Learning to Change: Evolution and Behavior

Natural Selection: Theory and Examples

• Definition: Natural selection is the tendency for characteristics that contribute to the survival of a species to persist, and for those that do not to disappear.

• Classic examples:

  • Beak length of finches in the Galápagos Islands (environmental variation favors certain beak sizes).

  • Melanin variation in humans (skin color variation has ecological and social implications).

• Core idea highlighted by the lecture: traits that improve survival and reproduction become more common over generations; deleterious traits tend to be weeded out (though not perfectly or instantly).

Race and Human Variation

• Topic heading: “Many or One Race?” by Nina Jablonski (noted in the transcript).

• Important implication: human variation exists, but the concept of race is debated in biology; the slide indicates a discussion around whether race is a meaningful biological category or a social construct.

When Variations are Maladaptive but Persist: Sickle Cell Example

• Sickle Cell Disease (SCD):

  • Occurs when an individual has two copies of the mutated gene; red blood cells become misshapen and function poorly.

  • Leads to multiple health problems and earlier morbidity; life expectancy improving with new treatments, but early health challenges persist.

  • Question posed: How does this persist under natural selection if it reduces fitness?

• Sickle Cell Trait (SCT):

  • Occurs when a single copy of the mutated gene is present; individuals usually have no symptoms because they have one normal gene and one mutated gene.

  • SCT is common in regions where malaria is prevalent.

  • Protective effect: SCT provides some protection against malaria, a disease that historically caused high mortality.

• Evolutionary insight: the heterozygote advantage maintains the sickle cell allele in populations where malaria is common, illustrating a balance between opposing selective pressures.

• Notation summary:

  • SCD (two mutated alleles) → disease and health complications.

  • SCT (one mutated allele) → generally asymptomatic and malaria-protective.

Learning and Behavior: Evolution on an Individual Scale

• Learning can be viewed as a form of evolution on the scale of an individual life.

• Note from the lecture: learning-related or “bad” behaviors may not be bad in themselves; the semester plans to discuss this further.

• Practical implication: not all adaptive changes are genetic; some are learned during an organism’s lifetime.

Artificial Selection: Humans Breeding for Traits

• Definition: Artificial selection is human intervention in animal or plant reproduction to improve the value or utility of future generations.

• Classic examples mentioned: Brussel sprouts, strawberries (cultivar improvements).

• Key idea: humans can steer the evolutionary trajectory of other species by selecting for desirable traits.

Artificial Selection is Not Exclusive to Humans

• Concept: Even when humans select traits, there is an underlying genetic basis producing those traits.

• Quotation cited in the slide (paraphrased): genetics constrain and enable the responses we see; selection acts on heritable variation.

• The point highlights that selection acts on genetic variation that exists in populations, not on arbitrary outcomes.

Side Note: Breeding in Laboratory Animals

• Many laboratory animals have been bred intentionally for research purposes.

• Example: Sprague-Dawley rats arose from breeding Wistar rats to hybrids of laboratory-derived and wild stocks.

• Implication: our experimental subjects often carry strong artificial selection histories, which can influence behavior and physiology.

Behaviors and Natural Selection: 3 Major Categories

• Innate behaviors with a genetic basis fall into three broad categories:

  • Reflexes

  • Modal Action Patterns (MAP) / instincts

  • General Behavioral Traits (GBT)

• The framework helps explain how some behaviors are universal or species-typical while others are flexible and context-dependent.

Reflexes

• Definition: A reflex is a relationship between a specific stimulus and a simple, automatic response; not the response itself, but the connection between event and response.

• Characteristics:

  • Involves very little variability across individuals.

  • Examples:

  • Patellar reflex: the relationship between tapping the patellar tendon and the consequent movement of the leg.

  • Withdrawal reflex: the link between heat stimulus and pulling away of the hand.

• Key point: reflexes are hard-wired, rapid, and show minimal variation.

Modal Action Patterns (MAP) / Instincts

• Definition: A largely inherited sequence of interrelated acts involving multiple systems, usually elicited by a particular stimulus (the releaser).

• Features:

  • More variability than reflexes, but still limited.

  • Historically termed “instincts.”

• Notable examples:

  • Cuckoo chick kicking out the eggs of Warbler (an interspecific MAP example).

  • Contagious yawning.

  • Grasping response in infancy.

• Controversy: there is debate about whether MAPs exist in humans.

General Behavioral Traits (GBT)

• Definition: General behavioral tendencies strongly influenced by genes; broad behavioral tendencies rather than discrete sequences.

• Examples in animals and humans:

  • In rabbits: “bold” individuals may hoard food but face a higher predation risk; in low-predator environments, boldness may be favored; in high-predator environments, boldness may be selected against.

  • In humans: introversion, general anxiety are cited as GBTs.

• Key properties:

  • Much more variable than MAPs.

  • Not dependent on a single releasing stimulus.

  • Help individuals cope in a wide variety of situations.

The Limits of Natural Selection

• Constraints on what natural selection can do:

  • Cannot protect you from invasive species (e.g., Asian carp invasion).

  • Cannot prevent mass casualties from pandemics (e.g., COVID-19).

  • Cannot insulate you from rapidly changing environments (e.g., Spotted moths changing color).

  • Cannot guard against abrupt genetic changes (e.g., albinism).

Not All Behaviors are Innate: The Role of Learning

• Humans and other organisms must adapt within their lifetimes; learning plays a crucial role in survival when environments or genetics change.

• Learning definition: a change in behavior that results from experience.

• Behavior definition: anything an organism does that can be measured.

• Critical distinction: acquiring a new behavior vs. changing an existing behavior.

A Very Simple Example of Learning: Habituation

• Habituation: a decrease in the intensity or probability of a reflex response after repeated exposure to a stimulus.

• Functional significance: habituation allows organisms to ignore non-threatening repetitive stimuli and focus on signals that matter for survival or opportunity.

• Illustrative example: moving to an apartment behind train tracks leads to adjusted behavior due to experience (reduced startle or annoyance toward train sounds, etc.).

Nature versus Nurture Debate

• A major historical barrier to studying behavior: which is more important in determining behavior—genetics (nature) or experience (nurture)?

• The modern synthesis emphasizes that behavior results from an interaction between genes and environment, not a simple either/or.

Integrated Takeaways: Natural Selection, Learning, and Behavior

• Natural selection favors survival traits that persist and maladaptive ones that disappear, but trade-offs can maintain certain alleles (e.g., sickle cell in malaria regions).

• Artificial selection demonstrates human-driven evolutionary change by choosing desirable traits in crops, pets, and lab animals.

• Innate behaviors (Reflex, MAP, GBT) have different levels of variability and dependence on stimuli, shaping how organisms respond to their environments.

• The limits of natural selection show that evolution cannot perfectly adapt organisms to all future challenges; rapid environmental changes or novel threats require learning and behavioral flexibility.

• Learning and habituation illustrate that significant adaptive changes can occur within an individual’s lifetime, bridging the gap between genetic predispositions and real-world experiences.

• The nature/nurture interplay suggests that behavior is best understood as an integrated product of genetics and environment, with each shaping the other across development.