Darwin’s theory of natural selection is not the only method of evolution. Research shows that some evolutionary traits in animals can occur from embryo development through to adulthood.
The researchers used a roundworm species. They changed just two genes: this caused females to become self-fertile hermaphrodites.
Self-fertile hermaphrodite roundworms were found to produce smaller sperm than their male counterparts. Because the animals were in a controlled laboratory setting, natural selection could not have played a part.Read more at University of Medicine and Dentistry of New Jersey