Science Magazine volume 280 mei 98
Genes Put Mammals in Age of Dinosaurs

The long-standing view from the fossil record is that mammals first appeared 225 million years ago as small, shrewlike creatures and that only after a mass extinction 65 million years ago at the end of the Cretaceous period killed off the dinosaurs were mammals able to evolve into everything from primates to rodents to carnivores. But in this week's issue of Nature, a pair of researchers compared genes from hundreds of vertebrate species and used the differences as a molecular clock to date when animal lineages originated. The molecules show, they say, that the modern orders of mammals go back well into the Cretaceous period, in some cases to more than 100 million years ago.

 

Science Magazine volume 280 mei 98
Genome Data Shake Tree of Life

The new wealth of microbial genome sequences is threatening to overturn evolutionists' "tree of life." In the current tree, a universal common ancestor gave rise to the two microbial branches, the archaea and bacteria (which lack cell nuclei), and the archaea then gave rise to the eukarya (all organisms that have cell nuclei). But the new sequences show that genes don't evolve at the same rate or in the same way, so the evolutionary history inferred from one gene may be different from what another gene appears to show. Even more perplexing, some genomes have been found to contain a mix of DNAs from both the archaea and the bacteria. Many evolutionary biologists are coming to believe that these mosaics arose because genes hopped from branch to branch as early organisms either stole genes from their food or swapped DNA with their neighbors. If this gene swapping was extensive enough, the tree's "base" may turn out to be indecipherable: a network of branches that merge and split and merge again before sprouting the modern kingdoms.

 

Nature mei 98
Why small populations of animals go extinct
A small butterfly living on the Åland islands off southwest Finland has provided the first convincing evidence from the wild that loss of genetic diversity in isolated populations is an important risk factor for extinction, according to a report in the 2 April 1998 issue of Nature. The Granville fritillary (Melitaea cinxia) is itself in no immediate danger of extinction, but many of its naturally scattered local populations go extinct each year; the empty sites are recolonized by immigration.

This situation provided Ilik Saccheri and colleagues from the University of Helsinki, Finland, with a living laboratory in which they could tease apart the factors that contribute to small populations of animals tending to become extinct.

The greatest danger facing many animal species today is the destruction of their habitat. When extensive areas of habitat become fragmented, the species can only hang on precariously as isolated small populations, sometimes as few as a single pair and their offspring, in the 'islands' of habitat that remain. There is no question about the fact that a small population is more likely to go extinct, but the exact reasons why have been the subject of debate. From experience with breeding endangered animals in zoos, it is clear that continued inbreeding within a small group of closely related animals eventually leads to the emergence of genetic defects and to a decline in reproduction and general fitness. This well-known genetic phenomenon of 'inbreeding depression' is due to a reduced genetic diversity within the population and the accumulation of deleterious genes in the offspring of matings between close relatives.

Until now, however, there was no hard evidence of a link between loss of genetic diversity and a greater risk of extinction in the wild. Some ecologists had argued that, in such small populations, inevitable adverse fluctuations in food supply, weather and even simple accident will lead to extinction before the effects of inbreeding could kick in, and so genetic factors could not be of any importance at all.

Saccheri and colleagues have now provided good evidence for a link between inbreeding and extinction. They studied the causes of local extinctions in scattered populations of the Granville fritillary butterfly living in dry meadows on the Åland islands. Each meadow represents a distinct, more-or-less isolated, population; some small populations represent the offspring of only a single pair of butterflies.

They had already surveyed the butterfly populations in 1,600 or so meadows between 1993 and 1996, and found that about 200 populations go extinct each year. The risk of extinction increased, as might be expected, with decreasing population size, decreasing numbers of butterflies in neighboring populations, and decreasing size of habitat patch. In 1995 to 1996 they combined their field survey with DNA analyses on individual butterflies from selected sites. In this way they could determine the genetic variation within each population and thus the degree of inbreeding.

By choosing a variety of isolated and less isolated sites, with populations of varying sizes, they were able to establish that, even allowing for all the other factors that influence the survival of a butterfly population, the reduction of genetic variation in a population, even quite a large population, significantly increased the risk of its going extinct.

In the case of the Granville fritillary, the damaging effects of inbreeding appeared as fewer larvae surviving to adulthood, shorter-lived adults, who thus laid fewer eggs, and eggs that were less likely to hatch successfully.