Selective Breeding Shows Populations Change Fast When Humans Pick the Winners
Definition
Selective breeding
Selective breeding, or artificial selection, is the process by which humans intentionally breed animals or plants to enhance specific traits.
In artificial selection, humans are the selective agent, choosing which individuals breed.
In natural selection, environmental pressures decide which individuals survive and reproduce.
Both processes shift allele frequencies across generations, which is why Darwin used breeding as evidence for evolution.
Hint
Domesticated species let us watch heritable change over years, not the millennia natural selection usually needs.
Breeders Repeat One Simple Loop Every Generation
A population already holds heritable variation from mutation and recombination.
Humans select parents that show a valued trait, such as high yield or docility.
These parents breed, and the offspring are screened for the trait.
Only offspring with the strongest expression breed again.
Over many generations the favoured alleles become fixed in the population.
Tip
The loop only works because the selected trait is heritable, so it passes to offspring.
Domesticated Animals Diverged Fast From Wild Ancestors
Dogs from gray wolves
Every modern breed descends from the gray wolf, Canis lupus.
Selection for size, coat, and behaviour produced the Siberian husky (bred for pulling sleds), the dachshund (bred for hunting burrowing animals), and the size gap between a chihuahua and a Great Dane.
Chickens from red junglefowl
Modern laying hens produce hundreds of eggs a year, while their wild ancestor lays only a few.
Case study
Belgian Blue cattle carry a mutation in the myostatin gene, which normally limits muscle growth.
Breeding individuals that carry the mutation produced cattle with greatly increased muscle mass for meat.
This rapid, targeted shift shows selective breeding can reshape a population in just a few generations.
One Wild Plant Can Become Many Different Crops
Maize from teosinte
Modern maize evolved from a wild grass called teosinte, which has small, hard kernels.
Farmers selected plants with larger, softer kernels until they had today's high-yield crop.
Many vegetables from one wild mustard
Wild mustard, Brassica oleracea, is the single ancestor of cabbage, broccoli, kale, and cauliflower.
Selection on different parts gave each crop: leaves for cabbage and kale, flower buds for broccoli, and dense flower heads for cauliflower.
Note
The same ancestral genome can yield radically different phenotypes when humans select for different traits.
Selective Breeding Is Direct Evidence That Populations Evolve
It shows heritable traits change over generations, fast enough to observe.
It shows allele frequencies shift when certain traits are favoured, exactly as in natural selection.
It shows small genetic differences accumulate into large phenotypic divergence between breeds and varieties.
It differs from natural selection only in the selective agent: human choice rather than the environment.
Theory of Knowledge
Breeding for extreme traits can leave some dog breeds with serious health problems.
Where should we draw the line on human intervention in the traits of other species?
Breeding Is a Miniature Model of Evolution
Heritable variation exists in populations.
Selected traits accumulate over generations.
New varieties emerge, showing that gradual change can have large evolutionary consequences.
Active recall
Why is selective breeding treated as evidence for evolution?
How did one wild mustard give rise to cabbage, broccoli, kale, and cauliflower?
What does the Belgian Blue example show about how quickly allele frequencies can shift?
How is artificial selection similar to and different from natural selection?
