It seems silly posting an essay for my zoology class on my blog, but I thought you all might find it interesting. The assignment was to explain homology to one of our idols who wouldn’t necessarily know what it was about. Part of this was using examples that would make sense to them. I chose Dr. William Sears and, of course, used breastfeeding as my example. So here you go! We learn something new every day, right?
I would first like to say, Dr. William Sears, that I am very honored to meet you. I have read many of your parenting books and strongly believe in attachment parenting. I could not have imagined how strongly I would believe in these ideals before I had my daughter, Dea. I would also like to say that I am honored to explain homology to you. I think that you will find the adaptations of breastfeeding, a very important concept of attachment parenting, particularly interesting.
So what is homology? In the most basic terms, it is similarity between two animals based on common descent. The problem with this definition is that it does not give us a true sense of how vital it is to understand homology. Homology is a very important concept in biology and we use this knowledge to help us in many aspects of life—scientific or not. The human brain works in a way such that it can find relationships between new concepts and the ones that we already understand, these relationships allow us to better understand the new concept. This organization of thoughts is what separates us from most other animals.
This is where homology comes in. Homology is how we can relate new species to the ones we already know, thereby forming a better understanding of the new species and giving a basis to which we can make hypotheses about the new species. Oftentimes, our initial assumptions about a new species are incorrect, which is the case with dolphins.
It is a common misconception that dolphins are fish. This misconception could be attributed to the fact that both fish and dolphins have a similar body shape. Rather than a closely shared ancestor, these two types of animals share a common environment—the water. The streamlined shape of their bodies allows for the most efficient movement in their environment, therefore both types of animals have separately evolved to have this body shape. When species share a trait because of shared pressures rather than common descent, it is called convergence. The streamlined shape of both dolphins and fish is a classic example of convergence.
When we delve deeper, we find that dolphins are actually more closely related to humans than the fish that share their environment. As you know, one of the cornerstones of attachment parenting is breastfeeding. It is the way that mammals are meant to feed their young, hence the name “mammary glands.” Of all the homologous traits that define an animal as a mammal, it is the only one that does not have an exception to the rule.
Since all mammals nourish their young with milk from their mammary glands, as opposed to fish larvae who are nourished via a yolk sac they carry with them, we can say that the common ancestor did the same. When I say common ancestor, we are saying a population from which all mammals were derived. As in the case of any population, there is variation from individual to individual. Without this variation we would not have evolved into all of the varying types of animals today that are classified as mammals. Some form of variation within the species must prove to be advantageous to at least a portion of the population for many generations for a species to evolve to represent that variation. Since the trait proves to be more advantageous time and time again, it is bred into the species by the fact that the individuals with this trait are better able to reproduce. This is called natural selection. Since the trait continues to be favored, the individuals with a more advantageous form of the trait will continue to be favored reproductively and therefore the trait will evolve to become better fit to their conditions. This is called adaptation.
Back to the example of dolphins, it may not initially be apparent that dolphins breastfeed their calves because it is different than what our species does to breastfeed. This is because of the variation in the population that proved to be beneficial to what eventually evolved to be a dolphin. Dolphin breast milk has a much higher fat content than human breast milk, which is logical considering that dolphins live in an environment where milk with a high water content would easily dissipate. This high fat content also allows the calf to breastfeed less frequently, thereby not putting the mother and calf at more risk than necessary considering they must stay mostly immobile and near the surface during feeding. The percentage of fat in dolphin breast milk can be as high as 50%, depending on the species of dolphin and the stage of lactation (West et al., 2006). Dolphins also have adapted their anatomy so that they are able to breastfeed while staying streamlined by hiding the nipple inside of a slit. Both of these adaptations, along with all the other adaptations that make a dolphin different from the common mammalian ancestor, must have evolved slowly over time. Hypothetically, if the fat content of the breast milk of the common ancestor was closer to 10%, it would have slowly gone up from generation to generation, possibly only by a fraction of a percentage point. Generation over generation must have continued to favor this increase in fat content for the adaptation to persist. So as you can see, this is a very slow and deliberate process.
Without homology we would have never understood the true evolution of dolphins and how they relate to humans and other mammals. But why does it matter to understand that dolphins are more directly related to humans than fish? Homology gives us a place to start. When we are able to see the relationships between different animals, we are better able to understand them and use this to our advantage. It could be something purely altruistic, like finding out how to preserve a species, or it could be something selfish, like finding new medicines to cure human diseases, or anywhere in between. There are so many different ways that this information can benefit ourselves and others and it all starts from the simple organization of homology.
West, K. L., O. T. Oftedal, J. R. Carpenter, B. J. Krames, M. Campbell, and J. C. Sweeney (2006) Effect of lactation stage and concurrent pregnancy on milk composition in the bottlenose dolphin. Journal of Zoology 273:148-160.