Human Evolution and Ecology - Week 3 Lecture Notes
Topic = The forces of Evolution and the Formation of Species and Sexual Selection
What is evolution?
Definition: A change in allele (gene) frequencies in a population over time.
Emphasizes population-level genetic change across generations.
Population genetics fundamentals
What is a population?
Breeding populations: groups that tend to mate within the group.
Human populations are often defined by geography or political boundaries.
Frequencies to measure:
Genotype frequency: relative proportions of genotypes within a population.
Allele frequency: relative proportions of alleles within a population.
Practical implication: to analyze evolution, you need to account for all individuals and genotypes in the population.
Hardy–Weinberg equilibrium
The Hardy–Weinberg (H-W) equilibrium relates allele frequencies to expected genotype frequencies in the next generation under certain conditions.
Assumptions of the H-W model:
Random mating with respect to the studied locus.
No new alleles arise by mutation.
No migration (gene flow) into or out of the population.
No evolution due to sampling (i.e., large population size and no sampling bias).
Interpretation:
If observed genotype frequencies differ from H-W expectations, at least one assumption is violated, indicating evolutionary change.
Null hypothesis viewpoint:
The H-W equilibrium represents a null model for a population that is not evolving.
The four evolutionary forces
Changes in allele frequencies originate from four forces:
1) Natural selection, 2) Mutation, 3) Genetic drift, 4) Gene flow (migration).Key roles:
Mutation provides new genetic variation (source of new alleles).
The other three forces alter allele frequencies over time.
Mutation as an evolutionary force
Mutation introduces new alleles into a population and shifts allele frequencies over time.
If there is no further evolutionary change after a mutation, allele frequencies would stabilize in future generations.
Mutations can revert (mutate back) to original forms, but such reversions are rare.
Mutations are crucial for evolution but occur at relatively low rates; by themselves, they do not drive large allele-frequency changes rapidly.
Nonrandom mating can affect the rate of evolutionary change but does not by itself cause evolution.
Selection and its effects
Selection filters genetic information and drives evolution by favoring certain variations.
Directional selection (a form of natural selection) pushes allele/phenotype frequencies in one direction.
Selection acts on phenotypes, selecting for traits that improve survival and reproduction.
It acts on randomly occurring variation, favoring variations that enhance reproductive success.
Terminology:
Directional selection: shifts populations toward one extreme.
Stabilizing selection: reduces extremes, favors intermediate phenotypes; discussed later with examples in birds and sparrows.
Directional selection example
Finch beak size as a classic example:
When food is plentiful, beaks tend toward one phenotype size.
When food is scarce, selection favors another beak size; directional pressure shifts the mean beak size in the population.
Stabilizing selection examples
Stabilizing selection in sparrows under cold winter conditions:
Keeps populations uniform by reducing extreme phenotypes (e.g., extreme large or small body sizes).
Human birth weight as a potential example of stabilizing selection:
High vs. low birth weights predispose to early mortality.
Stabilizing selection could favor middle-range birth weights.
Question posed: Is there stabilizing selection acting on human birth weight?
Evidence discussed: Birth weights around 3–4 kg are favoured globally, suggesting stabilizing selection on birth weight.
Human birth weight and obstetric dilemma
Question: Does stabilizing selection act on human birth weight?
Evidence: Birth weight around 3–4 kg is commonly favored worldwide.
Obstetric Dilemma concept (Cephalo-Pelvic Disproportion):
The conflict between large neonatal head size and maternal pelvic dimensions in humans.
Illustrated with comparative anatomy across great apes (orangutans, chimpanzees, gorillas) and humans.
Consequence: selection pressures may balance the advantage of larger-brained offspring with the risk of difficult childbirth.
Genetic drift and sampling error
Genetic drift: random fluctuations in allele frequencies across generations due to chance.
During meiosis, only one of two alleles at a locus is transmitted, introducing sampling effects.
Drift causes allele frequencies to drift randomly, with direction being random.
Over time, drift reduces genetic variation, especially in small populations.
Founders and bottlenecks:
Founder effect: when a new population is started by a small subset of individuals, their gene frequencies may differ from the source population.
Bottlenecks: drastic reductions in population size that reduce genetic diversity and alter allele frequencies.
Example discussions:
Amish and Ellis–van Creveld syndrome (a form of dwarfism with extra digits) illustrate founder effects and unusual disease frequencies.
Population bottlenecks can drastically alter genetic diversity and affect long-term evolution.
A volcanic eruption scenario is used as an analogy for population bottlenecks.
Visualizations and figures in the transcript show bottlenecks and their effects on population genetics.
Human evolution, diversity, and the Out-of-Africa model
Global human similarity and diversity:
All humans have low overall genetic variation; humans are highly similar relative to many other species.
The global human genetic diversity is roughly half that of chimpanzees when considering the entire genome.
Sub-Saharan African populations contain the greatest genetic variation.
Each migration out of Africa carried only a subset of variation, resulting in founder effects and reduced diversity in non-African populations.
Out-of-Africa model and bottlenecks:
Modern human diversity patterns support a recent African origin, followed by population bottlenecks and founder events as humans dispersed globally.
References include Relethford (1999) as a standard text on human diversity and evolution.
The idea that Africa contains most of human genetic diversity underpins arguments against simplistic racial generalizations in biology.
Australia during the Last Glacial Maximum (LGM) and refugia
The 2013 Journal of Archaeological Science study analyzes Aboriginal Australian refugia and human demographic patterns during the LGM (~23–18 ka) and Antarctic Cold Reversal (~14.5–12.5 ka).
Methods include radiocarbon dating, geospatial analysis, K-means clustering, and Minimum Bounding Rectangles (MBR) to map refugia and movement.
Findings:
Refugia existed across multiple regions (e.g., Gulf Plains/Einasleigh Uplands, Brigalow Belt South, Murray Darling Depression, Tasmanian Central Highlands).
Some refugia persisted between 25 and 12 ka; others appeared sporadically.
Population declines during LGM and ACR were up to ~60% in some areas, with resilience and cultural continuity in others.
The maps show refugia, barriers, and corridors for human occupation during LGM.
The Mismeasure of Man — Stephen Jay Gould
Gould argues against simplistic interpretations of human racial diversity and the mismeasures of human variation.
Main points (paraphrased from text):
Early narratives of “out of Africa” and racial hierarchies have been contested by accumulating genetic data.
Africans harbor greater genetic diversity, and non-African groups form a subset within Africa’s genetic variation.
The claim that Africans are more or less intelligent or have other fixed race-based traits is scientifically inappropriate because of greater intra-African diversity and the complexities of human evolution.
Bottlenecks and human genetic diversity (Proceedings of the Royal Society B)
Bottlenecks and expansion:
Large-scale human expansion out of Africa led to a gradient of decreasing genetic diversity with distance from Africa.
The BotTLENECK program detects historical events through patterns of heterozygosity excess relative to allele number after a population decline.
Findings by Amos & Hoffman:
Evidence for two primary bottlenecks: Out of Africa and a later bottleneck around the Bering Strait (entry to the Americas).
These bottlenecks correspond to regions with larger founder events and help explain global genetic diversity patterns.
Gene flow
Gene flow (migration) involves movement of alleles between populations.
When gene flow occurs:
Populations mix genetically and tend to become more similar.
New variation is introduced within populations when migrants bring new alleles.
Gene flow can spread advantageous mutations across a species, influencing genetic variation on a broad scale.
Five Fingers of Evolution
A mnemonic for five evolutionary forces (Paul Andersen): Five Fingers of Evolution.
Visual/educational aid: a thumbs-up sign helps remember the five processes that affect gene pools over time.
Link: YouTube explanation by Paul Andersen and Alan Foreman.
Evolutionary perspective on selection
Natural selection vs. artificial selection:
Natural selection acts in natural environments to shape traits that increase survival and reproduction.
Artificial selection (selective breeding) is a tool humans use to produce desired traits; historically used to introduce Darwin’s ideas to learners.
A brief note: In the context of the transcript, humans receive limited explicit discussion beyond recognizing both forms of selection.
Darwin’s problem and sexual selection
Darwin’s problem: Some perceived advantages in traits seemed disadvantageous for survival (e.g., peacock’s tail, antlers, lekking, mate competition).
Darwin’s solution: Sexual selection as a separate mode of selection acting on mating success rather than just survival.
Sexual selection defined (original scope):
Competition within a sex for mates or fertilizations; differential choice by the opposite sex.
Often emphasizes male-male competition and female choice, though broader definitions include a range of mating strategies.
Later critiques and expansions:
Trivers (1972) introduced parental investment and sexual selection, linking reproductive strategies to mating opportunities.
Subsequent debates expanded the scope to include female competition, mate choice, and non-traditional forms of sexual selection.
The classic example: Peacocks and other species display exaggerated male traits that seem costly but increase mating success; the trade-off is higher visibility to predators and energy costs, balanced by increased reproduction.
Visuals of sexual selection – classic examples
Birds of paradise and long-tailed manakins illustrate extreme male displays used to attract mates.
Female choice and male display co-evolve, creating elaborate traits.
Multimedia references (e.g., Attenborough videos) provide real-world demonstrations of these displays.
Critiques and contemporary views on sexual selection
Contemporary debates address bias and the scope of sexual selection definitions.
Present perspectives include: broad versus narrow definitions; inclusion of female choice and non-mate-related selection pressures.
Box on counteracting bias offers strategies for research planning, execution, interpretation, evaluation, and publication to reduce gender and other biases.
Key references include Box 1 strategies for mitigating bias in research planning, execution, interpretation, and publication.
Darwin’s third and fourth ideas on selection
Darwin’s Third Great Idea: Sexual selection.
The slide emphasizes that there is no widely accepted “fourth method of selection” beyond artificial selection as a recognized process in evolution.
Domestication and canids genetics
Complete mitochondrial genomes of ancient canids suggest a European origin of domestic dogs.
Key points:
Domestication processes may have started in Europe with hunter-gatherers and interacting canids.
Molecular dating places domestication onset in Europe between approximately 18,800 and 32,100 years ago.
Modern dogs are closely related to ancient or modern canids from Europe, suggesting joint history with European human groups.
Implication: Domestication is a major example of human-influenced selection and rapid morphological/behavioral diversification.
Species concepts and taxonomy
What is a species? The biological species concept defines species by reproductive isolation and the ability to produce fertile offspring.
Example: Horses and donkeys interbreed to produce mules, which are sterile; thus, horses and donkeys are separate species.
Linnaean taxonomy and hierarchical organization (Page 68): Kingdom, Phylum, Class, Order, Family, Genus, Species (examples given for humans, chimpanzees, tortoises).
Homologous vs. analogous traits (Pages 69–71):
Homologous traits arise from shared ancestry.
Analogous traits arise from convergent evolution, serving similar functions but not from a common ancestor.
Cladistics and derived traits (Pages 75–83):
Cladists group organisms by ancestral traits and then identify subgroups based on derived characteristics.
Derived characteristics include traits that evolved within a group (e.g., prehensile tails, jaw mechanics, digestion capabilities in primates).
Convergence (Page 72–74):
Convergent evolution results in similar structures (e.g., bird wing vs bat wing) due to solving the same problem (flying) rather than shared ancestry.
Taxonomy and evolutionary relationships:
Ancestral characteristics vs. derived characteristics define groups and subgroups.
Examples include primate-derived traits and the broader vertebrate classification schemes.
Speciation and geographic isolation
Allopatric speciation: speciation arising from geographic isolation.
Example framework: Spreading populations (e.g., chimpanzees and bonobos) diverged due to geographic separation in Central Africa, leading to different traits and niches.
Speciation models contrast with continuous gene flow and hybrid zones; allopatric speciation demonstrates how isolation can drive divergence.
Evolution tempo: gradualism vs punctuated equilibrium
Phyletic gradualism: evolution proceeds gradually through small changes over time.
Punctuated equilibrium (Eldredge & Gould, 1972): long periods of stasis punctuated by rapid evolutionary change during speciation events.
Implication: The fossil record may show gaps or rapid changes that correspond to speciation events rather than steady transformation.
Fossil record and missing links
Ernst Mayr’s allopatric speciation framework and the concept of missing links.
“Missing links” are common in fossil data; fossils are often intermediates in evolutionary transitions.
Example chain illustrating sea-to-land transitions in vertebrates:
Eusthenopteron → Panderichthys → Tiktaalik → Ichthyostega (illustrative progression toward terrestrial tetrapods).
Inclusive discussion of missing links emphasizes the incomplete nature of the fossil record and the continuous reconstruction of evolutionary history.
Inclusive fitness and kin selection
Inclusive fitness concept: an individual’s genetic success is determined by direct reproduction and by helping relatives share genes (kin selection).
Components of fitness:
Direct fitness: personal reproduction success.
Indirect fitness: impact on the reproduction of relatives due to helping kin.
Hamilton’s Rule (1964): altruistic behavior can evolve if rB > C, where r is relatedness, B is the benefit to the recipient, and C is the cost to the actor.
Examples:
Worker bees and queen bees: workers forgo reproduction to help sisters (other workers or queen) reproduce, thereby increasing inclusive fitness.
Practical takeaway: Kin selection explains altruistic behaviors in social species where individuals share genes with relatives.
Misunderstandings and tutorials
Five common misunderstandings about evolution (as discussed in The Conversation, 2016).
Tutorial resources and documentary links provided for further clarification:
PBS documentary: Evolution - Part 5 of 7 - Sexual Selection
Darwin’s Dangerous Idea and related media links
Summary of core concepts and connections
Evolution is a change in allele frequencies in a population over time, driven by multiple forces including natural selection, mutation, drift, and gene flow.
Hardy–Weinberg equilibrium provides a baseline null model for a non-evolving population; deviations indicate evolutionary processes at work.
Mutation introduces new genetic variation, but its rate is typically low; significant evolutionary change often requires other forces to alter allele frequencies.
Natural selection acts on phenotypes to increase reproductive success, with directional and stabilizing modes shaping trait distributions.
Sexual selection explains the evolution of exaggerated traits and mating preferences that may trade off with survival.
Genetic drift leads to random fluctuations in allele frequencies, especially impactful in small populations and during founder events or bottlenecks.
Gene flow tends to homogenize populations and introduce new variation, influencing adaptation across populations.
Human evolution shows complex patterns of diversity shaped by Out-of-Africa migration, bottlenecks, and founder effects; Africa is a major source of genetic diversity.
Species concepts and taxonomy rely on ancestry and derived traits; convergent evolution can produce similar features in unrelated lineages.
Speciation can be allopatric (geographic isolation) or occur through other mechanisms; tempo of evolution can be gradual or punctuated.
Inclusive fitness and kin selection explain altruistic behaviors in social species through genetic relatedness and indirect fitness benefits.
Modern debates address bias in sexual selection research and the broad vs. narrow definitions of selection mechanisms; strategies exist to mitigate biases in planning, execution, and interpretation of research.
Formulas and key quantities (LaTeX)
Hardy–Weinberg relationships (baseline null model):
Allele frequencies satisfy p+q=1
Genotype frequencies in the next generation are p^2+2pq+q^2=1 where p is the frequency of allele A and q is the frequency of allele a.
Inclusive fitness concept (conceptual framing): Direct fitness and indirect fitness contribute to an overall genetic success. A common formal representation used in kin selection discussions is Hamilton’s rule, rB > C where
r = coefficient of relatedness between actor and recipient,
B = benefit to recipient's reproductive success,
C = cost to actor's reproductive success.
Other key quantitative ideas (implicit from the transcript):
Changes in allele frequencies due to natural selection, mutation, drift, and gene flow; population genetics formalism often requires comparison of observed genotype frequencies to HW expectations to detect deviations.
Bottleneck effects reduce genetic diversity and alter allele frequencies, potentially detectable via excess heterozygosity relative to allele number in certain tests (as implemented in BOTTLENECK analyses).
Quick reference glossary (selected terms)
Allele: one of two or more alternative forms of a gene at a given locus.
Genotype: the genetic makeup of an individual with respect to a locus or set of loci.
Phenotype: observable traits resulting from the genotype and environmental influences.
Allopatric speciation: speciation that occurs when populations are geographically isolated.
Founder effect: genetic differences that arise when a new population is started by a small subset of individuals.
Bottleneck: a sharp reduction in population size resulting in loss of genetic variation.
Convergence: similarity in traits due to similar selective pressures, not shared ancestry.
Homology: similarity due to shared ancestry.
Analogy: similarity due to convergent evolution, not shared ancestry.
Inclusive fitness: total genetic contribution of an individual to future generations, including indirect effects via kin.
Kin selection: evolution of social behavior benefiting relatives due to shared genes.
Obstretric dilemma: evolutionary pressures balancing childbirth efficiency and infant survival with pelvic constraints.
Punctuated equilibrium: long stasis interrupted by brief bursts of rapid change during speciation.
Phyletic gradualism: slow, continuous evolutionary change.
Sexual selection: selection arising from variation in mating success; can involve male-male competition and female choice.