[Editor: Quick answers are 1. Yes double yolk eggs can be viable. 2. They can be fertilized. 3. Each yolk represents one chick and so each one gets fertilized. 3. There does not seem to be any evolutionary advantage.]
My understanding is that fertilization occurs before the yolk is sealed in the shell so it seems likely that both yolks could include a zygotic nucleus, leading to the production of twins. Since the materials stored in the egg are normally consumed by one developing chick that nearly fills the shell upon maturity, the resulting twins are likely to be runts or deformed, so there is unlikely to be any selective advantage; on the contrary.
Colleage Dr. Michael J. Smith, also a biology professor at SFU adds:
- Double yolks occur naturally but rarely; generally due to simultaneous ovulation or egg release from the ovary;
- Double yolks may be fertilized (i.e. fertilization occurs shortly after egg release from the ovary). Fertilization would be 'independent' in a sense (not twinning as we perceive it).
- Fertilized embryos generally do not develop; not enough room in the egg shell etc.
- I can see no evolutionary advantage to double yolkism because because failure of development.
[EDITOR: A google search ultimately yielded lots of answers. It seems that certain types of chickens do carry a hereditary trait to lay double yolk eggs. A minority of farmers breed these to get a higher percentage of double yolk eggs for sale. It strikes me as odd that the genetic trait for double-yolk-ism is an inheritable thing because the survivability of chicks from such eggs is low. According to sites such as Dr. Karl Kruszelnicki's Great Moments in Science, as a chick finishes development within the egg it must seek and find a particular spot on the shell to peck its way out. Apparently when there are two chicks, the contest to peck this hole usually results in one and often both of the chicks dying. There are other reasons why two chicks may die when developing together in one egg. In any case, I suppose enough of them survive (not in the wild but in controlled farmyard breeding conditions) so that this trait is preserved.]
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