Ongoing evolutionary changes
g massive extinction event such as the Cretaceous-Tertiary event has a low likelihood of occurrence during any given short time period, and that probability has fluctuated only moderately over the course of the history of Earth, at least before the advent of humanity. But even though such major events are unlikely in the near future, less dramatic environmental changes, many driven by our own activities, are taking place at the present time. Glaciers have advanced and retreated at least 11 times during the last 2.4 million years. The diversity of vertebrates before the onset of this series of ice ages was far greater than the diversity of vertebrates of the present time (Barnosky et al., 2004; Zink and Slowinski, 1995). More recently, human hunting resulted in a wave of large mammal and bird extinctions in the late Pleistocene (Surovell etal.,2005).
There has been a relatively mild decrease in the number of species ot mammal, compared with their great abundance in the early Ploicene. This decrease has resulted from Pliocene and Pleistocene climate change and from the human Pleistocene overkill. Now, the decrease is likely to become substantially steeper. It is clear that the relatively mild decrease in the number of species resulting from Pliocene and Pleistocene climate change and from the human Pleistocene overkill is likely to become substantially steeper. Some of the often discussed worst-case scenarios for the future range from the onset of an ice age of such severity that the planet freezes from poles to equator, to a series of nuclear wars or volcanic eruptions that irreversibly poison the atmosphere and oceans. If such terrifying scenarios do not transpire, however, we have a good chance of coming to terms with our environment and slowing the rate of extinction.
As we have seen, alterations in the environment can open up opportunities for evolutionary change as well as close them off through extinction. Even small environmental changes can sometimes have dramatic evolutionary consequences over short spans of time. Three species of diploid flowering plant (Tragopogon, Asteraceae) were introduced into western Washington State, North America, from Europe about 100 years ago. Two tetraploid species, arising from different combinations of these diploids, arose without human intervention soon afterwards and have thrived in this area (though such tetraploids have not appeared in their native Europe). DNA studies have shown that a variety of genetic modifications have occurred in these two tetraploids over a few decades (Cook et al., 1998). This report and many similar such stories of rapid recent evolution in both animals and plants indicate that evolutionary changes can take place within the span of a human lifetime.
New analyses of the fossil record suggest that recovery to former diversity levels from even severe environmental disasters may be more rapid than had Previously been thought (Alroy et al., 2001). Present-day ecosystem diversity may also be regained rapidly after minor disturbances. We have recently shown that the diversity levels of tropical forest ecosystems are resilient, and that while these forests may not recover easily from severe environmental disasters there is an advantage to diversity that can lead to rapid recovery after limited damage (Wills etal., 2006).
Nothing illustrates the potential for rapid evolutionary response to environmental change in our own species more vividly than the discovery in 2004 of a previously unknown group of hominids, the 'hobbits'. These tiny people, one metre tall, lived on the island of Flores and probably on other islands of what is now Indonesia, as recently as 12,000 years ago (Brown et al, 2004). They have been given the formal name Homo floresiensis, but I suspect that it is the name hobbit that will stick. Sophisticated tools found near their remains provide strong evidence that these people, who had brains no larger than those of chimpanzees, were nonetheless expert tool users and hunters. Stone points and blades, including small blades that showed signs of being hafted, were found in the same stratum as the skeletal remains (Brown et al., 2004). Using these tools the hobbits might have been able to kill (and perhaps even help to drive extinct!) the pygmy mastodons with which they shared the islands.
It is probable that the hobbits had physically larger ancestors and that the hobbits themselves were selected for reduced stature when their ancestors reached islands such as Flores, where food was limited. It was this relatively minor change in the physical environment, one that nonetheless had a substantial effect on survival, that selected for the hobbits' reduction in stature. At the same time, new hunting opportunities and additional selective pressures must have driven their ability to fashion sophisticated weapons.
The ancestor of the hobbits may have been Homo erectus, a hominid lineage that has remained distinct from ours for approximately 2 million years. But, puzzlingly, features of the hobbits' skeletons indicate that they had retained a mix of different morphologies, some dating back to a period 3 million years ago - long before the evolution of the morphologically different genus Homo. The history of the hobbits is likely to be longer and more complex than we currently imagine (Dennell and Roebroeks, 2005).
Determining whether the hobbits are descendants of Homo erectus, or of earlier lineages such as Homo habilis, requires DNA evidence. No DNA has yet been isolated from the hobbit bones that have been found so far, because the bones are water-soaked and poorly preserved. In the absence of such evidence it is not possible to do more than speculate how long it took the hobbits to evolve from larger ancestors. But, when better-preserved hobbit remains are discovered and DNA sequences are obtained from them, much light will be cast on the details of this case of rapid and continuing evolvability of some of our closest relatives.
The evolution of the hobbits was strongly influenced by the colonization of islands by their ancestors. Such colonizations are common sources of evolutionary changes in both animals and plants. Is it possible that similar colonization events in the future could bring about a similar diversification of human types?
The answer to this question depends on the extent and effect of gene flow among the members of our species. At the moment the differences among human groups are being reduced because of gene flow that has been made possible by rapid and easy travel. Thus it is extremely unlikely that different human groups will diverge genetically because they will not be isolated. But widespread gene flow may not continue in the future. Consider one possible scenario described in the next paragraph.
If global warming results in a planet with a warm pole-to-pole climate, a pattern that was typical of the Miocene, there will be a rapid rise in sea level by 80 metres as all the world's glaciers melt (Williams and Ferrigno, 1999). Depending on the rapidity of the melt, the sea level rise will be accompanied by repeated tsunamis as pieces of the Antarctic ice cap that are currently resting on land slide into the sea. There will also be massive disturbance of the ocean's circulation pattern, probably including the diversion or loss of the Gulf Stream. Such changes could easily reduce the world's arable land substantially - for example, all of California's Central Valley and much of the southeastern United States would be under water. If the changes occur swiftly, the accompanying social upheavals would be substantial, possibly leading to warfare over the remaining resources. Almost certainly there would be substantial loss of human life, made far worse if atomic war breaks out. If the changes occur sufficiently slowly it is possible that alterations in our behaviour and the introduction of new agricultural technologies, may soften their impact.
What will be the evolutionary consequences to our species of such changes? Because of the wide current dispersal and portability of human technology, the ability to travel and communicate over long distances is unlikely to be lost completely as a result of such disasters unless the environmental disruption is extreme. But if a rapid decrease in population size were accompanied by societal breakdown and the loss of technology, the result could be geographic fragmentation of our species. There would then be a resumption of the genetic divergence among different human groups that had been taking place before the Age of Exploration (one extreme example of which is the evolution of the hobbits). Only under extremely adverse conditions, however, would the fragmentation persist long enough for distinctly different combinations of genes to become fixed in the different isolated groups. In all but the most extreme scenarios, technology and communication would become re-established over a span of a few generations, and gene flow between human groups would resume.
Continue reading here: Changes in the cultural environment
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