Many parasites have complex life-cycles. Tremadodes, the flukes, are a parasitic class of flatworms (Platyhelminths), with over 20,000 species. Most of them infect molluscs in the first part of the life-cycle, and vertebrates in the second part. The biology of scrub typhus is even more complex. It involves these factors:
The occurrence of all these factors together would be limited in space, and brief in time. This is typical of the ecology of parasitic infections.
Both evolutionary rates and speciation rates can be high. Sibling species are very common in the bug "Erythroneura", in which about 150 transfers from one host to another has resulted in about 500 species in the genus.
The clearest evidence comes from the large size of many parasitic families.
Sometimes there is good evidence of the speed of speciation. For example, five or more species of the moth "Hedylepta" must have evolved within 1000 years in Hawaii, because they are specific to banana, which was only introduced then.
Adaptive radiation in parasites is extensive. Its development in each taxon (group) depends on:
Diversity of hosts is a big factor. If many related species of host are available, then many related species of parasite will evolve. Mites on Lepidoptera families or fleas on mammals and birds are good examples. Eichler's rule goes as follows:
The two British oaks support some 439 species of parasite directly, and indirectly many hundred more which parasitise these parasites. "It would certainly be an underestimate to say that the two British species of oak are the primary products for a thousand species of parasite". Obviously, the large size of these trees is a factor in the number of parasite species. In general, this holds whether the host is a plant or an animal. A larger bird will harbour more species of ectoparasite than a small bird.
One of the reasons large trees have so many parasites is that they may have lived a long time in a particular area compared with other types of plant. They have had longer to accumulate parasites.
Parasites can be most useful in sorting out the phylogenetic relationships of their hosts. Sibling host species have been discovered when their parasites diverged. Common ancestors of present-day parasites were themselves parasites of the common ancestors of present-day hosts.
As the host evolves defences, so the parasite evolves to cope with this. This is co-evolution.
Consider two families of host plants, the Umbellifers (fennel, cumin, parsley, hemlock) and the Gramineae (grasses). The umbellifers have many aromatic species, and are chemically diverse and pharmaceutically interesting. Their resins and oils are defences against herbivory and parasitism. On the other hand, grasses have one big defence to herbivores: their stony inclusions in their cells wear down the teeth of herbivorous mammals, but they have few chemical defences. Both families are attacked by leaf-miner flies. There are four times as many grass species as there are umbellifers, but there are twice as many leaf-miner species on the umbellifers.
This is speciation which does not require geographical isolation. Ernst Mayr, the chief exponent of geographical speciation, admitted that host races of phytophagous animals "constitute the only known case indicating the possible occurrence of incipient sympatric speciation".
Moment of silence
"There is also a video game The Moment of Silence, and a poem "Moment of Silence"".
A moment of silence is a short time when people do not make noise. A moment of silence shows respect for people who have died. Many countries observe a minute of silence after a tragic event. Moments of silence often last one minute, but other amounts of time may be chosen.
On November 11th, many countries observe a two-minute silence to remember those who have died in World Wars. The tradition was started in 1919 exactly one year after the end of World War I. It became an official part of the annual service on Remembrance Day or Armistice Day.
During the moment of silence, people often bow their heads, remove their hats, and do not speak or move. A person in charge of a group will tell everyone when the moment begins and ends. A moment of silence may come before or after other events with symbolic meaning. Examples of these events are the ringing of bells, the release of doves or balloons, or a bugle playing the "Last Post".
The first recorded instance of an official moment of silence dedicated to a dead person took place in Portugal on February 13, 1912. The Portuguese Senate dedicated 10 minutes of silence to José Maria da Silva Paranhos Júnior, baron of Rio Branco, Brazil, and Minister of the Exterior of the Brazilian government, who'd died three days earlier on February 10. This moment of silence was registered in the Senate's records of that day. In the same year, large parts of the United States kept a ceremonial silence to honour the dead of the Maine and the "Titanic".
In the United States, some people say that allowing prayer as part of a moment of silence means that moments of silence can make it hard to keep the separation of church and state (the idea that religion and government should not affect each other).
Moments of silence do not have to be time for prayers. They can be used for other thoughts that are not religious. Many people who want time for prayers in public schools and government meetings use moments of silence so that some people can pray and other people do not have to pray. Because they represent the government, and because the Constitution of the United States says that government cannot force people to do religious things, these people cannot tell other people to pray.
When public schools have a moment of silence, Buddhist students could meditate (relax and think calm thoughts), students with other religions such as Christianity, Islam, and Judaism could pray, and atheist students could think about the day ahead.
Colin Powell, a famous government leader, likes having moments of silence in schools. He has said that a simple moment of silence at the start of each school day is a good idea. He also has said that students could use this time to pray, meditate, think, or study.
Many people believe that prayer is not allowed in United States public schools, but this is not true. The Supreme Court ruled in 1962 that students can pray in school, but teachers and other school leaders cannot lead the prayers. Students can form clubs where they can pray, and they can pray alone, but they cannot lead prayers at school events. The reason prayer is not allowed at those times is because of the First Amendment. The First Amendment says that government cannot force people to do religious things, and public schools are part of the government.
In 1976, the state of Virginia allowed schools to have a moment of silence at the start of the school day. This moment would last one minute. In 1985, the Supreme Court said that a "moment of silence" law in Alabama would not work with the United States Constitution and could not be used. In 2005, the state of Indiana made a law that said all public schools had to give students time to say the Pledge of Allegiance and a moment of silence every day.
In April 2000, Virginia changed its law to say that all public schools in Virginia had to have a moment of silence (before this change, schools could choose not to have a moment of silence). In October 2000, a judge named Claude M. Hilton said that the "moment of silence" law was allowed by the United States Constitution. Judge Hilton said that the law has a secular (not religious) purpose, that the law does not make religion more important or less important, and that the law does not make government and religion be too close to each other. Judge Hilton also said, "Students may think as they wish," and that this thinking could be religious or not religious. He said that the only thing students had to do because of the law was sit and be quiet.