Type: Research Essays
Sample donated: Bernice Wells
Last updated: September 24, 2019
Evolutionary biology is a multi- disciplinaryfield with two main aims: understanding the origins and evolution of systemswithin complex organisms and how changes to these systems enable to theevolution of biodiversity at macro evolutionary level (Rolian 2014).
Multiplemethods can be employed to investigate evolutionary processes. Generally, thesecan be divided into two broad categories: bottom-up and top-down approaches. Thebottom-up approach uses genetic and developmental information to understand howthese give rise to the phenotypic characteristics of any given species, and howdifferences in these mechanisms lead to differences among species.
In contrast,integration, modularity and evolvability are top-down approaches, where thephenotype’s characters are used to infer the underlying forces responsible forthem. The two approaches form the basis of two complementary research agendawithin the field of evolutionary biology; the proposed study falls within thesecond category. ‘ Modularity and Integration are two inherently connectedconcepts that have become a central framework within evolutionary biology andrelated fields, providinga unifying theory of organismal structure and variation that can be applied atmultiple hierarchical levels (Klingenberg 2008; Melo et al.
2016, Wagner et al.2007). The concepts are similar to those used in system theory and networkanalysis, conceptualizing biological organisms as a system of parts workingtogether through a series of networks with interrelated elements/traits with varyingdegrees of interconnectedness between those elements/traits. Integration andmodularity represent the connecting building blocks that govern and are an expressionof the organisational pattern present in organisms. Integration refers to thelevel of cohesion, the tendency for traits to be interlinked and consequentlyvary in a coordinated manner, be it due to: pleiotropy – when one geneinfluences multiple phenotypic traits -, shared developmental pathways, and/orthe need for coordinated functionality.
Measurement of trait variation andcovariation have demonstrated that some traits are linked by strongcorrelation, whilst others do not (Goswami and Polly 2010). This heterogeneouspattern in organismal organisation is termed ‘modularity’, where modules are setof traits that are internally integrated by interaction amongst those traitsbut are relatively autonomous from other such modules (Elbe 2005). Bothintegration and modularity are identified through the examination of patterns ofcovariance, from which inner biological mechanism and evolutionary changes tothese mechanisms can be inferred.
Morphological integration and modularity are flexibleconcepts that can be used in different investigative contexts at multiplelevels, although most commonly used for the analysis of standing variation ofadult organisms and the study of developmental processes (Klingenberg 2008).Here, within the proposed research, the integration and modularity framework isused at a macro evolutionary level. At this level, integration manifest itself whenmorphological features evolve in aco-ordinately manner whereas modules can evolve relatively independentfrom one another (Goswami et al. 2014). Currently, two different – although notmutually exclusive – investigativeframeworks have been advanced for examining morphological integration andmodularity: the functional and the developmental model that form the basis ofset hypothesis. Within the functional model, modules are defined as a trait orgroup of traits that collectively serve a primary functional role, withdifferent modules serving different functions (Wagner and Altenberg 1996).Another approach, as advanced by Klingenberg (2008), places developmentalprocesses at the core with integration occurring in traits that sharedevelopmental processes and/or share ancestral origins.
Both the functional andthe developmental model propose a link to the underlying genetic structureresponsible for the actualisation of the coordination, but place a differentfocus on the cause of covariation. By investigating the patterns through thesehypotheses, it is possible to make inferences on the mechanism that generateevolutionary change. Gaining understanding of the integration and modularity patternare also of importance to understand how the organisational pattern may contributeto the evolvability – the ability of an organism to adapt – of that organism. Itis generally held that highly integrated organisms constrain adaptation andtraits evolve together whilst a more modular organisation of an organism tendsto facilitate its adaptive evolution since selection can act relativelyindependently on each module.
(Hansen 2003, Hendrikse et al. 2007, Wagner etal. 2007, Kurantani 2009, Goswame and Polly 2010, Goswami et al. 2014, Goswamiet al. 2015, Huseynov et al. 2017). The organisational pattern is alsosubjected to the forces of evolution, and may itself evolve.
Computationalevolution experiments, for example, where selection pressure is applied tomaximize network performance and minimize connection costs yield networks thatare significantly more modular and more evolvable than control experiments thatonly select for performance (Clune et al. 2013). Identifying an increase inmodularity or documenting a modular restructuring may thus indicate naturaldirectional selective forces being at work.
If directional selection is linked toan increase in modularity, studies by Melo and Marroig (2015) demonstrate that stabilizingselection results in a maintenance of its organisational pattern. The pelvis structure of humans and of our closest relatives,the primates, represents an interesting model system for the study ofmodularity and integration not least since it is within this context that thehuman obligatory bipedalism and rotational childbirth emerged. It pelvis iscomposed of several developmental units (ilium, ischium, pubis, and sacrum; thelatter being another composite structure) representing separatechondrification/ossification regions (Scheuer et al., 2000).
These elementsfuse during ontogeny, requiring developmental coordination. On the other hand,pelvic morphology serves a diversity functional needs also requiringcoordination, including locomotion and birthing. Taken together this results ina complex interplay between evolutionary, functional, developmental andenvironmental factors shaping pelvic morphology and influencing its modularityand integration pattern. Previousworks on the evolutionary role of morphological integration and modularity inprimates have focused almost exclusively on patterns within skull (e.
g. seeCheverud 1982, 1995 , 1996b; Ackermann and Cheverud 2000; Strait 2001; Marroigand Cheverud 2004a,2004b; Hallgrimsson and Lieberman 2008 ; Bastir and Rosas2009 ). In sharp contrast, almost no research has been undertaking toinvestigate the pelvic area and consequently currently there is littleunderstanding about the pattering within the primate pelvic griddle, andtheir potential role in shaping human and non-human primate evolution.
Theproposed research seeks to address this knowledge gap by testing severalhypotheses on morphological integration between the pelvis and the appendicularskeleton, and within the pelvis.