Species that use sex to reproduce mix the genetic information from the parents into the offspring, so whatever mixing around there is in the population as males and females pair off causes the entire group to retain genetic similarity.
If such a group is divided into isolated parts that don’t mix, the two groups maintain genetic similarity within but the groups drift apart and eventually are separate species.
How does it work with bacteria and hydras and algae and all the other life forms that don’t use sex? That is, why are e coli more similar to one another than either is to some other bacterium?
Isn’t every individual of an asexually reproducing species equally closely related to every other individual of all kinds of species, in the sense of the time that has passed since their entire tree of evolution started?
That’s an excellent question, and a key problem in identifying “species” among microorganisms.
However, many kinds of bacteria do undergo exchange of genetic material through a process called conjugation. This can result in the spread of particular genotypes throughout a population. There are other methods of “lateral gene transfer” (genes transferred between different lineages, even different kingdoms, by viruses.)
Studies of genetic diversity do sometimes show a large variation within what appears to be a single bacterial “species” based on morphology. On the other hand, there can be selection working on an entire bacterial population that will eliminate some divergent genotypes, thus keeping the population more genetically similar than what might be expected just based on mutation rates.
Algae also use sexual reproduction. They produce gametes (though meiosis) that pair and form a new individual. Pretty much the same as our system.
As a matter of fact, the proper question would be ‘is there any specie that does not use some kind of sexual reproduction (or horizontal genetic information transfer) ?’. And I think that the answer may pretty much be ‘no’
Even freakier is that not all genetic transfer is between members of the same bacterial species or strain. There have even been documented genetic transfers between plant species and resident bacteria.
Well, I’ll be damned. They are having conjugal visits and getting a pass.
Now I’m interested in the question Oukile raises, and in one other: what about viruses? Exchanging generic material sounds to me like it takes some kind of metabolic process going on to make work. What I am imagining is creating and emitting DNA, perhaps in some kind of wrapper, and also absorbing and adopting DNA from other bacteria. But viruses don’t actually do anything, right? They don’t really have behaviors, they just carry instructions for other kinds of life to do things for them, right? Can viruses exchange genetic information with one another?
Viral genetic transfer involves multiple viruses infecting the same cell (or cells transferring viral DNA from one cell to another), and then when little baby viruses are packaged up, either host or other viral DNA getting mixed in.
Ultimately, it’s because some, really the vast majority, of bacteria do not rapidly change. They could under the right circumstances, but mostly don’t, because they are very well-adapted.
Another interesting area of microbe research is the interaction of viruses and bacteria; bacteriophages.
I’m only a layman, so I’m sure it’s way more complex than my limited understanding, but from what I know there are interactions and genetic transfers going on between bacteria, viruses, and more complicated organisms all the time. Quite a bit of our “junk” DNA appears to be leftovers from various retroviruses that our ancestors encountered, for instance, and mitochondria are believed to have originally been symbiotic bacteria. Every living thing (and pseudo-living viruses) are linked. We all affect each other constantly and have been doing so since the very beginning of life on this planet. Fascinating stuff.