If evolution is based on genes, then why don't we have a smear of animals?

Factual Questions
1

OK, first off this is not to bash evolution. I firmly believe in its principle; this is just a question about mechanics.

We all [hopefully] agree that evolution is based on random mutations in DNA, which translate to changes in genes. Genes are typically defined as segments of DNA that encode for individual proteins. The Human Genome Project estimates that we have about 20-25,000 different types of proteins within our bodies.

So the question is - if a single gene / protein were to differ from parent to offspring, how noticeable would it be? Sure, there are single gene mutations that can be fatal or grow an extra body segment; other gene changes can be very subtle.

But most of the time, wouldn’t it take a combination of many different genes all together to create an organism that is visibly different from its predecessor?

ie, don’t each of Darwin’s finches have more than just one gene difference between them? Put it another way, if it’s just a single gene change between organisms, why don’t we see a smear of gradually transformed animals instead of the distinct groups that we call a species?

Or is it that our definition of a species is inaccurate / incomplete, and actually encompasses a wide range of genetic diversity within that group?

2

We *define *two descendant groups as being different species when they have become sufficiently genetically isolated that they can no longer effectively interbreed. IOW, the very definition of species means that we will rarely see intergrades between contemporaneous species.

For there to *be *two species, they have to be effectively incapable of interbreeding, which means that there has been at most limited gene flow for tens of thousands, more often hundreds of thousands, of years. During that time period, random fluctuations alone will mean that the two groups start to differ in appearance.

In the real world, we do see a “smear” within species. All sorts of organisms have considerable variability within species. In theory, if those forms became isolated they could develop into new species, but the period of isolation needed would mean that other differences will become much more pronounced.
If a population of organisms really did have just one gene separating it from its parents and siblings, then it wouldn’t be considered a separate species unless, by some unbelievable fluke, that gene rendered it incapable of back crossing.

Just occasionally we will find species that are phenotypically almost indistinguishable, usually where the two ancestral populations were physically isolated and then rejoined. In those cases you can get species that have only very, very minor differences and, at least potentially, will intergrade phenotypically even if they are not interbreeding at all. But that;s the exception, not the rule.

3

I hope this answers your question non-technically, if not please refine it.
Clearly many of our genes differ from our parents, since we look different from them. So I suppose your question is about a gene that is different from any in the genome of either parent. In that case the impact depends strongly on where the mutation is. It could be so major that the fetus dies in utero, or so minor that we’d never know - in fact I remember reading that the vast majority of mutations have no impact.

Speciation is when a sub-population can no longer successful breed with the parent population, which I would guess usually takes a bit of drift. Of course sometimes the mutation will make it impossible for the holder to breed with other member of the population. In that case the mutation is disadvantageous by definition.
If you parents had no children, you probably won’t have any either. :slight_smile:

4

Wow, that’s a brilliant reply. Thank you Blake.

Gotta love this site. Where else can you find an evolutionary biologist at midnight?

Would you then say that evolution happens within a species level then? That the little changes still convey survival advantage, but that they are not so significant that they would preclude breeding / transference with their surrounding members?

5

Have you looked at the topic of ring species? They are a smear, but at the ends of the ring they’re incompatible. They only exist due to a great geographical range of the species involved, and even then the whole ring is one particular kind of bird.

6

Yes - the short answer is that we do have a smear (which is a very apt word for the concept) - but in many/most cases, gaps eventually appear in the smear and it breaks into separate pieces, which are separate species (or separate groups of subspecies, etc*) - this is exactly what speciation is.

*The concept of ‘species’ is a man-made one - our attempt to impose a compartmentalised, granular catalogue upon the various smeared sections of nature that do not in fact care whether they are just one thing, or a smeared collection of intercompatible organisms.

7

One thing to note is that no one individual is ever going to be considered a new species in its own right - only an entire population would be considered such. One individual unable to breed with its nearest relation is not a new species - just unlucky :slight_smile:

Our definition of species allows for considerable genetic variability internally - you just have to look at plants to see how much variation is allowed.

Lastly, I would say evolution happens “within [the species level” (if, by that, you mean at the level of sub-populations). Speciation is not (all of) evolution, it’s just an easy way to say it’s definitely happening - but look at ring species for a nice illustration of why being a sensu strictu species pedant aren’t that helpful). But evolution covers everything from individuals to entire biomes, in some ways.

8

Another thing to note is that what we’re talking about here is the exact answer to the creationist argument about micro/macro evolution typified here.

9

Mutations happen all the time. Odds are some of your genes differ is some slight way from parents. If the difference is too serious, you may have trouble reproducing. Generally, the errors in “copying” in making your sperm/eggs show up as miscarriages, they never make it to a viable offspring.

For species separation like Darwin’s finches, the differences appear gradually and reinforce. SPecies typically take over “niches” or opportunities in the ecology. The Galapagos was a brand new opportunity. Seeds and nuts floated there, started growing, but nothing was eating them. Ditto insects.

Birds that were eating one thing, some found easy food elsewhere. (“Hey, I can eat these nuts!”) The ones with the variations best suited for eating that food tended to survive and thrive - the ones with say the powerful nut-crunching beaks or long thin insect-catching beaks. It’s like the giraffe and the abundant supply of higher-up leaves. (or basket-ball players) The ones with the right physique survive to breed. If they breed with similar, those survive.

Note the gradual separation - if long beak and strong beak mate, the offspring likely will be only moderately good at both, and be out-competed by the specialized types with both parents the same. We’re not sure how instinctive behaviour also evolves, but the finches instinctively recognize someone like them and mate with the most compatible.

(It’s as if the chunky girls are attracted to football players and the tall lanky ones attracted to basketball players, to get totally rude and extend the metaphor. Or… if we could mate with horses or lemurs - we’d end up with something unable to compete in either world; unlike a horse, they’d get less nutrition from grass. Being les smart and likely unable to use hands, they’d be useless as humans… unlike a lemur, we couldn’t climb trees as easily, need a lot more food to survive, and so on. )

At a certain point the two types are still able to mate, but don’t usually. Note that Darwin picked on the beaks (and colouring) but there’s also the other adaptions; Don’t know, but for example the type of digestive adaption for different foods. The same applies - a hybrid will be worse than the purebred in either situation.

So there’s always a “smear” of genetic options in a species. Either geography and time, or different opportunities (like different food, different camouflage, different environments, etc.) will cause one part of that smear to extend and separate from the original. The conditions that favoured that extreme will continue to operate, likely rewarding the individuals that extend the “smear” of genetic material toward better adaption to the new circumstances.

Once the differences are distinct enough, that branch may not be able to reproduce with the original.

10

Here’s an interesting example of Blake’s point about speciation. We have frogs in our back yard, which make quite a racket during the summer. I wanted to know what kind they are, and as it turns out, there are two species that fit the description: Common Gray Tree Frog and Cope’s Gray Tree Frog.

They’re identical in appearance, and their habitat range overlaps a bit, near where we live. But they can’t mate very successfully, because one kind has twice the number of chromosomes as the other. Sometime back, there was one species that got physically separated (possibly by glaciers.) For some reason, one of the populations developed “polyploidy”, with 4 copies of each chromosome rather than the normal two.

The two kinds can mate, but most of the offspring die young and the few adults are often infertile.

There is one distinguishing characteristic: the calls they make are similar but different. Most likely, the call differentiated so that they don’t waste their time and resources mating with the wrong kind. Now that they’re genetically separate, it’s likely that they’ll take different paths on survival strategies, rather than competing for the same ecological niche. That doesn’t happen by foresight, but as an effect of natural selection. One population will benefit from some new gene that helps them at one thing; the other will benefit from a different new gene that helps them at something else.

There are many kinds of plants where, unlike most animals, it’s purely a matter of opinion where one species ends and another begins. Being unable to “mate” (e.g., cross-pollinate) for biological reasons is a clear cut case of different species. But there are plenty of examples where plants could cross-breed but don’t, simply because they’re geographically separated, and they often can look quite different. Whether to call these varieties or species is subject to a lot of debate.

One of the definitions of “species” is that it “fits an ecological niche”. By this theory, two species never occupy the same niche – or at least, they don’t for long. If two varieties occupy a different niche, then they’re considered different species, even though they might interbreed, and especially so if the resulting cross-breed doesn’t fit well in either niche and hasn’t found one of its own.

The point here is that in plants, there is a lot of “smear”, but that “smear” tends to eliminate itself over time (varieties that drift into different niches continue to get more differentiated with successive generations).

With bacteria, it’s nothing but smear. There’s really no good definition of “species” for them, other than the ecological niche one, which allows lots of gray area. Bacteria that are quite unrelated can still exchange DNA, thanks to plasmids, which are small bits of DNA that can migrate easily between kinds of bacteria. In addition, DNA from plasmids can get inserted into the “main” DNA strands for a bacteria.

Life is messy and never quite fits our definitions or into our neat little boxes.

11

Heck, it doesn’t even need to be an advantage. Imagine a mutation that only lets people conceive in the summer; they could still interbreed with regular humans (in the summer). And imagine regular folks then produce another mutant, who can only conceive in winter: the, uh, Winter People can interbreed with regular humans, who can interbreed with Summer People – who can’t interbreed with Winter People.

12

Members of a species by definition can interbreed. Evolution - that is, directional genetic change - can happen within a species if there is some environmental factor (let’s say, increasing dryness) causing selection for a particular genetic combination.

However, many species occur across a range of environmental conditions. Because its members can interbreed, this precludes close adaptation to specific conditions. If some members of the species inhabit wet areas, and others inhabit dry areas, gene exchange through interbreeding between these groups will limit the amount of local adaptation that can occur (although some may).

However, if populations of the same species become isolated from one another (say on different islands or mountain ranges), so that there is little or no interbreeding and hence gene flow between them, they become free to adapt more closely to their own local environment. One population may adapt to wet conditions, and the other to dry. Over time they may also acquire behavioral or genetic differences that prevent them from mating, and thus become separate species. The majority of directional evolutionary change probably happens under this kind of scenario.

13

Essentially the “smear” does happen, but over geologic time scales. At any given moment, all you’ll see are individual species, but if you look over time, you’ll see the smear. And in some cases, the species are so effectively adapted, that they don’t change much, if at all, hence the term “living fossils” to denote species which are effectively the same as the ones from millions of years before. Things like alligators, horseshoe crabs, etc… fall into this category.

Other species, like say… cattle, derive from things like the Aurochs, and Bison derive from a different ancestor, both of which had a common ancestor about 10 million years ago. If you look at them today, they’re separate species, but if you look forward from that common ancestor, it’s a lot more of a “smear”.

14

Yes, remember that the “smear” is there, it can be spread out - but that the environment selects for certain characteristics. In the example of Darwin’s finches consider the nutcrackers - note that the ones that do best will feed well and live. WHen there’s a drought, or other shortage of nuts - the survivors will be the ones who can crack the difficult nuts. The others die off - selection.

Every characteristic “costs”. We are smart, but one statistic I read says that about 30% of our food goes to feed our brain. A cheetah, on the other hand, needs to be fast. Beyond anticipating a gazelle’s next turn, they don’t need “smart”. If smart costs food, then when the gazelles are hard to find, smart and fast will die while fast but not smart will eke out a living until the gazelles are plentiful.

Similarly, muscles are useless beyond a certain point for humans. Some things we need to do, a group can get together to do - communication is better. A really strong human is useless if a bunch of nerds with bows can pick him off from a distance.

So the key is that in the environment they are in, species may have a variation but nature selects for certain characteristics - to the point where some incredibly specialized types outcompete any less capable animals - giraffes not only have a food supply that half-a-giraffes don’t, they also can eat the half-a-giraffe’s lunch (which is why there are no half-height giraffe species in the same area). It’s like an arms race, one a species begins to specialize, the more specialized will win, thus becoming more different from their ancestors as the ones with beneficial mutations win the contest.

The caveat is - unless the niche they pick is sufficiently variable that it is safer not to specialize to heavily. A honeybee is useless if there are no flowers. A swamp-dweller is useless if the swamp dries up. A tree dweller (think spotted owl) is going to lose if there are no big trees to live in… and so on. The legendary small elephants of Malta(?) were the winners because the smaller the animal, the less food it needs. On a tiny island with limited resources, the winners are those that make do with less. On a giant continent, size will win.

Remember, as a part of the population starts down the road to specialization, the half-breeds again are not suffciently adapted compared to the purebreeds. One theory of behaviour suggests this is what we see as “beauty” - symettry indicates health. Ugly indicates improper development, poor genes, poor nutrition, and many other indicators that this person is not the optimal healthy person to contribute genes to your offspring. Presumably, other species recognize the same features. Indeed, the whole point of bright plumage in some male birds is because all they have to contribute for the female is an indicator as to how healthy they are, and bright large plumage is the best indicator.

15

This is quite wrong. In the standard case, all the genes someone has are genes that one or other of their parents had (about 50% from each, though males will actually get a few more from their mother than from their father). As far as genetic differences go, we look different from our parents (and differ in other ways) because we have a different mix of genes than does either parent, not because we have genes that neither of them has. (However, it is also important to realize that by no means all differences in looks (or other characteristics) are due to genes. Many other factors are involved, both pre and post natally.)

Mutations (which, when relevant at all, have normally occurred in the parental germ cells) only play a rather small role in making children different from parents, and probably, in most cases, none of a person’s genes are mutated from the ones their parents have.

16

Describe a “dog.” Now think of how many variations you can name. From toy poodle to Irish WolHound, from WolfHound to Mastiff, from Mastiff to English Bulldog.

We do have a smear, we just group them according to basic attributes, like “spinal cord.”

17

. . ., all the genes someone has are genes that one or other of their parents had . . .

Change “had” to “carried.” Big difference. Not all the genes I express were expressed in my Mom. Some are not expressed in either of them, but since they both carried them, I now have those traits. The range of possibilities is much wider than what is observable in the parents. for example, I can taste things that neither of my parents can taste. they both had a recessive gene which expressed to create those receptors, and so chemicals that they happily and ignorantly swallow make certain processed foods repugnant to me.

18

Correct of course. I was trying to get at variability and the fact that it is hard to tell variability from mutations from normal variability in most cases.

However it appears that the mutation rate is a lot higher than this - 200 - 300 per generation based on this article from Nature.. Very little of which involves things that will cause differences, of course.

19

I may have been underestimating the generation to generation rate, but that seems awfully high. Note however, that those numbers are derived from work specifically on the Y chromosome, which is a bit of a special case, and contains very few genes. The article at your link does suggest that these findings may not apply to mutation rates in other chromosomes. Anyway, even if the Y is mutating at that sort of rate (and I remain skeptical) presumably most of the mutations are in non-coding, junk DNA.

20

There are a lot of potentially correct answers to this question, in addition to the excellent ones already given.

There’s the fact that selection can often act to eliminate intermediate forms in favor of divergent, more specialized forms.

There’s the Dobzhansky-Muller model of speciation, which explains how two populations of one species can gradually develop genetic incompatibility, leading to separation into different species.

It’s a wide-open question. Perhaps if the OP could narrow it down a bit, if it hasn’t already been answered to his satisfaction.