Brain and Evolution

Brain and Evolution

1. "Evolve"

   • To develop gradually
   • The gradual change in the structure and physiology of plant and animal species as a result of natural selection
   • Traits change through successive generations over time
     • Change in traits of a population

2. Brain Size

   • May play a role in innovative behaviour, tool use and social learning
   • Species with larger brains maybe better able to establish themselves in novel environments
     • Attempts to introduce partridges (smaller relative brain size) into a novel environment for hunting were not successful (animals could not adapt)
     • Attempts to introduce barn owls (larger relative brain size) into a novel environment for pest control were successful (could adapt)
   • Comparisons are best made across related species
   • Characteristics to help humans compete with other species (e.g. colour vision, mastery of fire, bipedalism, linguistic skills) require a larger brain
   • Evolution produced a larger brain with an excess of neural circuits that could be modified with experience
     • Intelligence related to amount of nerve cells able to learn, remember, reason and make plans
       • Primate brains contain more neurons per gram than rodent brains
   • The size of the brain doesn't always increase proportionally with body size

3. Evolution 2-Step Process

   • Hereditary (genetic) variation between individuals
   • Selection made for the genetic variants that will be passed on most effectively to the following generations

4. Hereditary (Genetic) Variation

   • Based on two mechanisms
     • Spontaneous mutation of one variant into another
     • Sexual recombination - re-grouping of maternal and paternal genes during the formation of gametes (sex cells)
       • Leading to new gene combinations in the offspring
   • Alternative genetic variants cause differential reproductive success among individuals
     • Some genetic variants increase the likelihood that some organisms will survive and reproduce more successfully than organisms without those variants

5. Selection

   • May occur as a result of differences in survival, fertility, rate of development, mating success, or any other aspect of the lifestyle of an organism
   • Natural selection is quantified using 'relative fitness' - the probability that a genetic variant will be reproduced and therefore passed onto the next generation
     • Natural selection can be studied by analysing its effects on changing gene frequencies in a population and examining its effects on observable characteristics of individuals in the population
   • Types of Selection
     • Stabilising
       • Decrease in genetic variance
       • Trims both sides of the distribution
       • When natural selection favours an average phenotype and selects against extreme variations - the population stabilises on one particular trait value
     • Directional
       • A single phenotype is favoured
       • Trims the one side of the distribution
       • Allele frequency continuously shifts in one direction
     • Diversifying/ Disruptive
       • Increases genetic variation
       • Trims the middle of the distribution
       • When natural selection selects for 2 or more extreme phenotypes that each have specific advantages
     • Sexual
       • Involve preference for one sex shown by the other sex that exhibit certain traits that yield greater success in securing mates
       • E.g. Peacock's ornate exhibition (despite predator risk) - must be a sign of fitness

6. Brain Regions

   • Brain size and specific components of the brain have been shown to correlate with ecological variables
     • Morphological diversification is highest in those groups with the largest brains
   • There are differences between species relating to proportion of the brain dedicated to certain regions
     • There is structure conserved across species but there is also variability - can be accounted for by individual niches and pressures on different species
       • Cortical representation has evolved to meet species' ecological needs (e.g. bats with echolation due to lack of visual input)
       • Brain size and specialisations correlate with collection, storage and retrieval of food (depending on ecological niches)
         • E.g. Hippocampal volume in food-storing birds for memory and navigation for food storage
         • E.g Olfactory bulb whisker representation in rodents
         • E.g. Auditory representation in bats
     • E.g. mice have larger olfactory bulbs due to their sense of smell being more important to them than vision (as nocturnal creatures) compared to humans
   • Major cortical regions conserved across mammals
     • Somatosensory cortex (S1) - tactile information
     • Primary visual cortex (V1) - thalamic information about vision
     • Primary auditory cortex (A1)
   • Selective breeding can alter cortical physiology - can be modelled in the lab
     • Mice's whiskers are represented in S1 - mice selectively bred (mimicking selection pressures) to grow more whiskers develop additional whisker fields in S1
   • Neocortex volume relative to total brain volume is high in primates compared to insectivores

7. Human Cognitive Development

   • Cooperative breeding (looking after offspring that are not own) is novel in humans
     • Pressure to develop complicated cognitive skills to identify appropriate caregiver
   • Taxonomy: the classification of organisms into a hierarchy of groupings (from general to particular) that reflect evolutionary and morphological relationships
     • Kingdom - Phylum - Class - Order - Family - Genus - Species
   • Cannot comprehend ourselves or our nervous systems until we gain insight into a range of nervous systems (Theodore. H. Bullock)

8. Factors in Brain Evolution

   • Diet
     • Fruit-eating species have larger brains that species with other types of diet (e.g. leaf-eaters)
     • Larger brains may reflect evolution favouring cognitive abilities underpinning exploration (fruit is less predictably distributed than leaves)
     • Lifestyle often correlates with diet therefore it is important to find evidence from different taxonomic groups
   • Home Range Size
     • Will have more complicated tasks to solve
     • However, direction of causation could be that species with larger brains could more easily become established in new locations (decreasing chance of extinction) (Sol & Price, 2007)
   • Social Group Size
     • Larger brains may be selected for as they confer greater reproductive fitness when functioning in a group
     • Complicated social cognitive tasks not present when living alone
     • For increased social acuity/ keeping track of social rank/ ability to manipulate others within the group

9. Mosaic Evolution of Brain Structure

   • Brain structures with functional and anatomical links may evolve together - independent of evolutionary change in other structures
     • Brain components may 'grow together'
       • Revealing functional units (with a common function)
   • Neocortex (associated with intelligence and flexibility of behaviour) varies in size between species - varies even after accounting for scaling with the size of the rest of the brain
     • Substantial taxonomic differences exist (e.g. between primates and insectivores)
       • Independent of overall brain size
       • Cerebellum size correlates with neocortex size in primates
       • Cerebellum size correlates with dencephalon size in insectivores
         • Correlates with neocortex when diencephalon size is excluded (showing 3-way relationship)
     • Correlations found between structures (their volumetric evolution) linked by important functional and anatomical connetctions