Crossed signals, or evolutionary mismatches, occur when traits that were adaptive in an ancestral environment are no longer advantageous, or may even be detrimental, in a changed environment. This concept is commonly used in the context of evolutionary biology, particularly when examining the impact of rapid environmental changes on species.

These ecological or environmental imbalances arise due to evolutionary lags - situations where traits are not able to adapt quickly enough to keep up with the changing conditions. As a result, a trait that may have been beneficial in the past can become maladaptive in the present, leading to a decline in fitness or reproductive success.

Examples of crossed signals include:

- Melanism in urban environments: In many cities, darker-colored variants of organisms, such as moths or birds, have become more common due to industrial pollution. Initially, this darker coloration provided an advantage as it helped camouflage these organisms against the sooty environment, allowing them to avoid predators more effectively. However, as cities have become cleaner, the darker coloration has become a disadvantage, making them more visible to predators against a lighter backdrop.

- Antibiotic resistance: When antibiotics were first introduced, they provided a significant advantage against bacterial infections. However, the widespread and sometimes excessive use of antibiotics has led to the evolution and spread of antibiotic-resistant bacteria. Now, certain infections have become more challenging to treat due to the "crossed signals" created by antibiotic overuse.

- Diabetic complications: The modern human diet, often rich in refined carbohydrates and sugars, has resulted in an increase in the prevalence of type 2 diabetes. Although this dietary shift may have provided an advantage during times of scarcity by ensuring consistent energy intake, the high levels of sugar and processed carbohydrates in today's diet can lead to complications like heart disease, kidney damage, and eye problems due to an ancestral adaptation that is no longer suitable.

Crossed signals highlight the importance of adaptation in response to environmental change. As the environment changes more rapidly due to human activities, understanding and predicting these mismatches becomes essential for conserving biodiversity, managing ecosystems, and addressing public health challenges.