Can someone clarify the evolutionary theory for me?

Factual Questions
1

Is evolution:

Let’s say there is a fish which is common prey to sharks. Because of a random genetic mutation, one particular type of this fish is born with foul tasting skin. When a shark approaches this fish, it is turned away because it does not like the taste. This fish now has a greater chance of survival, so when it reaches breeding age, chances are it will pass on this ‘bad tasting’ gene to it’s young, which in turn will have a greater chance of survial then the more common type of this fish. Eventually this gene will become standard amongst all these fish, due to natural selection, and the end result is - evolution.

Is this correct?

If it is correct, my second question is this:

If only one fish is genetically mutated in to having this gene which results in bad tasting flesh, when it breeds, what is to stop the other fishes ‘good tasting’ gene to be passed on to the young? And after successive breeding through generations, wouldn’t this ‘bad tasting’ genetic mutation be ironed out because of all the fish which DON’T have it?

Thankyou to whoever takes the time out to answer one or more of these questions.

2

Hello? Isn’t there that big bad white shark roaming the waters somewhere? :wink:

3

“Hello? Isn’t there that big bad white shark roaming the waters somewhere?”

Yes, of course. But the shark isn’t going to get every nice tasting fish. This genetically mutated fish is going to have to breed with the more common type of nice tasting fish.

What is to stop the genes of the nice tasting fish being passed along?

What is to stop the genes of the bad tasting fish being ironed out through successive generations?

4

OK, I am not a biologist, but I’ll take a swing at it:

Your first question: That’s how I understand it as well. The foul-tasting fish has better chances of living to breeding age.

Nothing. But if a fish with the “foul taste gene” breeds with a fish with the “yummy taste gene”, the foul taste gene might “win” in all, some or none of the offspring.

If it’s in all, no problem. If it’s in none, bad luck. If it’s in some, those with the foul taste gene will breed much more than those without it. And thus the proportion of fish with the foul taste gene goes up.

Nope, because the fish without the mutation are falling out of the gene pool at a higher rate than those with it.

Or so I think.

S. Norman

5

Spiny Norman is pretty clever for a hedgehog. :wink:

It sounds from your question that there is one nasty-tasting fish. Of course, someone has to be first. Okay, now think of a lottery. The chances that someone will win several million dollars are quite good. But the chances of a particular individual winning the jackpot are millions-to-one. So. Given that there may be millions of fish, only one of which is foul-tasting, the chances of that particular fish being eaten are fairly small. Of course it does happen, in which case its genes are not passed along if it hadn’t yet bred.

If I am correct, the vast majority of offspring (particularly in the ocean) do not survive to adulthood. So the bad-tasting fish would have to survive against vast odds in order to pass on its genes. Doubtless, many mutations have been lost because the genes were not transferred. But over thousands or millions of years, it’s possible that they’d pop up again. Eventually one or more may survive to breeding age and if the mutation is beneficial then it will provide a survival “edge”.

Now, many of these mutants must be killed. One shark eating one fish might deter that shark from eating another fish of the species. But there are a lot of sharks and other creatures that feed on that type of fish. There has to be enough of the mutated fish that they can be tasted by enough fish etc. that whole groups of fish avoid the mutated fish. Maybe that’s why evolution takes so long.

Bacteria and viruses mutate very quickly. Their populations are very great and their reproductive cycles are very short, so their evolutions come faster.

On the other hand, I’m not an expert. Maybe it’s all a joke being played on us by a supreme being. :wink:

6

Welcome to the board, Client! :slight_smile:

Both the foul-tasting and the great-tasting fish will presumably be reproducing. The key is that the foul-tasting fish has a higher probability of successfully reproducing relative to the great-tasting fish. Fitness is always a relative measurement, compared to the other fish with the alternative phenotype (in this case, the exhibition of the tastiness trait).

For example, let’s say you have an average of 100 offspring per adult fish. 50% of the offspring of the great-tasting fish succumb to predation, while 30% of the offspring of the foul-tasting fish succumb to predation. Assuming that other causes of mortality (or just anything that would prevent reproduction, so this would include sterility and success in finding mates too) are the same for both types of fish, the foul-tasting trait will gradually become more and more common in the population.

It would also help this fish greatly if there is some outward demonstration of its taste so that sharks will veer away from the foul-tasting ones … perhaps a sign on their scales: ‘about to be served by Red Lobster’ or something. :wink:

(otherwise how will they know which are great-tasting and which are foul-tasting? The protection incurred by taste could either be diluted by the presence of indistinguishable great-tasting fish or it could extend to the great-tasting fish too, conferring an advantage on them as well. This is a problem faced by poisonous critters and their mimics)

7

Er… oh yes, Spiny Norman and Johnny L.A. are correct too.

8
  • The Origin of Species * Charles Darwin, Chapter IV:

[q] …*f a single individual were born, which varied in some manner, giving it twice as good a chance of life as that of the other individuals, yet the chances would be strongly against its surival. Suposing it to survive and to breed, and that half its young inherited the favourable variation; still, as the Reviewer goes on to show, the young would have only a slightly better chance of surviving and breeding; and this chance would go on decreasing in the succeeding generations. The justice of these remarks cannot, I think, be disputed. If, for instance, a bird of some kind could procure its food more easily by having its beak curved, and if one were born wsith its beak strongly curved, and which consequently flourished, nevertheless there would be a very poor chance of this one individual perpetuating its kind to the exclusion of the common form; but there can hardly be a dou;bt, judging by what we see taking place under domestication, that this result would follow from the preservation during many generations of a large number of individuals with more or less strongly curved beaks, and from the destruction of a still largern number with the straightest beaks. [/q]

It follows from that that if just one fish had the good mutation, that mutation would not survive even if the fish were not eaten. However, Darwin says that the mutation would, if favorable, begin to occur over many generations in many different fish.

9

Another important point is that it’s just as likely, through random mutation, for a fish to be born that’s espescially tasty, and thus sought out by sharks. Evolution does not cause “good” mutations to occur more often than “bad” ones; in fact, “bad” mutations are much more commen, since there’s so many things that can go wrong in an organism. Sometimes it’s not even clear which mutation is good: If, for example, people found out about that yummy fish, we might start breeding it in protected aquaria for our own dinner plates. Every fish ends up getting eaten, but only after reproducing, so that’s actually a success story for the fish.

10

I can add little here, but I’ll try. First, the fundamental concept that the OP proposed is correct: occasional “birth defects” work in favor of the newbie, and it’s possible that those genes will stay in the gene pool for a while. If another one like it occurs, and gets matched up with that one, it could be come much more common in the gene pool. But, when it comes to the question of foul tasting fish, I think that the possibility then arises that the sharks, finding fewer (tasty) fish to eat, could find their populations diminishing. This might have the effect of making in more likely that the tasty, as well as the non-tasty fish, have a little better chance at survival. It’s one of these deals that shows us how everything - EVERYTHING - is connected.

11

Not to take anything away from the mostly useful and accurate answers that have preceded mine, but it’s also useful to remember that evolutionary theories from Darwin on have emphasized the role of selection pressure in causing modification of species. In an environment where most of our hypothetical fish would survive to reproduce whether they are fair- or foul-tasting, the foul-tasting gene enjoys no particular advantage (or disadvantage) relative to the others, and might possibly become extinct quickly, or might (in a perhaps more likely outcome) remain present among a small minority of the individuals in the species without gaining or losing ground.

Change the environment so that many or most of individuals in this fish species do not survive to reproduce because of predation by the sharks, however, and there’s a strong probability (though by no means a certainty) that the foul-tasting individuals will make up an increasing percentage of those that do have offspring. Keep up the selection pressure long enough, and it’s possible, even likely, that foul-tasting individuals will supplant fair-tasting ones entirely.

The other key to modification of species not mentioned so far is reproductive isolation. Many events that increase selection pressure in favor of a certain trait in a population are local in their effects. So long as there remain populations of the species that are not affected by such pressures with which the local group can interbreed, any particular mutant trait is unlikely to become sufficiently dominant in the species as a whole to result in either a new species or a permanent change in the existing species. Prevent the gene pool from being replenished with fair-tasting individuals from the outside, however, either by geographical isolation or by some change in mating behavior that discourages or prevents fair- and foul-tasting individuals from interbreeding, and you have a scenario in which the foul-tasting individuals will soon make up most if not all of the population, and in which the foul-tasting subspecies may eventually become a new species, incapable of interbreeding with its long-lost cousins.

For further reading, I highly recommend Jonathan Weiner’s The Beak of the Finch, which details the work of Peter and Rosemary Grant on the finches of the Galapagos Islands, providing vivid and (to my mind) nearly irrefutable evidence of the mechanisms of natural and sexual selection at work in the modification of species, and counters the notions that evolution is too slow to have produced the myriad varieties of life on our planet and that it’s not subject to direct observation by scientists.

12

barbitu8 wrote:

Charles Darwin’s theory of evolution by natural selection was missing one vital piece of the puzzle, the existence of genes that carry inherited characters without dilution.

Without genes, innovations would in fact fail to spread into the general breeding population, because they would be ‘washed out’ over time.

But with genes, even a recessive trait can spread from one source. If both parents carry the recessive gene, due to both individuals being descended from the same original innovation, then ~1/4 of their offspring will express the innovation, and the new trait can start to spread more rapidly (if it is ‘desirable’).

Bill

13

I’d like to add one more piece to the good explanations here, by explaining genetic drift.

Suppose you have a population of six individuals, three of which have gene A, and three of which have gene B. There isn’t any advantage associated with either gene- the difference between the two genes is purely neutral as far as selection goes.

Now, after those six individuals mate to have offspring, it might just be the case, purely by chance, that you end up with four A’s and two B’s in the next generation. (You could just as well have two A’s and four B’s, depending on the roll of the dice.)

When that generation reproduces, there are more A’s having kids, so there’s more of a chance that the A’s will have a few extra children than there were in the last generation. I realize that this is a little oversimplified (in reality the genes A and B would be distributed throughout the population, and some individuals would have both genes, etc.,) but if you use a very famous equation from evolutionary biology called the Hardy-Weinberg equilibrium, you find that the chances of going from 4A, 2B to 5A, 1B are a little higher than the chances of going back to 3A, 3B because there are more A parents, so any statistical fluctuations in the number of offspring they have will tend to be amplified. Think of it this way: if there’s only room for 1 million people in america, and 90% of them are blonde and 10% are redheads, then it’s possible for the blondes to squeeze out the redheads just by having an average of 2.1 children per family rather than just 2. If the redheads have 2.1 children per family, they will only increase their percentage in the population ever so slightly.

In the next generation, there’s an even better chance that you will go from 5A 1B to 6A 0B. That means that A has become “fixed”; everyone has it now, so all the offspring will have A for all generations to come, until a new mutations appears. Meanwhile, B is lost.

Now, bear in mind that this would appear to make it even more likely that a beneficial gene could be lost, but that’s not quite so. The equations take into account the amount of selective advantage that a gene confers, so if a gene is beneficial, it has a better chance of snowballing from a single mutation to being fixed in the population. Also, as has been mentioned before, small populations tend to evolve faster. If you have a small population of organisms that are cut off from the rest, then that single B is a larger percentage of the total population, because there’s less A to compete with. That, too, makes it more likely to become fixed. This is presumably the reason behind punctuated equilibrium, and is generally referred to as allopatric speciation: a large population has a lot of genetic “inertia” that soaks up new mutations, so it remains unchanges for a long time. If a small population is isolated for some reason (for example, by the severing of a land bridge, or the introduction of transposons (genetic parasites, essentially) into the population which prevent some animals from interbreeding with others) then it will evolve very fast, as good mutations can be better fixed.

-Ben

14

We are assuming that the individuals who taste bad are eaten less often by their predators. Let’s think this through…
[WB]

Daffy: “Oh no you don’t, buster! I’m not going into that jungle firth-st! There’s tigerths out there! Man-eating tigerths! Duck-eating tigerths!”

Bugs: “Mmmm…I see yer point, Doc. Lissen, …::chomp chomp::, …I got just the thing right here, ::whips out spray can:: ‘Tiger Repellent Spray’. Guaranteed to make even the most succulent duck taste most fowl. Of course, now, if you’d rather wait here while I go to the treasure cave…”

Daffy: “GIMME THAT!!” ::spray spray spray:: “‘Wait here’, he says…who does he think I am?”

::stalks off into jungle wearing Indiana Jones hat::

::stops suddenly, one foot away from grinning salivating tiger::

Daffy: <gulp> “…TASTE??”

Tiger: <CHOMP> ::eyes cross:: “Ptuuu-ii! Bleeeah!”

::minus bite-shaped excision in midsection, Daffy stalks back to Bugs::

Daffy: “You’re disth-picable!”

15

I for one elected not to delve too deeply into the specifics of this. The OP did say that the shark “approaches” the fish and is “turned away because it does not like the taste”. While I agree that this is unlikely, it’s not critical to the point, which is more abstractly that some fish have quality X, and quality X causes fish who have it to be able to reproduce before they’re eaten by sharks more frequently than fish who do not have X. Substitute some form of camouflage that makes the X-fish more difficult for the sharks to find, if you prefer.

Did enjoy the Bugs and Daffy short, however.