create barriers when working to prove a hypothesis as being accurate.

Other evidence for the inclusion of both hypotheses is the consideration of how organisms relate and may interact. A species that diverged from another over time may still posses that characteristic that they don’t use. One the other hand, the other species might still use that characteristic. The species that doesn’t use the characteristic might begin to use it for an alternate function.[18]

Another avenue in which evolutionary characteristics are studied is within cancer research. There are many types of cancer where many similarities and differences can be identified. As with all other studies, consistencies can be found and applied to the many other areas within the greater study of how organisms evolve. This study compares phenotypic characteristics to genotypic characteristics.[19] The study concludes that genomic analysis supports both models are supported and highlights the importance of studying the relationship between genotype and phenotype.[19]

A similar study also looks at cancer to describe evolutionary change. This study challenges old gradual conclusions and also supports both models using more modern techniques.[20] A study that also studies cancer looks at breast cancer. This study focuses on genome analysis that some of the previous studies expressed the importance of doing.[21] The study highlights how dynamic the body can be during the progression of cancer.[21] The changes can be seen in cancer cells as they can show patters of punctuation and gradualism.[21]

Conclusion

The understanding of how species evolve over time can be interpreted in several different ways. Three hypotheses have been proposed and it has been concluded that phyletic gradualism and punctuated equilibrium of the hypotheses are relevant when considering species evolution. Some organisms are better understood than others and the correlations that can be drawn between species can vary. This is why it’s important to analyze a broad range of species and how they interact. Both of these models are very relevant to history and both methods can be applicable in the future. Working to understand what has happened in the past and what is happening currently leaves researchers in the best place to predict what may happen in the future.

Improvements

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Punctuated Gradualism is a theory in evolutionary biology relating to a mix between theory that species evolve gradually evolving over time and the theory that they evolve rapidly in periods separated by times of little evolution. Working to understand how species evolve over time is an important area of study within evolutionary biology. This means that unique methods have to be used to identify the changes over time.

Two theories are related to punctuated gradualism. One theory, punctuated equilibrium, describes one potential way that species evolve. This model can be described as a species undergoing multiple periods of rapid evolution split around periods of little evolution. A second theory, phyletic gradualism is one that differs from the previous one and can be explained by species that undergo gradual change over long periods of time. Punctuated gradualism can be described as a mixture of these two models stating that some species undergo periods of isolated rapid evolution, while others gradually evolve over time.

There is strong evidence supporting both gradual evolution of a species over time and rapid events of species evolution separated by periods of little evolutionary change. Organisms have a great propensity to adapt and evolve depending on the circumstances. Studies attempt to use evidence to predict how organisms evolved in the past and apply this evidence to the present. Both models of evolution can not only be seen between species, but also within species. This is shown in a study done on the body size evolution in the radiolarian Pseudocubus vema.[13] This study presents evidence of a species exhibiting punctuated and gradual evolution, while also having periods of relative stasis.[13] Another study also used body size and looked at both micro-evolutionary patterns and fossil records.[10] The study uses quantitative data to make conclusions and is an example of another study using body size as an indicator of evolution.[10]

One study focuses on how efforts to apply only one mode of evolution to a phenomenon can be inaccurate.[14] It supports how difficult it can be to show that only one mode of evolution is at play at any given time.[14] Another study also displays the importance of considering both models. The study supports that there can always be both models at play at any time.[15] Another related study focuses on the extent of undefined area when trying to compare the two modes of evolution making it difficult to isolate one model.[15]

There will always be variance in environments. Some environments present challenges that require quick adaptation for survival, while others are relatively stable. In addition, organisms differ in the amount of traits upon which selection can act. This along with replication time can create barriers when working to prove a single mode of evolution as being accurate. A study expresses the importance of defining the clear objectives before research is done. The study directly challenges phyletic gradualism and punctuated equilibrium. It shows how many factors can come into play when comparing the two modes of evolution.[17]

Other evidence for the inclusion of both styles of evolution is the consideration of how organisms relate and may interact. A species that diverged from another over time may still posses that characteristic that they don’t use. On the other hand, the other species might still use that characteristic. The species that doesn’t use the characteristic might begin to use it for an alternate function showing difficulty when trying to track evolution.[18] Fossils do not always show the evolution of function.

Another avenue in which evolutionary characteristics are studied is within cancer research. There are studies on many types of cancer where similarities and differences have been identified. One study compares phenotypic characteristics to genotypic characteristics.[19] The study concludes that genomic analysis supports both models and highlights the importance of studying the genotype, phenotype, and the relationship between the two.[19] One study looked at pancreatic cancer.[11] Pancreatic cancer is a rapidly progressing cancer.  This study examines the punctuated genomic change that results in the rapid progression of this cancer.[11] Cancer studies are compared to analyze modes of evolution.

A similar study also looks at cancer to describe evolutionary change. This study challenges old gradual conclusions and also supports both models using more modern techniques.[20] A study looks at breast cancer. This study focuses on genome analysis that some of the previous studies expressed the importance of doing.[21] The study highlights how dynamic the body can be during the progression of cancer.[21] The changes can be seen in cancer cells as they can show patters of punctuation, gradualism, and relative stasis.[21]

  1. ^ a b c Holman, E. W., & Wichmann, S. Ø. R. E. N. (2017). New evidence from linguistic phylogenetics identifies limits to punctuational change. Systematic biology, 66(4), 604-610.
  2. ^ a b Pearson, P. N., & Ezard, T. H. (2014). Evolution and speciation in the Eocene planktonic foraminifer TurborotaliaEVOLUTION OF TURBOROTALIA. Paleobiology, 40(1), 130-143.
  3. ^ a b c Aze, T., Ezard, T. H., Purvis, A., Coxall, H. K., Stewart, D. R., Wade, B. S., & Pearson,P. N. (2011). A phylogeny of Cenozoic macroperforate planktonic foraminifera from fossil data. Biological Reviews, 86(4), 900-927.
  4. ^ a b c Chaline, J., & Laurin, B. (1986). Phyletic gradualism in a European Plio-Pleistocene Mimomys lineage (Arvicolidae, Rodentia). Paleobiology, 12(2), 203-216.   
  5. ^ a b c d Christensen, W. D., & Hooker, C. A. (2000). Anticipation in autonomous systems: foundations for a theory of embodied agents. Int J Comput Anticip Syst, 5, 135-154.
  6. ^ a b c Tollis, M., Hutchins, E. D., Stapley, J., Rupp, S. M., Eckalbar, W. L., Maayan, I., ... & May, C. M. (2018). Comparative Genomics Reveals Accelerated Evolution in Conserved Pathways during the Diversification of Anole Lizards. Genome biology and evolution, 10(2), 489-506.
  7. ^ a b c Churchill, M., Geisler, J. H., Beatty, B. L., & Goswami, A. (2018). Evolution of cranial telescoping in      echolocating whales (Cetacea: Odontoceti). Evolution, 72(5), 1092-1108.
  8. ^ a b c d Hull, P. M., & Norris, R. D. (2009). Evidence for abrupt speciation in a classic case of gradual evolution. Proceedings of the National Academy of Sciences, pnas-0902887106.
  9. ^ Carrer, F., & Angelucci, D. E. (2018). Continuity and discontinuity in the history of upland pastoral landscapes: the case study of Val Molinac and Val Poré (Val di Sole, Trentino, Eastern Italian Alps). Landscape Research, 43(6), 862-877.   
  10. ^ a b c d e Landis, M. J., & Schraiber, J. G. (2017). Pulsed evolution shaped modern vertebrate body sizes. Proceedings of the National Academy of Sciences, 114(50), 13224-13229.
  11. ^ a b c d Notta, Faiyaz, et al. "A renewed model of pancreatic cancer evolution based on genomic rearrangement patterns." Nature538.7625 (2016): 378.
  12. ^ a b Reuveni, E., & Giuliani, A. (2012). Emergent properties of gene evolution: Species as attractors in phenotypic space. Physica A: Statistical Mechanics and its Applications, 391(4), 1172-1178.
  13. ^ a b c d Hunt, G. (2008). Gradual or pulsed evolution: when should punctuational explanations be preferred?. Paleobiology, 34(3), 360-377.   
  14. ^ a b c d von Vaupel Klein, J. C. (1994). Punctuated equilibria and phyletic gradualism: Even partners can be good friends. Acta Biotheoretica, 42(1), 15-48.
  15. ^ a b c Alters, B. J. (1994). Modeling modes of evolution: Comparing phyletic gradualism & punctuated equilibrium. The American Biology Teacher, 56(6), 354-360.
  16. ^ Ricklefs, R. E. (1980). Phyletic gradualism vs. punctuated equilibrium: applicability of neontological data. Paleobiology, 6(3), 271-275.
  17. ^ a b Johnson, J. G. (1982). Occurrence of phyletic gradualism and punctuated equilibria through geologic time. Journal of Paleontology, 1329-1331.
  18. ^ a b Joly-Lopez, Z., & Bureau, T. E. (2018). Exaptation of transposable element coding sequences. Current opinion in genetics & development, 49, 34-42.
  19. ^ a b c d Graham, T. A., & Sottoriva, A. (2017). Measuring cancer evolution from the genome. The Journal of pathology, 241(2), 183-191.
  20. ^ a b Cross, W. C., Graham, T. A., & Wright, N. A. (2016). New paradigms in clonal evolution: punctuated equilibrium in cancer. The Journal of pathology, 240(2), 126-136.
  21. ^ a b c d e f Sato, F., Saji, S., & Toi, M. (2016). Genomic tumor evolution of breast cancer. Breast cancer, 23(1), 4-11.
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