Evolution of Brain and Behavior
Evolution of Brain and Behavior (Ch. 6)
Objective: Understanding the evolution of brain and cognitive functions.
Notably, despite similarities with nearest ancestors, human brain size has increased rapidly in recent history.
Evolution provides insights into the study of different species.
Evolution - Definition and Concepts
Definition of Evolution: A gradual process where species undergo changes over time, leading to the emergence of new species.
This process is indifferent to whether the changes are beneficial or detrimental for the species.
Key Questions Addressed by Evolution:
How did species form?
How do species change into new ones?
The Scale of Nature
Historical attempts to classify and organize living organisms into categories.
Reference to Striedter GF (2005) on principles of brain evolution.
Classification of Species
Carl Linnaeus (1730s): Published Systema Naturae, which categorizes animals based on shared characteristics.
Hierarchical Classification of Living Organisms:
Species: Basic unit of classification; population or set of organisms capable of interbreeding.
Example: Domestic dog as Canis familiaris (around 400 breeds within this species).
Genus (plural: genera): Group of similar, closely related species.
Example: Canis includes dogs, wolves, coyotes, and jackals (approximately 8 species).
Family: A group of related genera.
Example: Family Canidae includes dogs, foxes (about 35 species).
Order: A group of related families.
Example: Order Carnivora includes species like dogs and cats (approximately 235 species).
Class: A group of related orders.
Example: Class Mammalia contains mammals (approximately 4300 species).
Phylum (plural: phyla): A group of related classes.
Example: Phylum Chordata encompasses vertebrates (approximately 40,000 species).
Kingdom: The broadest classification, including major groups of life.
Example: Kingdom Animalia (approximately 1 million known species); total species estimated up to 30 million.
Pre-Darwinian Ideas about Species Origin
Prior to the theory of evolution in the mid-1800s, belief existed that species were created separately.
The idea suggested that species would have different structures according to their locomotion needs, yet similarities were observed among mammalian limb structures.
Natural Observations Leading to Evolutionary Theory
Emerging ideas from naturalists noticed similarities in limb bones across different mammalian species.
Geological observations revealed the Earth's changes over millions of years; fossil discoveries provided evidence supporting evolution.
Understanding Species Formation and Change
Understanding Species Formation: Derived from fossil examination and cross-comparisons among extinct and extant species.
Paleontology: The scientific study of fossils.
Species Change Mechanism:
Charles Darwin's Research: His significant work in the Galapagos Islands led to the publication of On the Origin of Species by Means of Natural Selection (1859).
Alfred Russell Wallace: Conducted pivotal observational work in Indonesia.
Darwin's Observations of Evolution
Darwin's theory hinged upon four critical observations:
Reproduction can increase population size unless limited by external factors.
Individuals within a species exhibit variations.
Some variations are inherited.
Not all offspring manage to survive and reproduce.
The Role of Natural Selection
Definition of Natural Selection: The process where certain traits become more or less common based on differential reproduction rates.
Adaptations: Traits enhancing the likelihood of survival.
Fitness: Effectiveness of a species or individual in thriving within its ecological niche.
Convergence in Evolutionary Traits
Similar traits can develop independently in different species (e.g., birds and bats' flight).
Homology: Features derived from a common ancestor.
Homoplasy: Similar features arising separately due to environmental adaptation (e.g., aquatic adaptations in dolphins vs. fish).
Analogy: Traits that serve similar functions without implying evolutionary connection—caution against confusion with homology.
Genetic Heritability and Evolution
A critical gap in Darwin's theory was the hereditary mechanism underlying variations.
Mechanism of Inheritance: Genes for traits are encoded in DNA.
Initiated by Gregor Mendel’s laws of inheritance, culminating with Watson and Crick's discovery of DNA structure (1950).
Genetic Makeup:
Humans possess 23 pairs of chromosomes.
Some genes remain highly conserved across species; others vary significantly.
Despite phenotypical differences, genetic commonalities exist among organisms.
Natural Selection at the Genetic Level
Process:
Mutations generate variations.
Unfavorable mutations are selected against and diminish reproductive success.
Favorable mutations thrive and spread throughout populations.
Evolutionary Taxonomy in Light of Darwin
With Darwin's theory, Linnaeus's system now accounts for common ancestry among species.
Breakdown of the classification structure based on common relations.
Modern Classification Methods
Taxonomy: System for classifying organisms.
Phylogeny: The evolutionary history of species.
Modern genetics aids in classifying based on phylogenetic closeness.
Estimating Divergence Through DNA Analysis
DNA changes at a consistent rate over time, serving as a “molecular clock.”
Genomic differences can estimate the time of divergence from a common ancestor.
Important Notes on Evolution
Evolution is non-linear and not directed towards a goal.
Successful traits are contingent on selective pressures that evolve over time.
Misapplied terms such as “primitive” and “advanced” misrepresent all species' evolution.
Evolutionary changes occur over long timescales, often spanning hundreds of thousands to millions of years.
The Value of Studying Other Species
Comparative research yields insight into evolution.
Reasons to focus on specific species include:
Unique traits
Accessibility
Comparative analysis
Preservation in the wild
Economic implications
Disease treatment relevance
Model Organisms in Evolutionary Research
Drosophila Melanogaster (Fruit Fly):
Advantages:
Genetic analysis ease
Quick generational turnover
Simple nervous system (~20,000 neurons)
Cost-effective and minimal ethical concerns.
C. elegans (Nematode Worm):
Advantages:
Similar ease of genetic analysis and generational cycle as fruit flies
Even simpler nervous system (302 neurons).
Aplysia Californicus (Sea Hare):
Advantages:
Large neurons and simplicity in identifying them with few ethical concerns.
Rodents:
Advantages:
Close relation to humans (both being vertebrates and mammals).
Facilitation of genetic analysis and reasonably affordable housing and breeding.
Larger nervous systems advantageous for neuroscience research (especially in rats).
Significant ethical concerns.
Understanding Brain Evolution
Comparative analysis of brain structures across species helps highlight evolution.
Amount of brain dedicated to specific structures aligns with functional importance.
Specific Examples of Brain Function and Structure Correlation
Warblers: Brain size of a region (HVC) correlates with song variety.
Food Storing: Birds exhibiting larger hippocampal volumes show enhanced spatial memory reflective of their food-storing behavior.
Sensory Adaptation: Whisker representation in rat cortex is larger compared to that in squirrels, while the platypus has adaptations for its unique bill, reflecting sensory processing needs.
Homologies in Vertebrate Brain Architecture
Across vertebrates, several structural similarities exist:
Derived from a hollow dorsal neural tube.
Bilateral symmetry and segmentation seen in all.
Hierarchical control systems, distinct systems, and function localization are maintained.
Comparative Neural Structures in Vertebrates
Evidence highlights basic characteristics shared among vertebrates, with complexity increasing in higher species.
Analysis of Vertebrate Brain Complexity
Changes in function among brain regions across evolution:
Example: Superior colliculus transitions from visual processing in lower vertebrates to a reflex center in mammals.
Cortex Layers: Mammals typically feature a mix of allo- and neocortex, with more complex mammals showing a higher proportion of neocortex (>50%).
Reptiles possess a simpler 3-layered cortex.
Brain Size vs. Intelligence Correlation
Consideration of brain size must also factor in body size to assess intelligence accurately.