The most direct proof of evolution is furnished by the science of PALEONTOLOGY, or the study of life in the past through fossil remains or impressions, usually in rock. Additional evidence comes from comparative studies of living animals and plants, including their structure (COMPARATIVE ANATOMY), BIOCHEMISTRY, EMBRYOLOGY, and geographical distribution. Approximately 2 million different species of organisms are now living, but it is estimated that at least 99.9 percent of the species that have ever lived are now extinct and that some 2 billion species have evolved during the past 600 million years.
Changes occur in living organisms that serve to increase their adaptability, or potential for survival and reproduction, in the face of changing environments. Evolution apparently has no built-in direction or foreordained purpose. A given kind of organism may evolve only when it occurs in a variety of forms differing in hereditary characteristics, or traits, that are passed from parent to offspring. Purely by chance, some varieties prove to be ill adapted to their current environment and thus disappear, whereas others prove to be adaptive, and their numbers increase. The elimination of the unfit, or the "survival of the fittest," is known as NATURAL SELECTION because it is nature that discards or favors a particular variant. Basically, evolution takes place only when natural selection operates on a population of organisms containing diverse inheritable forms. Recently, natural selection was demonstrated for the first time outside of the laboratory when scientists observed guppies change their reproductive behavior over an 11-year period in direct response to being transferred to a new environment that had different predators.
A habitat is an organism’s environment. Related to the habitat is the niche of an organism, which is the function of that organism within a habitat. In other words, the habitat describes where a living thing dwells, while the niche tells what it does.
The word ‘population’ is used in ecology to describe a single species population. A group of individuals belonging to one species (of bacteria, fungi, plant, or animal) living in an area is a population. The subfield of ecology that deals with populations focuses on the number of organisms that live in a particular area and the biological and physical factors that sustain them.
A community is a group of populations of different organisms that interact with one another in a given habitat or area. Within a community, all the organisms are interdependent (that is, they depend on each other for survival).
An ecosystem is a large area where materials are exchanged between living communities and nonliving things. This exchange of materials depends on the decomposition of living organisms and the intake of inorganic materials into living organisms (to put it simply, this means eating). Balance is extremely important in an ecosystem, where disturbances and slight fluctuations in stability can threaten the lives of the organisms within the system. Ecosystems are basically self-contained energy and nutrient cycles. Soil, air, water, sunlight, minerals, and nutrients are all important components of a healthy ecosystem.
Finally, a biome is the largest ecological unit. It consists of many ecosystems which interact with each other in complex ways. Different biomes are distinguised from each other by variations in climate, rainfall, soil type, food sources, plants and animals. There are nine major terrestrial biomes: coniferous forest, desert, grassland, mediterranean, mountain, rainforest, savanna, temperate forest, and tundra. There are also aquatic biomes: coastal waters, coral reef, freshwater, and open ocean, to name only a few.
Unfortunately, human activity has caused significant damage to ecosystems and biomes worldwide. Pollution has destroyed many ecological habitats throughout the world beyond hope of repair. Myriad species of organisms have become extinct without ever being discovered and potential forms of medicine have died with them. The devastation that the human species has caused through the exploitation of natural resources and the sake of making money is sickening. Disturbing the global ecology of our planet will have serious repercussions in the future. For this reason, ecology is an important field of study. By studying the relationships between biotic and abiotic things, we will gain an understanding of and an appreciation for the incredibly complex world we live in. This understanding will help us improve the environment, manage natural resources and ultimately protect the health of the planet Earth.(http://library.thinkquest.org/C003763/index.php?page=planet04)
Chapter 24:
Birth rate (b) − death rate (d) = rate of natural increase (r).
birth rate expressed as number of births per 1000 per year (currently 14 in the U.S.);
death rate expressed as the number of deaths per 1000 per year (currently 8 in the U.S.);
So the rate of natural increase is 6 per thousand (0.006 or 0.6%).
Although the value of r is affected by both birth rate and death rate, the recent history of the human population has been affected more by declines in death rates than by increases in birth rates.
The graph shows birth and death rates in Mexico since 1930. The introduction of public health measures, such as
better nutrition
greater access to medical care
improved sanitation
more widespread immunization has produced a rapid decline in death rates, but until recently there was no corresponding decline in birth rates. In 2007, r is 1.7%. (Data from the Population Reference Bureau.)
Although death rates declined in all age groups, the reduction among infants and children had — and will continue to have — the greatest impact on population growth. This is because they will soon be having children of their own.
This situation, resulting in a rapid rate of population growth, is characteristic of many of the poorer regions of the world.
The Demographic Transition
Slowly declining birth rates following an earlier sharp decline in death rates are today characteristic of most of the less-developed regions of the world.
The shift from high birth and death rates to low birth as well as death rates is called the demographic transition.
This graph (based on data from the Population Reference Bureau) shows that the demographic transition began much earlier in Sweden than in Mexico and was, in fact, completed by the end of the nineteenth century. The spike in deaths in the interval between 1901 and 1926 was caused by the worldwide influenza pandemic of 1918–1919.
The birth rate in Sweden is now (2007) 12/1000; the death rate 10/1000, giving a rate of natural increase (r) of 0.2
No comments:
Post a Comment