Thale cress grown close in rows like this. If the rows were made up of plants from the
same family, their leaves would bend away from thr row, looking like they were trying
to get away from one another. In the rows where they were from different families they
didn't seem to do this so much. Bending away like this will decrease the amount
of light they're blocking for a neighbor, but they end up blocking light for themselves.
It looks like they're trying to help their neighbors at their own expense. But only for
relatives. It seems to be a plant... that cares about
family Let's look at some important things that happen
to DNA. Segments of DNA are used as the design for
proteins. And pretty much everything that happens in the cell is done by proteins, so
DNA ends up being the code for everything that a cell will be, do and build.
DNA gets copied. One copy of which is to go into another cell. And eventually DNA and
the cell will fall apart. Replicating is why we are all here. Why cells
don't just die and we all go away forever. It's the cells that can replicate more or
more reliably that we should expect to see more of.
A gene can code for proteins that can do any number of things. But if it does something
that helps the cell reproduce, like makes the cell chase after resources it can use
or run away from dangers, then that gene will be reproducing more. Because that cell is
reproducing more. Even though it may be having negative impacts on the reproduction of that
cell like using up resources to make that protein, if it has a net benefit on reproduction,
then we should expect this gene to reproduce more. And we should expect to see more genes
like that. In reality, how a gene is going to help or
hurt reproduction has to do with the environment it interacts with. Including the other proteins
of that cell. What works and doesn't work for reproduction is always changing and it's
hard to predict and quantify. Here we're just talking hypothetically.
Genes that code for proteins that have a net cost on reproduction, like making the cell
swim in a circle for no reason, or if this protein when excreted is followed by predators
like a bread crumb trail, those genes would be reproducing less and we should expect to
see less genes like that. So then how can we see proteins that are helping
other cells reproductively, at their own expense? That's a waste of resources. Cells with these
genes should reproduce less and that trait should go away. But if this mutation came
earlier and this cell was produced through mitosis and they were genetically identical.
Then this protein is reproductively helping DNA that designs that protein. As long as
the reproductive benefits are greater than the reproductive costs, that code should do
OK. Even though it's not helping the exact DNA molecule that was worked from to assemble
it. Let's say the proteins these genes code for
will guarantee a copy of the cell ... for whatever reason. Whether a protein is guaranteeing
a copy in the cell it came from, or a protein is guaranteeing a copy in another identical
cell, the same number of copies are made. The effect on the replication of a gene would
be the same. Because DNA is ultimately going to fall apart. Whether that DNA will continue
is about whether the protein it codes for helps it leave behind more copies. Both of
these genes would be accomplishing that and we should expect to see proteins that help
other cells just like we see proteins helping the cell they came from.
But if it guarantees a copy in this stranger cell, a cell that doesn't share the gene,
it's not helping itself reproduce. That gene isn't there when it tries to help that cell
and it's a waste of resources. The other cell needs to share that gene and we shouldn't
expect genes that help strangers to do very well.
One example is slime mould. Single celled protist, usually goes about doing its asexual
single cell thing. Eating. A-sexing. When the food runs out, they get together with
thousands of cells and form these fruiting body things. The cells at the top will produce
spores that can survive without food for a while. Being up a little stem means they can
be picked up easier by things like passing insects easier or ingested and excreted and
then deposited somewhere where there's food. In the right conditions the spores will form
single celled slime moulds again. The bottom cells of the fruiting body die
and won't reproduce anymore. But because the top cells are clones whatever genes are involved
in that behaviour are actually reproducing better because of it. So we should continue
to see that behaviour. From the perspective of reproducing code, a few cells dying doesn't
really matter It's kind of like the cells that make up humans.
Most of our cells aren't going to reprod uce for very long. But body cells will continue
to exist because they're helping the reproduction of the sex cells that carry the same DNA.
Reproductively our bodies are kind of like, slime mould bottom cells.
But the relationship is a little bit different when there's sex cells in the mix. Like between
parent's cells and children's cells. Chromosome pairs get split up with meiosis,
and combined with someone else's at sexing. For a chromosome, on average only half of
the offspring are going to share that same DNA. And the same goes for siblings, on average
only half of the siblings are going to share any given chromosome.
What this means for genes is that any given gene that's helping the offspring's cells
reproduce, is only helping a copy of itself on average half as often. Compared to a gene
that's helping the bunch of cells it came from. When it's not helping a copy it's basically
helping a stranger. It's a waste of resources. So a gene that tries to help a offspring or
a sibling, will have on average half the reproductive benefit. Because of all the times it's not
helping a copy of itself. Or maybe you can think about it like, if a
gene guarantees an offspring for this mass of cells, on average there is a 50% chance
that the offspring will have that gene. So for the gene, it's really only guaranteeing
on average half a copy. If it guarantees an offspring for one of its
offspring, without knowing which offspring shares that gene, it's really only guaranteeing
on average a quarter of a copy. Because on average only a quarter of those grandchildren
are going to share any given chromosome. A gene guaranteeing an offspring doesn't really
make any sense. But that doesn't matter. The idea is just that proportionally, a gene that's
helping an offspring, parent or sibling, is getting relatively less reproductive benefit
than it would helping itself. SO we wouldn't expect genes that help family
members to do as well. And they would be less common. Genes that help grandchildren, cousins,
grandparents and other more distant relatives would be even less prevalent. But still, more
prevalent than genes that help strangers. With mitosis, a gene can give equally good
reproductive benefit to its copies. But reproductive benefits are reduced between meiosis relatives
because chromosomes are getting split up. Would an offspring ever help their parents
reproduce? Offspring's cells are the ones that have to continue to code. So if cells
age and die, shouldn't the helping genes only go towards the offspring? And otherwise it
would be counter-productive to long term reproduction? But children's cells are the copies that need
to continue the genes. If cells age, surely the helping relationship should only go one way towards the children? Wouldn't siblings helping the parents reproduction be sort of
counterproductive to long term reproduction? It does make a certain sense. Especially for
us. We put a lot of work into taking care of our kids. We're mammals. We have appendages
that shoot out liquid child caring. Yes if a gene makes them help the parents
reproduce, and then never reproduce themselves, they would go extinct. But if some of the
offspring do reproduce at some point, then a helping gene can pass on in the same way
we've been talking about. And you're starting to get something that looks like the social
structure of bees, ants, termites and other animals.
With the European honey bee, the queen's worker children gather food for her and the hive.
A queen may live on average three to four years. And she'll create hundreds of thousands
of workers throughout her life. Workers will only live weeks to months and
most of them won't reproduce. New queen are produced young workers or from a queen's drones
that go off and mate with other queens. So reproduction in these bees is sort of carried
out queen to queen. The workers assist that relationship by helping
the queen and her drones. They also feed and raise the new worker. Those workers can then
go on to help the queen, the drones and new workers. So that those workers can go on to
help the queen, the drones and the new workers and so on.
OK, while you can trace the line of reproducers from mother queens to daughter queen, the
majority of the bees only live to help their parents and siblings and there doesn't seem
to be much "helping offspring" behaviours. They seem to be helping their parents live
and reproduce more reliably. Whether a protein results in the reproduction
of its gene in the cell that assembled it, or another cell. Whether those cells are attached
or separated they are causing the DNA that designed that protein, to reproduce.
This seems to be a big part of why we see family.
Why belding's ground squirrels will risk making alarm calls about ground predators much more
frequently when with family. Why White-Fronted Bee-Eater offspring will
often stay and help their parents with their new kids rather than leave and make a nest
of their own. Why a dominant male turkey's brother will
help scare away other male turkeys, and give a backup display for his brother, but never
mate with any of those females himself. It's genes in one cell that have had an effect
that reproduced those same genes in another cell.
But then, if my cells have the family caring genes, and your cells have the family caring
genes, why don't we just treat each other as family? Surely if a woman were to nurse
any other human not just their family, that's mammary glands helping the genes for mammary
glands reproduce? Why don't we see that? You know why are we talking about the probability
a gene will be shared? Why don't they just identify which offspring has that gene?
And otherwise how do proteins and cells recognize what other cells are family? How does that
dumb plant do it? And how did those bees become non breeding
workers? They're not clones of their future reproducing siblings the way slime mould cells
are? I'm going to leave you with these. There is
some additional information on this and some other stuff in the notes below.
This episode is brought to you by, Hamilton's Grass-fed Human Milk.
Huma Mumma Yumma Liqua Human Milk.