Punctuated Equilibrium and the evolution of languages.
Over the past few weeks the science-blog cadre has been squabbling about punctuated equilibrium (PE). Today, in a rather brilliant example of good timing - if I say so myself - some colleagues (Quentin Atkinson, Mark Pagel, Andrew Meade, Chris Venditti) and I have a paper showing that languages evolve in punctuational bursts.
Atkinson, Q.D., Meade, A., Venditti, C., Greenhill, S.J., & Pagel, M. (2008) Languages evolve in punctuational bursts. Science, 319, 588. (doi:10.1126/science.1149683)
.
More on this in a minute.
So, what is punctuated equilibrium?
Most orthodox views of evolution argue that new species arise out of a gradual accumulation of genetic changes. Eldredge and Gould (1972) disagreed, and argued for a model of punctuated equilibrium where evolution shows a pattern of "bursts" of change and periods of little overt change ("stasis").
This did not go down particularly well. Richard Dawkins and Daniel Dennett, for example, tried to explain it away. Dawkins, in The Blind Watchmaker complained that this was what everything thought all along, and no-one was really a gradualist in the way Eldredge and Gould had proposed. He argued that their conception of gradualism was akin to believing that in the biblical story of the children of Israel, the kids literally moved one yard an hour over the 40 years it took them to cross the Sinai desert. Dennett made a similar argument in Darwin's Dangerous Idea, which tried to explain away punctualism as an incorrectly scaled graph (i.e. increase the time axis and everything looks gradual!).
Others focused instead on the types of mutations that could cause punctuational evolution. Are there certain mutations which have a large effect on the phenotype? This argument has popped up repeatedly under various names like 'saltationism', 'hopeful monsters', and 'macromutations', etc. Most recently, last week, Olivia Judson revived it again, and was promptly stomped by Jerry Coyne.
However, punctualism does not require macromutations, and Gould largely saw these as separate debates. As T. Ryan Gregory points out, punctualism is more about the rates of change. Are there periods in evolutionary lineages where rates of evolution are fast, and are there periods where they're slow? If there are differences in rates of evolution over time, then we can detect these using phylogenetic methods.
Punctuation and Language Evolution
The punctualism debate has also entered linguistics - Dixon wrote a recent and extremely controversial book, The Rise and Fall of Languages, where he argues that language evolution is, in many cases, punctuational. Dixon's not alone - other linguists have also noted the potential usefulness of this idea (Claire Bowern has an excellent paper reviewing this debate: Punctuated equilibrium and language change (PDF)).
So, how do languages evolve? The languages we see in the world today are the outcome of an evolutionary process of descent with modification. Over time, languages accumulate changes in their grammar, phonology (sound system) and vocabulary. Compare, for example the differences between modern English, Shakespeare or Chaucer. Or, to take a simple example, the words for "back" (the body part) in the Tahitian language, recorded at different points in time:
- 1773 - etoo
- 18th Century - tue
- Modern - tua
Occasionally, new languages are born, perhaps as a result of some members of the population of speakers migrating elsewhere causing the language lineage to split in two. The two descendant lineages then begin to accumulate changes independently, and over time become mutually unintelligible. Eventually, these become languages in their own right. For example, when the Saxons crossed the channel from Europe, their language separated from the ancestral Germanic language and eventually became English.
Using phylogenetic methods, we can reconstruct the evolutionary history of these languages. In this study, we used word "cognates" to build family trees of the languages from three large language families. These cognates are words of similar form and meaning that have descended from a common ancestor. For example, the German "vasser" and Hittite "watar" are cognate with the English "water", but not with the French "eau". For an even bigger example, you can see some of the data used in this study below (and it's all on my research website). This shows some of the words meaning "hand" in a number of languages. Note the first cognate set of related words like "lima" found in languages like Bali and Sasak (all the entries coded as "1" in the right-most column). There's a second and third cognate set for the forms related to "tangan" (18) and "lenge" (19):
...and just to re-use one of my favourite images, here's a map of where these words are spoken. This time, it's words meaning "hand" in the Austronesian languages of Indonesia and Papua New Guinea. The cognate sets of related words are color-coded. More on this here: Rates of word evolution.
Our model of language evolution aims to find the (set of) family trees that best explains the distribution of these cognate related words. So, if two languages - say, English and German - share a lot of cognates, then they are closely related. If they share few cognates, then they're further apart on the tree and separated by longer branches.
In this paper, we analysed the cognate sets found in 200 items of basic vocabulary from hundreds of languages in three of the largest language families in the world: Austronesian, Bantu, and Indo-European. You can see a good chunk of the data at my research website here.
Here's the fun part - once we've built these trees, the length of the branches is proportional to the amount of change between languages: the longer the branch the more change has occurred. This gives us a "path length" from the root of the tree to each language, which represents the total amount of change that each language has undergone.
What we show (after a few statistical corrections to avoid node density artifacts), is that as the length of these paths increase, then there are more languages born along that lineage. This is indicative of punctuated equilibrium: splitting events seem to be promoting language change. In contrast, if lexical evolution was gradual, then the path lengths would be independent to the number of divergence events along that path. In fact, across the three families, we show that 31% of Bantu, 21% of Indo-European, and 9.5% of Austronesian vocabulary arose at or around the time that the languages diverged.
Ok, so what could be behind this? We argue that there are two possible reasons for this punctuational effect in language evolution. First - as languages diverge, they tend to split into smaller populations. Like small biological populations, small speech communities tend to be at the risk of things like founder events and drift, which increase the rates of change. To check this, we looked at the Polynesian languages, which underwent a long chain of founder events as they settled the Polynesian triangle. These languages showed a huge increase in this punctuational effect (33%) when compared to their parent family, Austronesian (9.5%)
The second possible cause of punctuational language evolution is that people tend to use their languages as a way of providing a social "emblematic" function. That is, people use their languages as a tool for promoting social cohesion and group identity (we speak like this, they speak like that). To leave the last word to Noah Webster, the lexicographer behind the American English Dictionary: "as an independent nation, our honor requires us to have a system of our own, in language as well as government".
More Links:
The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages
This looks like a cool paper - just out in PNAS - "The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages":
The nature of evolutionary changes recorded by the fossil record has long been controversial, with particular disagreement concerning the relative frequency of gradual change versus stasis within lineages. Here, I present a large-scale, statistical survey of evolutionary mode in fossil lineages. Over 250 sequences of evolving traits were fit by using maximum likelihood to three evolutionary models: directional change, random walk, and stasis. Evolution in these traits was rarely directional; in only 5% of fossil sequences was directional evolution the most strongly supported of the three modes of change. The remaining 95% of sequences were divided nearly equally between random walks and stasis. Variables related to body size were significantly less likely than shape traits to experience stasis. This finding is in accord with previous suggestions that size may be more evolutionarily labile than shape and is consistent with some but not all of the mechanisms proposed to explain evolutionary stasis. In general, similar evolutionary patterns are observed across other variables, such as clade membership and temporal resolution, but there is some evidence that directional change in planktonic organisms is more frequent than in benthic organisms. The rarity with which directional evolution was observed in this study corroborates a key claim of punctuated equilibria and suggests that truly directional evolution is infrequent or, perhaps more importantly, of short enough duration so as to rarely register in paleontological sampling.
Sunday pot-luck
Assorted interesting posts for nice lazy Sunday:
- John Hawks provides the best discussion of the recent identification of the FoxP2 gene in Neanderthals without any of the stupid journalistic hype that others have succumbed to (although he does use the term "neandercooties"). Go read his The amazing talking Neandertals.
-
Creek Running North has identified that
Evolutionary Psychology may be genetically hardwired...Biologists have long assumed that evolutionary psychology, a controversial branch of psychology that ascribes many common social behaviors to genetics, is a muddled blend of half-understood evolutionary biology, selective data mining and resentment of women’s changing roles in society.
A new study, published in today’s issue of the German publication Unwirklichen Genetikjournal, does not challenge that assessment. But it does suggest that some men may be genetically predisposed to believe in evolutionary psychology, a finding that may well suggest future methods of treatment of the psychological malady.
- Scientist debunks nomadic Aborigine 'myth':
Before white settlers arrived, Australia's indigenous peoples lived in houses and villages, and used surprisingly sophisticated architecture and design methods to build their shelters, new research has found.
- More ammunition inn the punctuated evolution debate, this time it's Punctuated genome size evolution in Liliaceae:
Most angiosperms possess small genomes (mode 1C = 0.6 pg, median 1C = 2.9 pg). Those with truly enormous genomes (i.e. ≥ 35 pg) are phylogenetically restricted to a few families and include Liliaceae – with species possessing some of the largest genomes so far reported for any plant as well as including species with much smaller genomes. To gain insights into when and where genome size expansion took place during the evolution of Liliaceae and the mode and tempo of this change, data for 78 species were superimposed onto a phylogenetic tree and analysed. Results suggest that genome size in Liliaceae followed a punctuated rather than gradual mode of evolution and that most of the diversification evolved recently rather than early in the evolution of the family.
- The Evolutionary Origins of Human Patience: Temporal Preferences in Chimpanzees, Bonobos, and Human Adults:
To make adaptive choices, individuals must sometimes exhibit patience, forgoing immediate benefits to acquire more valuable future rewards. Although humans account for future consequences when making temporal decisions, many animal species wait only a few seconds for delayed benefits. Current research thus suggests a phylogenetic gap between patient humans and impulsive, present-oriented animals, a distinction with implications for our understanding of economic decision making and the origins of human cooperation.
On the basis of a series of experimental results, we reject this conclusion. First, bonobos (Pan paniscus) and chimpanzees (Pan troglodytes) exhibit a degree of patience not seen in other animals tested thus far. Second, humans are less willing to wait for food rewards than are chimpanzees. Third, humans are more willing to wait for monetary rewards than for food, and show the highest degree of patience only in response to decisions about money involving low opportunity costs.
These findings suggest that core components of the capacity for future-oriented decisions evolved before the human lineage diverged from apes. Moreover, the different levels of patience that humans exhibit might be driven by fundamental differences in the mechanisms representing biological versus abstract rewards.
-
Finally, there's a fun review and historical perspective of neutral theory in the latest Journal of Evolutionary biology:
To resolve a panselectionist paradox, the population geneticist Kimura invented a neutral theory, where each gene is equally likely to enter the next generation whatever its allelic type. To learn what could be explained without invoking Darwinian adaptive divergence, Hubbell devised a similar neutral theory for forest ecology, assuming each tree is equally likely to reproduce whatever its species. In both theories, some predictions worked; neither theory proved universally true.
Simple assumptions allow neutral theorists to treat many subjects still immune to more realistic theory. Ecologists exploit far fewer of these possibilities than population geneticists, focussing instead on species abundance distributions, where their predictions work best, but most closely match non-neutral predictions.
Neutral theory cannot explain adaptive divergence or ecosystem function, which ecologists must understand. By addressing new topics and predicting changes in time, however, ecological neutral theory can provide probing null hypotheses and stimulate more realistic theory.