Domestication: Quiz 1

Holocene

- The current interglaciation period, extending from 11,000 years ago to present day

- Domestication takes off at the beginning of this period

- End of the Pleistocene (Ice Age)

Pleistocene

- 2.6 million years ago to 11,000 years ago

- Last Ice Age

Oasis Hypothesis (Climate Forcing)

- "The Holocene is nasty"

- Popularized by V. Gordon Childe

- During a dry spell in early Holocene called "The Younger Dryas", habitable ecosystems shrank

- Suggested that dry periods forced people, plants, and animals together, domesticating all three by propinquity

Holocene Climate Change Hypothesis (Climate Forcing)

- "The Holocene is nice"

- Popularized by Carl Sauer

- Productive landscapes became more common, so human societies became sedentary

- Sedentary populations understood local plants and animals better, allowing for more exerted control

Population Pressure

Existing food procurement strategies weren't cutting it, so people domesticated plants and animals as populations grew at the beginning of the Holocene

Aggrandizement (The Beer Hypothesis)

- Posits that grains were domesticated to make alcoholic beverages

OR

- To create surpluses for feasts, in order to boost one's social standing

Problem with the Beer Hypothesis

Seemed like everyone had equal access to the earliest domesticates

HOW did hunters and gatherers first domesticate plants and animals?

- Cultivation/tending

- Herd/pack management

- Social institutions

- Mobility, seasonality, communal labor, surplus, storage, etc.

- Ideology and ritual

- Taboos, sacrifices, notions of property, etc.

Human Niche Construction/Ecosystem Engineering

- Human modification of ecosystems to increase the abundance of predictability of valued resources

- Heart of the process of domestication

Wild progenitor

The ancestral species that gave rise to a domesticated species

Nikolai Vavilov

- Traveled the world to identify crop progenitors and areas where biodiversity was high

- Identified centers of origin for most important crops

Vavilov's centers of origin (differential phytogeographic method)

1. Create a botanical taxonomy for each crop

2. Delimit the distribution of each taxa

3. Infer center of origin is the region where there is the greatest diversity

Jack Harlan

- Worked as a plant breeder

- Founds the Crop Evolution Laboratory with deWet

- Revised Vavilov's "centers of origin" to "centers of diversity"

- Proponent of ex situ conservation in seed banks

The dump heap theory of domestication (theory of weed-origin)

1. Humans disturb an area

2. This area is colonized by weedy plants

3. Humans find something useful about these weeds and expand the disturbed area to increase their abundance

4. Evolution of domesticated species ensues in these nascent gardens

Adaptive syndrome of weediness (Harlan and deWet)

An adaptive syndrome which permits a species or variety to thrive and become abundant and difficult to eradicate within areas of human disturbance

Annuals

- Prolific seed production

- Germination heteromorphism

- Crop mimicry

- Developmental plasticity

Perennials

- Propagation by difficult to remove roots

- Robust plant protections (thorns, hairs, poisons)

Mutualism

- An ecological interaction between two species that is beneficial to both organisms involved

- When talking about benefit, we mean the fitness (# of surviving offspring)

Axes of variation (Zeder)

1. Intentionality

2. Control/power

3. Locus of change

Intentionality (axes of variation)

- Domestication is the incidental result of human activities

- Ex: wild manioc initially took advantage of human-created clearings, then lost the ability to thrive without this disturbance over time

- Domestication is the result of purposeful action

- Selection of desirable traits in plants and animals for breeding

Power/control (axes of variation)

- Plants and animals are "self-domesticated"

- Ex: cats were drawn to villages because they were full of mice and rats (niche construction)

- Human-directed

- Ex: early sheep and goat domestication involved penning/captivity, allowing people to control the population and direct evolution

Locus of change (axes of variation)

- Genetic isolation and fixed phenotypic change

- Ex: difference between a wolf and dog skeleton or gene

- Behavior and development, aka plasticity (including changes in human socio-economic organization)

Adaptive Syndrome

- A set of like characteristics in distantly related organisms that results from convergent evolution

- Convergent evolution: distantly related organisms independently evolve similar traits to adapt to similar necessities

- Parallel evolution: same traits evolving in closely related organisms

Foliage is the fruit hypothesis (adaptive syndrome example)

- Leaves function as fruit: they entice animal disperser to consume seeds that are indigestible

- Evolutionary advantages of animal poop dispersal:

- Reduced sibling competition

- Directed dispersal to nitrogen enriched openings (poop)

Annual seed crop

Crops that complete their life cycle in one year

Cereals

The seeds of annual plants, mostly grasses

How does the adaptive syndrome evolve?

- Planting is key

- "It is the seeds that escape the harvester that contribute to the next generation" (Harlan et al. 1970)

Seed predator

Exerts selective pressure on plant population to avoid getting eaten

Seed disperser

Opens up selective advantages for allowing seeds to be captured

Harlan et al.'s hypothesis about harvesting seeds

Assuming harvested seed is then planted:

- Anything that makes it more likely for a given seed to be "captured" will enjoy a selective advantage

- Anything that gives a seedling an advantage in a garden or field environment will be selected

Seed head becomes non-shattering (about harvesting seeds)

- Non-shattering means the seeds don't fall off

- Most of such seeds will be harvested and thus contribute to the next generation

- Shattering seeds will be "lost"

Plant architecture becomes more determinate (about harvesting seeds)

- Determinate = less branches

- Ex: if harvesting with a sickle, plants that make this easier by having fewer, bigger seed heads will be captured

- "Sunflower effect": less branches, less but bigger flowers

Seeds mature simultaneously (about harvesting seeds)

Regardless of harvesting method, seeds that mature before or after harvest time are not captured, thus these seeds will then not mature during harvest times

Seed size increases (about harvesting seeds)

The "food store" part of the seed increases

Germination inhibitors are reduced (about harvesting seeds)

- Seed coat can prevent seeds from germinating right away so it doesn't germinate at bad weather conditions

- Reduction of inhibitors can cause the seed to sprout more often

Problems with Harlan et al.'s seed harvest hypothesis

Their mechanism only works if you open a new field every year, otherwise both "lost" and "captured" seeds make up the next generation

How did the domestication syndrome of annual seed crops evolve?

A switch in seed dispersal drove changes such as increase in seed size and change in color due to loss of germination inhibitors

Dmitry Belyayev and the Fox Farm Experiment

Wondered if selecting for tameness would lead to morphological changes observed in many domesticated animals

Tameness

Focused more on behavioral attributes than morphological traits, such as tolerance to penning, a social structure based on dominance, and reduced wariness and aggression

What makes an animal easy to domesticate?

- Tolerance to penning

- Dominance hierarchies: trait where animals usually follow around a leader (human leader in this case)

- Will breed in captivity

- Little wariness or aggression

- Traits listed above allow certain wild animals to form commensal relationships with people

Domestication syndrome in mammals: Traits connected to behaviors important for domestication

- Are more docile

- Can become pregnant more frequently and/or non-seasonally

- Have less sexual dimorphism (less difference in body shape between males and females)

- Have endocrine and neurological differences

- Exhibit juvenile behaviors for longer/changes in early development

Domestication syndrome in mammals: less clear traits connecting to domestication

- Smaller brains

- Smaller teeth and differently shaped heads and faces

- Floppy ears and curly tails

- Multi-colored (piebald) coats and more variety in hair texture

Breed (noun)

A genetically isolated sub-population of the domesticated population, usually less than 50-100 generations old, with markedly reduced genetic diversity due to human action

Breeding (noun)

Selection of parents and restriction of mate choice (aka complete control over reproduction)

Village dog niche

- Scavengers in villages, towns, and cities

- Need to be un-afraid of people, then people aren't afraid of them

- Need to be able to digest human food, specifically grains

How dogs were domesticated: Self-domestication hypothesis

- Wolves were commensal scavengers

- More successful scavengers were those who were not afraid of people

- Over time, a pop. of early dogs not unlike feral village dogs evolved in proximity to people

How dogs were domesticated: Human-initiated hypothesis

- Hunters took pups from their dens (possibly after killing the parents)

- Pups with a greater ability to thrive when raised by humans survived

- Multiple generations with at least one parents who had undergone selection for these conditions

Pleistocene niche

- Evidence from Mezhirich, Ukraine

- Middens (dumps) located close to dwellings

- Wolf would have to get very close to the houses to scavenge

- Contextual evidence that dogs were cared for by Pleistocene people -> suggests close relationship with people

- Niche likely follows human-initiated hypothesis

Critical period of socialization

- A period during early development when exposure to a novelty results in long-term familiarity

- Commences with ability to explore, approach, and investigate

- Ends with avoidance of novelty

Critical period of socialization: Wolves vs. Dogs

- Wolves: 2-6 weeks of age, weaned later, can only smell and still need nursing at the beginning of the period

- Dogs: 4-8 weeks of age, weaned earlier, can smell, hear, and see and don't need nursing at the beginning of the period

- KEY IDEA: dogs don't need to be nursed by their mothers during the critical period, while wolves do

Human mothering an orphaned pup scenario

Would've worked on standing variation in wolf populations to select for wolves that had a later critical period of socialization and the ability to be weaned sooner

Is there a domestication syndrome in plants and animals? (KEY IDEA)

- Every domesticated plant or animal has it's own unique history, just as each has it's own unique biology

- Will never see an exact set of traits for domestication in animals

- Plants are exceptions, we DO see DS

Commensalism

An association between two organisms in which one benefits and the other derives neither benefit nor harm

DS in annual seed crops

- Seeds become non-shattering

- Fewer + bigger seed heads (more determinant growth)

- Simultaneous maturation of seeds

- Increase in seed size

- Seed protections reduced

Tef (annual seed crop that doesn't fit the DS plant model)

- The smallest domesticated grain

- Likely that ancient farmers selected for plants with many branches and small seeds to avoid lodging (moisture gathering around the plant, causing the seeds to rot)

Is there a domestication syndrome for perennial crops?

Yes, but traits are not as apparent and many as in annual crops

De novo domestication for perennials

The process of creating a new domesticated crop species by genetically modifying a wild plant

Tubers

- An underground storage organ of a plant (enlarged stem) that bears buds from which a new plant can arise

- Can reproduce themselves by creating clones vegetatively

- Ex: Taro, potato, yam

Proposed DS in tubers (characteristics)

- Partial or complete loss of ability to reproduce sexually

- Increase in predictability, size, and number of tubers

- Decrease in other plant parts

- Tubers are easier to harvest (clumped) and/or separate easily

- Loss or reduction in toxicity and other defenses

Advantages of tuber cultivation for people

You can make as many of the exact same plant as you want (instant domestication)

Instant domestication

The idea that a given root crop sprang from a single desirable mutant, cloned millions of times

How do people effect the genetics of a plant that is propagated through cloning?

Landraces and extensive agriculture

Landrace

A variety of a domesticated plant that has evolved in a particular place to suit particular cultural preferences and ecological conditions

Extensive agriculture (slash and burn, swidden)

Fields don't stay in one place all the time but rotate

Agroecosystem of Amazonian manioc

- Manioc landraces are grown in blocks within swidden plots

- Manioc allows for "live storage", can harvest whenever we want

- Bees facilitate cross-pollination between landraces

- When new fields are cleared, farmers propagate their landraces via stem cuttings, which are clones

Folk taxonomy

- A classification system composed of a hierarchy of groups

- Cross-pollination occurs between two landraces, producing new variety

- Farmers categorize traits on seedlings between landraces and determine if seedlings belong to something new or not

DS in tubers: The case of manioc

- Increase in predictability, size, and number of tubers

- Easier to harvest

- Unprocessed manioc is poisonous, sweeter varieties have reduction in cyanide

What is domestication in terms of evolutionary process?

An evolutionary process whereby human modification of ecosystems and selective pressures results in changes in the genomes, bodies, and behaviors of plants and animals.

How does Zeder define domestication?

A form of mutualism that is asymmetrically enhanced by the human ability to culturally transmit knowledge.

How does Lord define domestication?

A population that is commensal with humans or otherwise generally reliant on a human-modified environment for survival.

"Some thoughts on weeds" by Jack Harlan & J. M. J. deWet (1965)

- Main argument is that weeds should be defined in terms of their ecology

- Ecological characteristics of weeds:

- Pioneers of open soil and human habitations (plants that contest with man for possession of soil)

- Cosmopolitan (not picky)

- Disturbance and succession

- Weeds are products of evolution, arisen from vast disturbances of the earth via agriculture

- "If we define the concept of weeds to species adapted to human disturbance, then man is, by definition, the first and primary weed under whose influence all other weeds have evolved."

"Central questions in the domestication of plants and animals" by Melinda Zeder (2006)

- Domestication is a form of mutualism that is asymmetrically enhanced by the human ability to culturally transmit knowledge

- Human intentionally sets domestication apart from other forms of mutualism

"The History of Farm Foxes Undermines the Animal Domestication Syndrome" by Lord et al. (2020)

- The Russian Farm Fox Experiment (RFFE) does not support claims of a causal relationship (tameness, phenotypic changes)

- No consistent set of traits found in domesticates

- Lord's main argument: RFFE is well-suited for studying behavioral genetics, but not DS in animals

- An universal DS might not exist

"Straw Foxes: Domestication Syndrome Evaluation Comes Up Short" by Zeder (2020)

- Zeder argues against Lord's claim about RFFE

- The foxes came from Canadian foxes bred in captivity, so already have developed traits differently than wild pop.

- Zeder's main argument: traits defined by DS cannot be interpreted strictly, thus argues RFFE actually supports the existence of DS