It's all in da gene: muscles

"Horses ain't like people, man, they can't make themselves better than they're born. See, with a horse, it's all in the gene. It's the fucking gene that does the running. The horse has got absolutely nothing to do with it." --- Paulie (Eric Roberts) in The Pope of Greenwich Village.

Myostatin gene mutation and enhanced musculature. Guess where it's common, and guess where it is extremely rare (results below)?

The second link refers to the same gene in dogs, where the evidence for the phenotypical effect (increased speed, musculature) is stronger. But I suppose they're just dogs and, well, we're different...

Humans:

GNXP.com:

...this preprint [pdf] of a paper arguing for an ongoing recent selective sweep in the gene encoding myostatin, an inhibitor of muscle growth. The evidence for selection is based on resequencing of the gene in 146 people, which allows them to perform full-sequence-based tests for selection. They also look at the surrounding haplotypes, which are suspiciously long around the derived alleles (thus hinting at selection).

They speculate that the amino acid changes they find might lead to downregulation of the protein, thus leading to increased muscle mass. Notably, these variants seem to only have a large hold in Sub-Saharan Africa.

Bantu N.E., Kenya (12: 29%, 25%)
Bantu S.W., South Africa (8: 6%, 31%
Mandenka, Senegal (23: 24%, 20%)
Yoruba, Nigeria (24: 8%, 19%)
San , Namidia (7: 0%, 14%)
Mbuti Pygmies, Democratic Republic of Congo (14: 4%, 29%)
Biaka Pygmies , Central African Republic (36: 8%, 14%)
Mozabite, Algeria (Mzab)(29: 2%, 2%)
Orcadian, Orkney Islands (15: 0%, 3%)
Adygei, Russia Caucasus (17: 0%, 3%)
Russian, Russia (25: 0%, 0%)
French Basque, France (24: 0%,4%)
French, France (27: 0%, 2%)
North Italian, Italy (Bergamo) (14: 0%, 0%)
Sardinian, Italy (28: 0%, 7%)
Tuscan, Italy (8: 0%, 6%)
Bedouin, Israel(Negev) (48: 0%, 5%)
Druze, Israel (Carmel) (47: 0%, 1%)
Palestinian, Israel(Central) (49: 1%, 9%)
Balochi, Pakistan (25: 0%, 4%) Brahui, Pakistan (25:2%, 2%)
Makrani, Pakistan (25: 0%, 12%)
indhi, Pakistan (25: 0%, 4%)
Pathan, Pakistan (24: 2%, 0%)
Burusho, Pakistan (25: 0%, 4%)
Hazara, Pakistan
(22: 0%, 0%)
Uygur, China (9: 0%, 0%)
Kalash, Pakistan (25: 0%, 0%)
Han, China (43: 0%, 1%)
Dai, China (10: 0%, 0%)
Daur, China (10: 0%, 0%)
Hezhen, China (10: 0%, 0%)
Lahu, China (10: 0%, 0%)
Miaozu, China (10:0%, 0%)
Oroqen, China (10: 0%, 0%)
She, China (10: 0%, 0%)
Tujia, China (8: 0%, 0%)
Tu, China (10: 0%, 0%)
Xibo, China (9: 0%, 0%)
Yizu, China (10: 0%, 0%)
Mongola, China (10: 0%, 5%)
Naxi, China (10: 0%, 0%)
Cambodian, Cambodia (10: 0%, 0%)
Japanese, Japan (30: 0%, 0%)
Yakut, Siberia (24: 0%, 2%)
Melanesian, Bougainville (22: 0%, 0%)
Papuan, New Guinea (17: 0%, 0%)
Karitiana, Brazil (24: 0%, 0%
Surui, Brazil (21: 0%, 0%)
Colombian, Colombia (13: 0%, 0%)
Maya, Mexico (24: 0%, 0%)
Pima, Mexico (25: 0%, 0%).

Dogs!

GNXP.com:

...a mutation in dogs that leads to increased muscle mass and speed:

Here we describe a new mutation in MSTN found in the whippet dog breed that results in a double-muscled phenotype known as the "bully" whippet. Individuals with this phenotype carry two copies of a two-base-pair deletion in the third exon of MSTN leading to a premature stop codon at amino acid 313. Individuals carrying only one copy of the mutation are, on average, more muscular than wild-type individuals (p = 7.43 × 10-6; Kruskal-Wallis Test) and are significantly faster than individuals carrying the wild-type genotype in competitive racing events (Kendall's nonparametric measure, τ = 0.3619; p ≈ 0.00028). These results highlight the utility of performance-enhancing polymorphisms, marking the first time a mutation in MSTN has been quantitatively linked to increased athletic performance.

The myostatin gene in humans plays a similar role in muscle growth-- a deletion in the gene leads to extreme muscularity, as evidenced by the "Baby Superman". The gene has also been shown to be under recent selection in humans--variants presumed to lead to more muscularity are far more common in Sub-Saharan Africa.