In this lesson, we’ll first review evolution, then we’ll look at one of the main types of evidence for evolution, comparative anatomy. We’ll look at homologous, analogous, and vestigial structures, and go over examples of each.
What Is Evolution?
If you look around outside, you probably see incredible biodiversity, even in your own neighborhood. Insects crawl on plants, avoiding predators like birds and lizards. The birds themselves hide in the trees.
How did all these forms of life become so unique from each other? The answer is evolution. Evolution is defined as a genetic change in a population over time. Over millions of years, one single cell evolved into all life we know today. It seems astounding, but scientists have lots of evidence for evolution. One form of that evidence is comparative anatomy – but we’ll get to that later.
When organisms reproduce, some of their genes get shuffled or mixed around. This is why offspring look different from their parents. Sometimes, this shuffling causes a new trait to form that helps the organism survive. For example, let’s say giraffes descended from a species with a short neck. Those short neck giraffes might have had one offspring with a little longer neck. That giraffe could reach the trees better, so it got more food, and was able to survive and reproduce.
Now, the next generation of giraffes might have even longer necks. So the long neck trait is passed down because it helps the giraffe survive better. After thousands of years, all giraffes have long necks. The genetics of the population has changed and created a new species different from the short neck ancestors.This all sounds great, but how do we know this really happened millions of years ago? Well, scientists have developed enormous amounts of evidence that evolution occurs. Today, we’re going to focus on one type of evidence, comparative anatomy.
What Is Comparative Anatomy?
Comparative anatomy involves comparing the body structures of two species.
‘Comparative’ means to look at the similarities between two things, and ‘anatomy’ has to do with the structure of the body. Scientists can look at anatomical structures of seemingly unrelated animals to tell how related they are.
Homologous structures are structures that are similar in two organisms because they have a common ancestor. For example, birds, humans, bats, and even whales all have a similar arm bone structure.
At first glance, you wouldn’t think whales and humans are very closely related, but millions of years ago, there was one ancestor whom we are both related to. That ancestor had offspring that were all a little different, and different traits were selected for it through evolution. New species were created, and even newer species evolved from those species. However, the arm bone structure was advantageous to all the species, and so it remained in all the descendant species. Now, although we all look different, birds, bats, whales and humans all retain the arm bone structure from our ancestors.
Analogous structures are the opposite of homologous structures. Analogous structures are anatomical features of two species that look similar, or serve the same purpose, but the species are not closely related. An example of an analogous structure is bird wings and insect wings. Although both are considered wings used for flight, the anatomical structure is very different, and like we have seen, bird wings are actually more similar to human hands than to insect wings.Analogous structures are caused by convergent evolution. In convergent evolution, two species evolve the same traits to adapt to an environment side by side, but they didn’t come from the same ancestor.
Think of analogous structures like a choice in jewelry. You and your best friend might choose the same pink necklace because you have similar taste, or because you are both going to the same party, but that doesn’t make you related. On the other hand, imagine if you and your sister both inherited the same style of necklace from your great grandmother. In both cases you and another person are wearing the same jewelry, but you and your sister inherited them together, whereas you and your friend just happened to purchase the same item.
Ever wonder what your appendix is for? Sometimes this small structure branching off the small intestine can become infected and will need to be surgically removed.
It’s a good thing we don’t actually need it! The appendix is a vestigial structure, or an anatomical structure that we inherited from an ancestor, but don’t use anymore. The appendix is a remnant of the cecum, an intestinal appendage that helps herbivores digest their grassy diet. Humans have a cecum, but it is just a dead-end structure with no function. Animals that still need to eat plenty of grass maintain their full cecum, like rabbits. Thus, although we might not look or act very similar, somewhere along the evolutionary timeline humans and rabbits shared a common ancestor.
Evolution is a change in the genetics of a population over time. Comparative anatomy is the study of the relatedness of species through examining anatomical structures.
Some structures, like homologous structures, indicate two organisms shared a recent common ancestor, like the arm bones in humans, bats, whales, and birds. Analogous structures, however, are structures that have the same purpose in species, but the species are not closely related. Analogous structures arise from convergent evolution.
Vestigial structures are anatomical features that are shared by different species but no longer have a purpose in one, like the appendix in humans, showing relatedness between the species.