LEARNING GUIDE · ARTIFICIAL SELECTION

Designed

by us.
Lived by
them.

For centuries, humans have shaped plants and animals through artificial selection, choosing which traits are passed to future generations. For the billions of animals caught in industrial farming, those choices have profound consequences for their bodies and lives.

CHAPTER 1 · INTRODUCTION

Eight wildly different foods are all the same species.

Broccoli, kale, cabbage, cauliflower, Brussels sprouts, collard greens, romanesco, and kohlrabi are all forms of Brassica oleracea. 24, 25, 26

Wild form

Brassica oleracea

Cabbage

Selected for enlarged terminal buds

Broccoli

Selected for large flower-bud clusters

Cauliflower

Selected for dense clusters of growing tips

Brussels sprouts

Selected for enlarged lateral buds

Kale

Selected for larger leaves

Kohlrabi

Selected for swollen stems

Romanesco

Selected for spiral clusters of growing tips

Collard greens (Brassica oleracea).
Collard greens

Selected for broad leaves

How did humans turn one humble plant into so many different vegetables?

And if we have done this to plants, what has selection done to animals?

THE DEFINITIONS

Artificial selection is a story about human preference.

NATURAL SELECTION

The environment decides

Natural selection favors inherited traits that help individuals survive and reproduce in their environment.

ARTIFICIAL SELECTION

Human priorities decide

Artificial selection, also called selective breeding, is the process by which humans choose which plants and animals reproduce based on desired traits.[11]

TWO DIFFERENT PROCESSES
Selective breeding works through reproduction across generations. Genetic engineering alters DNA directly, without relying on successive generations of selective breeding.
SELECTION IN EVERYDAY FOOD

Artificial selection has shaped what we eat

By selecting heritable traits such as color, sweetness, and cold tolerance, people have developed different forms of carrots and other crops over generations.

THE TERMINOLOGY

Different form does not mean different species.

A species is a biological lineage. Human selection can create remarkably different-looking members within that lineage without creating a new species.

BIOLOGICAL LINEAGE

Species

Reproduction, ancestry, and genetics help scientists identify it.

HUMAN SELECTION
PLANTS

Cultivar

A named, deliberately maintained form of a cultivated plant.

ANIMALS

Breed

A human-defined population within a domesticated animal species, maintained for selected traits.

Red, green, yellow and multicolored apples showing variation among apple cultivars.
SAME SPECIES · DIFFERENT CULTIVARS

Many apple cultivars

Fuji, Gala, and Honeycrisp are among the 7,500 apple cultivars developed and maintained for different characteristics.[8]

Six dogs with different body sizes, leg lengths, coat types and face shapes, illustrating diversity produced by selective breeding.
SAME SPECIES · DIFFERENT BREEDS

Many forms. One species.

Dogs differ dramatically in size, coats, and body shape, but they are all domesticated dogs.

Kale and broccoli are different cultivars, not different species. Yorkshires and pot-bellied pigs are different breeds, not different species.

FROM TEOSINTE TO CORN

Selective breeding transformed the humble teosinte’s few fruitcases (left) into modern corn’s rows of exposed kernels (right).

Artificial selection in corn: a small teosinte seed head beside progressively larger ears with more exposed kernels.
Two dog skulls, one large with a long muzzle and one small with a short muzzle, illustrating differences in head shape.

SELECTION SHAPES ANIMALS

From wolf ancestors to Chihuahuas: artificial selection has radically reshaped animal bodies too.[12]

BEFORE AND AFTER SELECTION

One lineage,
very different bodies

Over thousands of years, across generations, humans have selectively bred other species of animals.

Domestication begins with wild ancestors. Repeated human choices can transform their descendants in appearance, behavior, and biology.

HOVER, TAP, OR FOCUS AND PRESS ENTER TO REVEAL.

WILD ANCESTORWild boar

Sus scrofa

SELECTED INTOPot-bellied pig

Sus scrofa domesticus

Selected over generations for smaller stature and a distinctive pot-bellied shape.

WILD RELATIVEGray wolf

Canis lupus

SELECTED INTODomesticated dog

Canis lupus familiaris

Selection amplified sociability, trainability, and an extraordinary range of forms and behaviors.

WILD ANCESTORRed junglefowl

Gallus gallus

SELECTED INTO“Broiler” chicken

Gallus gallus domesticus

Modern breeding prioritized rapid growth, feed food efficiency, and dramatically greater breast muscle. [1]

Molly Condit / Sinergia Animal / We Animals
CHAPTER 2 · THE SCIENCE
FOCUSED SELECTION · DRAMATIC RESULTS

Change begins with a difference.

Humans do not create a trait from nothing. They notice variation, choose a heritable trait, and influence who contributes to the next generation.

Imagine wanting redder apples. You save seeds from the trees with the reddest fruit, grow those seeds, then select again. No single choice transforms the population. Repetition does.

VARIATION + INHERITANCE + CHOICE × TIME
01
Variation in apple colorFour apples with different colors represent variation within a population.
START HERE

Variation

Individuals in every population differ. Color, size, growth, behavior, and countless other characteristics are traits.

02
Two illustrative inheritance examplesAn orange parent is followed by yellow, orange and red offspring. A green parent is followed by green, yellow and green offspring. These are illustrations, not predicted inheritance ratios or apple colors.

*Illustrative examples

WHAT CAN CONTINUE

Heritability

Some differences can be passed from parents to offspring. Only heritable variation can accumulate across generations.

Offspring can inherit traits without looking identical to their parents.

03
Choosing a desired traitA ring highlights the red apple among yellow, orange, red and green apples.
THE HUMAN DECISION

Selection

Humans choose which individuals reproduce, or whose reproductive material is used, based on a desired trait.

04
Selected traits across generationsA schematic pattern shows the selected red trait becoming more common over repeated generations. The dots are not measured data or predicted ratios.EarlierLaterLater still
ACROSS GENERATIONS

Repetition

As selection is repeated, chosen traits become more common and the population gradually changes.[13]

Selection is not one dramatic event. It is a direction, applied generation after generation.

SELECTED ACROSS GENERATIONS

The traits humans selected for

Humans have selected heritable traits since before the rise of human civilization, shaping animals’ bodies, bodily functions, and behavior. Across generations, these choices have produced the extraordinary range of domesticated animals we see today.

HOVER, TAP, OR FOCUS AND PRESS ENTER TO LOOK CLOSER.

Hairlessness

Hairlessness

By selecting for less hair, humans have developed breeds like the sphynx cat which are hairless.
DOMESTICATED CAT

Hair Color

Hair Color

Almost all breeds of domesticated sheep have been selected for fleece color, especially white.
DOMESTICATED SHEEP

Endurance

Endurance

Humans have selectively bred sled dogs for endurance over long distances.
SLED DOG

Milk Yield

Milk Yield

Humans have selectively bred many breeds of cows to produce more milk.
DOMESTICATED COW

Growth Rate

Growth Rate

"Broiler chicken" breeds like the Cornish Cross have been artificially selected for faster growth.
BROILER CHICKEN

Litter Size

Litter Size

Humans have selectively bred American Yorkshire pigs to have larger litters.
DOMESTICATED PIG

Temperament

Temperament

Humans often selected for docility in dogs, creating breeds like Beagles. This very temperament also has been exploited in animal testing facilities.
DOMESTICATED DOG

Eye Color

Eye Color

Humans have artificially selected Ragdoll cats for vivid blue eyes.[9]
DOMESTICATED CAT

Size

Size

Humans have selectively bred Chihuahuas for small size.[10]
DOMESTICATED DOG
PAUSE AND CONSIDER

A trait can be useful to an industry without being beneficial to the individual animal.

CHAPTER 3 · THE COST

But, who pays the price?

Humans choose the traits. Fellow animals live with the consequences.

WHEN “MORE” BECOMES THE MEASURE

If a human infant grew at the same velocity as a conventional Cornish Cross chicken, at two months old they would weigh more than 600 pounds.

A proportional comparison, not a prediction of human growth: the study’s 2005 broilers grew from 44 g at hatch to 4,202 g at 56 days, a 95.5-fold increase. Applied to a 7-pound newborn, that is about 669 pounds at eight weeks. Study data.

Artificial selection and broiler growth: chicken strains from 1957, 1978 and 2005 at 0, 28 and 56 days. At 56 days they weigh 905, 1,808 and 4,202 grams respectively.

Selection for growth from 1957 to 2005 produced much larger broiler chickens at the same ages. The figure compares the 1957, 1978, and 2005 strains at 0, 28, and 56 days.

Figure: Zuidhof et al. (2014). Study and source [1].

SPECIES BY SPECIES

The animals behind the system

In animal agriculture, the selected traits tend to share one goal: greater output in less time.

A mother cow who gave birth a few hours ago walks with a full udder and the remaining placenta hanging out. At this farm, calves are taken from their mothers within an hour of being born.
Image: Jo-Anne McArthur / We Animals.

01

Holstein cows

Selected trait · Milk yield

Humans have selectively bred cows such as Holstein-Friesians to produce ever more milk, placing enormous demands on their bodies. Selection for higher yields can also increase susceptibility to painful udder infections and lameness.[3][4] Dairy cows can live around 20 years,[23] but commercial dairy cows commonly live about 5 years.[22] Health and fertility problems, alongside commercial culling decisions, cut their lives short.[23]

At a glance

Milk Yield

the selected trait

4X

milk per U.S. dairy cow compared with 1945[20]

~28,000 lb

annual milk yield in recorded U.S. Holstein herds[20]

⚠ Documented health problems

Nearly 1 in 4 U.S. dairy cows experienced clinical mastitis in 2013, a painful inflammation of the udder.[21]

A Cornish Cross “Broiler” chicken struggles to stand inside an industrial chicken farm. Most are unable to place weight on their feet without soon collapsing, eventually resting on their joints.
Image: Molly Condit / Sinergia Animal / We Animals

02

Cornish Cross (“broiler”) chickens

Selected trait · Rapid growth and large breast muscles

Broiler chicken breeds like Cornish Cross have been selectively bred to grow rapidly and develop large breast muscles, producing more body mass in less time.[1] Their bodies bear the cost: rapid growth increases the risk of painful leg problems, difficulty walking, and heart and lung disorders.[2]

Breeding timeline

112 days

1925 — to market weight

63 days

1960 — to market weight

47 days

2020 — to market weight

⚠ Documented health consequences

Heart failure, leg deformities, chronic pain

A young hen in a cage-free factory farm.
Lukas Vincour / Zvířata Nejíme / We Animals

03

White Leghorn hen

Selected trait · Egg laying

Humans have selectively bred White Leghorn hens for intensive egg laying. Some selected White Leghorn lines lay around 320 eggs a year.[5] Forming eggshells draws calcium partly from their skeletons, and sustained laying compromises bone strength.[5] Fractures of the breastbone can cause pain and make everyday movements harder.[6]

At a glance

300+

Eggs per year
White Leghorn hen[5]

~10–15

Eggs per year
Red junglefowl in the wild [7]

⚠ Documented health consequences

Compromised bone strength, painful keel-bone fractures, and difficulty with everyday movement. [5] [6]

A Yorkshire mother behind metal bars, with her piglets in the neighboring space.
Image: Jo-Anne McArthur / Sibanye Trust / We Animals.

04

American Yorkshire pig

Selected trait · Large litters

Humans have selectively bred American Yorkshire pigs for large litters, heavier piglets, muscular bodies, and little backfat. The pork industry calls them “the mother breed.”[14] In modern commercial systems, litters average 14–17 piglets, while a mother pig generally has only 13–15 teats capable of nursing.[15] When piglets outnumber teats, competition for milk intensifies. Large litters are linked to lower birth weights, hunger, injuries, and increased mortality before weaning.[15][16]

At a glance

Large Litters

the selected trait

14–17

piglets per commercial litter

13–15

teats capable of nursing per mother pig

⚠ Documented health consequences

Large litters are linked to lower birth weights, hunger, injuries, and increased mortality before weaning.[15][16]

A female turkey reaches up to drink water from a water dispenser at an industrial turkey farm in Poland.
Image: Andrew Skowron / We Animals

05

Broad Breasted White turkey

Selected trait · Rapid growth and large breast muscles

Humans have selectively bred commercial turkeys such as the Broad Breasted White for rapid growth, heavy bodies, and exceptionally large breast muscles. Over roughly 60 years, commercial turkeys came to reach twice the body mass in half the time.[17] Their breast muscles grew disproportionately compared with their leg muscles. Selection for rapid growth and increased breast muscle is associated with leg weakness, skeletal disorders, lameness, and difficulty walking.[17][18] Their selected body shape also makes natural mating impractical. Nearly all turkeys raised for food in the United States are conceived through artificial insemination.[19]

At a glance

Rapid Growth

the selected trait

2× Body Mass

in half the time[17]

~100%

conceived through artificial insemination in the United States[19]

⚠ Documented health consequences

Leg weakness and deformities, abnormal gait, lameness, muscle damage, reduced mobility, and cardiovascular problems.[17][18]

CHAPTER 4 · OUR RESPONSIBILITY

Sanctuary bears witness and provides care

Most farmed animals are killed while they are still very young. At sanctuary, these survivors live long enough for their bodies to tell the story of what they were bred to produce.

A turkey supported by a padded sling on a wheeled mobility frame indoors.

Bill was a broad-breasted turkey, bred to gain an enormous amount of weight in very little time. His legs could not support the body humans had bred him to inhabit. A custom mobility device helped him move.

A pink pig with upright ears resting on straw inside a wooden shelter.

Blue-eyed Wilbur was born into a body bred for rapid growth and production. As he aged, severe arthritis increasingly prevented him from moving through that enormous body. Alert and engaged with the world, he ultimately died because his body could no longer carry him.

A white chicken with a red comb standing in a doorway.

Bixby was bred to grow rapidly, reaching in weeks a body size his bones and joints struggled to sustain. Even with sanctuary care, the body selected for him shortened his life.

They are not broken animals.

They are fellow animals living in bodies we asked too much of.
AT THE SANCTUARY

Here they get to be more than a “trait.” They get to be somebody.

At Odd Man Inn, farmed animals are known as individuals, not units of production. Their bodies may carry the legacy of selective breeding, but their lives are their own.

Sanctuary care means adapting environments, nutrition, mobility support, and veterinary care to meet each resident where they are. It also means paying attention to who they are: their friendships, preferences, curiosity, and joy.

Before you continue

Three ideas to carry with you

01

Human preferences compound across generations, reshaping animals’ bodies and the lives they can lead.

02

Industrial productivity is not a measure of wellbeing for the individual animals trapped within the system.

03

Understanding the system helps us make more compassionate choices and advocate for change.

CHAPTER 5 · TAKING ACTION

What will you do with what you know?

Wherever you’re starting, we’re here to help. No judgment.

Change begins when what we learn changes what we notice, and what we notice changes the choices we make. Here are a few places to start.

Choose Vegan

For farmed animals living with the consequences of artificial selection, choosing vegan means refusing to fund the system that breeds and exploits them for food and other human uses.

Adopt, Don’t Shop

Even when breeders provide good care, breeding places human preferences in control of other animals’ reproduction. Adoption offers another path: giving an existing animal a home rather than creating demand for more animals to be bred. If you’re drawn to a particular breed, look for a breed-specific rescue.

Keep Learning

Don’t turn away from discomfort. Keep asking questions, reading, and having conversations.

Share With Your Friends

Share what you’ve learned with the people in your life and help the conversation reach further.

CITATIONS

  1. Zuidhof, M. J., et al. (2014). Growth, efficiency, and yield of commercial broilers from 1957, 1978, and 2005. Poultry Science, 93(12), 2970–2982. ↑ Back to text
  2. EFSA Panel on Animal Health and Welfare (2023). Welfare of broilers on farm. EFSA Journal, 21(2), 7788; section 3.6. ↑ Back to text
  3. Pritchard, T., Coffey, M., Mrode, R., & Wall, E. (2013). Genetic parameters for production, health, fertility and longevity traits in dairy cows. Animal, 7(1), 34–46. UK Holstein-Friesian study. ↑ Back to text
  4. Heringstad, B., Klemetsdal, G., & Steine, T. (2007). Selection responses for disease resistance in two selection experiments with Norwegian Red cows. Journal of Dairy Science, 90(5), 2419–2426. ↑ Back to text
  5. Eusemann, B. K., et al. (2020). The Role of Egg Production in the Etiology of Keel Bone Damage in Laying Hens. Frontiers in Veterinary Science, 7, 81. ↑ Back to text
  6. Nasr, M. A. F., Nicol, C. J., & Murrell, J. C. (2012). Do Laying Hens with Keel Bone Fractures Experience Pain? PLOS ONE, 7(8), e42420. ↑ Back to text
  7. Romanov, M. N., & Weigend, S. (2001). Analysis of Genetic Relationships Between Various Populations of Domestic and Jungle Fowl Using Microsatellite Markers. Poultry Science, 80(8), 1057–1063; introduction, p. 1057 (general estimate for red junglefowl in the wild). ↑ Back to text
  8. Wikipedia contributors. List of apple cultivars. Accessed October 16, 2026. ↑ Back to text
  9. Wikipedia contributors. Ragdoll. Accessed October 16, 2026. ↑ Back to text
  10. Wikipedia contributors. Chihuahua (dog breed). Accessed October 16, 2026. ↑ Back to text
  11. Wikipedia contributors. Selective breeding. Accessed October 16, 2026. ↑ Back to text
  12. National Geographic Society. Artificial Selection. Accessed October 16, 2026. ↑ Back to text
  13. University of California Museum of Paleontology, Understanding Evolution. Artificial selection. Accessed October 16, 2026. ↑ Back to text
  14. National Swine Registry. About Yorkshire. Accessed October 17, 2026. ↑ Back to text
  15. Vande Pol, K. D., et al. (2021). Effect of rearing cross-fostered piglets in litters of differing size relative to sow functional teat number on preweaning growth and mortality. Translational Animal Science, 5(4), txab193. ↑ Back to text
  16. Rutherford, K. M. D., et al. (2013). The welfare implications of large litter size in the domestic pig I: biological factors. Animal Welfare, 22(2), 199–218. ↑ Back to text
  17. Stover, K. K., Weinreich, D. M., Roberts, T. J., & Brainerd, E. L. (2018). Patterns of musculoskeletal growth and dimensional changes associated with selection and developmental plasticity in domestic and wild strain turkeys. Ecology and Evolution, 8(6), 3229–3239. ↑ Back to text
  18. Kremer, J. A., Robison, C. I., & Karcher, D. M. (2018). Growth Dependent Changes in Pressure Sensing Walkway Data for Turkeys. Frontiers in Veterinary Science, 5, 241. ↑ Back to text
  19. Weaver, T. (1998, July 23). Turkeys: The Good, The Bad, and The Fertile. USDA Agricultural Research Service. ↑ Back to text
  20. Council on Dairy Cattle Breeding. Impact on U.S. Dairy. U.S. per-cow milk production compared with 1945; approximately 28,000 lb per cow annually in U.S. Holstein Dairy Herd Information (DHI) herds. Accessed October 17, 2026. ↑ Back to text
  21. USDA APHIS (2016). Dairy 2014: Milk Quality, Milking Procedures, and Mastitis on U.S. Dairies, 2014. September 2016, p. 46, table E.1.a: 24.8% of cows had producer-identified clinical mastitis at some point during 2013. ↑ Back to text
  22. Alberta Milk. How long does the average dairy cow live? Canadian dairy-industry guidance: typical average lifespan of five years, including the first two years before milking. Accessed October 17, 2026. ↑ Back to text
  23. Penn State Extension. Have Your Cows Repaid their Debts? Dairy longevity and commercial herd economics; approximately 20-year potential lifespan. Accessed October 17, 2026. ↑ Back to text
  24. University of Florida IFAS. Cole Crop Confusion; University of Washington. Brassica selection diagram, lecture slides, January 10, 2022. Selected leaves, stems, terminal buds, and lateral buds. ↑ Back to text
  25. Duclos, D. V., & Björkman, T. (2008). Meristem identity gene expression during curd proliferation and flower initiation in Brassica oleracea. Journal of Experimental Botany, 59(2), 421–433. ↑ Back to text
  26. Azpeitia, E., et al. (2021). Cauliflower fractal forms arise from perturbations of floral gene networks. Science, 373(6551), 192–197. Open author manuscript. ↑ Back to text

IMAGE CREDITS

  1. Chartreuse eye color (human). Squeaks41. CC0 1.0.
  2. Apples. Scott Bauer / USDA Agricultural Research Service. Public domain in the United States (U.S. government work). Rotated 90° clockwise and resized.
  3. Carrots of many colors. Stephen Ausmus / USDA Agricultural Research Service. Public domain in the United States (U.S. government work). Rotated 90° clockwise and resized.
  4. Cornselection. John Doebley. CC BY 2.5. Background modified.
  5. Unnatural selection, 2 heads, one species. Dmccabe. CC BY-SA 3.0.
  6. Dog morphological variation. Mary Bloom / American Kennel Club. CC0 1.0.
  7. Dzik na ulicy Basistówka w Krakowie, 20250301 1042 3390. Jakub Hałun. CC BY 4.0.
  8. Front view of a resting Canis lupus ssp. Gary Kramer / U.S. Fish and Wildlife Service. Public domain in the United States (U.S. government work).
  9. Gallus gallus, Khao Yai Lam Ta Khong, Thailand 477443293. dhfischer. CC BY 4.0.
  10. Chat Sphynx. Dmitry Makeev (Knopik-som), © SAMOLAZ.COM. CC BY-SA 4.0.
  11. A curious Welsh Mountain sheep (Ovis aries). Seraaron (SeraaronArt). CC BY-SA 4.0.
  12. Greenland 467 (35130903436). Markus Trienke. CC BY-SA 2.0.
  13. Nach rechts blickende Milchkuh. 4028mdk09. CC BY-SA 3.0.
  14. White Rooster (159577385). Kelly Fletcher. CC BY-SA 3.0.
  15. Two domestic piglets suckling. Wiki.cullin. CC BY-SA 4.0.
  16. Beagle puppy Cadet. Soccersmp at English Wikipedia. Public domain worldwide by the author’s release.
  17. Ragdoll cat DSC07500 (4021848667). Cássia Afini (CassiaA). CC BY 2.0.
  18. Chihuahua lala café claro. Banne27. CC BY-SA 4.0.
  19. Pug portrait. existentist (John Attebury); color and temperature correction by Elpoye. CC BY 2.0.
  20. Chicken growth comparison: Figure 1 from Zuidhof, M. J., Schneider, B. L., Carney, V. L., Korver, D. R., & Robinson, F. E. (2014), Growth, efficiency, and yield of commercial broilers from 1957, 1978, and 2005, Poultry Science 93(12), 2970–2982. © 2014 Poultry Science Association Inc. CC BY-NC 3.0. Reproduced in web image format.
  21. Black-and-white cows in a barn, with one standing cow looking back toward the camera. Jo-Anne McArthur / We Animals.
  22. White chickens on a litter-covered floor, with a central chicken facing the camera. Molly Condit / Sinergia Animal / We Animals.
  23. A sow behind metal bars with piglets in the neighboring space. Jo-Anne McArthur / Sibanye Trust / We Animals.
  24. White turkeys gathered around a drinking-water line indoors. Andrew Skowron / We Animals.
  25. Laying hen in an aviary system, Czechia, 2021. Lukas Vincour / Zvířata Nejíme / We Animals.
  26. Brassica oleracea wild. Kulac (Kurt Kulac). CC BY-SA 2.5.
  27. Starr 070730-7852 Brassica oleracea var. capitata. Forest & Kim Starr; contrast and levels adjusted by Maurilbert. CC BY 3.0. Resized for display.
  28. Broccoli close up (49200262862). Bruce Dupree / Alabama Extension. CC0 1.0.
  29. Cauliflower heads on a shelf. Jeffery Martin. CC0 1.0.
  30. Brussels sprouts – San Francisco, CA. Daderot. CC0 1.0.
  31. Curly kale (8148834278). Ruth Hartnup. CC BY 2.0.
  32. Brassica oleracea var. gongylodes (kohlrabi). Photographer unidentified; provided via rawpixel.com. CC0 1.0.
  33. Romanesco photo: Ivar Leidus / Wikimedia Commons, CC BY-SA 4.0. Cropped for display.
  34. Collard greens photo: Eric Bronson (I likE plants!), via Wikimedia Commons, CC BY 2.0. Cropped for display.

Resident photographs of Irish, Bill, Blue-Eyed Wilbur, Bixby, Molly Jo, Marsha, and Florence: Odd Man Inn Animal Refuge.