Three-spined stickleback fish live in lakes formed when the last ice age ended 10,000 to 15,000 years ago. In lakes where the sticklebacks are prey for larger fish, they develop 35 bony plates along their body as armor. In contrast, sticklebacks in lakes where there are no predators develop only a few or no bony plates. In crosses between fish of the two different morphologies, the lack of bony armor segregates as a recessive trait that maps to the ectodermal dysplasin (Eda) gene. Comparisons between the Eda-coding regions of the armored and nonarmored fish revealed no differences. How can you explain this result?

Key Concepts

Natural Selection

Natural selection is a fundamental mechanism of evolution where organisms better adapted to their environment tend to survive and reproduce more than those less suited. In the case of the three-spined stickleback fish, the presence of predators in certain lakes drives the development of bony plates as a protective adaptation, illustrating how environmental pressures can shape physical traits over generations.

Genetic Variation and Recessive Traits

Genetic variation refers to the differences in DNA among individuals, which can lead to different physical traits. In sticklebacks, the trait for bony armor is recessive, meaning that it only manifests when an individual carries two copies of the recessive allele. This concept is crucial for understanding how traits can segregate in offspring and how certain traits may not be expressed despite being present in the genetic makeup.

Eda Gene and Phenotypic Plasticity

The Eda gene is involved in the development of bony structures in fish and is a key player in the phenotypic plasticity observed in sticklebacks. Despite the lack of differences in the Eda-coding regions between armored and non-armored fish, variations in gene expression or regulatory elements may account for the observed phenotypic differences, highlighting how environmental factors can influence gene expression without altering the underlying DNA sequence.