Contingency and Convergence: Are the Gods Plagiarists?

There are two concepts regarding the process of evolution: contingency and convergence.

Contingency

This theory holds that species evolve, due to natural selection, randomly according to whichever new adaptation is useful. The present is contingent on random events that occurred in the past (such as an asteroid hitting the earth). There is no directionality toward more advanced organisms; there is no predictability; anything is possible. If one were to re-wind the clock and start Life over at Year Zero, the world evolving from that would look very different from our world today. There would be no mammals, fish, plants, or anything necessarily resembling them. As Stephen Jay Gould states, “We are the accidental result of an unplanned process …. The fragile result of an enormous concatenation of improbabilities, not the predictable product of any definite process.”

Convergence

This theory agrees that natural selection is the motor that drives evolution, but that it is largely influenced by circumstances that cause separate evolutionary lineages to arrive at the same conclusion. Circumstances may include gravity, water, climate, and the general environment/habitat (of co-evolving plants). For example, both ducks and the platypus evolved duck bills, a very specific adaptation for a similar ecological niche. Both Red-winged Blackbirds and Fan-tailed Widowbirds evolved to have males with black bodies and red shoulders (with white borders) and sing from reeds in marshes, while the females are streaked brown.  They have converged to become similar species, although they are quite unrelated.  In this way, many species independently evolve in predictable ways that are replicated in time and space around the world. They converge over time and look like each other.  Moreover, directionality is also evident. Species evolve from simple to complex, from blob to human. (Some have taken the theological step that convergence and directionality imply universal rules which suggest the work of a Divine Hand.  That discussion is beyond the scope of this blog– for now at least.) If one were to re-wind the clock and start over in Year Zero, we’d still end up with a world pretty similar to the one we have now, with a similar mix of species fulfilling similar functions. As Simon Conway Morris states, “Although there may be a billion potential pathways for evolution to follow from the Cambrian explosion, in fact the real range of possibilities and hence expected end results appear to be much more restricted.”  That is, there are only so many options and species will fit into those options.  Anything is not possible.

There are actually a range of theories that combine aspects of contingency and convergence, in differing amounts.

whitebrows

Left to right: White-browed Conebill, South America; Slaty-backed Chat-Tyrant, South America; Slaty-backed Hemispingus, South America; Snowy-browed Flycatcher, Asia; Indian Blue Robin, Asia; White-browed Woodswallow, Australia

If only contingency was the case, birds would evolve in random ways, developing ever more unpredictable and bizarre shapes, sizes, and colors. Bird identification would be easy, as they’d all be so different (except right when closely related species are separating from each other). If convergence was the rule, we’d expect the same patterns to recur time and time again. Bird plumages would be limited to certain patterns and colors, and probably correlated with certain behaviors, habitat preferences, and ecological functions. There would be birds from completely different families, very distantly related to each other and on different continents, which might evolve looking very similar to each other. As my son puts it, the gods of evolution would look like plagiarists.  I believe this is clearly the case, for reasons that are not always easy to understand.  For example, the six Passerines above, all inhabitants of woodsy areas but only very distantly related, have solid blue-gray upperparts, reddish underparts, and white eyebrows.  It’s not clear what purpose this serves, but clearly it’s a pattern they have all converged on.

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Of Bobolinks and Bishops: Rules for Conspicuous Grassland and Wetland Birds

meadowlark

Western Meadowlark, Utah

Low Open Grasslands

In the simplest terms, our story begins over 50 million years ago with a single species of bird.  This bird looked nothing like a meadowlark or a longclaw; those were still millions of years in the future.  This one species spread out across a wide geographic area and many habitats.  It evolved, through classic divergent evolution, into many separate species, eventually spawning finches, flycatchers, weavers, pipits, sparrows, warblers, thrushes, tanagers, vireos, and many more.  Millions of years later, meadowlarks (related to blackbirds and orioles) appeared in the New World .  In the Old World, longclaws (related to pipits) were born.  In one of the most famous examples of convergent evolution, these two species, distant cousins millions of generations removed, were nearly identical.  In fact, the Eastern and Western Meadowlarks of North America are so similar to the Cape and Yellow-throated Longclaws of Africa that Linnaeus first thought they were the same species.

longclaw

Yellow-throated Longclaw, South Africa

In addition to their nearly identical plumages, they both forage for insects on the ground in relatively low open grass, sing from low but prominent perches, walk methodically on the ground when foraging, and fly with short wing bursts interspersed with glides.  A comment in Wikipedia borders on the absurd, but highlights the prevailing view in science that evolution is largely random:  “As this exact pattern provides no obvious adaptive benefit compared to that of other meadowlarks and longclaws, it seems to have arisen twice by sheer chance.”  This quantity of nearly identical characteristics, clearly filling the same ecological role, suggests that something greater than sheer chance is at work.  Obviously, there is something about this pattern, the yellow underparts, the black “V”, the streaked back (clearly camouflage in the grass), the unique flight style, perhaps even the yellow eyebrow, that does provide an adaptive benefit.   We just don’t understand what it is.  Regardless, it is apparent that these species are following some rule that is causing them to converge to the same shape, color, and habits.  From an American perspective, it appears that meadowlarks have evolved independently on another continent, filling the exact same niche in similar habitat.  This niche, low open grassland, calls for meadowlarks!

longtailedmeadowlark

Long-tailed Meadowlark, Argentina

rosylongclaw

Rosy-throated Longclaw, Tanzania

If the meadowlark/longclaw convergence were the only one, “sheer chance” would be a reasonable explanation.  But the coincidences don’t stop with these species.  Sticking just to grassland songbirds that forage on the ground in low open grass and sing from prominent perches, the “rule” has more to it.  Yellow may be substituted with red (but rarely orange).  Research suggests that red evolved more recently in meadowlarks and blackbirds; they all started with yellow.  Red versions of the meadowlark/longclaw convergence are the Long-tailed Meadowlark of South America and the Rosy-throated Longclaw of Africa.  While the underparts are brightly marked, the upperparts are camouflaged with the grass.  Another bird that fits this grassland color pattern is the Dickcissel of weedy fields of the Americas, with its black “V” on the breast and yellow face.  While the sexes are similar with meadowlarks and longclaws, the female Dickcissel is a faded version of the male, lacking the black “V”.  Most of these birds have silver or metallic colored bills as well.

Black “V’s” or “U’s” also occur on the breasts of some plovers, wagtails, thrushes, wheatears, flickers, scrub-jays, and McCown’s Longspur, to name a few.  They are all species that spend some (or a lot of) time on the ground.

Skulking grassland birds, like the grasshopper sparrow and grasshopper warbler, follow other rules that will be explored later.

Tall Grass and Reeds

As we move into deeper cover, such as tall grass or tules where birds cling to long waving stems, we encounter a slightly different niche and a modified rule.  Black, yellow, and red remain our primary colors.  In this habitat, the conspicuous birds are sexually dimorphic, the males more gaudy, nearly entirely black and yellow (or red), while the females are more cryptic, often streaked like the meadowlark above, but without the bright colors below.  Research suggests these females started colorful and evolved their camouflage later.  We still keep pale or metallic bills for most species.

blackbirds

From left to right (males above, females below):  Red-breasted Blackbird (South America), Yellow-headed Blackbird (North America), Tricolored Blackbird (North America), Fan-tailed Widowbird (Africa), Red Bishop (Africa), Red-backed Fairy-Wren (Australia)

Examples include several blackbird species and the Bobolink in the Americas (all related to meadowlarks), the widowbirds and bishops of Africa (related to weavers), and even the Red-backed Fairy-wren of Australia.  The female Bobolink (below) is nearly identical to a female bishop, and behaves in the same way in similar habitat.  The males have black underparts (rare in birds), pale backs, and sport a yellow (or orange or red) back of head and nape and flare their back and nape feathers when singing.  Check out the Bobolink singing and compare with the unrelated Yellow-crowned Bishop.  The fairy-wren can also puff out the red feathers on its back.

bobolinkbishop

Male and female Bobolinks from the Americas (left) and Yellow-crowned Bishops from sub-Saharan Africa (right)

In several species on both continents, not at all related to each other, the males are black with red shoulders (with a pale border), and give showy display flights over wetlands.  From an American perspective, it appears that swampy meadows demand Red-winged Blackbirds, even if it is the completely unrelated African widowbird version.  Again the females are camouflaged.  Female widowbird species present just as much of an identification challenge in Africa as do Tricolored and Red-winged Blackbirds females in the US.

blackbirdwidowbird

Male and female Red-winged Blackbirds from North America (left) and Long-tailed Widowbirds from South Africa (right)

Skulking marsh birds, like the Marsh Wren and Zitting Cisticola, follow another rule that will be examined later.

Magnitude of Convergence

In the examples above, we have meadowlarks, blackbirds, and Bobolinks in the Americas (all Icterids); longclaws (pipits), widowbirds, and bishops (both weavers) in Africa; and the fairy-bird in Australia.  The last time these all had a common ancestor was over 50 million years ago.  See the chart below, with the timeline along the bottom (in millions of years).  Weavers and pipits are identified on the chart.  Icterids and fairy-wrens fall in with “other passerine lineages”.  By this time, there was an Atlantic Ocean (of sorts) and the Americas and Africa were separate.  These species evolved independently, converging upon their remarkably similar patterns in their respective habitats, presumably filling ecologically equivalent roles. birdtaxonomychart1

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Convergent Evolution: an introduction

Convergent Evolution is when different species evolve similar structures or functions despite the fact that their evolutionary ancestors were very different and very distantly related.  A homoplasy or homoplastic trait refers to the similar biological trait acquired by species from different lineages.

Evolutionary_trends

This diagram illustrates the differences between divergent, parallel, and convergent evolution. The latter is when two different (and relatively unrelated) organisms evolve to be similar to each other.

Some examples from Biology Online:

  • the wings of bats, birds, and insects evolved independently from each other but all are used to perform the function of flying;
  • the complex camera eyes of vertebrates, cephalopods (squid and octopus), cubozoan jellyfish, and arthropods (insects, spiders, crustaceans) evolved separately, but all perform the function of vision;
  • the smelling organs of the terrestrial coconut crab are similar to those of insects;
  • the very similar shells of brachiopods and bivalve molluscs;
  • prickles, thorns and spines have evolved independently to prevent or limit herbivory;
  • plant hormones such as gibberellin and abscisic acid of plants and fungi;

flight

A long list of examples of convergent evolution is available at Wikipedia.

This blog will explore these and many other examples of convergent evolution.

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