How would early propagation of a species start?

Factual Questions
1

I’m pretty hazy here on what I’m talking about, but can someone explain to me what the thinking is regarding the very earliest sexual reproduction of species? I’m obviously not imagining something correctly here, maybe it’s a chicken-egg thing that I’m all caught up in, but basically how would a species begin to progagate itself sexually?

You have an animal, let’s say it’s the first breakthrough animal that qualifies as one of its species. Are there millions of such animals that mutate but are unable to find a mate to reproduce with, but sometimes there just happen to be two such animals, and it just happens that one is a male and one is a female, so they get to start a species? And all descendents of every species consists of offspring of brothers and sisters from that original pair? Or does this work in some other way where I’m leaving out a few crucial steps?

2

Think of it in terms of a Chihuahua and a Great Dane. They’re both of the same species, can bread together, and are completely different.

Eventually though, Chihuahuas and Great Danes will grow too far apart and become even physically (rather than just feasibly) impossible to breed. But that’s okay because by the time the two lines grow far enough apart, there will be enough Chihuahuas to mutually do the nasty.

Species don’t just come into being in a day. For a long time humans could probably still breed with apes, but eventually we stopped needing to.

And also, the reason there are Chihuahuas and Great Danes is because if you take the same proto dog and expose it to different things, it’s going to develop differently. But that’s something that’s going to effect the whole pack. So the pack itself is going to evolve together. Being able to breed with other packs that have evolved in a different direction is just a fallback.

3

Bacteria can can swap genes via conjugation. It’s not part of reproduction, but if one cell organisms can exchange genetic material, then the evolutionary process will take it several steps further if it helps.

Just start following the links in the above article. The one on sexual reprodcution says: “The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years ago.” A lot of advances can be made with 1 billion years to spend.

4

The basic mistake here is thinking that mutation is a crucial step in the creation of species, or that species are created in a single step at all.

Speciation, like most things in evolution, is usually a very gradual process. It can be exceedingly difficult to tell when one form has become “different” from another to be considered a new species. This is demonstrated by constant disputes and differences of opinion over which populations represent subspecies of the same species, and which are different species.

The general consensus is that speciation typically takes place through a process called allopatric speciation, that is, via populations that live in different places. There has been some controversy whether sympatric speciation - that is, speciation within a single population - can occur at all. The general view is that it does occur on occasion, but is rare, at least in animals.

Typically what happens is that two populations become isolated from one another, and no longer exchange individuals through dispersal. This can happen, for example, if climate changes, and populations of a forest-dwelling species become isolated by the development of a intervening large area of grassland; or if a mountain range arises and blocks migration. It can also happen through dispersal, when wandering individuals colonize an offshore island or an isolated mountain peak.

Once populations become isolated, they begin to differentiate. This will happen even in the absence of mutation. If a population has been established by a small number of founding individuals, it will often have a different genetic makeup just by chance. This is known as the “founder effect.” Also, while the population is small, chance events can have an effect on its genetic makeup, even in the absence of selection. This is known as “genetic drift.”

Beyond this, isolated populations will very often be living in different climates, with different food sources, and different competitors. They will undergo natural selection in different directions due to these factors, which will also change gene frequencies.

All of these factors can cause populations to differentiate even without mutation. Of course, over a period of time, different random mutations will accumulate in the two populations and further serve to differentiate them. Over time, the two populations will diverge enough to become recognizable as separate species.

If climate or other factors change, two such isolated populations may come in contact once again. If they haven’t differentiated very much, they may interbreed, and will be considered subspecies of the same species. If they have changed quite a bit, they may be unable to interbreed, or do so rarely, and will be considered different species.

There’s more to it than that, but that’s the outline.

5

Possibly. Ignoring the fact that we are apes, and using that term as it is used in the vernacular… a “human” is a member of the genus Homo, which only arose about 2M years ago. But our line split from the chimp/bonobo line 5-6M years ago, which means the first “human” was several millions of years removed from his nearest ape relative. It’s certainly not out of the question that a Homo habilis could have successfully mated with his proto-chimp contemporary, but it’s not a slam dunk.

If you meant that our pre-*Homo *ancestors (Australopithecines and the like) could probably mate with proto-chimps, then yes, that’s likely. In fact, some biologists think the two lines split more than once-- ie, we went our seperate ways, then merged again for a time before separating permaently.

6

Were you asking how the first sexually reproducing organism evolved from an asexually reproducing ancestor? That is, in fact, one of the bigger puzzles of modern evolutionary biology…

7

This Wikipedia article on speciation isn’t all that great, but it at least gives an outline of different modes of speciation.

8

Read up on a phenomenon called Ring Species - this is when a local population of some organism is able to interbreed with the very slightly different population next door, which is in turn able to interbreed with it’s neighbour population (which is also slightly different, and so on - these chains of inter-fertile neighbours can go all the way around the world and back to where they started, but because each link in the chain is slightly different from its neighbour, the two ends that meet back together are incompatible, different species. This is speciation not lengthways through time, but widthways through geographical space.
If a natural disaster happens to destroy the middle of the chain, the two ends go their separate ways.

It’s only one method by which speciation can occur, but it’s a particularly easy one to visualise, because it’s all observable in real time right now.

9

IIRC, the OP is a proffessor in the humanities. Perhaps he can think of the analogy of how different languages come into being. Groups speaking the same language separate, and after a time find they can no longer communicate with each other. And even people who stay in the same place find that, after a considerable amount of time, they can no longer read the texts their own ancestors wrote. Yet every parent was able to speak with every child during the entire process.

Similar to ring species, you can find zones of dialect contintuity where every village can communicate with its neighboring villages, but people from far distant villages cannot communicate with each other.

10

You might find this book of interest – it covers a lot of the material that colibri and the others have summarized in its introductory sections.

11

Here is a very good example of the exactly the process the OP described…

12

I don’t know if this question is a hijack or a clarification. If it’s a hijack, I’ll make a new thread.

It’s my understanding that humans have 46 chromosones and chimps have 48. I understand the concept of slow speciation over time, but it’s not like a species can have 47.6 chromosomes. How do 2 species evolve from a common ancestor and have a different number of chromosones?

13

Two of the ancestral ape chromosomes fused sometime after our line split off from that of the chimps. We assume the chimp form is the ancestral one since gorillas share the same number of chromosomes as chimps.

14

I should have added that this is a pretty common phenomenon and we often see closely related species with differing chromosome numbers-- horses and donkeys are another common example.

15

An area where many different allopatric populations of birds come into contact is the Great Plains of North America. What were once single populations of forest birds became split into separate eastern and western populations when the Great Plains became more arid after the end of the Pleistocene (although there were probably previous climatic fluctuations and corresponding splites as well).

There are at least 14 species or species pairs of birds in which such populations meet in the Great Plains. In some of these, the two populations hybridize extensively, and are considered subspecies of the same species. In others, they hybridize rarely or not at all, or the hybrids are limited to a narrow zone, and the populations are considered to be separate species according to the Biological Species Concept.

Note that the level of hybridization considered to be sufficient to lump two populations as one species is a matter of opinion. Since the linked article was written in 1988, the Baltimore and Bullock’s Orioles, formerly lumped as Northern Oriole, have been re-split.

16

I vaguely recall some Discovery channel show (possibly Naked Ape) that claimed there were originally many “sexes.” By originally I mean microscopic sea critters, and by many I mean six or more. Eventually, two of the six or more sexes won out through natural selection, and thus animals are left with only two sexes today.

Can anyone confirm or refute this hazy memory of mine?

17

Some microorganisms have multiple “mating strains.” Mating can only take place between cells of different mating strains, serving to help prevent inbreeding. I don’t think I would characterize this as having multiple sexes. The basic difference between males and females is in the kind of gametes they produce, small and usually motile in males, large and non-motile in females. Mating strains are essentially identical except for the factors that prevent mating within strains.

18

Related species with different numbers of chromosomes is something you see even more commonly in plants. How those differences came about is often pretty different from the way changes in chromosome number happen in animals, though.

19

So, theoretically, at some time a 48 chromosomed momma ape gave birth to a baby with 46 (or is 47 possible)? If so, was this 46 chromosomed ape able to breed with 48 chromosome apes, or did he have to wait for a female with 46 (or 47) chromosomes?

To put it another way, is it necessarily impossible to mate (with viable offspring) with an animal with a different number of chromosomes?

(Note: I attended private Christian schools until college, so my education is evolution, and any science closely related to it, is limited to what I’ve learned on the internet.)

20

Yes, it is possible for two individuals with different numbers of chromospmes to produce fully fertile offspring.

The problem with having different numbers of chromosomes arises during meiosis. the kind of cell division that results in gametes (sperm and ova). In each pair of chromosomes, each one must line up with its partner before the division in order to produce a haploid gamete with a single set of chromosomes. If the pairs do not line up, then gametes with an unbalanced number of chromosomes can be produced, resulting in an inviable embryo after fertilization.

When a chromosome splits into two or fuses with another, the original chromosome is often still able to pair up with the split or fused versions. Therefore meiosis can take place normally and a viable embryo results. After time, the new versions of the chromosomes may accumulate mutations that prevent the pairs from lining up properly. Once this happens, hybrids may still be produced, but will be infertile.

One well known example of animals that can produce fertile hybrids despite different chromosome numbers is Przewalski’s wild horse, which has a diploid number of 66, and domestic horses, which have a diploid number of 64. In contrast, horses are unable to produce fertile hybrids with the donkey, which has a dipoid number of 62, but do produce sterile hybrids (mules).

Here’s an example of chromosome number polymorphism in a species of mouse.