By David Thill
Take a look, if you will, at the birds pictured here.
With their yellow throats and yellow breasts, their mottled feathers and the black streaks lining their wings like racing stripes – tiny cotton balls where their eyes once sat – they look like birds of a feather.
But they’re not of the same feather.
These birds share no ancestors, or at least no recent ancestors. (Remember, “recent” is relative. In evolutionary terms, we’re talking millions of years.) The one on the right, a yellow-throated longclaw, was retrieved in 1968 from the Dondo forest in Mozambique in Southeast Africa. The bird on the left, an eastern meadowlark, was retrieved in 1999 from 2301 South King Drive in Chicago – McCormick Place, where it presumably died after hitting one of the convention center’s windows.
They’re two of the 500,000 (and counting) specimens in the Field Museum’s bird collection.
If you think it’s at all interesting that these two unrelated birds look almost the same, you’re not alone.
These specimens, discovered more than 8,000 miles apart, are found side by side in what John Bates, an evolutionary biologist and the associate curator of birds at the Field, calls the “Gee, whiz!” drawer.
So why do they look so alike if they’re not related? That’s because of an evolutionary pattern called convergence. You may have heard of divergence, when one species separates into two, which then evolve in different ways. Convergence is almost, but not quite, the opposite. It’s when two separate species evolve similarly, often because they live in similar environments.
Despite living in different parts of the world, meadowlarks and longclaws both make their homes in open grasslands. Their plumage reflects the way they’ve adapted – in this case, adapted in the same way – to live in those grasslands, blending in with the ground where they build their nests and forage for insects.
This pattern of convergence isn’t just a longclaw-meadowlark phenomenon. It’s found throughout the living world. Take a vacation to the Caribbean, and you’ll find Jamaican lizards that look almost the same as Puerto Rican lizards, even though they’re not related.
Convergence happens on a microscopic level, too. Tuberculosis, a dangerous lung infection caused by the bacterium Mycobacterium tuberculosis, is treatable with antibiotics. The problem is, that bacterium continually becomes resistant to the antibiotics scientists make to fight it.
The thornier problem: M. tuberculosis has many strains, each of which is genetically different. But by comparing strains of the bacterium, researchers have found common gene mutations in resistant strains. Those common mutations indicate that even though the strains are genetically different, they evolved resistance in the same way, at least in some cases. That’s convergence. With this information, scientists can try to figure out how to stop those mutations and develop effective antibiotics.
Now back to the birds.
Fall is migration season, and as birds move across countries, continents and oceans, the Field Museum’s bird department sees an influx of new specimens. These specimens are mostly from local locations: If you’ve ever seen a dead bird on the ground, it might end up in the bird department. You could even bring it there yourself.
During the fall and spring migrations, the Field receives more than 150 birds on some days, according to Bates. These specimens are brought in from around the city by volunteers, many from the Chicago Bird Collision Monitors, a volunteer group operated by the Chicago chapter of the Audubon Society.
Once the deceased arrive at the museum, Dave Willard, the Field’s retired collections manager who now volunteers in the department, identifies the birds and records wing measurements and other details.
Then the feathers fly.
A team of staff members and volunteers prepare the specimens one of three ways:
Some are enclosed in jars of ethanol and taken to the museum’s basement level to be stored with similarly prepared fish, insects and other specimens. As technology like CT scanning advances, researchers can examine specimens kept intact this way in great detail.
Other birds are skinned and stuffed, like the meadowlark and longclaw pictured above. To envision this process, try imagining scalpels and forceps making tiny incisions to extract tiny ribcages and hearts out of tiny bird bodies. To be fair, though, some of the specimens are quite large. Big or small, the stuffed specimens find their way to the drawers, labeled and cataloged.
A third group of specimens is skeletonized – pared down to just the skeleton – which is an efficient way to store them for later study, said Bates. Essentially, after drying out the de-feathered carcass, staff place it in a tank of flesh-eating beetles, which eat away until only the skeleton remains. The fact that the beetles are flesh-eating isn’t special, Bates noted: Most beetles will eat dead flesh if given the opportunity. What is special about these beetles, he added, is that their whole life cycle revolves around decaying flesh.
No matter how the specimen is processed, staff collect a tissue sample, a tiny piece of the carcass, usually muscle. Since the department began collecting these samples in the 1980s, the collection has grown to 120,000 tubes of tissue samples from 75,000 birds. It’s these samples that can be used to analyze DNA in order to learn about evolutionary patterns like convergence.
But convergence is only one piece of a gigantic evolutionary puzzle.
A CASE STUDY IN WARBLERS
Bates studies birds primarily in the African tropics and the Neotropics, regions with some of the highest levels of species diversity in the world. One of the most recent papers he co-authored, led by Official University of Bukavu professor Charles Kahindo, focused on Bradypterus graueri, or Grauer’s Swamp Warbler.
The warbler lives in mountain swamps in the Albertine Rift, a group of mountains, lakes and valleys that covers parts of several African countries, including Uganda, Rwanda and the Democratic Republic of Congo. The warbler is endemic to the rift, meaning it’s found nowhere else in the world.
By looking at genetic patterns, Bates and his fellow researchers found that the Grauer’s Swamp Warbler comprises three distinct genetic populations living in neighboring geographic regions of the rift.
The populations began diverging during a period when the rift became drier and warmer. The change in climate may have resulted in forests, home to the warblers and their swamps, retreating up the mountain slopes and becoming fragmented. Once their homes were isolated, the warblers – which, the researchers noted, are weak fliers – were also isolated. That isolation could have caused the populations to diverge, each one becoming genetically different from the others.
Gorillas, on the other hand, can travel greater distances and are less constrained to a specific environment than the warbler is to its swamp. So Albertine Rift gorillas are less genetically differentiated than the warblers.
The warbler populations began diverging genetically about 170,000 years ago. That’s a long time. But it’s actually pretty recent in evolutionary terms, Bates noted.
These patterns – convergent and divergent, in warblers, meadowlarks, longclaws, gorillas, lizards and bacteria alike – are a look into a past that stretches back millions – really, billions – of years. And, Bates pointed out, it’s information that could help us in a future we can’t yet see.
He likes to quote Harry Truman: “The only thing new in the world is the history you do not know.”