Chapter 20: Competition, Predation, and Population Biology

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Welcome to the Deep Dive.

Today, we're getting into the thick of it, you know, moving past pure genetics into the messy real world of populations.

Exactly.

We're looking at how organisms actually interact out there, how they live, how they compete, cooperate, that sort of thing.

And our focus for this Deep Dive, drawing from Strickberger's Evolution Chapter 20 specifically, is how these interactions like competition, predation, symbiosis are really the engines of evolutionary change.

That's the core mission.

Understanding population biology, the numbers game, the interactions, that's where the evolutionary action is.

We'll focus on those

competition, predation, and symbiosis.

Okay, but first, let's set the stage.

We talk about population a lot.

What are we actually talking about here?

Right.

So a population is that whole group of interbreeding individuals, but the real like functional unit where evolution actually happens locally, that's the deem.

The deem.

Got it.

And within that deem, every organism is trying to find its place, its ecological niche.

Exactly.

Its role, its relationship to resources, to other species, the environment itself.

But here's the kicker.

That environment, especially the biological part of it, is always changing.

Other species are evolving too.

Ah, the Red Queen hypothesis.

You've got to keep running just to stand still.

Precisely.

It means a lot of the pressure forcing species to adapt doesn't come from, say, climate change alone, but from the other organisms evolving right alongside them.

It's a constant race.

Okay, so let's dive into that.

How does this race start?

I guess with the basics of population growth.

Yeah, that's the foundation.

Malthus pointed it out and Darwin and Wallace ran with it.

Life has this incredible potential for just

explosive growth.

Like unchecked.

Totally unchecked.

Exponential growth.

Two become four, four become eight, sixteen.

Just keeps doubling.

That inherent power to overproduce is what creates the struggle for existence.

There just aren't enough resources or space for everyone if growth goes wild.

But of course it doesn't go wild forever.

The real world steps in.

And we measure that with, what are the key terms again?

Two main ones.

First, the rate of increase, which we call R, is basically birth rate minus death rate.

Think of it as the engine speed.

Okay, R is the speed.

And second, carrying capacity, or K.

That's the environmental speed limit.

The maximum population size the environment can sustain long term given the available resources, space, whatever the limiting factor is.

So R is potential growth, K is the reality check.

You got it.

And ideally, if you plot population size over time in a stable environment, you often see this S -shaped growth curve.

Right.

I remember seeing that.

Like the yeast example in the text Saccharomyces cerevisiae starts slow, then zooms up exponentially.

Then it levels off as it hits that ceiling, that K.

In their lab setup, it was around 665 yeast cells.

A nice plateau.

But you said ideally, real life isn't usually that neat, is it?

Not at all.

Look at the data for the great tit population in Holland, for instance.

It's not a smooth S curve.

It bounces around sometimes wildly.

What causes those fluctuations?

All sorts of things.

Some are density independent stuff like a sudden cold snap, a bad winter.

Doesn't matter how many birds there are, it hits them all.

Okay, like external shocks.

Right.

But others are density dependent.

These factors bite harder when the population is crowded.

Think disease spreading faster or running out of food or predators finding prey more easily.

Their impact depends on how dense the population is.

And these different realities, stable versus fluctuating environments, hitting K versus booming and busting, actually shape how species evolve, right?

This leads to R and K selection.

Exactly.

It connects population dynamics directly to evolutionary strategy.

So tell us about rice selected species.

Okay, rice selected species are the opportunists.

Think bacteria, weeds, insects that appear in huge numbers suddenly.

They live in unstable, unpredictable environments.

Their strategy.

Maximize R, grow fast, reproduce early, have tons of offspring.

High fecundity is key.

Quantity over quality, you could say.

Just flood the zone and hope some make it.

Pretty much.

Now contrast that with K selected species.

These are your long lived organisms, often larger ones like elephants, humans, big trees.

They live in more stable environments, often near their carrying capacity, K.

So they're not focused on rapid numbers?

No, selection favors efficiency,

using resources wisely, competing effectively, investing heavily in fewer offspring, but ensuring they have a better chance of survival.

Think parental care, slower development, quality over quantity.

And selection can shift these strategies relatively quickly.

You mentioned guppies.

Yeah, the guppy studies, poecilia articulata are fascinating where predation pressure is high.

Lots of things trying to eat them.

Selection favors guppies that faster and pump out babies earlier.

They reproduce before they get eaten.

Basically, it shifts the life history towards earlier, more intense reproduction, even if it doesn't necessarily mean a longer life overall after reproduction stops.

The risk changes the optimal strategy.

Wait, so mortality risk is a direct selective pressure on life history traits.

And even population density itself can act as selection.

The fruit fly example.

Absolutely.

The Drosophila melanogaster's work on the foraging gene is a brilliant demonstration.

They found different versions alleles of this gene.

One leads to rover behavior flies that move around a lot looking for food.

The other leads to sitter behavior flies that tend to stay put.

Okay, rovers and sitters.

Now, which one is better depends entirely on the population density.

When density is high and food is scarce.

Being a rover makes sense.

You got to explore to find anything.

Exactly.

The rover allele is favored.

But if the population is sparse, density is low and food is easy to find right where you are.

Then moving around is just wasting energy.

Sitters win.

Right.

So the ecological condition, high or low density directly determines the fitness of these genetically based behaviors.

It's a beautiful link.

Wow.

Okay.

So density shapes things within a population.

But what happens when different populations or species bump up against each other, especially if they need the same limited stuff?

That brings us to competition.

Yes, competition.

When two or more groups rely on the same limited resources.

And this interaction, it forces populations down some really interesting evolutionary paths.

It's often a lose lose, or at least a less win for everyone involved initially.

Because fighting takes energy and carries risk, even if you win.

Precisely.

So evolution often favors ways to avoid direct, costly competition.

The first major consequence we see is resource partitioning.

Meaning they just split the resources.

Yeah, they specialize.

They find ways to use the resources differently, minimizing the overlap.

The classic example is those five species of warblers, the Dendroika birds, living in the same spruce trees.

How do five types of similar birds manage that in one tree?

They divide it up.

If you were watching carefully, you'd see some species mainly high up in the newest needles of the crown.

Others step to lower branches or closer to the trunk.

Some probe bark crevices.

Others glean insects off leaves.

They've partitioned the tree into different micro habitats or foraging styles.

So they're coexisting by not directly competing for the exact same bug on the exact same needle at the exact same time.

That's the idea.

And you see it again with the Anolis lizards in the Caribbean.

Where different species live together, they specialize.

You get crown giants living way up high, trunk crown species lower down, trunk ground species near the base, even grass bush specialists.

And they look different too, right?

Adapted to those specific spots.

Oh, absolutely.

The crown giant A.

Ricordii is large and green, blending in with leaves.

The trunk specialist A.

Disticus is smaller, flatter, often grayish or brownish to match the bark.

Their morphology reflects their partition niche.

Okay.

So partitioning is one outcome, but what if the competition is really intense?

Does it lead to more permanent changes?

It certainly can.

And this leads to the really fascinating phenomenon of character displacement.

This is where the physical traits, the morphology of competing species actually diverge because of the competition.

So the competition itself drives evolutionary change in body form.

Exactly.

The textbook example, again, is Darwin's finches on the Galapagos, particularly the work by Peter and Rosemary Grant.

They looked at geospeeds of Phylogenosa and geospeeds of Fortis.

The medium ground finches.

Right.

When these two species live on separate islands, their beak sizes are pretty similar, intermediate,

around 10 millimeters, maybe.

But on islands where they live together.

They specialize.

Dramatically.

G.

Phylogenosa's beak becomes smaller, maybe 8 millimeters, adapted for tiny seeds.

G.

Fortis's beak becomes much larger, like 12 .5 millimeters, for cracking big, tough seeds.

The presence of the competitor forces them into distinct feeding niches, and their beak morphology evolves to match.

They displaced each other's characters, in this case, beak size, to reduce competition.

Precisely.

And the source material highlights something truly profound here, linking this visible evolution to genetics.

Oh yeah, the BIMP4 gene.

BIMP4 gene.

Research found that the variation in beak size and shape in these finches is strongly correlated with when and how much this single developmental gene is expressed in the developing beak.

Just one gene having such a big effect on beak shape.

Largely, yes.

It suggests a relatively simple genetic mechanism can underlie significant, rapid morphological evolution.

It helps explain how the grants could actually observe evolution happening over relatively short time scales in response to environmental changes, like droughts changing seed availability.

That's incredible.

A direct line from ecological pressure through gene expression to evolutionary change in form.

Okay, so partitioning and displacement are ways to coexist.

But what if they can't?

What if two species need the exact same thing in the exact same way?

Then you get the ultimate consequence.

Competitive exclusion.

The rule being?

The principle is simple.

Two species cannot coexist indefinitely if they occupy the exact same niche.

If they use the same limited resources in the same way, one will eventually out -compete and eliminate the other locally.

Is there experimental evidence for this?

Oh, classic experiments.

Goss's work in the 1930s with two paramecium species, P.

aurelia and P.

caudatum, grown separately, both thrived, but put them together in the same test tube.

One drives the other out?

P.

aurelia consistently out -competed P.

caudatum.

It turned out caudatum was more sensitive to the waste products they both produced in the limited test tube environment.

Same niche, same limitation, one winner.

And this isn't just tiny protozoa in a lab, right?

This principle has real world applications.

Huge ones.

Think about agriculture and health.

The development of the pre -empty treatment for chickens is a direct application of competitive exclusion.

How does that work?

They basically culture a complex mix of harmless gut bacteria from healthy adult chickens.

Then they give this good bacterial mix to newly hatched chicks.

These beneficial bacteria rapidly colonize the chicks' cut, occupying all the available niches.

So when a dangerous bacterium like salmonella comes along?

There's literally no place for it to gain a foothold.

The established good bacteria competitively exclude it.

It's using ecological principles to prevent disease.

A really smart application.

That is brilliant.

Using competition for our benefit.

Now before we shift gears from competition, you mentioned earlier that even the idea of the noosh itself has evolved.

It really has.

It's a concept scientists keep refining.

Initially, Grinnell, back in 1917, saw the niche, almost like an arrest the habitat requirements the environmental box and organism fits into.

Like the California Thrasher belonging to Chaparral.

Habitat focus.

Right.

Then Hutchinson, around 1957,

shifted the focus more to the organism itself.

The niche as an n -dimensional hypervolume, defining all the conditions and resources an organism needs to survive and reproduce.

More abstract, but organism -centered.

And then came niche construction.

Yeah, Lewontin and others really pushed this idea more recently, around 2000.

Niche construction.

The revolutionary part is that organisms aren't just passively fitting into pre -existing niches.

They actively change their environment, and in doing so, they modify the selection pressures on themselves and other species.

Like beavers building dams.

The classic example.

Beavers don't just find wetlands.

They create wetlands by building dams.

This transforms the entire local ecosystem, altering resources, habitats, and thus, the niche is available for countless other organisms, including the beavers themselves.

Organisms as ecosystem engineers.

So niches aren't fixed boxes, but dynamic interactions, partly built by the organisms living in them.

Okay, that's a lot on competition.

If competition can lead to exclusion or divergence,

what keeps one super competitor from just taking over everything?

That must be where predation fits in.

Exactly.

Predation plays a crucial role as a regulator.

By keeping prey populations in check, often below their carrying capacity, k -way, predators can prevent the best competitor from monopolizing resources and excluding others.

So predators can actually increase diversity sometimes?

They can, by reducing competitive exclusion.

But the relationship between predator and prey is intense.

It's a life or death arms race, and it often leads to these coupled cycles, their population numbers.

Ah, the boom and bust cycles.

Right.

The source material describes a great example with mites in a lab setting.

You have herbivorous mites, the prey feeding on oranges, their population explodes.

Then the predatory mites, which feed on the herbivorous mites, start to increase because there's so much food.

And then the predators eat too many prey.

Causing the prey population to crash.

Which then leads to the predator population crashing because they've run out of food.

And then with fewer predators, the prey population can recover, and the whole cycle starts over again.

A tightly linked rise and fall.

Like the famous links and snowshoe hair cycles, those are more complex in the wild.

True.

Wild systems are more complex.

For instance, if a predator isn't reliant on just one prey species, its population can be buffered.

Buffered, meaning?

Meaning if one prey species crashes, the predator can just switch to eating something else.

This can stabilize the predator's numbers and also dampen the wild oscillations in any single prey population.

Complexity adds stability, often.

Does the type of predator matter too?

Absolutely.

The impact isn't always the same.

Studies on mule deer showed that mountain lions were a significant source of mortality, really impacting the deer population.

But wolves, in that specific study, didn't have the same effect.

Why not?

Don't wolves eat deer?

They do.

But analysis suggested the wolves were often taking very young, old, or weakened individuals, animals that might have died anyway, or had low reproductive potential.

So the wolves were culling, perhaps, rather than driving down the healthy breeding population like the mountain lions were.

Different predation styles, different impacts.

That's a subtle but important distinction.

And sometimes multiple factors work together with even bigger effects.

The snowshoe hair study you mentioned.

Yes.

The snowshoe hair study in Canada is a powerful example of interactive effects.

Researchers experimentally manipulated food supply and predator access for different hair populations.

What did they find?

Adding extra food or reducing predators each allowed the hair population to increase maybe two or threefold compared to controls.

Standard ecological effects.

But when they did, both added food and excluded predators.

The effect was bigger.

Way bigger.

An 11 -fold increase in hair density.

It wasn't just additive, 2x plus 3x equals 5x, it was multiplicative.

Food and safety together had a much larger synergistic effect than either factor alone.

It shows how different pressures interact in complex ways.

Wow.

Okay, so we've seen competition driving divergence and exclusion, and predation acting as a regulator and driving cycles.

What about interactions that aren't antagonistic?

Where does symbiosis fit in?

Symbiosis covers these more intimate, often long -term interactions between different species.

And they're not always negative.

The best known category is mutualism.

Where both species benefit.

Exactly.

Both partners gain something improved survival, growth, fitness.

The text gives the example of paramecium bursaria, a ciliate protozoan that houses chlorella vulgaris.

A photosynthetic green alga inside its cells.

The alga gets protection and materials.

The paramecium gets food from the alga's photosynthesis.

A microscopic, solar -powered animal.

Kind of.

And of course, the huge one is the microbiome in our own guts.

Hundreds of millions of bacteria, mostly mutualistic, helping us digest food, synthesize vitamins, even train our immune system.

We give them a home and food, they give us essential services.

Okay, so mutualism is win -win.

What about commensalism?

Commensalism means at table together.

It's a relationship where one species, the commensal, benefits, while the other species is essentially unharmed and unaffected.

Like barnacles on a whale.

Perfect example.

The barnacles get a free ride and a place to filter feed.

The whale probably doesn't even notice they're there.

Or birds that follow army ant swarms catching insects flushed out by the ants.

The birds benefit.

The ants are unaffected.

So, win -neutral?

Pretty much.

And there's a really intriguing specific type mentioned.

Related to plasticity.

Predator prey induced polymorphism.

That sounds complex.

How does that work?

It's fascinating.

In some aquatic systems, chemicals released by a predator can actually trigger the eggs of their prey, like water fleas or tadpoles, to develop into a different morph.

A different physical form.

A form the predator can't eat.

Often, yes.

Maybe they develop spines, or a different chase that makes them hard to handle or swallow.

The prey obviously benefits by surviving.

But interestingly, the predator might also benefit slightly by ensuring it doesn't completely wipe out his food

sourcely.

The chemical cue induces a defense that prevents over -exploitation.

Wow.

That's like chemical communication shaping development for mutual -ish benefit.

Could that be a step towards speciation?

It's possible.

If those morphs become reproductively isolated, it could be an initial stage.

It highlights how dynamic these interactions can be.

And speaking of fundamental interactions, we have to briefly mention endosymbiosis, right?

The ultimate partnership.

Absolutely.

We can't talk about symbiosis without acknowledging its role in the very origin of complex life.

Endosymbiosis, one organism living inside another, is how eukaryotic cells acquired key organelles.

Like mitochondria and chloroplasts.

Exactly.

Mitochondria were likely once free -living bacteria that got incorporated into an ancestral host cell, forming a mutualism that became permanent.

Same for chloroplasts, originating from photosynthetic bacteria.

This symbiotic integration was a pivotal step in evolution.

So symbiosis isn't just ecological interactions.

It's built into the very fabric of our cells.

Fundamentally, yes.

Okay, let's try to synthesize this, then.

We've covered a lot of ground from chapter 20.

Competition, predation, symbiosis.

And the core message, really, is that evolution isn't just about changes in gene frequencies and isolation.

It's profoundly shaped by these ecological interactions.

The struggle for resources, the dance of predator and prey, the collaborations of symbiosis.

These forces determine the fate of deems, the structure of populations, and ultimately the divergence of species.

So for you, the learner, thinking about evolution means thinking ecologically.

It means understanding how these pressures, competition, keeping species apart, or forcing change, predation regulating numbers and driving arms races, symbiosis creating novel partnerships, are constantly working right now, shaping life around us.

It connects the molecular level, like that BRUMP4 gene, to the entire ecosystem.

Absolutely.

And maybe a final thought to leave you with, something hinted at in the text.

When we see closely related species living in different places today, we often assume they've fought a competitive battle in the past to force them apart.

Like the finches.

Right.

But it's also possible they never really computed intensely.

Maybe they always had slightly different preferences or segregated into different niches early on, effectively avoiding the fight.

So the question becomes, are we looking at the scars of battle or the wisdom of avoidance?

A great question to ponder.

Did they fight their way apart or did they just agree to disagree from the start?

Something to think about until our next deep dive.

Thanks for joining us.