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Evolution and Interbreeding Species
Guffrey
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To start off, I'm a geology student in college, and also a dedicated Christian. Unlike some, I believe strongly in both an old Earth and evolution. However, today I was thinking, and saw a bit of a problem. Now I'm sure there is an easy explanation that I'm missing, and I'm having trouble putting my thoughts into google. A "species" is essentially a group of organisms that are capable of interbreeding, correct? I guess I'm seeing that as a restriction of evolution. Lets take the ancestor between humans and chimps. At what point along the tiny changes over millions of years did one finally become "chimp" or "human"? And then who did they mate with? Is there a certain "gray area" where they are well on the way to being a different species, but can still breed with the previous? I apologize if I'm being confusing, or missing the obvious. Most of my time has been spent with rocks/minerals, I'm just now taking my first paleo course. Thanks.
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It would be incredibly wrong to think of Neanderthals as "more ape-like humans", as they evolved concurrently with Homo Sapiens as either a subspecies or a separate species, depending on which anthropologists you talk to. There is evidence of inter-species genetic exchange between Neanderthals and Homo Sapiens, too.
The long and short of it is that it's not an overnight thing. I remember reading somewhere that humans now are taller on average than they were hundreds of years ago. This change is completely imperceptible to us because we only have two or three generations to look at and compare, but if you dig up bones from a thousand years ago you can clearly see the difference. An average growth of a few inches can take thousands of years, and evolution as a process from an ancestor species into child species can be thought of as hundreds of simultaneous changes like that taking place over tens or hundreds of thousands of years. We didn't evolve from whatever over a single generation; there was no "click," just a gradient of imperceptibly small changes over time.
Mitochondrial DNA studies are better, but even that has limitations, as seen with various examples of inter-species breeding/hybridization (where the mitochondria are shared between two compatible species).
Height is also a crazy example. It's funny we were just talking about height trends in D&D earlier. Height doesn't follow the same simple evolutionary mechanisms that you might learn in high school or an undergraduate course. Just to quote Winky from earlier this afternoon, "Height genetics is also retardedly complicated and relies on (iirc) nearly one hundred distinct genes and forward reverse up down double reach-around inheritance patterns."
@Guffrey: There isn't a single solid consensus on how human speciation occurred. However, a lot more is known about how other speciation events have taken place.
Basically, if one population within a species gets genetically isolated from another population, then those two populations might diverge over time and become different species.
One really solid example involves the shrimp that live on each side of the Panama Canal. Today, the canal is an artificial construction, but about 3 million years ago, there was natural water flowing through it. Around that time, it closed, causing the ocean on the eastern side to be isolated from the western side. So for 3 million years, those two species of shrimp have been evolving separately. They still have the same basic body shape and anatomy (when a biologist refers to body shape, they use the term 'morphology'), however their mating behavior has diverged and when the two species are reintroduced to one another in a lab, they don't mate.
When there's a geographical isolation between two species, that's called "allopatric speciation." There are a few other types of speciation, too. Sympatric speciation is where there's a genetic isolation within the same geographical space.
Apple flies are a good example. The apple tree as we know it today is not native to the United States. They were brought over by Europeans during colonization. There is a very distant relative of the apple tree that is native to the US - the Hawthorne tree.
Well, the flies that lay their eggs on the Hawthorne tree can also lay their eggs on the apple tree. However, regardless of whichever eggs they hatch from, the young flies prefer the eggs they ate as larvae. (They'll eat the other one, but only if their preferred fruit is not available.) So when the apple tree was introduced to the United States, some of the flies laid their eggs on apple trees. Those larvae developed a slight preference for apples, so their offspring were also laid on apples, and so forth for uncountable generations.
Today, Hawthorne tree flies and apple tree flies do not cross-mate. They stick to their own kind, even when artificially introduced to each other. Due to a slight behavioral preference, the hawthorne flies speciated into a new species of apple fly.
Exactly how humans diverged from ancestor species is, as far as I know, still a major topic for debate. But I hope those examples give you an idea of how it might have occurred with humans and how it does occur with other creatures.
the "no true scotch man" fallacy.
This is in essence how speciation occurs. There's a split for some reason, geography is the usual culprit, and two or more populations that are _capable_ of breeding don't anymore. To my mind they really stop being a species when they stop breeding; the _inability_ to breed is just an inevitable consequence of not breeding. If they stay separated long enough they will change too much to breed.
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The idea that species 'branch off' is called cladogenesis (this is as opposed to the idea that species form 'in a line', which is called anagenesis).
Speciation is thought to be caused when parts of a group (meaning a species) are cut off from reproducing with the other members of the group in some fashion. These usually take the form of what's called pre-zygotic (before fertilization) barriers and post zygotic (after fertilization) barriers.
these barriers can include: (pre-zygotic)
ecological isolation - when the population occupies a different habitat within a region, preventing interaction between groups (say, a natural disaster isolates a group from the rest of the population)
temporal isolation - when matings take place at different times (either different parts of the season or different parts of the day)
ethological isolation - individuals from different populations meet but do not mate for behavioral reasons ( For example, a female bird meets a male from another population, but that female prefers the brightly colored feathers of the males nearby, thus she won't choose the dull feathers of the outsider)
mechanical isolation - when matings between individuals of different populations occur but no transfer of gametes takes place (due to the mechanics of, say, genital forms not allowing for transfer)
gamete mortality/incompatability - when matings between individuals of different populations occur, and transfer of gametes takes place, but eggs are not fertilized (due to different pH requirements or something of that sort).
post-zygotic:
inviability - hybrids are born that have a reduced capacity for survival
sterility - hybrids are born that have reduced fertility
hybrid breakdown - hybrids are born, but backcross with initial population or later generation hybrids have reduced viability or fertility.
the mechanism for introducing these barriers is genetic mutation. The phenomenon of genetic drift results in allele frequencies among isolated populations to become more pronounced, leading to speciation. genetic drift occurs because in an isolated population it is very unlikely that the allele frequency in the (usually smaller) group is adequately representative of the entire species.
i hope this cleared it up for you, if you have any more questions just ask
And, for what it is worth, the linear and iconic "descent of man" image is most assuredly incorrect, as many of the hominds it depicts co-existed with one another, and split off from one another reproductively for a variety of (mostly) unknown reasons.
Speciation, the event where two populations of organisms encounter barriers to sharing genetic information, is exceptionally subtle.
Heck, biologists still argue about what really constitutes a "species". If you want a really technical read, you can pick up Jerry Coyne and Allen Orr's book Speciation, which explores this concept in depth. It is worth noting that Coyne and Orr are extremely biased towards the "biological species concept", i.e. -"The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance." This quote is taken from here, and I really recommend this site as an intro to the nuances of evolutionary theory.
Of particular note are the alternate species concepts it lists. It isn't a complete list, but it is enlightening. Take the "recognition species concept", for example. This website defines it as: a species is a set of organisms that can recognize each other as potential mates.
Now, in the context of modern humans, which one applies? The biological or the recognition concept?
And which of these concepts applies to our ancestors?
The answer is, of course, "it depends on the question and the researcher/s".
For example, under a variety of species concepts (biological, recognition), Homo neanderthalis could be considered conspecific (that is, they belong to the same species as) Homo erectus. Under the phylogenetic species concept, the argument doesn't hold as much water, as you can separate populations of neanderthalis and erectus by geography and theoretically by ancestry. On a molecular level, these two also have a few differences, even though there is strong evidence we interbred.
Here is a fun example- Many wasps can reproduce asexually in a process known as parthenogenesis. Some researchers established two colonies of wasps: one had only females, and one had males and females. After 20 years the wasps that only reproduced asexually were introduced to males of the other colony, and less than 20% of the "asexual" female wasps mated with the males.
Are these two groups of wasps now different species or not? And how much longer do we need to isolate the populations before they can no longer interbreed?
I think this got off on a tangent, but the TL;DR is this, @Guffery:
You said
The answer to this is "yes, for the most part," which should also answer your initial question.
If you have any more questions about this, feel free to send me a message. I am an entomology PhD student who works in evolution and development, and I can talk about this all day.
But yeah, the thing I run into is simply the time involved. I think humans have a hard time reconciling the length of time for genetic variations to result in new species, because we have a difficult time thinking of time on the terms of thousands of years, let alone millions.
Throughout the discussion, he kept saying 'i'm being scientific- i'm asking questions!', and then he dropped the bombshell:
'If evolution exists, how did the moon evolve??? Tell me that!'
lol
Evolution on a macro scale such as that happens with wear and tear, an asteroid probably impacted into the earth, or, it was a collection of space dust that orbited the earth. That's how it "evolved."
Biological evolution is something different from evolution of landscape or the universe as a whole.
but they're listening to every word I say
And evolution doesn't pick "the best" traits, but merely slowly picks towards what works better.
So while three eyes may have been better, we evolved two just because it was "good enough." It's also why you still see a huge range in speciation too.
Consider wolves and domestic dogs. There are 39 subspecies of wolves, of which the domestic dogs are one. Despite 15,000 years of directed breeding by humans, domestic dogs are still capable of interbreeding with not only Gray Wolves (from which they're descended) but all (I believe) of the other Canis lupus subspecies as well. Perhaps in another hundred thousand years (assuming neither goes extinct) wolves and dogs won't be able to interbreed anymore. It just takes a really, really long time for enough mutations to build up and be carried between generations that fertilization can't happen anymore.
Of course, wolves and dogs could probably already be there if the people doing the domestication had purposefully selected for dogs that had lower rates of successful birth with wild wolves than they had with other dogs in the pool of directed-breeding ones rather than focusing on making poodles and schnauzers and so forth.
But seriously, all good stuff. Trying to explain evolution is hard because evolution is many things.
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We do see evolution happening, mostly in other species. In some cases very quickly. (We can even see it happen in humans, but that's a lot more subtle.)
Antibiotic resistance in bacteria is probably the best example I could use.
You have a population of bacteria - say, staphylococcus aureus - that causes an infection. You give a person with that infection some antibiotics - say, penicillin. After taking penicillin for a while, most of the bacteria will die, but a few might survive. Those few are the few that have some random gene that makes them slightly more resistant to penicillin.
If the patient's body can't kill off those resistant-penicillin stragglers naturally, then they'll multiply. That human might pass off some of those stragglers to another person. Then that person gets an infection, takes penicillin, and the non-resistant bacteria die while the resistant ones survive.
Repeat across millions of generations. (Millions of generations of bacteria, not millions of generations of humans.) Eventually, the bacteria develop better and better resistances to penicillin, until penicillin doesn't work anymore.
This isn't just some imaginary thing that somebody thought up. This is happening and has been happening every single day since penicillin was discovered, and is a huge challenge in hospitals (and anywhere else lots of sick people might be housed).
An example of evolution in humans? Lactose intolerance. Or, more specifically, lactose tolerance (technically called "lactase persistence"). Most mammals, including apes, can only comfortably digest milk when they are infants. After infancy, they stop producing the enzymes that let them digest milk. Some individuals (of different species) keep producing that enzyme, in small amounts, for whatever reason. Well, humans are the first beings to cultivate livestock for milk-drinking any time we want. Consequently, humans who kept producing milk a little longer into adulthood got a bit of a nutritional advantage. For some populations (mostly in Europe, but some communities in other continents too) who relied on cow meat as a major source of nutrition, the advantage of being able to digest milk as well as meat was huge. Consequently, lactose intolerance has become relatively rare in people of European stock, while it is still common among Native Americans and Asians.
Note that neither of these are examples of speciation. Penicillin-resistent staph is the same species as non-resistant staph. Lactose intolerant humans are the same species as lactase-persistent humans. But they are good examples of evolution (within a species) that we have observed in the real world.
the "no true scotch man" fallacy.
For instance, some viral dna seems to have been incorporated into the human genome and I recall reading an article some years back about reptile dna being found in cows or something crazy like that. There it is.
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Remember how I said that height is a crazy example?
In high school, a lot of us learned the basics of genetic inheritance and recessive and dominant genes. If you have two genes for blue eyes, you have blue eyes. If you have a gene for blue eyes and a gene for brown eyes, you have brown eyes. This is because the blue eye gene isn't expressed if there's a brown eye gene present.
Height is a little like that, only in a much more complicated way. Nobody knows exactly how many genes affect height. Winky said "over a hundred." I've seen an article that estimated 200. Some of those genes turn the expression of other genes on or off.
But here's the kicker: what happens in the environment can turn genes on or off. This includes nutrition. For example, in livestock, adding folic acid supplements to feed can turn on certain genes related to muscle mass development and fur growth. In humans, your diet in early childhood can activate genes related to diabetes.
When something in the environment (like nutrition, or climate) activates or deactivates the expression of a gene, that's called epigenetics. The intersection of nutrition and epigenetics is a very young (and very fertile) field of study right now. This shit gets ridiculously complicated - the expression of one gene might cause a another gene to fail to express, but only in the presence of a third gene that sits adjacent to it, and only if that third gene came from the mother rather than the father. Consider that for the 3 billion DNA base pairs in the human genetic code. As complicated as these interactions can be, scientists have only been studying them for a couple of decades, and it takes years for an entire team of geneticists to unravel even one single epigenetic interaction. (Studying the human genome in detail wasn't really possible until computers got small, cheap, and powerful enough to crunch the vast amounts of data involved.) On top of that, the human genetic code is full of 'junk data' - stuff that isn't expressed at all, so scientists have to figure out which genes are never expressed and which genes are only expressed in limited circumstances. Most epigeneticists have to pick one tiny little piece to study, and they can spend most of their careers studying it. Consequently, we've barely even scratched the surface of what we might learn.
Out of those 100 or 200 genes that affect height it's pretty certain that some of them can be turned on or off by early-life nutrition (even if scientists haven't pinpointed which ones and what their triggers are).
So it is a little inaccurate to use a phrase like "dietary as opposed to genetic" in this context. Epigenetics bridges the nature vs. nurture division - for any given trait, whether we're talking about height or diabetes or just the color of a mouse's fur, both nature and nurture (including but not limited to nutrition) interact.
the "no true scotch man" fallacy.
On the other hand, donkeys and horses produce viable but economically useless offspring.
It's the biggest problem with evolution as a theory. Because it wasn't really a theory in the way we currently define things. It was a whole set of hypotheses based on observations. I would bet more recent academic works have better definitions, but Darwin's kind of the guy, so he's who we usually go back to.
--LeVar Burton
Doc: That's right, twenty five years into the future. I've always dreamed on seeing the future, looking beyond my years, seeing the progress of mankind. I'll also be able to see who wins the next twenty-five world series.
I don't like the way this post is set up. Both the allusions to the fact that evolutionary theory has "big problems" and the throwback to Darwin as the de facto authority on the subject.
I would start by researching the modern evolutionary synthesis for "recent* academic works" on the definition of evolutionary theory, as well as paying attention to Massimo Pigliucci's early work trying to redefine the synthesis a bit to account for epigenetic influences and phenotypic and developmental plasticity.
Sorry if I am coming off as a snooty academic in my ivory tower, but you don't really know what you are talking about here, and if you do you didn't explain yourself very well.
*(although this really came about in 1945, so calling it "recent" is a stretch)
I'm not sure what you are getting at here, and your post ignores all of the complexity surrounding the debate on how to properly apply species concepts. It seems you are applying a sort of molecular species concept, which may or may not hold for oceanic bacteria. Certainly the idea of a "species" isn't "useless", but you perhaps have to choose how to define "species" a bit more carefully in this instance
Per the modern synthesis, evolution is defined as "a change in gene frequencies in a population over time," mostly thanks to the integration and dependence on Hardy-Weinberg Equilibrium as a base model in population genetics**. In that sense, it isn't that amazing to claim that "evolution can happen overnight", especially in regards to bacteria.
What this has to do with species concepts and speciation, I don't really understand, because as pointed out a few times evolution and speciation events aren't the same thing, and you can theoretically get a new species without a change in gene frequency.
**Now, I personally think the dependence on population genetic models to account for evolutionary change is a bit silly, and others have even called this approach "the tyranny of population genetics". This makes me slightly of a radical when it comes to evolutionary biology, but there it is.
A mild example is the common livebearing toothcarp (genus Poecilia and Xiphophorus). The fish sold are less species than they are hybrid complexes. Xiphophorus is usually sold as three species (variatus, maculatus, and helleri) but what you see in pet stores are actually extensive hybrids of these three and over a dozen other species, crossed and recrossed to the point that you can actually buy platys, breed them, and occasionally find males that have tail spikes like a swordtail. Poecilia is a bit less widely crossed (mollies and guppies have been hybridized quite a bit, but most of it's been with very closely related species. Guppies and mollies can breed, but it's difficult to manage and the result is often ugly prone to spinal deformities). However, there's an example in Poecilia of how a wild hybrid can become its own species. The Amazom molly is believed to have originally been a hybrid. It's a female-only species that technically reproduces asexually, but needs to mate with a male from another livebearing species first. Which is easy, because male livebearers are absolutely ridiculous when it comes to mating. They don't care if its the same species... they usually don't even care if it's a fish, if it has a hole and holds still long enough they'll give it a try.
A more extreme example are the Central/South American cichlids. Many of them have been known to hybridize outside their own genus, often without needing any artificial encouragement. One of the results is the Flowerhorn, a series of increasingly complex hybrids. Some of the more recent strains supposedly involve as many as 20 species. Many strains are not just fully fertile, but well enough equipped for life in the wild. In parts of Malaysia where new world cichilds had already devastated native fish, low grade flower horns dumped for "disposal" still managed to be devastating invaders. And then there's the Blood Parrot, which is probably the closest man has actually come to breeding an entirely new species. Not as viable as flowerhorns (fertile males are rare and they're not able to survive in the wild), but by the time they appeared for sale in the west, they'd been hybridized so extensively and for so long that going on eighty years later we're still not actually sure exactly how they were bred.
Stuff like this is why biologists argue about exactly where the species distinction should be drawn. No matter how wide you're willing to draw it, nature finds two things that even you won't call the same species and has them give you viable offspring. The more you move away from classification and into selection and evolution, the less important the distinction becomes. From the evolutionary perspective, species are fluid states, not fixed things.