Core Concepts in Evolutionary Biology

Species Concepts

• Biological Species Concept (BSC)

  Species are groups of interbreeding natural populations that are reproductively isolated from other groups. This means no viable, fertile offspring with members of other species.

• Phylogenetic Species Concept

  A species is the smallest group that shares a common ancestor and can be distinguished by unique traits.

• Ecological Species Concept

  Defines a species by the ecological niche it occupies, including interactions with the environment and other species.

• General Lineage Concept

  Considers species as independently evolving metapopulations.

Isolating Barriers

• Geographic (Extrinsic) Barriers

  Landscape features physically prevent gene flow (e.g., mountain ranges, oceans).

• Reproductive (Intrinsic) Barriers

  Biological traits prevent successful interbreeding (e.g., behavior, gamete incompatibility).

Reproductive Barriers

These are intrinsic biological traits that prevent successful interbreeding.

• Premating Barriers

  Examples include behavioral isolation, ecological separation, and mechanical incompatibility.

• Postmating-Prezygotic Barriers

  Gametic incompatibility, where sperm or pollen cannot fertilize the egg.

• Postzygotic Barriers

  Hybrids form but have low fitness (e.g., sterile mules or inviable offspring).

Modes of Speciation

• Allopatric Speciation

  Geographic separation leads to divergence and reproductive isolation.

• Parapatric Speciation

  Adjacent populations diverge with limited gene flow.

• Sympatric Speciation

  Speciation occurs within a shared habitat, usually through behavioral or ecological isolation.

Key Speciation Concepts

• Reinforcement

  Selection favors traits that increase prezygotic isolation to avoid costly hybrids.

• Ecological Speciation

  Reproductive isolation arises due to adaptation to different environments.

• Speed of Speciation

  Can be rapid in plants (e.g., via polyploidy) or slow in mammals.

Coevolution Defined

Coevolution is reciprocal evolutionary change between species that interact and influence each other’s evolution.

Coevolutionary Relationship Types

• Mutualism (+/+)

  Both species benefit (e.g., pollinators and flowers).

• Parasitism/Antagonism (+/–)

  One benefits, one is harmed (e.g., parasites and hosts).

• Commensalism (+/0)

  One benefits, the other unaffected.

Coevolutionary Mechanisms

• Geographic Mosaic Theory

  Coevolution strength varies by location, with “hot spots” of strong selection and “cold spots” of weak or no selection.

• Arms Races

  Continuous escalation of adaptations (e.g., garter snakes evolving TTX resistance to toxic newts).

• Mimicry

  • Müllerian Mimicry

    Two harmful species resemble each other (reinforces warning).

  • Batesian Mimicry

    Harmless mimics harmful species (can collapse if too many mimics).

Other Coevolution Concepts

• Mutualism Vulnerability

  Susceptible to “cheaters” (e.g., species that gain benefits without reciprocating).

• Diversifying Coevolution

  Can lead to speciation and increased biodiversity.

Tinbergen’s Four Questions

1. Mechanism

   How does it work?

2. Development

   How does it arise in the individual?

3. Function

   Why is it beneficial?

4. Evolution

   What is the evolutionary history?

Key Principles of Behavior

• Behavior Evolves

  Behavior evolves just like physical traits.

• Proximate Explanations

  Address immediate causes of behavior.

• Ultimate Explanations

  Deal with the evolutionary function and origin of behavior.

Innate vs. Learned Behavior

• Innate Behavior

  Genetically encoded (e.g., reflexes).

• Learned Behavior

  Depends on experience and neural plasticity (e.g., birdsong learning).

• Behavioral Trade-offs

  Learning is adaptive but costly (e.g., energy, time).

Social Evolution

• Kin Selection

  Helps relatives; governed by Hamilton’s Rule: r × B > C (Relatedness × Benefit > Cost).

• Inclusive Fitness

  Calculated as Direct + Indirect fitness.

• Altruism

  Can evolve if it increases inclusive fitness.

Game Theory in Evolution

• Evolutionary Stable Strategy (ESS)

  A strategy that cannot be outcompeted by alternative strategies.

• Example: Side-Blotched Lizards

  Show a rock-paper-scissors dynamic in mating strategies.

Key Trends in Human Evolution

• Bipedalism

  Evolved early (~6 mya), before brain enlargement. Associated with habitat shifts to open savannas.

• Tool Use

  • Oldowan Tools

    Simple stone flakes (~2.6 mya).

  • Acheulean Tools

    More sophisticated hand axes (~1.8 mya).

• Brain Enlargement

  Follows tool use and social behavior expansion. May be linked to diet (e.g., meat-eating) and endurance running.

• Cultural Evolution

  Manifests as art, language, burial practices (Neanderthals), and jewelry.

Hominin Lineages

• Australopithecus

  Small-brained, bipedal hominins.

• Homo habilis

  Known as the “Handy Man,” associated with early tool use.

• Homo erectus

  Characterized by migration out of Africa and more advanced tools.

• Neanderthals

  Sophisticated hominins who interbred with Homo sapiens.

• Denisovans

  A related group of hominins, also interbred with Homo sapiens.

• Homo sapiens

  Modern humans, originating ~300,000 years ago, with global expansion.

Local Human Adaptations

• Lactase persistence
• Skin pigmentation variation
• Immune system variation (e.g., MHC)
• Altitude tolerance

Why Humans Are Vulnerable to Disease

1. Pathogens Evolve Faster

   Pathogens have shorter generation times, allowing them to evolve more rapidly than humans.

2. Natural Selection Lags

   Natural selection is slow to adapt to rapid environmental changes in modern society.

3. Evolutionary Trade-offs

   Some traits that are beneficial early in life may increase disease risk later (e.g., testosterone and cancer risk).

4. Historical Constraints

   Evolution modifies existing structures, leading to imperfections rather than optimal designs.

5. Reproductive Success vs. Health

   Traits that boost fitness may inadvertently increase disease susceptibility.

6. Symptoms Can Be Defenses

   Symptoms like fever, cough, and vomiting are often evolved defenses that aid recovery.

Pathogen Evolution

• Virulence

  The balance between harming the host and maximizing transmission.

• Transmission Mode

  Affects virulence evolution (e.g., airborne viruses can afford to be more virulent).

• Horizontal Gene Transfer

  Spreads antibiotic resistance rapidly among pathogens.

Antibiotic Resistance

• Evolved within hosts and across populations.
• Overuse in medicine and agriculture accelerates resistance.
• Resistance genes often spread via plasmids.

Cancer and Evolution

• Somatic Evolution

  Mutated cell lines replicate unchecked within the body.

• Oncogenes

  Mutated proto-oncogenes that promote uncontrolled cell growth and cancer.

• Tumor Suppressor Genes

  Normally prevent uncontrolled growth; mutations in these genes can lead to cancer.

• Short-Sighted Evolution

  Cancer cells gain a short-term advantage by replicating rapidly, but ultimately harm the host and themselves.

The “Old Friends” Hypothesis

• Immune systems evolved with regular exposure to microbes and parasites.
• Reduced exposure due to modern hygiene may increase autoimmune diseases (e.g., asthma, Type 1 diabetes).

Q&A: Evolutionary Concepts

Q: How does reinforcement contribute to speciation and provide an example?

A: Reinforcement occurs when natural selection favors stronger prezygotic isolation to avoid producing low-fitness hybrids. This accelerates divergence between populations. Example: when closely related frog species with overlapping ranges evolve distinct mating calls to avoid hybridization.

Q: Compare allopatric, parapatric, and sympatric speciation, and explain what makes sympatric speciation challenging.

A: Allopatric speciation happens with physical separation; parapatric involves partial overlap and limited gene flow; sympatric occurs in the same space. Sympatric speciation is challenging because it requires strong disruptive selection and nonrandom mating despite no geographic barrier.

Q: What is an evolutionary arms race? Describe one example from nature.

A: An arms race is when two species exert reciprocal selective pressure, leading to escalating adaptations. Example: rough-skinned newts produce TTX toxin; garter snakes evolve TTX resistance. Both escalate in response to the other, creating extreme traits.

Q: How does geographic variation shape the coevolutionary process?

A: The Geographic Mosaic Theory states that coevolution varies across space: some populations (hot spots) show strong reciprocal selection; others (cold spots) show weak or no interaction. This leads to uneven coevolution across a species’ range.

Q: Use Tinbergen’s four questions to explain a behavior like bird song.

A: Mechanism: brain and hormone control; Development: learned during critical periods; Function: attracts mates, defends territory; Evolution: evolved through sexual selection in ancestors. This framework connects proximate and ultimate causes.

Lecture 8: The Origin of Species

This lecture explains the process of speciation and the barriers that lead to it. Species can be defined using the biological, phylogenetic, ecological, or general lineage species concepts, depending on the organism. Isolating barriers—either geographic or reproductive—prevent gene flow and drive speciation. Barriers can be premating (behavioral, mechanical) or postzygotic (sterile or inviable hybrids). Different modes of speciation include allopatric (physical separation), parapatric (adjacent ranges with some gene flow), and sympatric (within the same range). The lecture emphasizes that speciation is complex and variable in speed—fast in plants (via allopolyploidy) and slower in mammals. Microbial speciation presents unique challenges due to horizontal gene transfer, and often relies on ecological niche differences and the Stable Ecotype Model.

Lecture 9: Coevolution

Coevolution is the reciprocal evolutionary change between species that interact—mutualists, parasites, predators, or competitors. Relationships range from mutualism to antagonism, and selection intensity can vary across landscapes (the Geographic Mosaic Theory). Key examples include the TTX arms race between toxic newts and resistant garter snakes, and mimicry systems: Müllerian (shared warning signals among toxic species) and Batesian (harmless mimics harmful). Coevolution can increase biodiversity, drive divergence (e.g., pollinator shifts), or be disrupted by extinction of partners in highly specialized mutualisms. Parasites and hosts can engage in arms races that escalate (or decelerate) based on costs and trade-offs.

Lecture 10: Brain and Behavior

This lecture explores the evolution of behavior, starting with Tinbergen’s four questions (mechanism, development, function, evolution). Behavior evolves like any other trait and can be studied at both proximate and ultimate levels. Even organisms without brains (e.g., slime molds, plants) show behavior. Vertebrate brain structure is conserved, but regions evolve with ecology. Behavior can be innate (genetic, automatic) or learned (experience-dependent), with learning tied to synaptic plasticity and trade-offs. Social behavior evolves through kin selection (Hamilton’s Rule: rB > C), and game theory explains strategy evolution (e.g., side-blotched lizards’ rock-paper-scissors dynamic). Group selection exists, but individual selection usually dominates.

Lecture 11: Human Evolution

Humans are primates and share ancestry with apes, with fossils and DNA placing our divergence from chimpanzees around 7–8 mya. Major trends in hominin evolution include bipedalism (before big brains), tool use (Oldowan and Acheulean), and eventually language, art, and culture. Early species like Australopithecus walked upright but had small brains. Homo habilis and Homo erectus show larger brains and better tools. Neanderthals buried their dead, made jewelry, and interbred with Homo sapiens. Modern humans originated ~300,000 years ago and expanded globally. DNA evidence shows hybridization with Neanderthals and Denisovans, and recent adaptations include lactase persistence, pigmentation variation, and high-altitude genes.

Lecture 13: Evolutionary Medicine

This lecture explains why natural selection hasn’t eliminated disease. Six key reasons include: rapid evolution of pathogens, mismatch with modern environments, evolutionary trade-offs (e.g., early reproduction vs. cancer), constraints from evolutionary history, and defenses mistaken for disease (e.g., fever). Pathogens evolve quickly via high mutation and horizontal gene transfer. Virulence evolves as a trade-off between host harm and transmission. Antibiotic resistance evolves rapidly, especially with overuse. Cancer is framed as somatic evolution—mutated cell lines compete within the body. The “Old Friends” Hypothesis” links immune disorders to reduced exposure to microbes. Modern issues like obesity and autoimmune diseases reflect evolutionary mismatches between past and present environments.