Natural Selection and Adaptation

Three Necessary Conditions for Natural Selection:

1. VARIATION • Individuals in population differ in traits • Variation must exist for selection to act upon • Without variation, all individuals identical → no selection possible • Source: mutation, sexual reproduction, recombination

2. HERITABILITY • Variations must be heritable (passed to offspring) • If trait not genetic, cannot evolve • Example: Bodybuilder's muscles not passed to children • Only genetic variation matters for evolution

3. DIFFERENTIAL REPRODUCTIVE SUCCESS • Some variants survive/reproduce better than others • Differential fitness based on traits • "Struggle for existence" - limited resources • Individuals with advantageous traits leave more offspring

Why ALL THREE are Required:

• Variation alone: Differences exist but don't matter • Heritability alone: Traits passed on but all equal fitness • Differential success alone: Some survive better but trait not inherited

Together:

1. Variation provides raw material
2. Heritability allows favorable traits to accumulate
3. Differential success determines which traits increase

→ Result: Evolution by natural selection!

Example - Antibiotic Resistance:

1. Variation: Some bacteria randomly have resistance genes
2. Heritability: Resistance genes passed to offspring
3. Differential success: Resistant bacteria survive antibiotic; sensitive ones die → Population evolves resistance

ADAPTATION (Evolutionary): • Genetic change over many generations • Result of natural selection • Heritable trait that enhances fitness • Population-level change • Permanent (in population) • Takes many generations

Example: Arctic fox white fur • Genetic trait for white coat color • Evolved over thousands of generations • Provides camouflage in snow • Inherited by offspring • Population adapted to arctic environment

Other examples: • Cactus spines (water conservation) • Bird wings (flight) • Antibiotic resistance in bacteria

ACCLIMATION (Physiological): • Phenotypic change within individual's lifetime • Response to environmental conditions • NOT heritable • Individual-level change • Reversible • Occurs within one generation/individual

Example: Tanning from sun exposure • Increased melanin production • Occurs within individual's lifetime • Not passed to offspring genetically • Reverses when sun exposure decreases • Individual acclimates to sunny environment

Other examples: • Red blood cell increase at high altitude • Calluses from physical labor • Muscle growth from exercise • Seasonal coat thickness changes

Key Distinction: • Adaptation = GENETIC, HERITABLE, EVOLUTIONARY • Acclimation = PHENOTYPIC, NON-HERITABLE, PHYSIOLOGICAL

Common misconception: "Giraffes stretched their necks to reach leaves and passed longer necks to offspring" ← This is WRONG! (Lamarckism)

Correct: Giraffes with longer necks (genetic variation) survived better and reproduced more → adaptation

Types of Natural Selection:

1. DIRECTIONAL SELECTION Effect: • Favors one extreme phenotype • Mean shifts in one direction • Variation may decrease initially • Eventually may increase with new mutations

Example: Antibiotic resistance • Before: Range of resistance levels • Selection: Antibiotics kill sensitive bacteria • After: Mean resistance increases • Population shifts toward high resistance

Other examples: • Darker moths during Industrial Revolution • Larger beak size during drought (Darwin's finches) • Pesticide resistance

1. STABILIZING SELECTION Effect: • Favors intermediate phenotypes • Extremes selected against • Mean stays the same • Variation DECREASES • Most common in stable environments

Example: Human birth weight • Babies too small: lower survival (health problems) • Babies too large: difficult delivery, complications • Intermediate weight (7-8 lbs): highest survival • Selection maintains optimal weight

Other examples: • Clutch size in birds (too few or too many eggs = lower fitness) • Body size in many organisms • Number of bristles in Drosophila

1. DISRUPTIVE SELECTION Effect: • Favors BOTH extremes • Intermediate phenotypes selected against • Mean may stay same • Variation INCREASES • Can lead to two distinct groups (bimodal distribution) • May lead to speciation

Example: African seed-cracker finches • Small beaks: efficient at eating soft seeds • Large beaks: efficient at cracking hard seeds • Medium beaks: inefficient at both • Population has two peaks (small and large beaks)

Other examples: • Black-bellied seedcracker bird beak sizes • Darwin's finches on islands with two seed types • Some insect populations with host-plant specialization

Summary Table:

Type | Mean | Variation | Favored Phenotype --------------|---------|-----------|------------------ Directional | Shifts | Decreases*| One extreme Stabilizing | Same | Decreases | Intermediate Disruptive | Same | Increases | Both extremes

*Initially decreases; may increase later with mutations

Sexual Selection: A type of natural selection where traits evolve because they increase mating success, NOT necessarily survival.

Difference from Natural Selection for Survival: • Survival selection: Favors traits that help organism survive • Sexual selection: Favors traits that increase mating success • Can work AGAINST survival (peacock's tail is burden) • Explains "ornaments" and "weapons" that don't aid survival

Two Types:

1. INTRASEXUAL SELECTION ("within sex") Mechanism: • Competition between members of SAME sex (usually males) • Direct competition for mates • "Male-male competition" • Fighting, displays of dominance

Traits favored: • Larger body size • Weapons (antlers, horns, tusks) • Aggressive behavior • Strength, fighting ability

Examples: • Male elephant seals fighting for beach territory and females

• Larger males win fights
• Winners mate with many females
• Size increases over generations

• Male deer antlers

• Used in combat with other males
• Larger antlers = more wins = more mates

• Male dung beetles with horns

1. INTERSEXUAL SELECTION ("between sexes") Mechanism: • Mate choice by one sex (usually females) • "Female choice" • Females select males based on traits • Males compete to be chosen

Traits favored: • Bright colors • Elaborate ornaments • Complex songs/calls • Courtship displays

Examples: • Peacock's tail

• Females prefer males with larger, more colorful tails
• Despite survival cost (predation risk, energy)
• "Good genes" or "runaway selection"

• Bird of paradise elaborate plumage and dance

• Males perform complex displays
• Females choose based on display quality

• Firefly flash patterns • Frog mating calls • Bower bird nest decorations

Why Does Female Choice Evolve?

1. Good genes hypothesis: • Ornaments indicate male health/quality • Females choosing healthy males get "good genes" for offspring

2. Runaway selection: • Positive feedback loop • Females prefer trait → males with trait reproduce more → sons inherit trait AND daughters inherit preference • Escalates even if no survival benefit

3. Sensory bias: • Pre-existing preference in nervous system • Males evolve to exploit this preference

Key Insight: Sexual selection can favor traits that DECREASE survival but INCREASE mating success. Net effect on reproduction is what matters!

Why Natural Selection Cannot Produce Perfection:

Constraints on Evolution:

1. HISTORICAL CONSTRAINTS • Evolution works with existing structures • Cannot start from scratch • "Tinkering" with what already exists

   Example: Human back problems • Spine evolved for horizontal posture (quadrupedal) • Adapted for upright walking but not "designed" for it • Results in back pain, herniated discs • Better design would be different structure, but evolution can't start over

   Example: Recurrent laryngeal nerve in giraffe • Nerve goes from brain down neck to heart and back up • ~15 feet of "unnecessary" nerve • Historical artifact from fish anatomy

2. LACK OF GENETIC VARIATION • Selection can only act on existing variation • If no genetic variation for a trait, no evolution possible • Mutation is random, not directed toward needs

   Example: Antifreeze proteins in fish • Antarctic fish need antifreeze • Had to wait for random mutation to provide variation • Can't evolve trait if genes don't exist

3. TRADE-OFFS AND CONFLICTS • Adaptations for one function may impair another • Finite resources/energy • Pleiotropy (one gene affects multiple traits)

   Example: Peacock tail • Good for mating (sexual selection) • Bad for survival (predation, energy cost) • Compromise between opposing selection pressures

   Example: Reproduction vs. survival • Energy invested in reproduction not available for growth/maintenance • Salmon die after spawning (extreme trade-off)

4. RANDOM EVENTS (GENETIC DRIFT) • Especially in small populations • Beneficial alleles can be lost by chance • Harmful alleles can increase by chance • Overrides selection when drift is strong

5. TIME LAGS • Environment changes faster than evolution • Organisms adapted to past conditions • Always "catching up"

   Example: Human craving for sugar/fat • Adaptive when food scarce • Maladaptive in modern environment (obesity) • Evolution hasn't caught up to abundance

6. PHYSICAL AND CHEMICAL CONSTRAINTS • Laws of physics and chemistry limit possibilities • Cannot violate thermodynamics • Material properties limit design

   Example: Flying organisms • Must be lightweight (limits size) • Largest flying birds near physical limit • Cannot evolve to be arbitrarily large and fly

7. COMPROMISES IN DEVELOPMENT • Developmental programs constrain adult form • All tetrapods have 4 limbs (ancestral constraint) • Could benefit from 6 limbs but development locked in

Key Principle: Evolution produces organisms that are "good enough" to survive and reproduce in their current environment, not perfect organisms. Natural selection is a process of satisficing (satisfactory + sufficing), not optimizing!