h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hi Muzaakboy,

Those are sensible questions but I regret I don't have answers for you. (I suspect the flies started using the apples because they were newly introduced to the area and represented an ecological niche - just as grass quickly starts to grow on a brand new patch of soil).


I do have a question for Ste though:

Ste; you say you teach evolution so could you explain how a species can change the number of chromosomes it has? As a genetic shift it's pretty radical, affecting as it does the genetic material itself and the reproductive process.

F

Hi Adib

Should be writing a paper, but hey ho, nevermind, here I am on h2g2.

"What I came up with is that I do not think that they are a distinct species. This is because the two types of have many similarities as I know would be expected from a species developing from another."

But the similarities are not the issue here. We're **VERY** similar to chimps, we are not one species. However, the flies are developing differences because of the reproductive isolation, they have only been apart for 100 years, which is a very small amount of time. The point is, is they're temporally isolated from each other, therefore breeding is not taking place (apart from at the very extremes of the two where the emergence times overlap in the hybrid zone).

The two flies are reproductively isolated in time, not physical space. Usually, speciation is thought of when a geographical barrier disrupts a species into two groups, and those two groups differentially adapt to their different local environments. It's the same here, but the barrier is not a mountain or a lake it's time, specifically emergence time, which is differentially synchronised to different fruiting times of the different plants.

I talked about hybridisation in my previous post...

The actual detection of the hawthorn fruit is genetically controlled. Those that have a copy of a certain functioning gene can smell the hawthorn fruit. Those without the gene are a lot less fussy about what fruit they will breed on.

The apple trees were introduced by man in the 1850s remember, so this represented a new unexploited ecological niche. All those juicy apples and noone feeding upon them. Speciation happens when ecological niches open up, for example, after all the major extinction events in the history of life, an explosion of species always follows. Look at mammals replacing dinosaurs in their niches after the K-T asteroid collision 65Mya.


"This is because the apples while effectively a safer place for there maggots are not more nutritious and so the only ones that will survive are the ones with the ability to survive on less nutrition than they used to have. Meaning that the things that make living on an apple easier would become more prominent while the other genes would disappear.
Meaning the apple maggot flies would end up being to specialized."

I think that's generally correct, apart from the nutrition part. The apple flies *have* become specialised, in the space of 100 years. Apples are larger than hawthorns, and the organ that is used to deposit fertilised eggs into the fruit by females (the ovipositor) is significantly longer in the new apple species compared to the hawthorn ancestor.

The flies that happened to emerge at the right time to take advantage of the apples, which fortuitously had their emergence time synchronised with the fruiting of the apples were naturally selected. That group grew in size because of the huge previously unexploited resource it was now using.

To answer your specific questions:

"Why did they start to change fruit in the first place?"
I explained this above, apple trees were introduced and a new ecological niche (read: "role in an ecosystem") opened up.

"Was there a shortage of hawthorns at that time?"
Nope.

"Were there less hawthorns because man had destroyed them to plant the apple trees there instead?"
Not that I know of.

"If the apples where not present in that area before could the flies have thought it was a bigger version of the hawthorn fruit?"
If certain genetically different flies that cannot detect hawthorns so well (via smell) then they wouldn't care. Or know. Flies "know" nothing, they just are.

"How many eggs do they hatch on average?"
Around 300.




Fathom

"... how [does] a species can change the number of chromosomes it has? As a genetic shift it's pretty radical, affecting as it does the genetic material itself and the reproductive process."

Genomes are pretty messy things. They also change all the time, they are very fluid and dynamic. One of these changes is spontaneous changes in chromosome number. Most mammals have two sets of chromosome, we have 23 *pairs* of chromosomes. Rice (the plant) has two sets also, but maize (aka corn) has four. Maize and rice are very closely related (common ancestor ~50Mya), so somewhere along the line maize has doubled it's entire genome. This can happen during cell division (mitosis), when all the DNA in a cell is doubled up, then neatly packaged into two cells. Sometimes that goes wrong, and you end up with a cell with twice the DNA. It's called "polyploidy", and happens often in plants. Plants that have (essentially) more genomes are more robust, healthy with larger yields. This is because deleterious, harmful recessive genes are more likely to be shielded by copies of the genes on the other genomes. So, polyploidy is the most common way an organism changes it's chromosome number.

Chromosomes are also quite fragile in places. They break sometimes (often causing disease) giving a new chromosome number. That's another way. Also chromosomes may not segregate evenly between two new cells that are dividing.

Ste

'"How many eggs do they hatch on average?"
Around 300.'

Oops, they *lay* 300, not sure about hatching...

Thanks Ste,

The problem is; if one individual undergoes a change in the number of chromosomes it carries, should this not render it unable to reproduce with another member of its species? This would stop the change dead in its evolutionary tracks. Clearly this isn't the case or all species would have to carry the same number of chromosomes but I was taught (many years ago!) that organisms with different numbers of chromosomes could not interbreed. Perhaps that is true but for a different reason so is a generalisation rather than a cause.

F

Don't potatoes have the same number of chromosones as we do?

I'm not sure as I did read it in a Harun Yahya book, an you know what he is like.


Adib

Just to add his motto is if evolution is 2+2 = 5 then proving creation is simple 2+2+5 - 8 =0

Or inother words rubbish.

Hi Fathom,

Good question. when an organism has an odd number of chromosomes, it is sterile. Their cells cannot divide equally and messes up sex cell formation (meiosis). Mules are sterile offspring of horses. Mules have 63 chromosome, horses have 64 and donkeys 62 (we have 46). People whith Down's syndrome are sterile because they have three copies of chromosome 21 (therefore 47 chromosomes).

The thing is, that few few life forms only ever have one offspring at a time, like humans do. If this kind of change is going to stick and not become an evolutionary dead-end, then it would have to happen in more than one organism; more than one offspring at once. This could be achieved by the error occurring in a parent (sex cell formation, again). So when you get a number of individuals with this chromosomal error, either inbreeding or self-fertilisation has to occur to rescue the population from becoming a dead-end. Inbreeding in small "founder" populations, as it turns out, is an important factor in evolution.

So, if a plant had 50 offspring with the same error, then they would only be able to breed with each other, or themselves. Notice what this means? They would be reproductively isolated. Over time the individuals would diversify and diverge and the effect of inbreeding depression would lessen.


Adib,

Spuds may have the same chromosome number as us, but I don't see too many people getting it on with potatoes. Lions and tigers have the same number of chromosomes, but live in entiresly different parts of the world, you can even get them to make offspring. Because they are different species because they never come into contact with each other. Even if they did, there would be no guaruntee they'd want to mate (probably wouldn't).


Ste

Er I just said in one of those of hand matter of fact ways, I did not mean anything by it.

I know chimps have a different number to us though.

Didn't they report a hybrid lion and tiger cross a few years ago?

I'm sure they made some sort of large cat hybrid a few years ago.

Adib

http://english.peopledaily.com.cn/200208/28/eng20020828_102249.shtml

Yeah I know, I was being pedantic. I had also just woken up.

Here's a picture of some strange folk with a 'liger' http://www.scumpa.com/~art/king-richards-sep02/liger.jpg

Ste

Its the offspring of a male lion and a female tiger then. A Tigon has its parents being a male tiger and a female lion.

If species do not evolve then why do many species not appear in the fossil record unitl a certain point. Why do some animals disapear at around the same level that similar but identifibly different animals appear in the fossil record.

The numbers of scientists who still see creation as being real and evolution being false are tiny by comparison to those who judge that the evidence show the complete opposite to be true. These are people who understand and work in science.

Many people ultimately abadoned their faith because what the evidence shows to be true shows that the object of their faith was inpart unfounded conjecture.

Atheists are different from Muslims in that if evidence for what we see as true were superceded tomorrow we would be able to test and accept this new evidence, it is the one thing that atheists *believe* in, everything else we check, we believe that examination is better than faith and trust.

Faith says that animals are immutable. Science shows us reptiles slowly transforming into birds by progressive random mutations these mutations being alowed to flourish and be carried forward by their fitness for survival or snuffed out by the environment where they are not suited to give an advantage.

Faith has always explained the things we cannot not which is why Ra needed a boat to cross the under world. Though later religions did not need sun gods to explain night and day the concept stayed in egypty even after people understood astronomy. Christians who can and do accept the truth of evolution adapt the story of creation because they still need to beilieve in a god. It is a personal thing but can not change what is evident.

one love

"I know chimps have a different number to us though."

Does anyone have any support for this? I find it hard to believe considering how they claim that Chimps are about 99.5% geneticly identical to humans.

The 99.5% similarity number is based up DNA seuqence similarity. You can have the same actual DNA sequences contained in slightly different packages. Chromosomes can split and join with little effect upon the actual DNA sequence or genes. One human chromosome looks just like two chimp chromosomes lined end to end.

Ste

Thanks, Muzakboy for bringing that up, and Ste for explaining it.

http://www.godless.org/sci/sr_talk.html

Dr. Norman F. Hall

Another essential characteristic of science is that it must make a critical assumption about the very nature of the universe. Since science is based on experimentation and observation, it must assume that the universe is not out to trick us, and that it will not lie to us; indeed, we assume that it cannot lie to us. The biochemist Jaques Monod had a name for this assumption -- the postulate of objectivity. Note that this says nothing about human observers being objective. What it refers to is the universe itself. If the universe has an axe to grind, an agenda, a set of chosen people, a story we are supposed to believe even if it is not evident to our senses, an elite priesthood to which it will reveal itself, and to no others, then science is impossible.

Some of the old-time creationists had a wonderfully naive story called the Omphalos argument. Omphalos is greek for navel, or belly button. Did Adam have one? Sure, said the creationists, since God wanted to create the world as a going concern. Adam was created as an adult human being, complete with a navel (which was never employed as the attachment site for an umbilical cord, since Adam had no mother, and no birth.) If Adam was created in the shade of a tree, did that tree have rings? Yes, even though the tree was created ex nihilo and had no growth seasons. Furthermore, the grassy bank contained layers of sediment that were never laid down by seasonal floods, and may even have contained, in deeper layers, fossils of creatures that never lived.

You can see how the science of natural history, under such conditions, becomes a joke. So too with laboratory science. Did the contents of the test tube turn blue because protein in the sample underwent the same reaction as in the controls, and so the blue color is evidence of the presence of a certain amount of protein? Or did the All Powerful Creator and Sustainer of the Universe have some other reason to want me to think that there was protien in the sample?

There is a Warner Brothers cartoon with Wiley Coyote, once again out to do mayhem to the road runner, in which Wiley tries to do a controlled experiment. He has a rope tied to a tree overhanging the road, and he ties a stick of dynamite to the other end and swings it out into the road. He tries longer and shorter fuse lengths, until the dynamite explodes just as it reaches the center of the road. When the road runner comes along, the rope and dynamite, with the same length of fuse that worked perfectly in the test runs, swings out over the road, over to the far side, and then back into Wiley Coyote's face before it blows up. Sorry, Wiley. In a universe beholden to an intelligent creator, be that creator God or Friz Freleng, the postulate of objectivity is a bad bet, and science, with its controlled experiments, is a crock of nonsense.

When scientists abandon the postulate of objectivity, they are led into the most bizzare contradictions.

one love

Ste, is that 99.5% same thing a fair test though?

As Scientists have not mapped the whole chimp DNA code and instead compared a small amount of the code with the human genome code.

Could this mean that if they did map the entire genome then the results could vary from those results using the above method.

Adib

"As Scientists have not mapped the whole chimp DNA code and instead compared a small amount of the code with the human genome code."

Ste would know for sure, but my understanding iss that they can compare it without reaading it. Basicly, they take a single strand of chimp DNA and a single strand of human DNA and line them up. Then they try to get them to pair the way two strads in a human cell do to form a double helix.

Where the DNA is the same, they combine, but they stay seperate where it isn't. By watching what happens, they can see what percent of the DNA is the same.



Ste, is this right?

I think the method is right, but I don't think they have done it with the whole DNA sequence.

Adib

Salaam,

I know its of subject, but does any one know where I can get some good pictures of early earth being hit by a meteroite.

I mean computer graphic not real thing of course.

Got to make some poster for the exherbition and need a good picture.

Adib

If you'll settle for Jupiter instead of Earth there should be some on the NASA site of Shoemaker-Levy's plunge to destruction. That's usually a good place to look for all things astronomical.

Libraries get asked for all sorts of hard-to-find things and our local one was once asked if they had photographs of dinosaurs. It took some explaining before the customer realised there weren't any actual photographs.

F