The one glaring gap in evolutionary theory that persists to baffle us is exactly how we went from asexual (meiosis) to sexual (mitosis) reproduction. There is many theories floating about in the scientific community but we have still not "nailed it down" definitively. I am studying evolution as a layman and I know the religious community loves to point out this "gap" in our current understanding. Of course a lack of understanding is not grounds to jump to conclusions in the form of automatically assigning supernatural causation- the God Did It complex. They have it so easy on the other side. If anyone else has delved into this subject I would be open to suggestions about various information sources to check out. I did find one location at a Xtian apologetics website where they discussed current secular theory on sexual reproductive origins. It was an interesting read because they stayed with the facts for the most part and left their own commentary to a minimum. It is astonishing that the individuals who contributed to the article have doctoral degrees but still cling to the "young earth creationist" theory. I verified this by emailing them as I was unsure at the time if they were really proponents of young earth theory. And I know it is irritating to even embellish notions of a young earth argument with the title of "theory."
Some of these sexual origin theories are no longer as popular as they once were. I believe the "red queen" theory is still looked at favorably. At any rate I find it all very engrossing and intriguing.
What will be the breakthrough that gives us the additional insight into just what happened a billion or so years ago that set the stage for sexual reproduction to gain a foothold and become the preferred way to ensure our DNA was being carried forward faithfully? Inquiring minds want to know.....
Treating meiosis and genetic recombination as more or less synonymous, for example, leads to confusion.
If you look at the way RNA and then DNA function, and look at sex-like exchanges, such as horizontal transfers between bacteria...which then confer advantages from one to the other, you quickly see that the ability to adapt to new conditions more quickly is a major advantage.
If an individual or group that has developed an adaptation for a set of conditions meets another group adapted for different conditions, something that is more likely to happen at a transition zone between the two for example, the ability to exhange genetic information is an advantage.
Take a warming sea for example, and pH changes due to associated acidification. If a southern group is migrating north, due to conditions they are adapted to now being present further north than before...
...and a northern group is spreading south, for analogous but opposite conditions...they tend to overlap at the border of these conditions. If genes that code for an ability to calcify shells in more acidic waters, or to metabolize a protein in colder waters, etc, are exchanged, both can benefit.
When conditions of change slow, such as when tectonic plate movements move an organism's environment from say tropical to arctic conditions or visa versa, and stay there long enough, organisms tend to become further optimized for THOSE conditions.
Often, a STAGE of their development might be best suited, and, they start to spend more time in one phase than others...as THAT stage was simply better suited for overall survival of the involved genes.
Specialization can then start to occur, and some features of the organism start to become more adapted to a set of conditions that are present...often, at the sacrifice of function at other conditions.
You probably already know that the differences are on a single chromosome. So, just as a chromosome involved with, say, your height, can be passed on so as to increase your decendant's height, say a Watusi, or, decrease it, say as a pygmy...so can the ones that were related to genes that coded for the EXCHANGE of genes.
So, over time, it can be advantageous to have some of the species code for a slightly different exchange than others.
Over millenia, these differences simply multiply in response to the conditions...just as the other chromosomes did.
Dimorphism is simply a development along the same lines. It would be analogous to an amphibian spending its time in what used to be it's larval form, because the larval form was better adapted to the conditions...until it could actually reproduce in its larval form, which then in turn, over time, becomes an adult form, etc.
So, an early transition of SOME genetic information between bacteria, which is as close to sex as they get, can then evolve into exchanges of genetic code that is divided into matched chromosome sets.
The sets were always matched, but the WAY they matched evolved as manipulated by their environments.
Coding for "sex" as in gender takes a long time to evolve when you consider that even for fairly modern critters, such as gators, etc....the sex can change in response to the temperature the eggs are incubated at.
So, the species can further adapt to environmental conditions by having different versions of the same species. Males do better if there are "average conditions", but you may get all female's hatching if its exceptionally hot OR cold...for turtles, cold = females warm = males, and so forth.
They don't HAVE X/Y chromosomes dictating the gender...their genetic coding is still temperature based.
Some intermediate species use BOTH X/Y and temperature, and so forth.
So, if you go back far enough, just think of the "gender" as one of MANY types of that species that could be produced depending upon the circumstances.
So, this is not in conflict with evolution as a known biological fact. Just as with any scientific field, there can be details that we may never know specifically, without that negating what we DO know specifically.
If soft squishy stuff was better at leaving us fossils, sure, we could answer some questions that are open...but, not knowing EVERYTHING doesn't mean we know less than we do know.
Evolution is a fact. There is overwhelming data that can ONLY be explained by it, and nothing contradicts it.
Essentially, to simplify the evidence:
1) Sediments settle on the earth, both under water, and on land.
2) Things that settle on top of other things, settled after those things.
3) That means the stuff on the bottom was there before the stuff on top.
4) If the stuff on the bottom was there before the stuff on top...the stuff on the bottom is older, and, the stuff on top is newer.
5) There are many ways of estimating the age of stuff. Carbon dating is one way, if the stuff is made of carbon. There are many other isotopes that are used instead of carbon, or in addition to it.
6) There are some layers of sediment that are quite consistent, and, indicate that the same thing occurred over a small or large area, or even the entire planet. A layer if iridium for example is planet wide, and thicker in one area, and less so progressively further away...indicating a large asteroid strike for example.
A layer of lava would be associated with when the volcano erupted. A layer of sand could be from when a shore was present, and so forth.
7) If a fossil is found in a deep layer known to be billions of years old, then that thing was there that long ago.
8) If a thing is found in layers between certain ages, it was there between those ages.
9) If a thing that was being found, stops being found, and we see no modern examples, etc, we consider it to have gone extinct by around whatever the last dates it WAS found in.
10) If we see that older stuff is more primitive, and, as we get closer to the top/newer stuff, there is a progression in complexity or new features...it means the stuff is changing. So, if old fossils of things that look like horses seem to get more like camels or modern horses over time, that's evolution.
11) If we don't see a critter that appeared a 100 million years ago, in rocks before that, we can say it appeared by that date.
12) If we later find an even older fossil, we adjust the data, and say, hey, we found one 102 million years ago, and so forth.
13) In the course of doing all of this, we have NEVER seen a modern animal appear at an earlier period than the critters it evolved from.
14) Geology, etc, provides a way to understand that rocks from at the bottom of a deep ocean, due to tectonic plate motion for example, can end up being a mountain range...and a rock that was horizontal millions of years ago, could no be thrust up vertically in a mountain range.
15) So, evolution is evident in the fossils, and, in the genetic exploration of, all living things. There are no exceptions where there are dinosaurs when there were people, etc. (Despite misidentified examples of other things, etc)
16) So, if you can't see how we got from horizontal gene transmission to chromosome based zipping, etc, its OK, it doesn't change that evolution is a fact.
Some gaps in our knowledge may remain shrouded in evolutionary history forever. Another example is where did cows come from? We have fossil animals showing steps along the way from a pre-dog to a dog or a pre-horse to a horse. Try to find anything similar related to cows. I've been googling for years for a chart like the one below that shows the evolutionary history of the cow. I found this one by googling on "chart evolution of the horse." Try "chart evolution of the cow" and get ready to laugh.
Yes, searching for "Cow Evolution" is pretty funny.
Searching for the ancestor of the cow, you'd be better served by looking at ancestral Bovidae. That gets you back to primitive antelopes, some with modern descendants, as well as to the early versions of yaks and buffalo, etc. According to DNA studies, the antelopes and modern cows share early lineage.
I think TJ's right, as nearly always. I've just looked in my "big book of animals" and modern Bovidae includes: duikers (tiny little antelopes), hartebeest, wildebeest, impala, antelopes of all strange varieties, gazelles, goats, ibex, sheep, musk ox, bison, yak, kudu, eland. So that gives you an idea.
Antelopes are different from deer. Deer are allied with moose, caribou and elk. Antelopes have horns while deer have antlers.
You would think that given all that, deer and Bovidae may share a common ancestor.
It is sometimes hard to realize accept that we just don't have the answers to certain phases of evolutionary development, especially during it's more primitive stages. How organisms eventually developed specialized cells/organelles for advancing the sperm/egg form of reproduction is perplexing. We may never know for sure. Just as the origins of this particular universe may remain shrouded in darkness for eternity. It is fun trying to understand the "cutting edge" theories currently being offered for consideration. Maybe some day we will have the capability to reproduce in a lab something similar to what happened back then. In the same spirit of discovery as:
TJ, TL;DR at first.
Then I read it. TJ, that's awesome!
I just came back to point out how DNA sequencing across species is giving us the most precise (when known) and most probable (as comparisons are calculated) taxonomy, or tree of life. I.e., we'll keep getting better at tracing our evolution from single celled organisms, just by cataloging DNA sequences of all known species.
Here, several bacteria that have been sequenced, after which ancestral relations to each other could be shown. (Click to enlarge.)
By Ivica Letunic: Iletunic. Retraced by Mariana Ruiz Villarreal: LadyofHats - The image was generated using iTOL: Interactive Tree Of Life, an online phylogenetic tree viewer and Tree Of Life resource. SVG retraced image from ITOL Tree of life.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3633804
Apologies in advance but the problem with the evolution of the cow is that there were too many genetic mootations. I just had to milk that one for all it was worth.
Hmmm, that inspired me to understand what the source of the problems many christians seem to have when they simply assume that their parents' beliefs were the only logical set...
..its Cross Breeding.
Ironically, its also a version of ideological inbreeding.
So, where is the chart showing the variations along the way?
A cow is simply not a 'natural' choice for teaching evolution. That is what those simplified charts are used for.
Plenty of charts on whales, since they went back to the sea.
Ideological inbreeding... excellent phrase!