I learned a new word: vicariance. Although the related word vicarious means "on behalf of another" or even "second hand", the emphasis on this word's coinage is the separation of a biological population into two or more parts by some change in environment, leading to new species. The principal connotation, however, is on the "second hand" experience of members of the population that are thus separated from and even carried away from their fellows and thus physically prevented from breeding across the new barrier. And, AND, the principal emphasis of "vicariance biogeography" has been upon describing the distributions of related species and genera in terms of relict populations from the separation of continents that began about 200 million years ago with the breakup of Pangaea and then Gondwana.
Pangaea was the most recent supercontinent (there have been at least 5 over some 3 billion years or so), comprising just about all the land surface of Earth, that formed some 300 million years ago when earlier, separated continents were driven together. It first split in two, forming Laurasia in the north and Gondwana in the south, then these each split further. The rift that became the Atlantic Ocean began forming in the north about 140 ma ("ma" is the abbreviation for "million years ago"), but the southern half did not begin to open until something like 70 ma or so. The closest distance across the Atlantic is currently 1,800 miles (2,900 km), but it was half that roughly 35 ma, a fact to which we will return.
I was a geology undergraduate in the 1960s, just as the old "shrunken apple" explanation of mountain building I'd learned as a child was being thoroughly replaced by the new paradigm of plate tectonics. It was an exciting time to learn geology. I had the most interest in paleontology (fossils), and much was made of the correspondences of late Paleozoic and early Cenozoic fossils across the Atlantic between the facing continents of South America and Africa. This naturally led the few biogeographers of the time to describe the distribution of nearly all living things as being a consequence of continental motions.
As described in The Monkey's Journey: How Improbable Journeys Shaped the History of Life, by Alan de Queiroz, for some this concept became dogma. "Nearly all" became "all" in their minds. When new evidence showed that continental breakups do not explain everything about the distribution of species, they were, and in some cases still are, unable to assimilate the new information.
As it happens, the last rift in Gondwana opened about the time of the dinosaurs' demise; it is now obligatory to add, "except some of the flying dinosaurs, that we now call birds". To the point: major speciation by continental separations ended about 70 ma. Lesser events, such as desertification of formerly temperate regions, re-routing of rivers, or mountain chain development, must be called upon for any later splitting of gene pools due to vicariance. Of course, strictly speaking, vicariance refers to anything that divides a population, even the removal of several members to a different island or continent, but "vicariance biogeography" discounts "dispersal" mechanisms.
The Monkey's Journey describes the rise and fall of vicariance biogeography as the principal theory of species distributions. Prior to the new paradigm of plate tectonics, several competing theories about the dispersal of plants and animals competed for academic attention. Charles Darwin had done much work to understand how chance dispersal across oceans and other significant barriers could take place. Plate tectonic knowledge was more than a century in his future, and he didn't want to just posit "land bridges" rising and falling wherever it would be convenient. So then, just how did some similar kinds of living things come to populate far-flung continents? It is easy to see that, once continental motions were known, they were grasped upon like a holy grail, to explain absolutely everything about both geography and biogeography.
Of course, that "bio-" part is a problem. Rocks can't walk, swim, burrow, fly, or crawl. Animals can, and plant seeds and even certain plants can either do some of these things, or be carried along when animals do so. So it stands to reason that purely mechanical motions of continents and other landforms cannot explain everything when it comes to creatures with volition.
By the 1980s, significant evidence had accumulated to call into question many "of course" assumptions of the vicariance crowd (nearly all the biogeographers). Hawaii is a case in point. None of the Hawaiian islands was ever even close to any continent. Neither were any of their precursors, including the Emperor seamounts that used to be islands. Every species on any island of Hawaii that was not brought there by the Polynesians or later humans somehow crossed a few hundred miles of ocean, or is descended from a species that did so. The Galapagos islands are similar: perpetually oceanic. There are other examples. Pangaea didn't include absolutely all the land on Earth.
DNA sequencing, begun in the late 1980s, seemed to provide a way of confirming the various species-splitting events, and it was expected to confirm vicariance theories. However, the early "molecular clock" techniques were a bit of a joke. The various "clocks" were notoriously inaccurate and unsteady. Depending on the bit of DNA used, the "tick" could vary over a range of thousands to one. But time marches on, and scientific progress with it. Molecular biochemists have learned a few things in the 30+ years since, including ensemble methods to have a group of "molecular clocks" correct for each others' instabilities. Nobody can yet pin down any event millions of years in the past to the nearest hundred or thousand years, but the difference between 20 million and 200 million years is readily discernible, and that between 20 and 50 million can be cleanly determined.
Considering just animals, island ecosystems typically show a range of species diversity that matches ease of travel by different kinds of animals: many kinds of birds and flying insects, fewer kinds of endemic mammals and reptiles, and very few amphibians or even none. But there are some astonishing cases of animals you'd never expect could cross an ocean, living on oceanic islands. The most astonishing case is that of the monkeys, but this is not about islands. Unless you recall that, until 3 ma, South America was an island, just a very big one, and it became an island nearly 70 ma.
Three million years ago, the Atlantic Ocean was almost as wide as it is today, and the Pacific was much, much wider than that. So vicariance biogeographers have been very diligent to find a way to get primates between Africa and South America, without leaving a single fossil in North America, at some time before 70 ma. The plausible explanation was that monkeys could simply walk west out of future Africa to future South America. The trouble came after molecular dating showed that New World monkeys and Old World monkeys and apes did not split from one another until some time between 20 and 40 ma, with the most likely date close to 35 ma.
At that time, the closest bit of African land was 900 miles (more than 1,400 km) from the closest bit of South American land. So the new explanation came to this: posit land bridge (a proto-Panama) about that time, and some similar connection between northern Africa or Europe and North America, plus a long period of warming so monkeys would be willing to go to, say, Spain, to get to America. Then somehow they walked south to South America, leaving not a single monkey fossil behind. Considering the thousands of fossils of Eocene and Miocene horses, tapirs, and sloths found throughout North America, the absence of monkey fossils is telling. Yes, "absence of evidence is not evidence of absence", but such a case as this is pretty close, pretty close. Monkey fossils, later than 35 ma, are abundant throughout South America…
So somehow, a major "floating island" sort of raft got ejected by an African river and made its way to South America before all the animals on it died. Every chapter of the book ends with an anecdote, and here is a relevant one from the end of chapter 4:
In July 1892, a natural floating island was spotted off the northeastern US coast, at about the latitude of Philadelphia and some 300 miles from the nearest land. The island was roughly 9,000 square feet in area, contained living trees 30 feet tall, and is said to have been visible from 7 miles away. The same island was again seen in September, by which time the Gulf Stream had pushed it more than 1,200 miles northeast of its previous position. (Powers, Sidney, 1911, Floating Islands, Popular Science Monthly 79, 313-307)Let's see, 1,200 miles in two months. Had a similar island become caught up in the equatorial current between Africa and Brazil after being ejected by a proto-Senegal River, it might have made the crossing in 5-6 weeks. It would have brought, not just monkeys, but dozens or hundreds of species of all kinds, plant and animal. Not all would survive as colonists, but some would. It only had to happen once, once over a span of millions of years.
The vicariance diehards pooh-pooh such explanations, as much too unlikely. Well, so was the origin of life. But that also had to happen only once, and considering how early it happened, it was only "moderately unlikely". So was this. People tend to think of things that are very unlikely as "miracles" if they occur anyway. The "once in a million chance" is a venerable staple of storytelling, and the classic example is a hole-in-one in golf. Few golfers have seen one, fewer still have done one. Jack Nicklaus did one a couple of weeks ago, at age 75. Of course, he has done 20 of them, in competition, but then, he has probably hit several million drives that were not holes-in-one! But even if the monkey-bearing raft is not just one in a million, but one in a billion, a lot can happen in the 70 million years that South America was an island.
A major point of the book is, that the biota of nearly everywhere is much more a product of long-distance dispersals across all kinds of barriers, more than we had thought of. So much so that relict Gondwanan species pairs, or genus pairs, are actually rather hard to find! But that's not the only point. The author brings to life for us many of the principal players in biogeography, details the steps of study and reasoning that were key to the shifting winds of understanding over the century-and-a-half that have elapsed since Darwin released The Origin of Species in which he also discussed species dispersal mechanisms. "Dispersalism" is proclaimed as the resulting paradigm. Of course, there are a few pesky Gondwana relicts around, and other evidence that dispersal on all scales, over all time scales, has gone on. Scarce, chance travels by unlikely travelers have led to Earth's lands becoming a story of "everybody came from everywhere". The proper answer to any question, "Which way did these plants and animals get here" is, "All of the above". Leave dogma to the world religions where it belongs. Science is about discovering what is, and to some extent, how it came to be, based on evidence.
So THE major point of the book, that bears repeating endlessly (especially to certain mis-named "scientists" I know) is this: Evidence must drive theory, not the other way around.