Recent warming has caused changes in species distribution and abundance 1±3 , but the extent of the effects is unclear. Here we investigate whether such changes in highland forests at Monteverde, Costa Rica, are related to the increase in air temperatures that followed a step-like warming of tropical oceans in 1976 (refs 4, 5). Twenty of 50 species of anurans (frogs and toads) in a 30-km 2 study area, including the locally endemic golden toad (Bufo periglenes), disappeared following synchronous population crashes in 1987 (refs 6±8). Our results indicate that these crashes probably belong to a constellation of demographic changes that have altered communities of birds, reptiles and amphibians in the area and are linked to recent warming. The changes are all associated with patterns of dry-season mist frequency, which is negatively correlated with sea surface temperatures in the equatorial Paci®c and has declined dramatically since the mid-1970s. The biological and climatic patterns suggest that atmospheric warming has raised the average altitude at the base of the orographic cloud bank, as predicted by the lifting-cloud-base hypothesis 9,10 .This hypothesis builds on evidence that rising sea surface temperatures (SSTs) have altered the climates of tropical mountains. Enhanced evaporation from warm ocean surfaces has generated large amounts of water vapour, and latent heat released as this moisture condenses has accelerated atmospheric warming 5 . Because vertical thermal pro®les have tended towards a moist adiabatic lapse rate, the decline in temperature with increasing elevation has diminished, amplifying the warming in the highlands relative to the lowlands 11±13 . Freezing heights have shifted upwards 11 , and glaciers on high tropical mountains are rapidly melting 14 . If temperature-dependent relative humidity surfaces, and thus cloudformation heights, have likewise shifted upwards 10 , organisms may be affected in various ways. Monteverde's stratus±stratocumulus bank, which forms as the trade winds meet the Caribbean slope of the Cordillera de Tilara Ân,¯ow upwards and cool adiabatically, in¯u-ences several key ecological processes. A lifting cloud base should alter regional hydrology by reducing critical dry-season inputs of mist (low-intensity windblown precipitation) and cloud water (non-precipitating droplets deposited onto vegetation) 15,16 .To examine climate trends, we have analysed patterns of precipitation, stream¯ow, air temperatures and SSTs. The rainfall and air-temperature data (collected by J.H.C.) are from leeward cloud forest (1,540 m; ,1 km west of the Monteverde Cloud Forest Preserve headquarters and ,3 km west of the continental divide). The weather station lies on the western boundary of our 30-ha study plot for anoline lizards, which overlaps a 40-ha plot for birds. Both plots lie within the 30-km 2 anuran study area. The stream-¯ow data (from the Costa Rican Electrical Institute) are for the Rõ Âo Can Äas at Lõ Âbano (300 m; ,23 km northwest of Monteverde). The SST data (from NOAA) are ...

The multigene family is a unit of chromosomal organization. Its gene members are closely linked, homologous in sequence, and have overlapping functions. Multigene families can be divided into three catagories: simple-sequence, multiplicational, and informational-by a variety of structural and functional criteria. Multigene families exhibit two novel evolutionary features-coincidental evolution and rapid change in family size-that suggest that they all share one or more evolutionary mechanisms. Natural selection cannot act directly upon individual genes in a family because of their identical or overlapping functions; hence selection must operate upon the family as a whole or upon blocks of genes within the family. The mechanism(s) for coincidental evolution expands out variant genes within a family so they can be acted upon by natural selection and, accordingly, permits multigene families to evolve adaptively. The control mechanisms in multiplicational families appear to promote the rapid expression of many gene copies. In contrast, the regulatory mechanisms of informational families promote the selection, expression, and amplification of appropriate units of information. The close linkage of the genes in a family appears to be a consequence of the fact that their control and evolutionary mechanisms may only operate on tandemly linked genes. New multigene families may evolve from a single gene or from other multigene families. In addition to evolving new functions, the latter mode of evolution generates a new multigene family whose members are preadapted to interact with those of the old family. These family interactions can lead to the evolution of more sophisticated molecular machines or to the regulation of one family by a second. Multigene families may be large or small. The three catagories of multigene families allow potential multigene families to be identified, and they suggest specific experimental approaches for the study of new families. Some of the most interesting genetic systems under the investigation today are known or potential informational multigene families. This is not fortuitous in that many of the most interesting aspects of phenotype are complex ones with correspondingly complex genetic, evolutionary, and regulatory requirements. One of the frontiers in modern genetics is the identification, characterization, and understanding of informational multigene families.

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