The Numbers That Really Matter

Authors


‡ Address correspondence to F. W. Allendorf

We are drowning in information while starving for knowledge.

E. O. Wilson (1998)

Scientists get excited about numbers. We use numbers to detect patterns in nature and advance scientific debates. Sometimes, numbers embody ideas and viewpoints, taking on larger meanings. Here are a few numbers that are likely to have importance for readers of Conservation Biology:

  • •   6 billion and counting: the estimated number of people on Earth at the turn of the millennium
  • •   7.9 billion: the most optimistic U.N. estimate of world population in 2050
  • •   10.9 billion: the most pessimistic U.N. estimate of world population in 2050
  • •   10 million: a conservative estimate of the total number of species on Earth
  • •   1.4 million: the number of described species
  • •   25%: proportion of the 50,000 vertebrate species currently at risk of extinction

These menacing numbers define our time. They symbolize the threats human activities pose to our continued existence and the existence of all life on Earth.

Stephen Jay Gould and the national media got excited last year about the number 30,000. According to his essay in The New York Times (19 February 2001), Gould departed from his intended lecture schedule for only the second time in 35 years of teaching at Harvard to share with his students this “great day in the history of science and human understanding in general.”

This “great day” arose from the latest landmark of the Human Genome Project: the news that humans have between 26,000 and 38,000 protein-coding genes, rather than the long-standing estimates of up to 140,000 genes (Venter et al. 2001). The finding that a mere 50% increase in gene number is sufficient to “progress” from a lowly roundworm (Caenorhabditis elegans) to humans has created quite a stir. Even more surprising, humans appear to have only five times as many genes as some bacteria (Claverie 2001). The number 30,000 signified to Gould that the old reductionist model of “one gene, one protein” is dead: “the collapse of the doctrine of one gene for one protein, and one direction of causal flow from basic codes to elaborate totality, marks the failure of reductionism for the complex system that we call biology.”

The view that 30,000 genes is somehow not large enough to explain the incredible complexity of a human or any other vertebrate organism is overly simplistic. As pointed out in an accompanying article in Science, the relationship between gene number and organismal complexity is neither obvious nor linear (Claverie 2001). According to a simple model in which each gene is either “off” or “on,” the human genome is 103000 times more complex than the genome of C. elegans (Claverie 2001). Certainly this number is large enough to accommodate any opinions about the expected increase in complexity from roundworms to humans.

Why these findings signify the death of reductionism is unclear. The number 30,000 is itself an achievement of reductionism. Participants in the Human Genome Project knew that the first step to understanding a larger structure—our genetic mechanism—was to inventory the parts, just as reductionists have done for other systems for hundreds of years. They sequenced approximately 95% of the genome from the DNA of two males and three females: one African-American, one Asian-Chinese, one Hispanic-Mexican, and two Caucasians. They found 26,588 protein-coding transcripts and estimated that there may be as many as 12,000 additional ones based upon computational analysis; the function of 42% of the identified genes is unknown.

We recommend the 48-page paper (“The Sequence of the Human Genome”) by Venter and some 300 other authors (2001) as a surprisingly good read for all biologists. The paper contains a fascinating discussion of the evolutionary implications of this work and the genome-wide pattern of genetic variation. Venter et al. found remarkable heterogeneity in the distribution of genetic variation throughout the genome. They also report an astounding amount of genetic variation in the human population. They estimate that a random pair of human gametes would produce a zygote that is heterozygous at 1 out of 1250 base-pairs for so-called single-nucleotide polymorphisms (SNPs). Such a zygote would be heterozygous for over 2,000,000 SNPs throughout the entire genome; however, less than 1% of all SNPs affect protein-coding regions.

The excitement over discovery of the size and complexity of the human genome should not be diminished by pointless controversy over the relative virtues of reductionism and holism. Science advances in many ways. The reductionism of Linnaeus was an essential precursor to the synthesis of MacArthur and Wilson. We need both. Personally, we marvel at the rampant diversity of both the human hereditary system and global ecosystems. As scientists, we have never doubted that the many parts of each interacted with one another.

There are many parallels between the structure and function of the human genome and the structure and function of any complex system. Gould talks about the “glorious ramifications of their irreducible interactions” of our 30,000 genes, but he could as well have been speaking of the organisms of the uncounted species in any ecosystem in the world. Ecology is defined in terms of the interactions among organisms and their environments. The relationships and their “glorious ramifications” matter as much as the individual units, whether genes or organisms. As conservation biologists, we are disappointed that the catalogue of global biodiversity remains woefully incomplete 250 years after Linnaeus began to compile a list of species. In contrast, the interval between Watson and Crick's work and publication of the human genome sequence was less than 50 years. Ecologists and taxonomists have not been able to get society as excited about counting our fellow species as geneticists have been at generating excitement about counting our genes.

In defense of the larger social vision of geneticists, we draw attention to a 1993 interview of James Watson (once of the Human Genome Project) and Frances Crick (  Jaroff 1993). When asked if they worried about the potential harm that could arise from gene therapy, they answered quite simply. Francis Crick said, “All the worries about genetic engineering pale in significance with the question of what you are going to do about there being so many people in the world and the rate at which they increase.” James Watson agreed: “Yes, that's what I worry about — overpopulation.” We think Crick and Watson are concerned about a number that really matters: 6,000,000,000.

The Human Genome Project is a great achievement with important significance for genetics, development, and evolution. Nevertheless, historians will not remember 2001 as a great year “in the history of science and human understanding” because we catalogued the human genome. Instead, they are far more likely to remember that in 2001 our society did not pay enough attention to the numbers that really matter, the size of the ever-increasing human population and the decline of the uncounted species on the Earth. They will remember that we had the will to count our own genes, but not the understanding and wisdom to control human population growth and to catalogue the species with which we share this planet. Those numbers should grab headlines and the attention of the world. They are numbers that are worth canceling a lecture for.

Ancillary