Pseudogenes: Whales have lost the ability to discern colors

Vision in vertebrates (fish, lizards, birds, mammals, etc.) begins in the back of the eye in a layer called the retina. The retina is composed of cells that are intrinsically sensitive to light. When light hits these cells, it changes the energy present in light into electrical energy, which travels through nervous tissue to the brain where it is processed and interpreted as what we call vision.

The light-sensitive cells come in two general varieties: rods and cones (Figure 1). Rods are used in dim-light conditions, as they are highly sensitive, and provide you with coarse, colorless vision, while cones are used in bright-light conditions and facilitate our ability to see color. An example I like to use to help people distinguish their cones from their rods involves movie theaters. Ever notice that when you walk into a movie theater, your vision goes from crisp and colorful to coarse and relatively devoid of color? You probably also notice that initially it’s very difficult to see where you’re walking, but slowly over time your vision gets better. This is your eyes switching over from using your cones to your rods, and since bright light saturates rods, rendering them nonfunctional, they need to regenerate when you’re in the dark. The bright movie screen, however, is activating populations of your cones, hence why you’re able to see crisp, colorful images on the screen, but not in your periphery.

Sagschem

A diagram showing an eye with an inset showing the retina. On the right of the retina inset, rods are shown in black, and cones are shown in red, green and blue.

So why does color vision exist only for cones but not for rods? There are a few reasons, but at the most basic level, it’s because rods have only one kind of light-sensitive pigment, whereas your cones have three kinds that are specialized for different colors of light. Basically, since your cones have multiple inputs of color to compare whereas your rods have just one, your cones are able to facilitate color vision but your rods do not. These pigments, called opsins, are proteins bound to a molecule derived from vitamin-A, and like all proteins, it’s coded for by a gene composed of DNA.

Though humans have three cone pigments, most mammals only have two. The idea that dogs, cats and horses can’t see colors is a myth. They can see colors, just far fewer than we can (Figure 2). So far, all hoofed mammals that have been examined have these same two pigments, blue opsin (coded by the gene SWS1) and red opsin (LWS), so called because they are sensitive to blue and red light, respectively. Since, evolutionary biologists believe whales have descended from hoofed mammals, as I’ve described in many of the posts in this series, one may wonder if whales have the same complement of cone pigments.

Protanopia-Color-Spectrum

At the bottom of this image is how most mammals see a rainbow.

McGowen et al. [1] highlights that all whales have actually lost the blue opsin, demonstrated by molecular studies and the presence of SWS1 pseudogenes. This means that all whales are color blind, even when their cones are functional, since they only have one cone pigment and therefore do not have multiple inputs to compare. In a paper I co-authored [2], Rob Meredith demonstrated that all toothed and baleen whales, respectively, have the same inactivating mutation present in their SWS1 gene. This suggests that this gene was lost in a common ancestor of each of the lineages, soon after both lineages had fully adapted to an aquatic lifestyle.

Why might this have occurred? In the monochromatic conditions of the open ocean, having the ability to discriminate colors probably did not have any advantages. As a result, when mutations occurred in the ancestors to toothed and baleen whales, respectively, the resulting pseudogene spread through the population and was inherited by all of those species’ descendants.

Additionally, in some other whales that frequently dive deep to feed on prey, such as sperm whales and beaked whales, they lost their red cone opsin as well, as evidenced by LWS pseudogenes [2]. Shared mutations can also be found in some of these whales. This means that these whales lack cones entirely, resulting in pure rod retinae, which is likely an extreme adaptation to low-light conditions.

Questions for creationists

Why would God create whales with nonfunctional versions of the blue and red cone opsin? Would it not have been simpler for God to create whales without these genes entirely?

References

1. McGowen, M. R., Gatesy, J., & Wildman, D. E. (2014). Molecular evolution tracks macroevolutionary transitions in Cetacea. Trends in ecology & evolution,29(6), 336-346.

2. Meredith, R. W., Gatesy, J., Emerling, C. A., York, V. M., & Springer, M. S. (2013). Rod monochromacy and the coevolution of cetacean retinal opsins.PLoS genetics9(4), e1003432.

Advertisements

2 thoughts on “Pseudogenes: Whales have lost the ability to discern colors

  1. Pingback: Pseudogenes: Subterranean mammals lose vision genes | Evolution For Skeptics

  2. Pingback: Pseudogenes: Sloths and armadillos cone pseudogenes suggest subterranean history | Evolution For Skeptics

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s