Depending on how online you are, you may have seen the plethora of articles and memes about carcinization. This is the phenomenon of various crustacean lineages eventually evolving into crabs. In Maine we have species of “true crabs,” like the finely-speckled Atlantic Rock Crab (Cancer irroratus). Elsewhere in the world, however, many other animals in the order Decopoda have evolved round, crab-like shells and the habit of scurrying sideways. This is convergent evolution, wherein species independently evolve similar traits to adapt to shared environmental pressures or to exploit the same resources. Examples of adaptations evolving two or more times litter the animal and plant kingdoms, from winged flight (bats, birds, many insects), to echolocation (bats and whales), to agriculture (humans and ants!) Here are several fun occurrences of convergent evolution that you can witness here in Maine.
Downy and Hairy Woodpeckers
There are a few tricks to differentiating Downy and Hairy Woodpeckers. Downys (Dryobates pubescens; pictured above) have smaller beaks that are about half the length of their head from front to back. Hairy Woodpeckers (Dryobates villosus) have beaks that are about the same length as their heads. Another sure-fire way to tell the difference would be to sequence their genomes, should you have the tools to do so! Despite looking almost identical to each other, the two species are not closely related. Their lineages diverged about 6.5 million years ago, which is around the same time hominids and chimps diverged. Eventually, they evolved to look almost exactly alike. While shared climate and habitat pressures can lead to similar plumage adaptations, it’s believed that their resemblance is a case of mimicry. Hairy Woodpeckers are larger than Downys and known for their dominance at the bird feeder and elsewhere. It’s been suggested that by resembling Hairys, Downy Woodpeckers can intimidate the other species with whom they share resources without having to be aggressive themselves. In other words, they look tough, but don’t actually have to get in the ring. In a species comparison, the only other distinguishing field note besides beak and overall size is some limited spotting on the outer tail feathers of the Downy Woodpecker. This and the fact that other birds may also confuse the two should make birders feel better about struggling to tell the difference!
Opposable thumbs
Did you know that o-possums have o-pposable “thumbs”? Together, humans and Virginia Opossums are the only mammals in North America with a digit that can move independently of the rest to grasp and manipulate objects. Our opposable thumbs descend from far back in our primate lineage and allowed us to climb trees for food and safety. The specialized bone structure and powerful muscles which allow precision grip evolved closer to 2 million years ago. Opposums can’t thread a needle, but they and their marsupial relatives have been grasping branches with opposable digits on their hind feet for more than 65 million years (yes, technically they have opposable toes!). This toe, which sticks out to the side, makes their tracks easily recognizable.
Birds also have an opposable “thumb” of sorts, although it’s not technically a thumb either. Fun fact: birds technically walk on their toes. What looks to us like their shin is actually fused, elongated foot bones, and what looks like their thigh is actually their shin. Birds’ femurs are usually hidden up in their feathers. All of this is why what appears to be their knee joint bends “backward.” It’s actually their ankle!
The ancestors of modern birds had five digits like many vertebrates, but that evolved into a variety of different toe arrangements. The most common arrangement is three toes facing forward and a hallux (the equivalent of our big toe) facing backwards. This opposite arrangement allows them to securely grasp branches and wiggly prey like fish. If you’ve ever wondered, as I have, how birds maintain their grip while sleeping on telephone wire, the answer is a neat little feature called flexor tendons. These tendons run the length of their legs, and constrict when a bird bends its knee, automatically curling its toes. This grip isn’t released until they stand up.
Lichen
I recently learned a great little mnemonic story about lichen from a participant on a Nature Walk: “Freddy fungus and Alice algae took a “lichen” to each other and got married. Freddy provided shelter, and Alice provided food. You could say that their marriage was a little ‘on the rocks.’”
This story represents two of the symbiotic partners that make up a lichen. Perhaps the marriage is rocky because the story is missing a third partner that was only discovered in 2016. While the fungi provides the physical structure of a lichen and algae make nutrients through photosynthesis, yeast (another fungus) that lives in the lichen’s outer cortex likely protects the organism against microbial threats. This partnership has evolved independently over ten times with different species of fungus and algae.
When you think about the complexity of each of these relationships and how long it took for them to form, that’s almost unbelievable. It makes more sense, however, when we look at where lichens live. Lichens can survive in habitats that are too dry, cold, hot, or windy for most plants and animals. There are a lot of places on the earth that fit one of those descriptions either year-round or seasonally, but no matter where you are on this planet, fungi and algae are probably not too far away. Life finds a way!
Eyes
Making eye contact with an octopus is reported to be a moving experience, one based on mutual curiosity. If you’re willing to dive to the rocky floor of the Gulf of Maine, you could find one of several octopus species living there to see for yourself. Despite being remarkably similar to each other, vertebrate eyes and those of cephalopods (squid, cuttlefish, and octopuses) evolved after our lineages split from one another. Eyes as a general concept have actually evolved a bunch of times. Vertebrates, cephalopods, insects with compound eyes, and even worms with simple light-sensitive spots all evolved from an eyeless common ancestor that lived in the ocean more than 700 million years ago. That animal possessed some of the chemical components of eyes, although they were utilized for very different functions. Many of that animal’s descendants found novel uses for those molecules through chance mutations and eventually arrived at “eyes.”
Although they follow the same general blueprint for high-resolution vision, vertebrate and cephalopod eyes evolved in very different ways. Ancient vertebrate fish (our ancestors) evolved eyes from brain tissue. Cephalopod eyes evolved from skin tissue. We both somehow ended up with camera-type eyes with a pupil that controls the amount of light that enters the eye, a lens that projects light onto a retina, and photoreceptor cells which transmit that information to nerves. This is a very functional and very specific way to sense visual information. The differences are in the details. For example, the two designs use different proteins and molecules to accomplish the same functions. Each has some surprising features: vertebrate eyes have a “blind spot” where photoreceptor cells are blocked by nerves (our brains compensate for it). Cephalopods are thought to lack color vision.
Our aquatic cousins have a leg up (eight legs up?) in one regard: they can see the polarization of light, which is the angle at which light waves are wobbling. This enhances contrast underwater, which may help them spot predators and prey, and accurately camouflage to their surroundings. Both types of eyeball are complex and relatively “expensive” for the body to make and maintain, so high-resolution vision must have been a game-changer for ancestors both spined and spineless.
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