While every species of animal and plant is unique, some species share structural features that serve a common purpose, i.e. wings for flying.
This is part of a phenomenon known as analogy, and these structures are known as analogous structures, or convergent structures.
For example, although their evolutionary journeys are different, humans and octopuses have similar eye structures, and our eyes even look alike!
Just like us, octopuses have stereoscopic vision, but while we are vertebrates, octopuses belong to the order Cephalopoda.
Since the very first organisms, convergent evolution has played a crucial role in the development of life on Earth.
Below, we’ll take a look at some of the most surprising examples of analogous structures, from wings, to fins, limbs, plants, and even food!
We’ll also take a look at examples of analogous structures that concern the behavior of different species, as analogous structures don’t just present themselves physically.
But first, let’s further define analogous structures by comparing them to homologous structures.
Analogous Vs Homologous Structures: What’s The Difference?
The main way we can distinguish between analogous and homologous structures is with ancestry and function.
So analogous structures are concerned with features that two species share that perform the same function, but have different ancestry.
If you consider what an ‘analogy’ is in the literary world, it’s the act of comparing two things based on what they have in common.
Meanwhile, homologous structures are features found in species that share a common ancestry, but are used in different ways.
Returning to the literary world, homologous structures are similar to ‘homonyms.’ This is where two words sound similar, but have different definitions.
So for example, the bones found in human fingers originated from a common ancestor between all mammals.
Dogs, bats, and even whales have these bones, but while we use our fingers to pick up and clutch objects, bats use these bones to spread their wings, dogs use these bones for walking, and whales do not use these bones at all as they are found inside their fins.
Homologous structures are used as evidence to further prove the theory of evolution.
After all, why would a whale have the same bones in its fins that we do in our hands if we didn’t inherit them from a shared ancestor?
Example #1 Of Analogous Structures: Wings
If you take a closer look at the wings of different species you can clearly see the analogous structures that are present.
For example, while the wings of a bird and insect have the same function (it lets them fly or move through the air), insects have evolved in a completely different way to birds.
In fact, it is a widely known fact that birds are the ancestors of dinosaurs!
Another good example of the analogous structures found in wings are bats and butterflies.
These two species have very different wings (as bats are mammals and butterflies are insects), but they still enable flight.
Example #2 Of Analogous Structures: Limbs
The limbs of arthropods and tetrapods are analogous to each other, and both evolved following the Cambrian explosion that occurred around 530 million years ago.
Tetrapods are the ancestors of fish, and arthropods evolved from terrestrial invertebrates (i.e. invertebrates that live on the land).
Therefore, their analogous structures evolved independently of each other.
Vertebrates and insects use their legs for the same purpose, but they have different structures and different histories of evolution.
Example #3 Of Analogous Structures: Fins
One of the easiest examples of analogous similarities to spot is that of the fins of various species.
For example, the fins that are found on mammals like dolphins and the fins found on penguins have the same function. They let these animals move through the water.
However, while dolphins and penguins are both vertebrates, their evolutionary history is completely different.
Despite this, they both developed anatomical features that look the same, and have pretty much the same function.
You can also see this similarity in the fins of more comparable animals – dolphins and sharks.
Sharks belong to the fish family, but while dolphins are mammals they too reside in the water.
Both use their fins for movement in water, but as far as their evolution is concerned they are completely unrelated to each other.
Example #4 Of Analogous Structures: Plants
Those belonging to the orders of plants Astrophytum and Euphorbia look strikingly similar.
They both have round bodies that are ball-shaped and are divided into eight equal segments.
They also have pointy, hard thorns that protrude in a row along the center of each wedge.
This protects them from curious animals who might want to eat them. If you didn’t know any better, you would think these plants belong to the same species!
What makes these plants noteworthy though is that these genii do have distant relations, but they are found in opposite parts of the world!
For example, every member of the Astrophytum are cacti that live in the southwestern deserts of North America.
Meanwhile, Euphorbia is a plant genus that includes poinsettias and some cacti that grow in the African deserts.
North American and African cacti conserve their water by reducing their surface area.
They shrink into a round ball and develop a thick skin with a waxy texture, sprouting prickles on its most vulnerable areas that deter animals from trying to eat them.
What you have is two planets that look pretty much identical, but have a completely different ancestry.
Behavioral Examples Of Analogous Structures
Research into analogous structures has found homoplasy (similarities) between the behavior of certain organisms as well similarities in their anatomies.
A notable example is when two birds with different origins are kept alongside each other, they may start to mimic the other’s songs.
But this has only been observed in a laboratory setting.
Speaking of birds, both Old World and New World vultures look rather similar. Neither have feathers on their necks and heads, and both live off carrion.
But despite these similarities, their origins are in different families. New World vultures belong to the order Cathartidae, while Old World vultures belong to the order Accipitridae.
Although both types of vultures live off carrion, they also hunt.
New World vultures hunt their prey using smell and vision, while Old World vultures rely on their sense of vision alone for hunting their prey.
In fact, they can spot a 3-foot prey from around 4 miles away! But while New World vultures have incredible vision, New World vultures have a more powerful sense of smell.
Another excellent – and one of the most fascinating – examples of behavior based analogous structures is the duck-billed platypus.
When discovered by an Australian explorer, scientists at a British museum were sure that the animal was fake and couldn’t possibly exist in nature.
In fact, they were convinced the explorer had simply stuck a duck’s bill onto a creature resembling a beaver! But the duck-billed platypus was absolutely genuine.
An egg-laying mammal, the duck-billed platypus is a curious creature, as egg-laying is a behavior usually associated with birds or reptiles (when it comes to vertebrates at least).
However, while a duck-billed platypus lays eggs and has a bill, these are the only characteristics shared by duck-billed platypuses and birds.
Analogous Structures In Food… Yes, You Read That Right!
One of the most common examples of homoplasy regarding an inherent aspect of an organism is the similarities between a potato and a sweet potato.
These vegetables are different species, with different evolutionary histories but share the analogous structure of storing their food inside their tissues.
Furthermore, potatoes take the form of a stem underground (much like cacti), while sweet potatoes are root vegetables.
So how similar are potatoes to cacti? Potatoes are able to grow in agricultural fields, meanwhile cacti grow primarily in the desert.
But while cacti and potatoes have undergone a different evolution, in their stem form both store food inside their tissues.
Analogy is rather distinct from homology with similar structures, because they have embryonic origins in common.
There are a few reasons why some animals share a resemblance to each other.
For example, two insects of the same species may have a similar appearance because the spots that have been passed down from their ancestors are the same color.
While humans, whales, and lizards live in extremely different habitats and have wildly different lifestyles, they actually have skeleton structures in common.
We also use our limbs in different ways.
We use our limbs to pick up objects (thanks to opposable thumbs), while whales use their fins to move through the water, and lizards mainly use their limbs to climb.
However similar our skeletal structures are though, there are differences in the details of our morphology.
This is why evolutionary biologists would deem these structures ‘analogous.’
Comparative anatomy helps evolutionary biologists understand how evolution works.
We associate evolution with events that happened millions of years ago, but evolution is happening right before our eyes.
Species are constantly evolving to meet the demands of our changing world.
With biogeography, fossil records, and molecular records evidence of analogous structures can be studied, and it’s thanks to methods like these that we know that analogous structures come about not because of embryonic origin, but because animals have similar needs in order to live.
Birds and insects, for example, are two completely different types of animals, but they face similar challenges.
Species then evolve so they can better meet these challenges, and if two species face similar challenges then these analogues structures are formed, even if everything else about the two species are different.
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