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Understanding the evolution of Mammalian brain structures; the need for a (new) cerebrotype approach.

Willemet R - Brain Sci (2012)

Bottom Line: In other taxa, no clear pattern is found, reflecting heterogeneity of the species' lifestyles.These results suggest that the evolution of brain size and composition depends on the complex interplay between selection pressures and constraints that have changed constantly during mammalian evolution.Because it forms homogenous groups of species within this complex "space" of constraints and selection pressures, the cerebrotype approach developed here could constitute an adequate level of analysis for evo-devo studies, and by extension, for a wide range of disciplines related to brain evolution.

View Article: PubMed Central - PubMed

Affiliation: 105 Chemin de la Salade Ponsan, Toulouse 31400, France. r.willemet@gmx.com.

ABSTRACT
The mammalian brain varies in size by a factor of 100,000 and is composed of anatomically and functionally distinct structures. Theoretically, the manner in which brain composition can evolve is limited, ranging from highly modular ("mosaic evolution") to coordinated changes in brain structure size ("concerted evolution") or anything between these two extremes. There is a debate about the relative importance of these distinct evolutionary trends. It is shown here that the presence of taxa-specific allometric relationships between brain structures makes a taxa-specific approach obligatory. In some taxa, the evolution of the size of brain structures follows a unique, coordinated pattern, which, in addition to other characteristics at different anatomical levels, defines what has been called here a "taxon cerebrotype". In other taxa, no clear pattern is found, reflecting heterogeneity of the species' lifestyles. These results suggest that the evolution of brain size and composition depends on the complex interplay between selection pressures and constraints that have changed constantly during mammalian evolution. Therefore the variability in brain composition between species should not be considered as deviations from the normal, concerted mammalian trend, but in taxa and species-specific versions of the mammalian brain. Because it forms homogenous groups of species within this complex "space" of constraints and selection pressures, the cerebrotype approach developed here could constitute an adequate level of analysis for evo-devo studies, and by extension, for a wide range of disciplines related to brain evolution.

No MeSH data available.


Related in: MedlinePlus

Phylogenetic Principal Component Analysis of prosimians’ brain structure proportions. Regression on independent contrasts of logged absolute brain size onto independent contrasts of position on PC1: Multiple R-squared: 0.489, F-statistic: 13.39 on 1 and 14 DF, P = 0.002575; PC2: Multiple R-squared: 0.143, F-statistic: 2.327 on 1 and 14 DF, P = 0.1494. Families are: black: Cheirogaleidae, red: Daubentoniidae, green: Galagidae, light blue: Lemuridae, dark blue: Indriidae, purple: Lepilemuridae, yellow: Lorisidae.
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brainsci-02-00203-f004: Phylogenetic Principal Component Analysis of prosimians’ brain structure proportions. Regression on independent contrasts of logged absolute brain size onto independent contrasts of position on PC1: Multiple R-squared: 0.489, F-statistic: 13.39 on 1 and 14 DF, P = 0.002575; PC2: Multiple R-squared: 0.143, F-statistic: 2.327 on 1 and 14 DF, P = 0.1494. Families are: black: Cheirogaleidae, red: Daubentoniidae, green: Galagidae, light blue: Lemuridae, dark blue: Indriidae, purple: Lepilemuridae, yellow: Lorisidae.

Mentions: The preceding section suggests that mammalian brains evolve in a characteristic manner between taxa. This section further examines this issue by using multivariate analyses. Several variables can be used in multivariate analysis for examining brains of different species. Using absolute volume can provide an insight into the allometric relationships between brain structures (see [20]), as can using absolute volume in regression analysis (Figure 1 and Table 1). The use of relative proportions of brain structures (see [12]) can be particularly appropriate in analyzing precise specific adaptations in brain structure size, because it considers structure sizes independently of brain size. Finally, using absolute proportions is an efficient way to characterize species brain composition [15]. Each method is useful and possesses its own advantages, and we can expect a lot when these three methods are used simultaneously in future detailed analyses of cerebrotypes. Using absolute proportions has the advantage of producing results directly comparable with those from Clark et al. [15]. The following analysis represents the evolution of the brains inside taxa by using three dimensions phylogenetic principal component analysis on brain structure proportions (3D Phylogenetic Principal Component Analyses (PPCA), Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Table 2, Supplementary Figure S1–S4 and Table S1). The variations of the slopes observed in Table 1 suggest that in most of the taxa, the relative size of each structure could vary with brain size. For this reason, correlations between positions of the species on the two firsts principal components have been calculated and are indicated in the figures legends. Also, in order to graphically represent the evolution of brain composition in the different taxa, a third dimension (symbol size) representing species’ relative brain size compared to the other species has been added to the graphs (see Methods).


Understanding the evolution of Mammalian brain structures; the need for a (new) cerebrotype approach.

Willemet R - Brain Sci (2012)

Phylogenetic Principal Component Analysis of prosimians’ brain structure proportions. Regression on independent contrasts of logged absolute brain size onto independent contrasts of position on PC1: Multiple R-squared: 0.489, F-statistic: 13.39 on 1 and 14 DF, P = 0.002575; PC2: Multiple R-squared: 0.143, F-statistic: 2.327 on 1 and 14 DF, P = 0.1494. Families are: black: Cheirogaleidae, red: Daubentoniidae, green: Galagidae, light blue: Lemuridae, dark blue: Indriidae, purple: Lepilemuridae, yellow: Lorisidae.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4061787&req=5

brainsci-02-00203-f004: Phylogenetic Principal Component Analysis of prosimians’ brain structure proportions. Regression on independent contrasts of logged absolute brain size onto independent contrasts of position on PC1: Multiple R-squared: 0.489, F-statistic: 13.39 on 1 and 14 DF, P = 0.002575; PC2: Multiple R-squared: 0.143, F-statistic: 2.327 on 1 and 14 DF, P = 0.1494. Families are: black: Cheirogaleidae, red: Daubentoniidae, green: Galagidae, light blue: Lemuridae, dark blue: Indriidae, purple: Lepilemuridae, yellow: Lorisidae.
Mentions: The preceding section suggests that mammalian brains evolve in a characteristic manner between taxa. This section further examines this issue by using multivariate analyses. Several variables can be used in multivariate analysis for examining brains of different species. Using absolute volume can provide an insight into the allometric relationships between brain structures (see [20]), as can using absolute volume in regression analysis (Figure 1 and Table 1). The use of relative proportions of brain structures (see [12]) can be particularly appropriate in analyzing precise specific adaptations in brain structure size, because it considers structure sizes independently of brain size. Finally, using absolute proportions is an efficient way to characterize species brain composition [15]. Each method is useful and possesses its own advantages, and we can expect a lot when these three methods are used simultaneously in future detailed analyses of cerebrotypes. Using absolute proportions has the advantage of producing results directly comparable with those from Clark et al. [15]. The following analysis represents the evolution of the brains inside taxa by using three dimensions phylogenetic principal component analysis on brain structure proportions (3D Phylogenetic Principal Component Analyses (PPCA), Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Table 2, Supplementary Figure S1–S4 and Table S1). The variations of the slopes observed in Table 1 suggest that in most of the taxa, the relative size of each structure could vary with brain size. For this reason, correlations between positions of the species on the two firsts principal components have been calculated and are indicated in the figures legends. Also, in order to graphically represent the evolution of brain composition in the different taxa, a third dimension (symbol size) representing species’ relative brain size compared to the other species has been added to the graphs (see Methods).

Bottom Line: In other taxa, no clear pattern is found, reflecting heterogeneity of the species' lifestyles.These results suggest that the evolution of brain size and composition depends on the complex interplay between selection pressures and constraints that have changed constantly during mammalian evolution.Because it forms homogenous groups of species within this complex "space" of constraints and selection pressures, the cerebrotype approach developed here could constitute an adequate level of analysis for evo-devo studies, and by extension, for a wide range of disciplines related to brain evolution.

View Article: PubMed Central - PubMed

Affiliation: 105 Chemin de la Salade Ponsan, Toulouse 31400, France. r.willemet@gmx.com.

ABSTRACT
The mammalian brain varies in size by a factor of 100,000 and is composed of anatomically and functionally distinct structures. Theoretically, the manner in which brain composition can evolve is limited, ranging from highly modular ("mosaic evolution") to coordinated changes in brain structure size ("concerted evolution") or anything between these two extremes. There is a debate about the relative importance of these distinct evolutionary trends. It is shown here that the presence of taxa-specific allometric relationships between brain structures makes a taxa-specific approach obligatory. In some taxa, the evolution of the size of brain structures follows a unique, coordinated pattern, which, in addition to other characteristics at different anatomical levels, defines what has been called here a "taxon cerebrotype". In other taxa, no clear pattern is found, reflecting heterogeneity of the species' lifestyles. These results suggest that the evolution of brain size and composition depends on the complex interplay between selection pressures and constraints that have changed constantly during mammalian evolution. Therefore the variability in brain composition between species should not be considered as deviations from the normal, concerted mammalian trend, but in taxa and species-specific versions of the mammalian brain. Because it forms homogenous groups of species within this complex "space" of constraints and selection pressures, the cerebrotype approach developed here could constitute an adequate level of analysis for evo-devo studies, and by extension, for a wide range of disciplines related to brain evolution.

No MeSH data available.


Related in: MedlinePlus