Social Interactions and Evolutionary Strategies in Biology

Cooperation (Interaction Type)

A social interaction where both the actor and the recipient receive a fitness gain. An example is the communal nesting of Greater Anis, where all females in larger groups have higher overall reproductive success despite initial egg-tossing behavior.

Selfishness (Interaction Type)

An interaction where the actor gains fitness and the recipient loses fitness. An example is the cannibalism in cane toad tadpoles, which eat the eggs of their own species to gain nutrition and reduce competition.

Spite (Interaction Type)

A social interaction where both the actor and the recipient experience a fitness loss. Hamilton proposed it could occur if the recipient is negatively related to the actor, while Wilson suggested it could be favored if the act also benefits positive relatives.

Altruism (Interaction Type)

A behavior that results in a fitness gain for the recipient but a fitness cost for the actor. An example is the 'trill' alarm call of Belding's ground squirrels, which increases the caller's risk of predation while warning others.

Hamilton's Rule

The mathematical formula that explains when an allele for altruistic behavior will spread: Br - C > 0. Where B is the benefit to the recipient, C is the cost to the actor, and r is the coefficient of relatedness. Altruism is favored when the benefit to a relative, weighted by relatedness, outweighs the cost to the actor.

Inclusive Fitness

An individual's total evolutionary success, calculated as the sum of their direct fitness (from personal reproduction) and indirect fitness (from the enhanced reproduction of relatives due to the individual's actions).

Greenbeard Effect

A 'selfish gene' concept where an allele causes a recognizable phenotype (the 'green beard'), allows the bearer to recognize it in others, and prompts altruistic behavior toward them. It's a form of kin selection that acts at the level of a single gene or linked genes, as seen in the csA allele of slime molds.

Parent-Offspring Conflict

A conflict of fitness interests between a parent and its offspring. Each offspring is related to itself by r=1 but to its parent by r=0.5, so the offspring demands more parental investment than the parent is selected to provide, as the parent must balance resources among all of its current and future offspring. This leads to phenomena like weaning conflict.

White Fronted Bee Eaters & Altruism

An example of parent-offspring conflict where fathers coerce their sons into forgoing their own reproduction to help at the parental nest. Sons comply because breeding opportunities are limited and the indirect fitness gain from helping to raise many siblings (r=0.5) is a better fitness outcome than likely failing to breed on their own.

Blue-footed boobies & Siblicide

An example of kin selection and parent-offspring conflict. In blue-footed ******, older chicks will kill their younger siblings during extended food shortages, a behavior parents try to prevent. In the related masked ******, siblicide is obligate and parents do not intervene, showing species differences in parental and offspring behavior.

Reciprocal Altruism (Evolution)

Cooperation between unrelated individuals can evolve if the cost to the actor is less than or equal to the benefit to the recipient, and if individuals who fail to reciprocate are punished. It is most likely to evolve in long-lived, intelligent, social species with good memories and repeated interactions.

Evolutionarily Stable Strategy (ESS)

A strategy that, if adopted by most members of a population, cannot be outcompeted by any alternative strategy. In the context of the Prisoner's Dilemma, 'always defect' is the ESS in a single interaction, but reciprocal strategies like '***-for-tat' can be an ESS in repeated interactions.

Vampire Bats & Reciprocity

Vampire bats share blood meals with groupmates that have failed to feed.

Blood sharing

Predicted by both the degree of genetic relatedness and the degree of prior association, indicating that both kin selection and reciprocal altruism are at play.

Manipulation (as Altruism)

A form of apparent altruism where the donor is tricked into behaving altruistically by the recipient, exemplified by brood parasitism.

Mutualism vs. Reciprocity

In mutualism, both individuals gain an immediate fitness benefit from the interaction; in reciprocity, there is a time lag between the altruistic act and the returned favor.

Eusociality (Characteristics)

A 'true social' system defined by three key characteristics: 1) Overlap in generations between parents and offspring, 2) Cooperative brood care, and 3) Specialized castes of non-reproductive individuals.

Haplodiploidy Hypothesis

An explanation for the evolution of eusociality in Hymenoptera, where males are haploid and females are diploid, leading sisters to share an average of 75% of their genes.

"Best of a bad situation" Hypothesis (Eusociality)

An ecological explanation for eusociality, suggesting it arises when the prospects for successful independent breeding are very poor.

Paper Wasps & Eusociality

An example of facultative eusociality where females can choose to start their own nest, help at a relative's nest, or wait for a breeding opportunity.

Naked Mole Rats & Eusociality

A mammal exhibiting true eusociality, driven by high levels of inbreeding within colonies, resulting in a very high average coefficient of relatedness (r=0.81).

Life History Analysis (Definition)

The study of how organisms allocate energy to growth, maintenance, and reproduction over their lifetimes to maximize total reproductive success.

Virginia Opossum (Life History)

A case study demonstrating changing energy allocation throughout life, with juveniles focusing on growth and adults on reproduction and maintenance.

Sand Cricket (Life History)

A case study demonstrating a trade-off between dispersal and reproduction, with long-winged crickets allocating more energy to flight capability.

Senescence

The late-life decline in an organism's fertility and probability of survival; also known as aging.

Rate of Living Hypothesis (Aging)

An early hypothesis that aging is caused by the accumulation of irreparable damage to cells and tissues, predicting a correlation between aging rate and metabolic rate.

Telomeres and Aging

Telomeres are repetitive sequences at the ends of chromosomes that shorten with each cell division, leading to cellular senescence.

Telomerase

The enzyme that can repair telomeres but is not active in most somatic cells, possibly due to a trade-off between aging and cancer suppression.

Evolutionary Hypothesis of Aging

The current leading hypothesis that aging evolves because of a failure to completely repair damage due to either mutation accumulation or antagonistic pleiotropy.

Mutation Accumulation

A concept where deleterious effects of mutations only appear late in life, contributing to aging.

Antagonistic Pleiotropy

Genes that are advantageous for reproduction early in life but have deleterious effects later in life.

Collared Flycatcher (Aging)

A case study demonstrating a trade-off where birds that breed early have smaller clutch sizes later in life.

Opossum Study (Aging)

A natural experiment showing that island opossums evolved delayed senescence and lived longer compared to mainland opossums.

Lack's Hypothesis (Clutch Size)

The hypothesis that natural selection favors the clutch size that produces the most surviving offspring, often failing due to ignoring critical trade-offs.

Great *** (Clutch Size)

A case study that found the most common clutch size in nature was smaller than the size that produced the most surviving young.

Parasitoid Wasps (Clutch Size)

An application of Lack's hypothesis where females lay smaller clutches than the calculated optimum due to trade-offs in egg-laying.

Principle of Allocation

The concept that energy used for one function reduces the energy available for other functions, leading to trade-offs.

Smith and Fretwell Model (Offspring Size)

A model predicting that a parent's optimal strategy is to produce offspring of intermediate size due to a trade-off between offspring size and number.

Seed Beetle (Offspring Size)

A case study showing adaptive phenotypic plasticity where females lay larger eggs on poor-quality hosts and smaller eggs on good-quality hosts.

Genomic Imprinting (IGF-II)

A conflict of interest between parents at the genetic level regarding the IGF-II gene, balancing resource flow during embryonic growth.

Campions (Biological Invasions)

A case study of the snowy campion plant that became more invasive in North America after escaping its native enemies.

Cholera (Case Study)

A disease illustrating epidemiological principles, notably traced by John Snow's 1854 map of London to a contaminated water pump.

John Snow's 1854 map

Traced an outbreak to a contaminated water pump.

Modern phylogenetics

Traced the 2010 Haiti outbreak to a strain from Nepal.

Germ Theory

The theory, proposed by Louis Pasteur, that contagious diseases are caused by microscopic germs (pathogens). It was a foundational concept for modern medicine.

Flu Virus (Evolution)

The flu virus evolves rapidly in an arms race with the human immune system. Its surface proteins, hemagglutinin and neuraminidase, have antigenic sites that are under strong positive selection to change and evade host immunity. New pandemic strains can arise from reassortment (gene-swapping) between viruses from different hosts like birds and pigs.

Antibiotic Resistance (Evolution)

The evolution of resistance in bacteria is a classic example of rapid natural selection. The use of antibiotics creates strong selective pressure, favoring resistant individuals. The prevalence of resistant strains is directly correlated with the intensity of antibiotic use.

Evolution of Virulence (Hypotheses)

Four main hypotheses: 1) Recent Entrance (pathogens evolve to be benign; largely unsupported), 2) Coincidental (virulence is an accident, e.g., tetanus), 3) Short-Sighted (pathogen harms host for short-term gain, hurting long-term transmission, e.g., polio in nervous system), and 4) Trade-off/Transmission Rate (pathogens evolve optimal virulence based on a trade-off with transmission ease; this is the leading hypothesis).

Tissue Evolution (Cancer)

The principle that cells within an organism can be units of selection. Cancer is an example of tissue evolution, where mutated cells reproduce, compete, and evolve within the host body, leading to tumor progression.

COVID-19 (Origin)

The disease caused by the SARS-CoV-2 virus, which is a zoonotic pathogen that jumped from an animal to humans. Phylogenetic evidence points to an origin in bats, possibly with a pangolin as an intermediate host, with the jump to humans facilitated by a mutation in the virus's spike protein.

Lactase Persistence (Human Evolution)

An example of recent human evolution. While most mammals lose the ability to digest lactose after weaning, human populations with a history of pastoralism have evolved the heritable trait of lactase persistence, allowing them to drink milk in adulthood.

Fever (Evolutionary Hypotheses)

Two hypotheses explain fever: 1) It is a manipulation by the pathogen, or 2) It is an adaptive defense by the host. Evidence from iguanas, which seek heat when infected and have better survival rates at higher temperatures, strongly supports the adaptive defense hypothesis.

Mayr's Biological Species Concept (BSC)

Defines a species as a group of interbreeding populations that are reproductively isolated from other groups. It is theoretically appealing but impractical for asexual species, fossils, and allopatric populations.

Morphological Species Concept

Defines species based on physical similarity. It is practical and widely used but fails with cryptic species, polymorphism, and sexual dimorphism.

Phylogenetic Species Concept (PSC)

Defines a species as the smallest diagnosable monophyletic group (clade). It is logically rigorous but can lead to a large increase in the number of recognized species.

Evolutionary Species Concept

Defines a species as a lineage evolving separately from others with its own unique evolutionary trajectory. It is broadly applicable, covering fossils and asexual organisms.

Steps of Speciation

A three-step process: 1) Population isolation (cessation of gene flow), 2) Divergence of traits and genes (via drift and selection), and 3) Evolution of reproductive isolating mechanisms.

Allopatric Speciation

Speciation that occurs due to a geographic barrier separating two populations. It is considered the most common mode of speciation.

Sympatric Speciation

Speciation that occurs without a geographic barrier, within the same area.

Parapatric Speciation

Speciation between adjacent populations that maintain gene flow across an environmental gradient.

Peripatric Speciation

A form of allopatric speciation where a new species arises from a small founder population isolated on the periphery of the ancestral range.

Genetic Drift

A mechanism of evolution that involves random changes in allele frequencies in a population.

Polyploidy

A rapid mode of sympatric speciation where an organism has more than two sets of chromosomes.

Secondary Contact

When two populations that have diverged in allopatry meet again.

Reproductive Isolating Barriers

Mechanisms that prevent gene flow, including prezygotic and postzygotic barriers.

Reinforcement Hypothesis

The hypothesis that when hybrids have low fitness, natural selection will favor the evolution of prezygotic barriers.

Hybrid Zone Outcomes

The four potential outcomes for a hybrid zone: 1) It can remain stable; 2) Reinforcement can lead to full reproductive isolation; 3) The populations can fuse back into one species; 4) The hybrids can become reproductively isolated and form a new species.

Haldane's Rule

In hybrids between two species, if one sex is sterile or inviable, it is almost always the heterogametic sex.

Human's Closest Relative

Chimpanzees and bonobos (genus Pan) are our closest living relatives based on genetic evidence.

Introgression

The movement of genes from one species into the gene pool of another through repeated backcrossing of a hybrid.

Sahelanthropus tchadensis

The earliest known hominid, dating to 6-7 mya, showing a mix of primitive and advanced features.

Australopithecines

An early group of bipedal hominins with small brains, including gracile and robust forms.

Key Homo Species

H. habilis (first tool user); H. erectus (first to leave Africa); H. neanderthalensis (archaic human in Europe/Asia); H. sapiens (anatomically modern humans).

Denisovans

An archaic human population from Asia known from a few fossils but extensive genomic data.

Homo floresiensis

A species of dwarfed hominin descended from H. erectus on the island of Flores, Indonesia.

Origin of Modern Humans Models

1) Out of Africa (Replacement); 2) Multiregional; 3) Hybridization/Assimilation.

Green Study (Leaky Replacement)

The study that sequenced the Neanderthal genome and found evidence of interbreeding with modern humans.

Evolution of Tool Use

The oldest tools are 2.5-2.6 million years old, possibly made by early Homo or late australopithecines.

Evolution of Language

Language likely evolved gradually, with evidence including brain size increase and anatomical changes in hominins.