In the Light of Evolution Volume VI: Brain and Behavior (2013) / Chapter Skim
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6 Cortical Evolution in Mammals: The Bane and Beauty of Phenotypic Variability--Leah A. Krubitzer and Adele M. H. Seelke
6 Cortical Evolution in Mammals: The Bane and Beauty of Phenotypic Variability--Leah A. Krubitzer and Adele M. H. Seelke
Pages 91-112
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From page 91... ...
We examine sources of variability and the constraints that limit the types of changes that evolution has and can produce. Finally, we underscore the importance of how genes and genetic regulatory networks are deployed and interact within an individual, and their relationship to external, physical forces within the environment that shape the ultimate phenotype.
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From page 92... ...
. Survival is based on selective advantages that particular phenotypic characteristics or behaviors confer to some individuals within a given environmental context.
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From page 93... ...
PHENOTYPIC SIMILARITY AND VARIABILITY ACROSS SPECIES There is a general plan of neocortical organization that has been observed in all mammals investigated. This includes a constellation of cortical fields involved in sensory processing, such as primary visual (V1)
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From page 94... ...
. Cortical fields within a sensory domain can also vary, both in their overall size and in the size of the representation (or cortical magnification)
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From page 95... ...
Sensory domain allocation C Relative size of cortical fields D
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From page 96... ...
H Seelke Cortical Magnification A
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From page 97... ...
The fin has undergone several important morphological changes including a transition from bone to soft cartilaginous tissue, elongated digits with additional joints (hyperphalangy) , atrophied triceps, immobilization of most of the joints, and lack of most connective tissue structures (Cooper et al., 2007)
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From page 98... ...
. In species in which the neocortex has been explored and related to such extraordinary morphological specializations, corresponding alterations have been noted, including cortical magnification within sensory
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From page 99... ...
First, hand morphology associated with specialized use covaries with cortical sheet size, cortical field addition, and corticospinal connections. Second, the independent evolution of these striking features of the morphological, behavioral, and cortical phenotype suggests that there are strong constraints on how complex brains and behaviors evolve.
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From page 100... ...
WITHIN-SPECIES VARIABILITY Phenotypic variability within a population is the cornerstone of evolution by natural selection, yet most studies of neural organization and connectivity underscore the similarities across individuals within a group rather than their differences. As a result, there are few studies that directly examine and quantify naturally occurring differences in features of nervous system organization within a species.
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From page 101... ...
. Individual differences have also been observed in smaller units of organization within a cortical field, termed modules.
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From page 102... ...
H Seelke A Mouse motor maps B Macaque area 5 maps area 5 map 1 area 5 map 2 motor map 1 motor map 2 1 mm 500 µm C Squirrel monkey ocular E Owl monkey face dominance columns isomorphs 5 mm S1 face map 1 D Owl monkey hand 1 S1 hand map 2 S1 hand map isomorphs 2 mm S1 face map 2 1 mm FIGURE 6.5 Examples of intraspecies variability for motor cortex in mice (A)
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From page 103... ...
However, examination of certain aspects of organization, such as the size and cellular composition of the primary visual area, are correlated with diel patterns and lifestyle of an animal. These, in turn, are linked to alterations in the visual system, such as the emergence of two-cone color vision and a highly laminated lateral geniculate nucleus in the highly visual, diurnal squirrel [see Campi and Krubitzer (2010)
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From page 104... ...
Stochasticity of gene expression often results in more variable phenotypic characteristics of the individual, whereas robustness of a gene regulatory network often, but not always, results in less variability of a phenotypic characteristic. Not surprisingly, fundamental biological functions, such as the cell cycle, cell growth, and transcription, are generally governed by robust regulatory networks, suggesting that high variability for these key functions is nonadaptive.
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From page 105... ...
. Numerous studies have shown that transcription factors and their downstream target genes covary with aspects of cortical organization, such as cortical field size, location, and connectivity [see O'Leary and Sahara (2008)
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From page 106... ...
This study establishes a clear link between genes, cortical field size, and behavior and demonstrates how alterations in patterns of expression of transcription factors and their downstream targets can generate relatively large degrees of phenotypic variability in the cortex, which in turn generates variability in the target of selection. Genes extrinsic to the neocortex can also affect cortical organization.
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From page 107... ...
. Thus, loss of sensory receptor arrays, loss of sensory-driven activity, or reduced patterns of activity can alter cortical domain allocation, cortical field size, connectivity, and neuronal density.
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From page 108... ...
Environmental Context 108 low high few many few many Wind velocity High energy prey with number of photons distinct auditory emission down up regulation of Gremlin small large poor good decrease increase auditory discrimination interdigit membrane size small large apoptosis of Size of A1 interdigit membrane Targets of selection down up short long Cortical phenotype regulation of of BMPs length of forelimb good poor Developmental visual discrimination process slow Genetic Event fast response time to changes Shallow Steep in air pressure extended restricted small large Slope of Emx2 gradient Size of V1 small large down up Proximal to distal Genetic Event limb growth regulation of fgf8 in small large apical ectodermal ridge Size of S1 Body Shallow Steep Brain extended restricted Slope of Pax 6 gradient Inherited genetic profile/ Current genetic profile/ selected phenotype phenotype Gaussian distribution within a population Optimal phenotypic characteristic within a population Genetic profile that co-varies with phenotype/developmental event
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From page 109... ...
. The light gray shading on the left corresponds to factors associated with the forelimb morphology, and the light gray shading on the right corresponds to factors associated with brain organization.
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From page 110... ...
CONCLUSIONS We have discussed phenotypic variability across and within species and conclude that the ways in which animals and brains change are limited and predictable. Further, we show that a specific characteristic, such as the size of a cortical field, can be generated by different genetic mechanisms and/or activity-dependent mechanisms.
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From page 111... ...
These networks can vary to produce phenotypic change in cortical field size, relative location, and connectivity within individuals in a population. This in turn generates changes in sensory-mediated behaviors, and as in the example above, it is behavior, not genes or features of cortical organization, that are the targets of selection (Fig.
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