Eli5: We have a dna correction system to avoid variations, but, sexual reproduction to promote variations. So, Are variations good?
In: Biology
The three main ideas amongst biologists about the way sexual reproduction helps are:
* It ensures that the population remains genetically diverse, so that alleles that are useless now but might be beneficial later will survive, or that new useful combinations of alleles can occasionally come up (eg, the genes for “6 foot 5” and “trust fund”, each individually rare, might come together at some point).
* It evolved as a way to “out-evolve” fast-evolving parasites, by ensuring that each new generation presents a new problem for the parasites to solve (eg, parasites this year may have evolved to successfully take advantage of “6 foot 5, trust fund” individuals, but the next generation those alleles are all mixed up and the parasites have to evolve a new strategy).
* Neither of these ideas are satisfactory, and this is an unsolved problem in biology.
I am in no way trained or learned in biology other than highschool and what I picked up online.
We WANT diversity between organisms as it makes it harder for diseases to attack a whole population.
We DON’T WANT diversity within the same organism as it may be seen as a foreign attacker (disease for simplicity) and attack ourselves.
Some variations are good; most variations are bad.
If you were to make any random change to our DNA, it’s likely the modification will be harmful, or even lethal. If the DNA stays identical, there is a higher risk of either being out competed or harmed by environmental changes.
A good balance between these is to try and minimize random changes during life, while encouraging changes during reproduction. A lot of the permutations produced will be non-viable, but you can keep trying until you get a viable offspring, so long as you remain in a healthy state. In humans, this means the vast majority of permutations of eggs and sperm will be non-viable, often before fertilization, though sometimes these will result in miscarriages or infants that die shortly after birth.
Let’s take a look at two fairly extreme examples: Bananas and Sickle Cell Anemia. The first is an example of how lack of variation is bad, and the latter is an example of how variation is dangerous, but possibly beneficial.
The most common type of banana found in grocery stores is the Cavendish. All of the Cavendish banana trees are grown from cuttings, and the trees are basically clones, with identical DNA. This means that the fruit is very similar, but it also comes with a major risk. All the Cavendish banana trees are vulnerable to the same diseases, and are constantly at risk of being wiped out by fungus. This actually happened to the previous most common/popular banana variety, the Gros Michel. It is only with great effort and constant vigilance that bananas don’t get wiped out by fungus. That’s the danger of having no variation. If there’s a disease, or environmental change, that every member of a group is especially vulnerable to, then that group can get wiped out easily.
Now consider Sickle Cell Anemia. This is a genetic disorder that causes red blood cells to grow malformed, and not carry oxygen as well as they should, and break down easier. People who have it live shorter lives, even with medical treatment. However, people who carry the gene that causes it are also resistant to Malaria. So this is a genetic variation that is both a negative and a positive, depending on a number of variables. For people living in climates where Malaria is endemic, having one copy of this recessive gene gives them a better chance at surviving a serious disease, but they also have a chance to have children with two copies of the gene, giving them a debilitating disease (and also making them more vulnerable to Malaria.) For people living in other climates, the gene is always a negative, and few people descended from ethnic groups living in cold climates have it.
Lets explain it with cars!
A random change in DNA does a single tiny thing which still could have vast consequences:
A different color. A missing screw (is it crucial?). An extra screw fixing that annoying wobble. And 5 tiny screws that do absolutely nothing. Maybe only 3 tires. 5? How about rectangular ones! An extra seat on the top… or on the bottom.
Some of those are pretty bad. Many are mostly inconsequential, slightly annoying at worst. And some few might turn out to be actually helpful. That’s **genetic mutations**, random undirected changes to see what works out and what not.
**Sexual reproduction** is then taking two old cars, taking parts from both, and make a new one with them. Cars tend to have 4 tires, so the child will, too. Only one steering wheel, right? Seats… there’s some variance here. And the color is pretty much a matter of preference. We aren’t inherently changing the individual parts, we only copy them from earlier models.
If we design a new car, we might want to take those “parents” that worked particularly well. Maybe we liked one color more. And the idea of putting the steering wheel _on the left_ was really just better. Oh and please don’t use that fifth wheel idea anymore.
The decision what is kept in the next generation is mostly decided by two aspects:
– **Survival**: did it survive long enough?
– **Sexual selection**: Did they like it?
The safest car won’t sell if it is really uncomfortable or expensive. Nor will a car that is known to be a death trap. And people probably laugh about a seat on top. Without the second part, sexual selection, we mostly measure if it can drive well enough to not immediately crash; but with sexual reproduction, there’s now a lot of new secondary factors.
DNA is copied for each cell division, trillions of which happen each day. Without regulation, the probability of having a deleterious mutation would be way too high.
Reproduction happens a couple times per generation so the rate of mutations happening is much lower.
Also, the variation during reproduction will produce diversity in phenotypes of the resulting individual (it’s good to have an individual that out competes others in a population) whereas variation during cell division will result in diversity of cells within a single individual (it’s bad to have cells that outcompete other cells within an individual).
Moreover, the two copies of genes from different parents are in theory from successful individuals whereas a random DNA mutation during cell division is not guaranteed to produce anything viable.
Yes. Unintentional variations within dna can lead to extremely problematic phenotype (physical appearance & physiology), like disabilities or even death if it’s something like a developmental organ defect.
Promoting variations through sex ensures that the variations that are passed down are benign or beneficial, but especially not harmful. This works because in order for a variation to be passed on, the organism needs to:
1. Live to an age old enough to reproduce.
2. Be strong enough to live to that age (outlast predators, compete for resources, etc).
3. & be attractive to the opposite sex, meaning other organisms view them as strong & a viable surrogate for children.
It’s a bit paradoxical, but basically sex works as a fail safe for keeping shitty genes out of the gene pool bc anything that is too weak will die before it can reproduce, or at the very most they will have weak children who will die early.
As living things we need variation in order to stay alive as a species because our environment is constantly changing. If we never changed, something external (like a shortage of a particular plant or prey) could change, & wipe everyone out because they can’t find food.
We want *non-harmful* variations. Variation is good as long as it doesn’t result in harm, because it makes a population more resilient. The problem is that most random mutations are harmful, because you’re far more likely to break a thing by jumbling it up than randomly improving it.
With sexual reproduction though, the variation you’re adding to your lineage isn’t random. Even on the most basic level you can guarantee that your partner’s DNA is capable of surviving to adulthood. And most organisms have some form of additional selection process to increase their chances of breeding with the partners that have the most successful genes they can.
An additional consideration is that you *really* don’t want cells that are already part of your body to start mutating. If your gametes have a bad mutation, most of the time the embryo will just die before it matures, and you can make a new one. But if your somatic cells mutate, there’s a good chance of them becoming cancerous, and that can kill you. So for multicellular organisms there’s a strong benefit to resist mutation in general.
The exception for this are microbes like bacteria and viruses. Microorganisms reproduce quickly and don’t have to worry about cancer, so sacrificing one out of a hundred cells to experiment on the off-chance they stumble upon a good mutation is a worthwhile gamble. So they typically have far weaker DNA correction systems than multicellular organisms.
To be more precise, we have mechanisms in place to prevent any errors when replicating our DNA. When copying the DNA from 1 cell when it splits into 2, we want the DNA to be exactly the same in both cells. Any errors in the replication might result in faulty cells that dont do what they’re supposed to. Or worse, cancerous cells. Its like building a brick wall, you want each brick to be an exact replica of each other so you can build a stable, precise wall. You dont want them to change shape and composition between each brick you pick up.
Where variation does have its advantage is across different individuals within a community. A genetically diverse population is much more resilient to diseases and other threats to the population. To achieve this variation we have 2 main mechanisms. First, some of the chromosomes in our gametes (sperm and eggs) do a little mixing and matching within itself so that they’re not an exact identical copy of the DNA in th rest of the body. If you imagine the X X chromosomes as 2 people, you can imagine them mayne swapping their left shoes, or swapping watches, so the X X on your gamete looks different from the original source XX.
Second, when you reproduce you take 1 chromosome from each parent and match them together, creating a mix and match that creates a unique offspring. The2 mechanisms combine such that even the same pair of parents having lots of children can still produce a relatively diverse group of offspring.
We’re talking different kinds of variations here. Let’s say that you need bread to live. So you find a recipe to make your own bread. But what if you’re unable to get one of those ingredients. So you get multiple different recipes that have different ingredient substitutions so you always have a way of making bread. So if something happens where there’s a shortage of an ingredient, you can still produce the thing you need to survive.
That’s one kind of variation. Let’s look at another. In this variation we randomly change some of the letters that makes the ingredients and or instructions unintelligible. Now you can’t make bread at all and you die.
The variations in our genome that are beneficial are of the first kind. They are different ways of doing things that allow us to survive and provide more ways of overcoming random things that might happen to us in life. Mutations are of the second variety and the most common effect is that they just make DNA not work at all.
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