Ch.6 Evolution of Brain and Behavior

Evolution

A gradual changing of one species into another

Indifferent (Evolution)

Change occurs over time regardless if it is helpful or harmful to the species

Evolution attempts to answer 2 questions:

How did species form? How do species change into new ones?

Scale of Nature

The attempt to classify things in nature into categories

Carl Linnaeus

Publishes Systema Naturae: Classification of animals based on similarities

Pre-Darwinian Ideology

Species are created separately. Different functions mean a different plan.

Early evidence for evolution

limb bones from all mammals were similar, regardless of lifestyle

How was evolution derived?

examination of fossils of no longer living species, and some comparisons across ones still in existence

Charles Darwin

Published On the Origin of Species by Means of Natural Selection (1859)

4 Observations of Darwin’s Theory

Reproduction increases population size unless factors limit it

4 Observations of Darwin’s Theory

Individuals in a species are not identical

4 Observations of Darwin’s Theory

Some variation among individuals is due to inheritance

4 Observations of Darwin’s Theory

not all offspring survive to reproduce

Theory of Evolution

Variation among individuals affects the probability of surviving & reproducing, and thus for passing on those characteristics.

Natural Selection

Traits become more or less common based upon differential reproduction over time

Adaptations

Traits which increase the probability of survival of the organism

Fitness

How well a species or member of the species “fits“ into its niche

Homology

features based on common ancestry

Homoplasy

Similar features that evolved separately

Analogy

Similar functions that evolved separately

Gap in Darwin’s Theory

1. Inability to Isolate the source of variation by natural selection (Mendel)

2. The mechanism of inheritance (3D structure of DNA)

Genes/DNA

Source of heritability

Conserved (Genes/DNA)

How unchanged genes are across species

Classification in light of Darwin

Linneaus’s classification now requires a common ancestor

Taxonomy

Classification system of organisms

Phylogeny

evolutionary history of species

Molecular Clock

DNA changes at a similar rate over time regardless of species; can be used to estimate how long ago species diverged from one another.

Dorsophila Melanogaster

Fruit Fly

Why study a fruit fly/nematode worm?

• short generational cycle

• simple nervous system (~20,000 for fly) (302 for worm)

• small, cheap

• few ethical concerns

C. Elegans

Nematode Worm

Aplysia Californicus

Sea Hare

Why study the sea hare?

• simple nervous system (~15,000)

• neurons are large and easy to identify

• few ethical concerns

Why study Rodents?

• similar to humans

• easy for genetic analyses

• cheap to house/breed

• larger nervous system  for neuroscience experiments

• HOWEVER significant ethical concerns

Brain Evolution

Birds with more brains can sing more songs/better at storing food

Brain structures in vertebrates

basically the same, although some regions’ functions have changed/been altered

(ex. superior colliculus responsible for visual processing in lower vertebrates → has become visual reflex center in mammals, while the occipital cortex is more important for visual processing)

Encephalization Factor

Each class’s (mammals) deviation from the slope of the ratio of brain : body weight

Recent Brain Evolution

Cortex size increases with brain size, but not other regions, as the expansion of the cortex and neuron complexity occurs in more recent evolution.

Brain development during gestation/development

regions that develop later become larger, suggests that small changes in genes involved in the later stages of brain development can translate to big changes in the brain.

Why is the human brain so large?

Rapid brain expansion indicates a strong advantage for survival. Likely multiple sources of pressure were likely to have favored bigger brains in hominid brain evolution.

Social Brain Hypothesis

Larger cortex is favored for handling the cognitively complex task of maintaining social relationships.

Sexual selection hypothesis

Natural selection for abilities to attract attention, stimulate, and surprise a potential mate; also provides an evolutionary basis for understanding human traits of humor, art, language, and creativityC

Costs of a large brain

• long gestation; burden on mother and childbirth

• much of brain development occurs following birth, requiring prolonged care

• uses majority of metabolic energy at rest

• complex genetics are required in development, prone to errors leading to behavioral disorders

ASPM gene

Gene specific to brain development. Very different between humans and other animals, and deficiencies in humans leads to severe disability and small brain size.

mRNA differences

gene expression (mRNA) differ more in brain than other tissues