We get mail

Recently the PT crew received an email with the subject line “A legitimate question about Evolution with no agenda.” As you might expect, the dual disclaimers–“no agenda” and “legitimate”–immediately raised a few eyebrows. “No agenda”? Hmmmmm. Well, I suppose it’s possible, though numerous previous encounters with creationists’ faux naivete have left me a dab cynical.

The email reads

Subject: A legitimate question about Evolution with no agenda
Date: Sun, 5 Jun 2011 16:48:13 -0700
From: [redacted]
To: thecrew@pandasthumb.org

Dear Panda’s Thumb crew:

I’m not a scientist, I’m a retired history teacher with a masters in that field.

I’m not writing because I have any agendas. I’m trying to get my questions answered and I’m having trouble doing it since I don’t know any evolutionary biologists whom I could ask. Those I have written to do not reply. I’m asking for the perspective of an evolutionary biologist who might answer a student with questions who is not hostile to evolutionary biology.

If you don’t have the time to reply, or don’t want to, please write me and tell me that.

Here are my questions about macroevolution. My goal is to understand how scientists explain how macro-evolution works in a real life situation, in this case between reptiles evolving into birds, since this is postulated as occurring:

``` Reptiles Birds Lay eggs lay eggs fly fly

have feathers? have feathers cold blooded? warm blooded```

Would being cold blooded show up in the fossil record? If not, how and why would a reptile adapt over millions of years into warm-blooded? How would anyone know whether a feathered reptile was now a bird if one is/may be cold blooded and one is warm blooded? Where is the proof?

Same topic different question: We know that horses and donkeys can interbreed to produce a mule, which is sterile. Using this explanation for cross breeding, how does that fit with macroevolution? In other words, could a flying, feathered semi-reptile mate with a full bird (or any other combination), and not be sterile? Even over millions of years, since there would be no progeny and the variant would die.

Third question: If a reptile/bird evolved, wouldn’t it also need a reptile/bird to mate with to carry on the new species? If one, a male, for instance evolved, and no female evolved at the same time and in the same place, wouldn’t that end the cycle of macroevolution?

Thank you for your time.


You see the difficulties, and it seems clear why the evolutionary biologists to whom he claims to have written haven’t bothered to reply. I see three main reasons.

First, while our correspondent claims to have no agenda, the general tenor and content of the questions suggests otherwise. While he may be telling the truth, the questions have a distinctly creationist flavor and over the years I’ve learned to be wary of such self-professed innocence. Like all who have been in this game for a couple of decades, I’ve seen it before too often. But we’ll go on anyway.

Second, a number of the specific questions are underlain by misconceptions about science in general and biology in particular that render them incoherent. In order to address the incoherent questions one would first have to address the underlying misconceptions. Most working evolutionary biologists don’t have that kind of spare time. (John Scalzi has some apposite remarks in a somewhat different context.)

Finally, to all appearances our correspondent has expended little effort to find answers for himself. An easy and obvious way of responding is by pointing the correspondent to some web resources. For example, there’s the Berkeley evolution site which is rich in resources for learning about evolution. Then there are search engines for the processional literature. For example, a search on Google Scholar for [birds dinosaurs evolution] produces 25,600 hits. Searching on [evolution endothermy birds] produces 4,860 hits. On the first page of each search there are several useful and trustworthy references. (I emphasize that these are Google Scholar searches, not plain Google searches.) And of course there’s PubMed, where both searches get some hits mostly having to do with genetic, metabolic, or molecular research.

Of course, the reader has to exercise some judgment in discriminating among the Google Scholar hits, and a long-time teacher like our correspondent should know that. But there’s no indication in his email that our correspondent has looked for answers on his own; he wants a working professional to take the time to spoon-feed him. I hope that in his teaching career he taught students do a better job of self-directed research than he’s done here.

I won’t attempt a comprehensive answer to the questions here. Doing so would require require tens of thousands of words, most of them devoted to clearing away the underbrush of misconceptions (“proof”?) and I prefer that our correspondent do some of his own work. Rather, I’ll address just a few of the misconceptions underlying the questions and provide some resources so he has a leg up on the work necessary to find answers if he’s as genuinely interested as he claims to be.

Consider the second and third sets of questions in the email. Our correspondent asks

Same topic different question: We know that horses and donkeys can interbreed to produce a mule, which is sterile. Using this explanation for cross breeding, how does that fit with macroevolution? In other words, could a flying, feathered semi-reptile mate with a full bird (or any other combination), and not be sterile? Even over millions of years, since there would be no progeny and the variant would die.

Third question: If a reptile/bird evolved, wouldn’t it also need a reptile/bird to mate with to carry on the new species? If one, a male, for instance evolved, and no female evolved at the same time and in the same place, wouldn’t that end the cycle of macroevolution?

Shades of Kirk Cameron and Ray Comfort and their crocoduck!. PZ Myers dealt with the ‘where’s the mate?’ topic a couple of years ago.

Those questions illustrate three common misconceptions. First, of course, they presuppose that speciation occurs only in giant steps–a ‘reptile’ morphed into a ‘reptile/bird’ and thence into a ‘bird’ in giant steps, and each new step is so different from its parents and siblings that it couldn’t breed with them. Second, they illustrate typological thinking, the notion that an individual must be a member of one or another crisp class and that there’s a chasm between the classes. Third, they illustrate a pervasive Inability to think in population terms. In general, populations evolve, not a single individual. Our correspondent uses the singular–“reptile” and “bird”–which masks the reality that (in the cases he cites, at least) it’s populations that evolve.

Let me expand just a bit in hopes that our correspondent reads this; I’ve notified him of this post.

For the most part, animal evolution occurs incrementally in populations

First, I ask our correspondent to consider this: Every generation of a sexually reproducing population is a member of the same species as its immediate parent generation, and yet after many generations–hundreds or thousands of generations or more–the last generation of the sequence of generations could be a different species than the first generation.

The correspondent’s questions imply that we must be able to classify one generation (or one individual member of a generation) as a new species, reproductively isolated from its immediate predecessor generation, somewhere along the line of those hundreds or thousands of generations, but that would be an arbitrary labeling decision and would not accurately map the continuity of evolution. It does not recognize the biological reality that the two generations, the parent generation and its immediate offspring generation, are not reproductively isolated–are not members of different species in the sense that the offspring generation surely could successfully interbreed with the immediate parent generation. An expert given the entire generational sequence from dinosaurs to birds would be utterly stumped in trying to find a sharp species boundary between any pair of succcessive generations, and that is precisely the point (this very appropriate formulation stolen from Andrea Bottaro of the Thumb). Natura non facit saltus, while not universally true, is a good rule of thumb in evolution, whether via natural selection or genetic drift. If our correspondent understands that he’ll be on his way to understanding why his questions make little sense.

Consider an obvious analogy. There was never a time when a child in an ancient Vulgar Latin-speaking population could not converse with its parents and siblings and peers in a mutually comprehensible language (teenagers’ slang aside!), yet over centuries/generations the (geographically dispersed) population of Vulgar Latin speakers incrementally diverged into separate populations of Italian speakers, French speakers, Spanish speakers, Portugese speakers, and speakers of dozens of other Romance languages. Today, after many centuries/generations, those languages are largely mutually incomprehensible (and many are extinct, along with–AFAIK–the original Vulgar Latin) yet there was never a time when a pair of Vulgar Latin-speaking parents suddenly produced a Latin/Spanish-speaking child unable to comprehend its parents’ or siblings’ language. Over centuries/generations the languages (populations) diverged until they became mutually incomprehensible (they ‘speciated’), but there was never a time when an offspring generation couldn’t speak with its parent generation.

About that giant-step speciation

In general, speciation occurs incrementally as two populations diverge over lots of time/generations. If a subpopulation becomes isolated from its parent population, say by some geological event, then over time/generations (via natural selection and/or processes like founder effects and genetic drift) the isolated subpopulation may incrementally diverge from the original parent population. If the isolation is extended, that incremental divergence can widen and may result in speciation in the sense that if the geological barrier is subsequently eliminated, the two populations will have become sufficiently different as to no longer successfully interbreed, and the subpopulation would then be classified as a new species (on the Biological Species Concept definition of “species”).

However, there are exceptions. Speciation may not necessarily require geological or geographic isolation. For example, sympatric speciation may be occurring in Rhagoletis pomenella right now in my backyard (I have both apple trees and haws there). See here for a recent genetic analysis of the two populations. The “races” (subspecies?) of R. pomenella are becoming reproductively isolated by their different hosts, and host-related behavioral and genetic changes are accompanying and producing that reproductive isolation, leading to incipient speciation. But for a contrary view see this recent paper for data that contradict the hypothesis that the host-shifting radiation of R. pomonella is due just to recent sympatry but rather is likely based on long-standing genetic variation generated allopatrically in the deeper past. So while the radiation of subpopulations in the (sexually reproducing) R. pomonella instance is recent, it may have deeper genetic roots produced by geological/geographic separation. For another candidate instance of sympatric speciation see here (PDF). (See also here; hat tip to Wesley Elsberry for flagging it to me.) I’ll also mention that in spite of our correspondent’s mule example, not all hybrids are sterile, and hybridization is one way speciation can occur.

But that’s a side issue. In some circumstances speciation can occur in a single step of a single individual offspring if the organism can reproduce itself without benefit of sexual interaction with other members of the species. That’s not uncommon in plants–polyploidy in self-fertilizing plants can give rise to a new species in one step of a single offspring–and (more rarely) it can happen in parthenogenic animals. A Google Scholar search on [polyploidy speciation] turns up 11,400 hits while a search on [parthenogenic speciation] gets 5,740 hits. Just last month I wrote a brief note on the rapid creation in the laboratory of a new species of parthenogenic whiptail lizard.

But no one argues that the evolution of birds from small carnivorous dinosaurs occurred in one step via polyploidy. It was an incremental process over many tens or hundreds of thousands of generations. A professional review by Kevin Padian (though a tiny bit dated) is here (PDF). It’s worth noting that the review addresses in passing our correspondent’s question about endothermy and fossils. For a fairly recent pop-science overview of the evolution of endothermy in general see here.

There’s much more that could be said, of course.

While our correspondent focuses exclusively on the fossil evidence for ‘macroevolution’ of birds from reptiles, the most powerful evidence for “macroevolution” is from comparative genetics and molecular biology, and a complete answer would require considerable expansion of that evidence. Fossils aren’t irrelevant, of course. Donald Prothero’s Evolution: What the Fossils Say and Why It Matters has a dozen pages in Chapter 12 on the evolution of birds. But there are other examples, too. Take, for instance, the evolution of mammals. So continuous is the fossil record of the transition from reptiles to mammals that we’re reduced (again!) to an almost arbitrary separation, with mammal-like reptiles on one side of the transition and reptile-like mammals on the other.

It’s disappointing to have to respond like this to one who has the academic credentials our correspondent claims. While I have not publicly identified the correspondent here, some research (well, single-digit seconds of Googling) found him, and judging from his writings elsewhere on the web he’s not quite as agenda-free as he claims. But never mind. Just some modest effort and the URL of Google Scholar would have provided him with the professional resources to address the questions, and the URLs of the Berkeley/NCSE evolution site or the TalkOrigins Archive would provide the background necessary for a lay person to understand the misconceptions underlying our correspondent’s questions. But that requires intellectual effort, and there’s no evidence that our correspondent has expended that effort. This post is over 2,000 words now and it barely scratches the surface of our correspondent’s misconceptions . I hope it’s enough to stimulate him to do his own research and thinking, but I have to say I’m not optimistic.