Unit 7: Natural Selection

Reviews mechanisms of evolution (natural selection, genetic drift, gene flow, mutation, non-random mating), Hardy-Weinberg equilibrium and its assumptions, types of natural selection (directional, stabilizing, disruptive), speciation, phylogenetic trees, and evidence for evolution.

Imagine you are you're watching a classic nature documentary the camera pans across the savannah Oh, I know exactly the scene you mean right you see a lion stalking through the tall grass Locking eyes with a herd of gazelles the chase kicks off dust goes flying everywhere and the lion takes down the slowest gazelle in the group you just sit back on your couch and think a Survival of the fittest yeah, it looks like a perfectly clean simple equation of you know speed strength and survival he's actually It feels incredibly intuitive we watch the fastest survive and the strongest win and we just assume we are witnessing the Well the entirety of evolution playing out right there on the screen like the organism with the best physical traits just gets to live another day But that classic documentary equation is actually missing the most critical variable completely missing it because surviving the lion taste is great for that individual gazelle But if that's surviving incredibly fast gazelle doesn't do one other very specific Thing its evolutionary impact is absolutely zero. So today we are doing a massive deep dive We are digging to a huge stack of biology sources video mega review transcripts study guides We even have a lab experiment on population genetics a lot of great material to go through our mission today is to shortcut you to a rock solid understanding of evolutionary biology We are going to unpack how populations actually change the mathematical forces driving those changes and How we map the massive history of life on earth and critically we are specifically dodging the most common traps that trip people up on AP exams Okay, let's unpack this because everyone thinks they understand evolution as survival of the fittest But is that phrase just a huge trap? Does it just mean the biggest and strongest survive? It is a massive trap and it's where a lot of misunderstandings begin especially on exams in everyday language fitness implies You know hitting the gym cardiovascular health or just being the biggest guy in the room right like physical physics Exactly, but biological fitness is entirely different. It is strictly defined by two interconnected achievements survival and reproduction So if an organism has some like Miraculous mutation that allows it to live to be a thousand years old. It outruns every predator. It survives every drought its immune system defeats every disease Yeah, but it never has any offspring you are saying its biological fitness is zero zero from an evolutionary standpoint that individual is a dead end of Evolution is defined as a change in the genetic makeup of a population over time, right? If you do not pass your alleles your specific gene variations on to the next generation You haven't contributed to that ongoing change at all the genetic blueprint that made that organism so resilient just dies with it Oh, wow, so the most fit individuals in a biological sense are simply the ones who leave the most viable offspring in the next generation It makes me think of a video game surviving the level dodging all the obstacles that just keeps you On the board. That's a great way to put it. You don't actually score any points or get on the leaderboard until you successfully pass your genes forward and The environment acts as the ultimate filter deciding which traits actually get those high scores The environment is a relentless filter and this filtering process is natural selection Depending on what the environment throws at a population natural selection generally pushes the genetic makeup in one of three distinct directions Okay, let's hear them Directional disruptive or stabilizing selection. Let's ground those directional selection makes sense intuitively, right? The environment favors one extreme of a variation exactly if we look at the classic peppered moth during the industrial revolution The trees in their habitat became covered in dark soot From coal pollution, right? So the background completely changed. Yeah, so the darker moths which used to stick out against light bark Suddenly had the camouflage advantage the lighter mocks just got eaten by birds And the rock pocket mouse follows that exact same pattern populations living on dark lava flows evolved dark fur to blend in Oh, right But the same species living on light sand maintained light fur the environment applied a singular pressure and the population shifted directionally to meet it Okay, what about disruptive selection? What's fascinating here is how disruptive selection is a bit more chaotic This happens when the environment favors both extremes of a treat but heavily penalizes the intermediate trait wait penalizes the middle so imagine a population of rabbits living in an area characterized by very dark volcanic rocks interspersed with very light quartz sand But absolutely no gray background perfect example the dark rabbits can hide perfectly on the lava rock The light rabbits vanish against the sand but a gray rabbit a gray rabbit sticks out everywhere right the predators basically form a search image For the gray rabbits because they are by far the easiest to spot Wow, okay the dark and light extremes survive and reproduce while that middle ground gets totally wiped out over time the population splits into two distinct groups then stabilizing selection would be the complete inverse of that the middle ground is the safest place to be Yes, human birth weight is a brilliant historical example of stabilizing selection before the advent of modern medicine and cesarean sections birth weight was heavily filtered by the environment because babies that were too small struggled to maintain body temperature and like fight-off infections exactly But babies that were too large caused life-threatening complications for both the infant and the mother during delivery Oh man, so both extremes were super dangerous exactly because both extremes carried massive survival risks the intermediate weight was heavily favored the genetic variants narrowed right around the safest middle ground Okay, but everything we just discussed implies that evolution is a logical response to a problem it creates a puzzle like soot on a tree and natural selection solves it by favoring the traits that fit right wait Does a population only evolve when it's actively adapting to something? What if an evolutionary change is just well? What if it's just lucky you are identifying a massive force in evolutionary biology right there genetic drift while natural selection is driven by environmental pressure genetic drift is a change in the allele frequencies of a population driven entirely by random chance just totally random it has absolutely Nothing to do with how advantageous or fit a trait is let's visualize this imagine a massive bucket filled with a million red beads and a million white beads representing a gene pool Okay tracking with you if you pull out handfuls to start a new generation The laws of probability say you'll likely pull out roughly half red and half white right because the sample size is huge But if your bucket only has ten beads five red and five white and you blindly pull out four to start a new population You might easily grab three reds and one white or even four reds Not because red is better just because of statistical noise The mathematical reality of genetic drift is that it has a profound sometimes devastating effect on small populations In a population of millions those random fluctuations just average out but in a small group in a small population a random event can eliminate an allele entirely Drift actively decreases the genetic diversity of a population which reduces its ability to inhabit to future challenges Examples really highlight the sheer brutality of genetic drift the first is the bottleneck effect This occurs when a disaster strikes a massive wildfire a novel disease or intense over hunting and just wipes out the vast majority of a population the cheetah is arguably the most famous victim of a severe bottleneck Cheetah populations have been decimated by historical climate changes habitat destruction and of course human hunting Yeah, the cheetahs alive today are descended from a remarkably small handful of survivors and the crucial point here is that those survivors didn't necessarily live because they had superior genes You might have just been on the side of the mountain that didn't catch fire. Exactly. They were just lucky So the surviving gene pool is essentially a random sample of the original population If the original population had 20 different variations of a gene related to the immune system But the five surviving cheetahs only happen to carry two of those variations those other 18 variations are just gone forever Wow Okay, what about the other form of drift? The other major form is the founder effect The bottleneck happens when the environment forcefully shrinks the population The founder effect happens when a small group of individuals voluntarily breaks off from a larger population to establish a new isolated colony The demographic history of the old order Amish in the US and Canada is a really striking look at the founder effect in humans This entire population originated from A remarkably small founding group roughly 200 immigrants from Switzerland and Germany in the 18th century right and due to their religious and cultural practices They generally marry strictly within their own community. They formed a totally closed genetic loop They did now out of those 200 original founders It just so happened by pure chance that a few individuals carried a recessive allele for a condition called Ellis found Kravald syndrome Which causes a specific form of dwarfism and heart defects, right? Yes in the broader global population This allele is incredibly rare, but because the Amish community grew entirely from that small pool of 200 founders the frequency of that specific recessive allele was artificially inflated So today the incidence of that syndrome is significantly higher there than anywhere else in the world precisely it wasn't natural selection favoring that trait It was just the geographic lottery of who happened to get on the boat exactly now to round out the engines of micro evolution We have genetic drift causing random changes natural selection filtering for Advantage gene flow, which is just individuals migrating and mixing their genes with neighboring populations and finally mutation Wait, let's contrast those yeah drift flow and selection just kind of shuffle things around right They only shuffle filter eliminate the alleles that already exist in a population mutation is the bedrock of it all because it creates something new yes a random error in DNA replication a mutation is the singular mechanism that can introduce a brand new allele into the world it is the raw material of evolution okay, but if we have all these engines running simultaneously Genes mutating animals migrating disasters wiping out populations. How do biologists ever manage to actually measure that change? It is tricky It sounds like trying to measure the movement of a single drop of water inside a turning blender Well, you solve that problem by defining the impossible you measure evolution by calculating exactly what a populations genetics would look like if evolution completely Stopped. Oh, this is the Hardy Weinberg equilibrium Yes, the famous Hardy Weinberg theorem to prove a population is not evolving that its allele frequencies are locked in perfect stasis generation after generation There are five incredibly strict conditions that must be met and you have to meet all of them right there can be absolutely no mutations There can be no gene flow meaning no one enters or leaves the population must be infinitely large so genetic drift doesn't occur mating must be 100 percent random Meaning no one is choosing a maid based on advantageous traits and there can be zero natural selection. That's the list wait I have to challenge the utility of this those five conditions Basically never exists simultaneously in the wild. Oh, they definitely don't mutations are a constant chemical reality Organisms are incredibly picky about their mates if this equilibrium never actually happens Why bother with the equation? It sounds like trying to measure the speed of a car by assuming it's parked the genius of the Hardy Weinberg Equilibrium is precisely that it is a parked car It serves as a mathematical null hypothesis biologists know the population is evolving right but to figure out how fast and in what direction they first calculate the expected genetic makeup of that theoretical perfectly parked population Then they go out into the field sequence the DNA of the actual living population and compare the two oh So the distance between the theoretical parked car and the actual moving car tells you the velocity of evolution it gives You the exact magnitude of genetic change, but understanding the mathematics behind this the whole p squared plus 2qq plus q squared equals one equation Requires a massive shift in how you think about genetics. Here's where it gets really interesting This is the big AP math trap Yes, it is people constantly conflate the frequency of an allele with the frequency of an actual walking organism Let's break down the mechanics in the Hardy Weinberg equations the single letters p and q represent the individual alleles floating around in the Total gene pool p represents the frequency of the dominant allele and q represents the recessive allele imagine dissolving a population down to just a massive vat of individual genes okay I'm picturing it if 60% of the alleles in that vat are the dominant p and 40% are the recessive q Those single letters represent the overall probability matrix of the gene pool, but biological organisms are deployed We carry two alleles for every trait one from our mother one from our father We aren't single letters. We are pairs of letters, right? So when the equation squared those variables to create p squared 2pq and q squared It is calculating the probability of assembling an actual living organism from that gene pool Let me make sure I have this if you reach into that vat of alleles where 40% are recessive in q is 0.4 and you randomly pull out two alleles to build an organism the probability of pulling two recessive alleles is 0.4 multiplied by 0.4 exactly that gives you 0.16 so q squared is 0.4 that means 16% of the actual physical individuals in the population will exhibit the homozygous recessive trait the logic collapse happens when someone looks at a population sees that 36% of the animals have the recessive trait and mistakenly assumes that q equals 0.36 that is the fundamental trap if 36% of the animals have the recessive trait that 36% represents the physical organisms that is your q squared oh man you mistakenly assign that value to the single allele q every subsequent calculation you make about the genetic makeup of that population will be mathematically shattered because if q squared is 0.36 you have to take the square root of that to find the frequency of the single allele in the pool the square root of 0.36 is 0.6 so the recessive allele makes up 60% of the gene pool even though only 36% of the animals physically show the trait it's a beautiful elegant piece of probability logic once you see the difference between the pool of genes and the actual bodies of those genes build it really is and once we track these shifting allele frequencies over thousands of generations we eventually hit a tipping point the genetic makeup of a population changes so drastically from its original form that we move from microevolution into macroevolution we reach speciation speciation is the tearing of one lineage into two distinct new species and the threshold for that break is defined by the biological species concept which states that a species is a group of populations capable of interbreeding and producing viable fertile offspring so the moment a population becomes reproductively isolated from its ancestors or neighbors a new species is born exactly the geography of how this happens usually falls into two categories allopatric or sympatric speciation allopatric means other homeland this is speciation driven by a physical geographical barrier the classic illustration is the squirrel population divided by the formation of the Grand Canyon oh yeah perfect example right eventually the genetic differences compound to the point where even if you built a bridge across the canyon the North Rim squirrels and South Rim squirrels could no longer successfully made two now sympatric speciation is conceptually much stranger it means speciation occurs without any physical barrier at all the populations live in the exact same geographic area share the same physical space but they become reproductively isolated anyway wait how does an invisible wall build itself in the middle of a shared habitat it happens through behavioral or temporal isolation imagine a population of birds living in the same forest a slight genetic mutation alters the mating song of a small group of males the females of the broader population don't recognize this new song so they just ignore those males only a few females with a corresponding mutation respond to it instantly an invisible genetic barrier is erected that's wild they share the same trees eat the same food but they're no longer mixing their genes exactly over time that initial behavioral split widens into complete genetic incompatibility or temporal isolation where the timing is off a mutation causes one group of plants to release their pollen in early April while the rest of the population releases pollen in late May right they are sitting right next to each other in the same soil but they are reproductively worlds apart now to track all these splits these millions of divergent paths over billions of years biologists construct massive family trees phylogenetic trees and cladograms map out shared derived traits to visualize how different lineages trace back to recent common ancestors which are marked by the nodes or the branch points on the diagram this is a huge trap on the AP test if I'm looking at a diagram and species A is printed at the very top tip of a branch right next to species B my brain instantly assumes they are like evolutionary cousins your brain is finding a pattern where none exists the physical layout of the tips is just an arbitrary choice made by the person drawing the diagram the only mathematical reality of a phylogenetic tree exists at the nodes the visual trick is to imagine the tree is a hanging mobile over a baby's crib you can grab any node any intersection and spin the branches 180 degrees yes I love that trick the species that was printed on the far left swings over to the right the physical order changes completely but the evolutionary meaning of the diagram hasn't altered by a single nucleotide to determine relatedness you must trace the lines backward from the tips down to the closest node where the paths of two species intersect that node represents their most recent common ancestor so the fewer nodes you have to travel through to connect to species the more closely related they are regardless of how far apart they are printed on the it's all about the nodes okay so we've laid out the theoretical frameworks we've built the mathematical models and drawn the sprawling family trees let's look at the physical receipts how do we prove these ancestral nodes actually existed well our understanding of evolution rests on several massive pillars of tangible evidence starting with anatomical structures we have homologous vestigial and analogous structures let's break those down homologous structures provide a physical map of divergent evolution right yes these are anatomical features found in entirely different species that share the exact same underlying blueprint but have been heavily modified by natural selection for different environments if you look at the skeletal structure of a human arm a dog's front leg a bird's wing and a dolphin's flipper the external functions couldn't be more different throwing running flying swimming but strip away the muscle and skin and you find the exact same architectural sequence in all of them one heavy upper bone two parallel lower bones a cluster of wrist bones and articulated digits that underlying blueprint was not designed from scratch for each environment it was inherited from a single ancient mammalian ancestor as lineages diverged into the ocean the air and the savannah natural selection simply stretched compressed and modified the existing homologous architecture to fit the new environmental filters now vestigial structures are even more haunting to me they are the evolutionary leftovers structures that serve a vital function in an ancestral species but have been rendered entirely useless by subsequent adaptation the most profound example is a whale is a masterpiece of hydrodynamics it has no hind legs and yet entirely detached from its spines just floating uselessly inside its flesh are the ghostly remnants of hip bones and thigh bones because the genetic instructions to grow those bones are still buried in the whales DNA it's a persistent receipt from its ancient land dwelling tetrapod ancestors we must be careful to contrast homologous and vestigial traits with analogous structures though analogous structures look similar and perform the same function but they evolved completely independently think about the wing of a bat and the wing of a bee they both saw the aerodynamic puzzle of flight but a bat is a mammal relying on bones and stretched skin while a bee is an insect relying on a kitten exoskeleton they didn't inherit the blueprint of a wing from a recent shared ancestor the environmental pressure of the air simply forced two entirely different lineages to arrive at a similar structural solution this is convergent evolution while the fossil record and anatomical structures are compelling the indisputable ultimate proof of evolution is found at the molecular level isn't it if we connect this to the bigger picture anatomy tells us a story but DNA provides the mathematical proof when we look down at the molecular machinery operating inside the cells of every living organism on earth we find an eerie universal consistency every living cell from the bacteria in the soil to the neurons in your brain uses the exact same metabolic pathway glycolysis to break down sugar for energy every organism utilizes DNA and RNA to store and transmit genetic information every eukaryotic organism whether it's a blade of grass a fungus or a human being organizes its DNA into linear chromosomes housed within membrane bound organelles the sheer statistical improbability of these hyper complex molecular systems evolving independently in millions of different species functionally zero the universality of the genetic code points to a singular inescapable conclusion l-u-c-a the last universal common ancestor every living thing on this planet is reading from the exact same fundamental biological instruction manual just with billions of years of edits mutations in environmental filtering applied to the text we're all descendants of a single origin point we have covered an immense amount of ground today we really have we completely dismantled the pop culture definition of fitness redefining it as the grim arithmetic of survival and reproduction we explore the chaotic random forces of genetic drift capable of rewriting a gene pool overnight we walk through the mathematical brilliance of the Hardy Weinberg null hypothesis making sure to keep the gene pool strictly separate from the physical organisms we spun the nose of phylogenetic trees and we sifted through the molecular evidence proving our shared ancestry right down to those whale pelvises yeah what does this all mean well it is a staggering elegant system but looking at the fossil record and ancient common ancestors can create a false sense of distance it is easy to trick yourself into viewing evolution as just a historical event something that happened in the deep past the process that built the world and then quietly stopped the reality is that the engine of evolution is running right now and in many cases it is accelerating when we think of natural selection we think of the environment applying the pressure today humanity is the heaviest environmental filter on the planet oh wow when industrial agriculture blends it's a field with chemical pesticides it is applying a massive instantaneous directional selection exactly we annihilate ninety nine percent of the insect population but that one percent that survives happens to carry a rare mutation granting them chemical resistance and they're the only ones left to breed the next generation isn't just a population of bugs it is a population composed entirely of resistant survivors we trigger the exact same evolutionary sprint when we overuse herbicides on weeds or critically when we misuse antibiotics on bacterial infections or utilize chemotherapy drugs on cancer cells we are forcing these organisms through brutal genetic bottlenecks we are acting as an unprecedented selective pressure forcing the evolution of hyperresistant organisms in real time right in front of our eyes the mechanisms we discussed today aren't theoretical models of the past they are the terrifying immediate realities of the present the profound question is whether our scientific ingenuity can outweigh pace the hyper evolution we are forcing into existence it fundamentally changes how you perceive the world around you the next time you watch that documentary you won't just see a lion chasing a gazelle across the savannah you will see a legal frequencies violently shifting in real time you will see genetic drift waiting to strike and billions of years of molecular receipts running through the tall grass you're not just watching survival you're watching the relentless ongoing algorithm of life itself