Dobzhansky

Spring has finally come to us here in the North, and it's time to start thinking about planting. So, when I went to a seminar yesterday by John Willis, it was with dual motives.

Naturally, I was interested in hearing about his work relating the evolutionary ecology of Mimulus species to their genomics. As Willis and his many former and current lab members made clear in a recent review article in Heredity, monkeyflowers have become a really interesting model system for studying the dynamics of natural selection on genomes -- particularly, with relation to local ecological adaptation, and also with relation to speciation.

But I was also thinking about whether I could find a nice flower variety for my garden. I'm not particularly excited about peas, and I tolerate Arabidopsis when it comes up, but let's face it, it's not exactly a show flower. I'd love to get one of the prettier hawkweeds going (these have eponymical appeal as well as botanical interest) but the common ones are pretty boring.

Well, Willis's lab has been a center of development for Mimulus genetics. They have developed a store of SNPs and other markers (available at the Mimulus evolution website) for QTL mapping, and are using them to find genes responsible for ecological adaptations in different wild Mimulus populations. In the talk, Willis featured some of his collaborators' work finding genes involved in wet versus dry habitat adaptations and in early versus late flowering. These traits are connected to each other, as well as to other life history, plant size and flower size.

I left having my prior belief abundantly confirmed: botany is awesome. I mean, think about it. You can go outside, in your own neighborhood, and study biology. You can uproot your subjects and transplant them somewhere else, to watch how well they do. If they die, well, that's a data point, not an ethical emergency! Worried about gene-environment interactions? No problem, just put samples of all your subjects in the same greenhouse and wait. Need to isolate a QTL against a uniform genetic background? Cool, just repeatedly backcross it into an inbred line for a few generations, selecting for the trait each time. Want to study genetic correlations? Well, you can breed a thousand plants and select for any trait you want!

Oh, and if you want to, you can clone them.

Let's look at an example, from the Heredity review:

Recent work on floral evolution demonstrates that fundamental evolutionary questions can be addressed in Mimulus through the combination of field experiments and modern genomic approaches. Bradshaw et al. (1995, 1998) pioneered the application of genome mapping to study of ecologically important traits in Mimulus using RAPD and allozyme markers to map floral QTLs underlying the divergence between red-flowered, hummingbird-pollinated M. cardinalis and pink-flowered, bee-pollinated M. lewisii. The initial mapping experiments, with hybrid phenotypes measured in controlled greenhouse environments, revealed QTLs with major effects on virtually every floral character studied, from coloration and morphology to nectar production. To determine the effect of these QTLs on pollinator visitation and discrimination, Schemske and Bradshaw (1999) moved the genotyped hybrids to a field site near one of the few regions where the species coexist, and observed bee and hummingbird visitation behavior. Amazingly, the M. cardinalis allele at a single QTL, YELLOW UPPER (YUP), was responsible for an 80% loss of visitation by bee pollinators, and the M. cardinalis allele at a QTL responsible for variation in nectar production doubled hummingbird visitation (Schemske and Bradshaw, 1999). Bradshaw and Schemske (2003) subsequently created near-isogenic lines (NILs), where heterospecific alleles at YUP were reciprocally introgressed into the parental genetic backgrounds, and evaluated the response of pollinators to the NILs in the field. They observed an even clearer pattern of pollinator discrimination due to this locus, with a 74-fold increase in bee visitation in M. cardinalis NILs that carried the M. lewisii YUP allele, and a 68-fold increase in hummingbird visitation in M. lewisii NILs with the M. cardinalis YUP allele. Although the ecological context, in this case the community of potential pollinators, is certainly important to the evolution of new pollinator associations, these results also demonstrate that single genomic regions can have a large effect on major evolutionary transitions (Wu et al. 2008: 224-225).

The talk was mostly focused on the Mimulus guttatus complex, where some of the most pressing issues are life history, drought tolerance, and tolerance of high mineral concentrations, such as salt or copper. They were able to trace many QTL's of small effect with relation to the major differences in life history and moisture requirements in ecogeographic races of M. guttatus, to show that the within-population variation for these traits is caused by high-frequency (likely balanced) alleles rather than mutation-selection balance or rare alleles, and to find the correlated responses to selection of different plant traits based on different QTL's.

With respect to the genetics of speciation and ecogeographic race formation, they are helped by a long history of research on Mimulus. For example:

Macnair and Christie (1983) performed the first direct genetic analysis of hybrid incompatibilities in Mimulus. While studying the genetic basis of copper tolerance in California populations of M. guttatus, they noticed that some crosses between plants from the copper mines and certain other populations resulted in F1s that died as young seedlings. Further crossing studies revealed that the F1 lethality was caused by a deleterious epistatic interaction between the copper tolerance allele from the mine populations (or a gene tightly linked to it) and alleles at an unknown number of different loci from the other populations. Such deleterious interlocus interactions, usually referred to as Dobzhansky–Muller (D-M) incompatibilities, are thought to be the major cause of low hybrid fitness in plants and animals (reviewed in Coyne and Orr, 2004). Remarkably, it appeared that natural selection for copper tolerance had indirectly resulted in the evolutionary origin of the hybrid incompatibility (Wu et al. 2008:226).

So yes, say what you want, botany is awesome. Plus, there's one more thing: I sat through an entire lecture about natural selection and ecological differentiation of species and races, and never once heard the word, "bottleneck." It was like traveling to some kind of bizarro world where biologists still read Darwin!

So we come down to the really difficult question: which variety am I going to plant? Mimulus glabratus is native here in Wisconsin, including Dane County, but it is not very showy, and prefers wet habitat. That makes it a poor fit for my native plant patch, which is dry/mesic, and which I never water unless the black-eyed Susans and bee balms start to wilt. Mimulus ringens is prettier, with bigger, lavender flowers, but also likes it wet.

I guess I'll have to keep looking. M. lewisii is a pretty variant, if I can find a good source for it, and I can keep it in one of the wetter corners of the yard. I would try for M. cardinalis, since we have hummingbirds sometimes, but I'd like to get Lobelia cardinalis going also, and it's a lot easier to find. Besides, it hardly looks like a monkey!

References:

Wu CA, Lowry DB, Cooley AM, Wright KM, Lee YW, Willis JH. 2008. Mimulus is an emerging model system for the integration of ecological and genomic studies. Heredity 100:220-230. doi:10.1038/sj.hdy.6801018

I'm doing some research for an essay, which relies quite a bit on the work of Dobzhansky and a few of his contemporaries. There are some great quotes that I won't be using, but thought it would be worth passing on. Probably to the greatest extent among the architects of the Synthesis, Dobzhansky concerned himself with the relationship between genetic evolution and human cultural evolution.

In 1963, he published an essay in Current Anthropology addressing the relationship of anthropology and the natural sciences -- part of a forum that also addressed the relation of anthropology with the social sciences and humanities.

Man is a highly variable polytypic and polymorphic species. The genetic variability affects behavioral traits no less than physiological and structural ones, and it is false to imagine that these three categories are clearly separable. The chief reasons why so many people are loath to admit the genetic variability of social and culturally significant traits are two. First, human equality is stubbornly confused with identity, and diversity with inequality, as though to be entitled to an equality of opportunity, people would have to be identical twins. Human diversity is not incompatible with equality. Secondly, it is futile to look for one-to-one correspondence between cultural forms and genetic traits. Cultural forms are not determined by genes, but their emergence and maintenance are made possible by the genetically conditioned human diversity. The division of labor in human societies is primarily a cultural rather than a genetic phenomenon, but could it be sustained in a population consisting of persons genetically as similar as identical twins? This is not entirely a vain question, since at least one great geneticist has recently envisaged the possibility of bringing about such genetic uniformity (Dobzhansky 1963:147).

Later in the essay, Dobzhansky raised the problem of an excess of success -- namely, that human population growth and technology made it possible to avoid mortality that once selected against various "bad" genes. Various beliefs about this trend gave the impetus to early-20th-century eugenics, and were a continuing concern for "well-thinking" people. Would humans become victims of their own success? Dobzhansky responded in two ways. First, by noting that natural selection is hardly a savior:

Neither do I need to retell here the story of the alleged relaxation or suspension of natural selection in civilized mankind. The dangers from this source, although not necessarily exaggerated, have often been presented in a wrong perspective. A notion, which is less frequently stated explicitly than implied in many writings, is that the progress of mankind would be safe and even irresistible if only the natural selection were permitted to operate unobstructed by civilization and its amenities. This notion does not stand critical examination. Natural selection does not even insure that the species on which it acts will survive, let alone that it will improve, in any sense of the word "improvement." Dinosaurs became extinct, despite their evolution having been piloted by natural selection, quite unhampered by culture, medicine, or charity (Dobzhansky 1963:148).

Second, Dobzhansky addressed the real question: whether the current direction of genetic change is desirable:

Reproductive fitness is assuredly not the only virtue which we admire in men and would like them to possess. By its maintenance of the reproductive fitness, natural selection brings results which we, men, do not necessarily hold desirable. But to say that natural selection is suspended in mankind because we are not sure that man's biological evolution has assumed a direction to our liking, is to make the word "natural" selection biologically meaningless. Natural selection is automatic, mechanical, blind. It has brought about the evolution of the living world and the emergence of man with his capacity for culture, but it has no purpose because purposes are human prerogatives (Dobzhansky 1963:148).

He ends the essay on the most remarkable note -- well, read it for yourself:

Being an anthropologist only by avocation, I may perhaps venture to claim for anthropology more than most anthropologists dare claim for themselves. The ultimate function of anthropology is no less than to provide the knowledge requisite for the guidance of human evolution (Dobzhansky 1963:148).

Shades of Hari Seldon, to be sure, but especially humorous in light of later events in the field. Still, one wonders how an anthropology directed toward this goal would be organized...

References:

Dobzhansky T. 1963. Anthropology and the natural sciences -- the problem of human evolution. Curr Anthropol 4:138+146-148.

I found this passage in the discussion following T. Dale Stewart's paper, "The problem of the earliest claimed representatives of Homo sapiens," from the 1950 Cold Spring Harbor Symposium on Early Man.

DOBZHANSKY: The great variability of the Neanderthaloids, so ably described by Drs. McCown and Stewart [McCown's paper immediately preceded the one under discussion], bears upon one of the basic problems of human descent. It is now clear that the Neanderthaloids and the so-called sapiens type were at no time two reproductively isolated species, but rather component races of a single species. Some modern populations may carry genes that were present in the Neanderthaloids, and other moderns may not carry such genes. But this does not mean, of course, that mankind consists of races descended from Neanderthaloids and other races which came from the sapiens type contemporaneous with the Nenderthaoids. In general, the old anthropological alternative of monogenic versus polygenic descent of man ceased to exist when considered from the vantage point of the present evolution theory. Different populations (races) of a polytypic species may be descended largely from different races of the ancestral species and may differ in some genes in which these ancestral races differed. And yet, a polytypic species may still evolve as a single genetic system. Favorable mutants or gene combinations arrived at in one part (race) of such a species may, under the influence of natural selection, eventually spread to all other parts and thus become a common property of the entire species. Thus, local autonomy of the gene pools of racial populations does not preclude retention of a basic unity of the species as a whole. I would like to point out that this view agrees quite well with the conclusions reached by the late Weidenreich on basis of purely morphological analysis of pre-human populations. This is worth while [sic] stressing because Dr. Weidenreich has sometimes used expressions which seemed to put him close to the old-fashioned polygenist camp, which he actually rejected absolutely (Dobzhansky, following Stewart 1950:106-107).

I love these discussions, which often included exactly the people whose opinions you would like to see, and sometimes some surprising ones. For instance, after Dobzhansky in this particular discussion, Joseph Birdsell and Stewart had an exchange about the implications of the fluorine dating of Piltdown for interpreting variability within modern humans (Birdsell's point being that such an apelike jaw must extend the variability of Homo sapiens even further if it is actually recent! Ha!)

And they often jumped off on tangents, like the Piltdown tangent, which remind you of the other things that people cared about besides the immediate topic. I think it's the closest thing to science blogging that the 1940's and 1950's had to offer!

References:

Stewart TD. 1950. The problem of the earliest claimed representatives of Homo sapiens. Cold Spring Harbor Symp Quant Biol 15:97-107, comments following.

I ran across this paper from a few years ago by John Avise and DeEtte Walker, which considers the implication of reticulation-based species concepts for mtDNA-generated phylogenies.

After quoting Dobzhansky on natural categories, they point to the central problem with using mtDNA phylogenies to define species: a clonally inherited gene does not easily lend itself to testing horizontal gene transfer:

In this same spirit, we ask here whether biotic discontinuities as seen through the eyes of laboratory-based mitochondrial geneticists tend to bear resemblance in number and composition to the biological units currently recognized as taxonomic species. There are additional reasons for interest in the outcome. First, discontinuities might be evident in local biotas (the nondimensional species perception) but may blur when geographic variation is taken into account. Molecular phylogeographic studies address this issue, because they explicitly analyze spatial variation (6, 7). Second, under the biological species concept (BSC), a sexual species usually is perceived as a reproductive community whose gene pool retains coherency primarily via the bonds of interbreeding and genetic exchange (1, 8); however, mtDNA molecules are transmitted asexually, and matrilines are nonreticulate. Thus, any genuine unities within (and discontinuities between) groups of organisms in mtDNA genotype cannot be attributed to "horizontal" patterns of contemporary lineage anastomosis via mating per se. Instead, they must be caused by "vertical" connections (and partitions) in matrilineal phylogenies. However, vertical connections themselves are functions of the demographic histories of population units demarcated by temporally extended patterns of interbreeding and gene flow.

I think this passage puts the situation more direly than deserved -- after all, every gene is vertically inherited. Mitochondrial DNA is no exception. It can be transferred by gene flow just as surely as any autosomal gene.

No, the key difference is that clonal inheritance leaves mtDNA with a greatly reduced effective size compared to autosomal (or X-linked) genes. This means that a given amount of gene flow is vastly less effective at dispersing mtDNA variants. Hence mtDNA (and Y chromosomes) have much higher F_ST (at equilibrium) than other genetic markers.

In other words, longstanding populations within a species will tend to look more divergent considering only their mitochondrial DNA than considering their autosomal genes. We can see this pattern when considering differences among subspecies of chimpanzees and other hominoids. The subspecies are highly distinct from each other considering only their mtDNA, with long divergence times ranging higher than a million years. The other uniparentally inherited genetic system, the nonrecombining portion of the Y chromosome (NRY) shows a similar pattern -- subspecies of chimpanzees are highly distinct, sharing no NRY lineages (Stone et al. 2001). In contrast, there is substantially more sharing of variants at autosomal sites (Fischer et al. 2004). Chimpanzee subspecies share many fewer autosomal variants than are shared among human groups, but they share many more autosomal than mtDNA or Y chromosome variants. Gorilla genes follow a similar pattern: mtDNA indicates very strong divergence between western and eastern gorillas, while autosomal genes show evidence for recurrent gene flow between them up to 150,000 years ago (Thalmann et al. 2007).
Avise and Walker compared mtDNA phylogenies for vertebrates with commonly accepted taxonomic species, finding roughly twice as many deep mtDNA phylogroups as taxonomic species. They consider that these generally represent historical patterns of demography and constrained gene flow within species.

Coalescent patterns in gene trees are related intimately to historical patterns in population demography (7, 21, 22). In particular, tight connections among nonanastomose [nonreticulating] genotypes suggest recent lineage coalescence to a shared ancestor, likely because of relatively small evolutionary effective population sizes that cause extant lineages to have shallow temporal depth. Conversely, large genetic gaps between gene-tree branches suggest long-standing historical population separations. In support of this likelihood, nearly all of the deep phylogenetic disjunctions registered in the intraspecific mtDNA gene trees in this review involved regionally separate populations.

This is basically saying that regional differentiation within species is an important source of genetic variability. They mention that male-mediated dispersal would create patterns not easily tested with mtDNA; this is one factor but broadly, any single gene will create a phylogeny that is potentially discordant with others in various ways.

References:

Avise JC, Walker D. 1999. Species realities and numbers in sexual vertebrates: Perspectives from an asexually transmitted genome. Proc Nat Acad Sci USA 96:992-995. Abstract

Fischer A, Wiebe V, Pääbo S, Przeworski M. 2004. Evidence for a complex demographic history of chimpanzees. Mol Biol Evol 21:799-808. doi:10.1093/molbev/msh083

Stone AC, Griffiths RC, Zegura SL, Hammer MF. 2002. High levels of Y-chromosome nucleotide diversity in the genus Pan. Proc Nat Acad Sci USA 99:43-48. doi:10.1073/pnas.012364999

Thalmann O, Fischer A, Lankester F, Pääbo S, Vigilant L. 2007. The complex evolutionary history of gorillas: insights from genomic data. Mol Biol Evol 24:146-158. doi:10.1093/molbev/msl160

On a bit of a writing junket for his book, Mankind Evolving, in 1963 Theodosius Dobzhansky put an essay in Current Anthropology titled "Anthropology and the Natural Sciences -- The Problem of Human Evolution."

He spends the first half of the essay expositing a dual inheritance theory of human evolution, with both biological and cultural systems included, and puts forward an argument for a synthetic vision of the cultural and biological approaches to studying humanity.

Like any well-written essay, it is really in the second half that this argument builds up steam. This passage concerning the recent force of natural selection as a result of cultural changes is a good example:

The radical changes in the ways of life of our generation compared to those of our parents and grandparents must have been largely cultural rather than genetic. This only proves again the absence of one-to-one correspondence between genetic and cultural changes; this does not prove that the biological evolution of mankind has stopped or that it is irrelevant to the cultural evolution. It is admittedly difficult to prove that mankind has changed biologically since, let us say, the days of the ancient Greeks and Romans, if by "proof" you mean demonstration of sizeable gene differences. We cannot test the genes of Pericles or Caesar or their contemporaries. But neither was Darwin able to "prove" organic evolution in this sense. The evidence is indirect, inferential, but nevertheless, I think, conclusive. Paradoxically, it is precisely because we know that mankind changes so greatly culturally that we can be so confident that it changes to some extent also genetically. When the environment changs, the only other necessary condition for the occurrence of genetic evolutioanry chagne can be defined. This is the presence in human popluations of genetic variants, some of which confer upon their carriers a higher fitness in the older, and other variants in the incoming environments. Despite all the inadequacies of our present knowledge of human genetics, this can scarcely be doubted. What is more, since the environment in which man lives is in the first place his sociocultural environment, the genetic changes induced by culture must affect man's fitness for culture and hence may affect culture. The process thus becomes self-sustaining. Biological changes increase the fitness for, and the dependence of their carriers on culture, and therefore stimulate cultural developments; cultural developments in turn instigate further genetic changes. This amounts to a positive feedback relationship between the cultural and the biological evolutions. The positive feedback explains the great evolutionary change, so great that it creates the illusion of an unbridgeable gap between our animal ancestors and ourselves.... Those who believe that man no longer evolves biologically might contend that our species has entered upon such a period. Here we must, however, proceed with the greatest caution. The potentialities for rapid evolution of the human species have not been depleted, since hte environment continues to change and the genetic variance remains plentiful. Mankind assuredly continues to evolve, both culturally and biologically (Dobzhansky 1963:147, emphasis added).

This really became the conventional view within a certain school of human evolutionists (which ultimately encompassed my own training), although not always so clearly expressed. It is behind the notion that culture is the "human niche," although not going so far as the ultimate expression of the "niche" idea, which came to hold that no other cultural species could have existed alongside ours.

I would say that Dobzhansky represents the rational conclusion about human evolution from the perspective of the neo-Darwinian synthesis. After having begun from the premise that morphological evolution must be explicable in population genetic terms, there is a problem for the synthetist: human behavior (and morphology itself to the extent that it is influenced by behavior) includes a learned component that can induce behavioral stability across genetic variants in time or space. In population genetic terms, this is an environmental effect, but it is clearly a determining effect for much of what interests us about humans. And culture is not readily incorporated into a synthesis in which the population genetic mechanisms are the basis of change.

So Dobzhansky attempts to bring the fields of biological evolution and cultural "evolution" together with three basic assumptions:

(1) Human exceptionalism -- "Man is the sole product of evolution who has achieved the knowledge that he came into this universe out of animality by means of evolution" (p. 148). The effect of this is to cordon off cultural evolution as a special mechanism applying to humans.

(2) Separate causes. By assuming that the causes of cultural evolution are different from the causes of biological evolution, Dobzhansky envisages a feedback process between the two. If they had the same causes (i.e., maximization of Darwinian fitness), then cultural evolution could simply supplant biological evolution. Nor would there be any possibility of a "conflict" between the two, as he goes on to discuss.

(3) Biological variation is value-neutral. This is the anti-racist assumption that marks a strong difference between Dobzhansky and many of his contemporaries.

These latter two claims both reflect the orthogonality of cultural and biological variation. In other words, the factors that cause and maintain cultural variability are not the same as the factors that cause and maintain biological variability, and the two are (potentially) independent. (We may observe that to the extent that a "feedback" process occurs, it is by inducing some correlation between the factors underlying cultural and biological variation). The two assumptions are marked by the following quote from page 147:

The chief reasons why so many people are loath to admit the genetic variability of socially and culturally significant traits are two. First, human equality is stubbornly confused with identity, and diversity with inequality, as though to be entitled to an equality of opportunity, people would have to be identical twins. Human diversity is not incompatible with equality. Secondly, it is futile to look for one-to-one correspondence between cultural forms and genetic traits. Cultural forms are not determined by gnes, but their emergence and maintenance are made possible by the genetically conditioned human diversity. The division of labor in human societies is primarily a cultural rather than a genetic phenomenon, but could it be sustained in a population consisting of persons genetically as similar as identical twins? This is not entirely a vain question, since at least one great geneticist has recently envisaged the possibility of bringing about such genetic uniformity.

I find it interesting that in comparison with most exponents of dual inheritance theory, Dobzhansky does not accept a fourth assumption that cultural evolution applies to rapid change and genetic evolution applies to slow change -- in other words, an assumption of "different rates." He knew all too well that the pace of genetic evolution is often rapid, and follows the implication of that view to quite logically conclude that cultural evolution could induce rapid biological evolution within humans, even asserting the logical necessity of accepting genetic evolution of humanity "since the Greeks and Romans."

After the passage cited above, Dobzhansky goes on to describe the perils of recent cultural and biological evolutionary trends in our species, which have tended toward population explosion and the potential hazards therein. Then he proceeds to point out that worries that evolution has been "suspended" are wrongly directed:

Neither do I need to retell here the story of the alleged relaxation or suspension of natural selection in civilized mankind. THe dangers from this source, although not necessarily exaggerated, have often been presented in a wrong perspective. A notion, which is less frequently stated explicitly than implied in many writings, is that the progress of mankind would be safe and even irresistable if only the natural selection were permitted to operate unobstructed by civilization and its amenities. This notion does not stand critical examination. Natural selection does not even insure that the species on which it acts will survive, let alone that it will improve, in any sense of the word "improvement." Dinosaurs became extinct, despite their evolution having been piloted by natural selection, quite unhampered by culture, medicine, or charity (Dobzhansky 1963:148).

Of all the major population geneticists of the synthesis, Dobzhansky was probably the least sympathetic with the aims and practices of the eugenicists. And he spends several paragraphs in this paper describing -- in not so many words -- why the problems addressed by eugenicists were wrongly posed.

But even in spite of this skepticism of aims, Dobzhansky is more or less accepting of the potential for human controls to deliberately control natural selection on humankind -- as he wrote (p. 148): "Man, if he so choose, may introduce his purposes into his evolution."

The paper ends with this (I would offer, "apocalyptic") vision of anthropology:

Being an anthropologist only by avocation, I may perhaps venture to claim for anthropology more than most anthropologists dare claim for themselves. The ultimate function of anthropology is no less than to provide the knowledge requisite for the guidance of human evolution. Human evolution has arrived at a crossroad from which there is no turning back and no escape. Our animal past is irretrievably lost -- we could not go back to it even had we wished. The choice is between a twilight, cultural as well as biological, or a progressive adaptation of man's genes to his culture, ans od man's culture to his genes. But to fulfill its function, anthropology cannot belong entirely either to biological or to social sciences or to humanities. It must, in the fullness of time, become a synthesis of all three (148).

Well, I think he probably could have predicted that the mantle of planning human destiny was not really what anthropology was about, even in 1963. In fact, reflecting on that date, this is probably about as close to a Kennedy-esque vision of anthropology as ever was expressed!

References:

Dobzhansky T. 1963. Anthropology and the natural sciences - The problem of human evolution. Curr Anthropol 4:138+146-148.

Lawrence Krauss has commentary in the NY Times about the recent Kansas State Board of Education elections:

But perhaps more worrisome than a political movement against science is plain old ignorance. The people determining the curriculum of our children in many states remain scientifically illiterate. And Kansas is a good case in point.

The chairman of the school board, Dr. Steve Abrams, a veterinarian, is not merely a strict creationist. He has openly stated that he believes that God created the universe 6,500 years ago, although he was quoted in The New York Times this month as saying that his personal faith "doesn't have anything to do with science."

...

A key concern should not be whether Dr. Abrams's religious views have a place in the classroom, but rather how someone whose religious views require a denial of essentially all modern scientific knowledge can be chairman of a state school board.

Krauss points out that the origins of the earth and other "contentious" scientific problems are connected in a web of theory and results with the way the world works everyday:

To maintain a belief in a 6,000-year-old earth requires a denial of essentially all the results of modern physics, chemistry, astronomy, biology and geology. It is to imply that airplanes and automobiles work by divine magic, rather than by empirically testable laws.

Although Krauss refers to the problem as "illiteracy", I think it is not so simple. Sure, everyday believers in creationism have little idea of the scientific evidence for the antiquity of the world, the record of evolutionary history, or the scope of biological diversity. But making it to elected office as an evolution-skeptic requires quite another level of knowledge about science. These folks have one thing down cold -- they know that scientific "facts" are often contingent, subject to revision or overturning, and open to challenges. Many of them probably have spent time thinking about the value of the scientific method, and may frankly prefer a philosophy of certainty, with tenets not subject to overthrow. You can teach an illiterate person to read. No one can teach Steve Abrams to abandon young-earth creationism.

What most people lack, and what these evolution-skeptics depend on, is a lack of deep understanding of the connectedness of scientific ideas. It is one thing to propose that the earth is 6000 years old. It is quite another to understand the full magnitude of physical and geological results that would have to be overturned to accept the young-earth theory. It is one thing for a scientist to say that young-earth creationism is akin to "airplanes and automobiles work[ing] by divine magic", and another to know what that even means.

Science works because it uncovers hidden connections in nature. Evolution is one of the richest sources of those connections. This is why Dobzhansky famously wrote that "nothing in biology makes any sense except in the light of evolution." The shame is that today people are so obsessed with the idea of hidden secrets in everyday life, but don't look to the real world of science that is uncovering those secrets every day.

If hybrid zones are transient, what are the likely outcomes? One possibility is that hybrid zones represent secondary contact and neutral diffusion, that the differentiated populations will fuse, yielding a single, possibly polymorphic, species. Alternatively, hybrid zones might represent the "wave of advance" of a superior competitor, resulting in the eventual extinction of one of the two hybridizing taxa. In fact, fusion and extinction are not mutually exclusive outcomes because the fusion of two taxa might involve either selective or random extinction of alleles from each of the parental types (Harrison, 1990).

In this sense, "extinction" means the disappearance of one of the parental types, not necessarily the disappearance of its alleles from the subsequent population.

(continued) If certain recombinate genotypes produced by hybridization and backcrossing persist as local "hybrid swarms" or "stabilized introgressants," the product of fusion may be considered a distinct species.

One of the most controversial issues surrounding hybrid zones is whether they are sites of "reinforcement" -- the evolution of prezygotic barriers to gene exchange in response to selection against hybrids. A mode of speciation originally championed by Dobzhansky (1940, 1941), the "reinforcement model" has met with considerable criticism in recent years (Patterson 1978, 1982; Butlin, 1987, 1989)....

This strain is picked up in the third chapter of the volume by Howard (1993).

(continued) Finally, selection within hybrid zones can lead to the weakening (rather than the strengthening) of barriers to gene exchange. Selection may favor those variants that show the least reduction in viability and fertility when crossed with either of the parental types.

Two hypotheses, discussed by Burke and Arnold (2001):

The role of epistasis in adaptive evolution has been a controversial issue ever since Sewall Wright and R.A. Fisher first formalized their views in the early 1930s. According to Wright (113, 114), natural selection retains favorably interacting gene combinations. Therefore, as a result of the highly integrated nature of the genome, selection may lead to the production of what Dobzhansky (43) has termed "coadapted" gene complexes. In contrast, Fisher (48) argued that natural selection acts primarily on single genes, rather than on gene complexes. In Fisher's view, therefore, selection favors alleles that elevate fitness, on average, across all possible genetic backgrounds within a lineage. Such alleles have been termed "good mixers" (75). Regardless of the role of epistasis within lineages, however, negative epistasis in a hybrid genetic background, or hybrid incompatibility, is fully consistent with both the Wrightian and Fisherian worldviews. This is because allelic fixation occurs in any one lineage without regard to the compatibility (or lack thereof) of new alleles with those in any other lineage. Hybridization then produces a vast array of recombinant genotypes that have never before been subjected to selection. On average, these genotypes will be less well adapted than their parents, giving rise to some level of selection against hybrids.

Hybrid breakdown, or the reduction in fitness of segregating hybrid progeny that often results from intercrossing genetically divergent populations or taxa, has long been taken as evidence of unfavorable interactions between the genomes of the parental individuals (e.g., 39, 42, 43, 75, 80). The most widely accepted genetic model for the occurrence of such incompatibilities was first described by Bateson (15, as cited in 83), and later by Dobzhansky (39) and Muller (79, 80). In short, the Bateson-Dobzhansky-Muller (BDM) model assumes that an ancestral population consisting solely of individuals of the genotype aa/bb is broken into two parts that are temporarily isolated from each other. In one subpopulation, a new allele (A) is then assumed to arise at the first locus. Meanwhile, a new allele (B) is assumed to arise in the other subpopulation. Because individuals of the genotype aa/bb, Aa/bb, and AA/bb can interbreed freely, the A allele can then spread to fixation in the first subpopulation; likewise, individuals of the genotype aa/bb, aa/Bb, and aa/BB can interbreed freely, and the B allele spreads to fixation in the second subpopulation. However, although A is compatible with b, and B is compatible with a, the interaction of A with B is assumed to produce some sort of developmental or physiological breakdown, such that hybridization between the two subpopulations leads to the production of offspring with decreased levels of viability and/or fertility. Although this model focuses on negative interactions between differentiated regions of the nuclear genome, similar interactions between one or more regions of the nuclear genome and some component of the cytoplasm (e.g., the chloroplast or mitochondrial genome) could also play an important role in hybrid incompatibility. Unfortunately, the BDM model does not provide any mechanistic explanation as to how mutations that are neutral (or beneficial) within a given lineage will produce strongly disadvantageous incompatibilities when combined in a hybrid background (Burke and Arnold 2001, emphasis added).

References:

Burke JM, Arnold ML. 2001. Genetics and the fitness of hybrids. Annu Rev Genet 35:31-52. DOI link