EEB 122.?Principles of Evolution, Ecology and Behavior

Lecture 01. The Nature of Evolution: Selection, Inheritance, and History?

https://oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122

Biological evolution has two big ideas. One of them has to do with how the process occurs, and that's called microevolution. It's evolution going on in your body?right now.?You've got about 10^13?bacteria in each gram of your feces, and they have enough mutations in them to cover the entire bacterial genome. Every time you flush the toilet, you flush an entire new set of information on bacterial genomes down the toilets. It's going on all the time.

Now, the other major theme is macroevolution. This process of microevolution has created a history, and the history also constrains the process. The process has been going on for 3.8 billion years. It has created a history that had unique events in it, and things happened in that history that now constrain further microevolution going on today.?The?process of microevolution is going to be the first thing we examine. It's the nuts and bolts. It's what's really created the patterns. But the patterns of macroevolution are also very important because they record the history of life on the planet and they constrain the current process.?

So where did this idea of evolution come from??

• You can go back to Aristotle and find elements of evolutionary thought in Aristotle. But really it's a nineteenth century idea, and in order to see how it developed let's go back to about 1790 or 1800; so at the end of the Century of the Enlightenment.?

• When Alexander von Humboldt, who?was certainly a creature of The Enlightenment, sets out to explore South America, he thinks that he might encounter some of those strange fossils, that the French have been turning up in the Paris Basin, on top of Tepuis in Venezuela. So he really thought that there was a lost world. Of course, Arthur Conan Doyle later wrote a novel about that.?

• They thought that adaptations were produced by divine intervention. They did not think that there was a natural process that could produce anything that was so exquisitely designed as your eye.

• By the time that Darwin published his book in 1859, people thought that the world is hundreds of millions of?years, not?yet in the billions range.?We now know about four-a-half billion, but at that point, based on the rate of erosion of mountains and on the saltiness of the ocean, assuming that the ocean had been accumulating salt continuously, and that it hadn't been getting buried anywhere, which it does, people thought hundreds of millions of years.

• They knew that fossils probably represent extinct species. That was Cuvier's contribution. He did it for mammal fossils in the Paris Basin. Geoffrey Saint-Hilaire had had a big debate with Cuvier about homology, and that was in 1830.?Basically it was about the idea that Geoffrey Saint-Hillaire had had that if my hand has five fingers then--and a bat's wing has five fingers and the fin of a porpoise has five fingers--that that indicates that we all got those five fingers from a common ancestor, and therefore we are related because we had a common ancestor.?

• Malthus's book had come out in 1798. Malthus said basically that populations grow exponentially but agriculture grows linearly. Therefore populations will always outstrip their resource base. This convinced Darwin that all organisms are in a competitive struggle for resources, and that that must inevitably be the case.?

So now I'm going to give you a brief overview of microevolution and macroevolution. Here's Natural Selection; here's Darwin's idea. In a population, there is variation in reproductive success, which basically means that?different families have different numbers of offspring, or different individuals have different numbers of offspring. Then there has to be some variation in a trait. There is a non-zero correlation between the reproductive success and the trait. Then there has to be heritability for the trait.?We can turn natural selection off by violating any of these four points.?

1. If there's no variation in reproductive success--for example, if there is lifetime monogamy and a one-child policy, there will be zero-variation in reproductive success if everybody just has one child; of course some people will still have zero, but that's about as close as you can get.

2. If there's no variation in the trait--if the trait is like five fingers; there are very few people with six fingers; there are some, but very few.

3. If there's a non-zero correlation between reproductive success and the trait; if there is a zero correlation between reproductive success and the trait. We'll go into all the conditions for that. That results in neutral evolution and then?things just drift.

4. ?If the trait is not heritable, if there's no genetic component to it, then it won't evolve.?

Natural Selection does not necessarily happen. It only happens under certain conditions. Both?are driven by variation in reproductive success. The difference is in whether there's a correlation between the variation of the gene or the trait and the variation in reproductive success.?

• If there's variation in the trait, represented on the X-axis, and there's variation in reproductive success, based on the Y-axis, and there is a correlation between the two, represented by the fact that I can just about draw a straight line between these points, Natural Selection will occur and it will push the trait to the right.?This situation produces adaptation, it produces all of the fantastic biology that you're familiar with. It's produced?meiosis; it's produced your eye; it's produced your brain.

• If all of these conditions, except the correlation, occur--so you have variation in the trait, variation in reproductive success but no correlation--then you get random drift. The random drift situation, is what connects microevolution to phylogenetics, and it's what allows us to use variation in DNA sequences to infer history. We?need to have a process of drift in order to generate a kind of large-scale regularity that gives us timing and relationship in macroevolution.?

Drift isn't such a morphologically or artistically beautiful thing. It's a mathematically beautiful thing. Drift happens whenever there is no correlation between reproductive success and variation in a trait, and it produces patterns like this.?

• Here we start off with 20 populations, and we start them all with a gene frequency of 0.5, and we let meiosis--which is like flipping a fair coin--and we let variation and reproductive success take their course.

• We just run these populations for 20 generations, and you can see that there's just about an equally likely distribution of end-states out here.?

• If any of these populations happens to get up to 1, or down to 0, in terms of gene frequency, the process will stop, because those are absorbing states.

• If the frequency becomes 1, then everybody's got it and there can't be any change, and if the frequency becomes 0, then nobody's got it and there can't be any change. So that's what's meant by absorbing state.?

The themes of microevolution are selection and drift. Natural selection is driven by variation in reproductive success. The strength of selection is measured by the correlation of variation in a trait with reproductive success. When there's no correlation, there's no systematic change, and then things just drift.

• We see design in the whole organism and?we see noise in the genome--to a rough cut; lots of?exceptions. Whole organism traits are the products of Natural Selection, maybe not in the immediate past, but usually at some point in the history of life. There are DNA sequences that have clear selective value.?

• A?lot of DNA sequences have been shaped by drift.?There are whole organism traits that have no apparent selective value; for example, the chin. It could just be a byproduct of something that was going on, basically from the mouth up. The developmental process that originally produced them didn't have to be adaptive.

Now macroevolution; the big scale process, the big picture.?There's one tree of life. Everything on the planet had a common origin and?is related to everything else, with the possible exception of the viruses, whose genomes?are too small for us to decide.?The branch points in the tree, speciation events--that's when new species were formed.?This history is marked by striking major events, such as?mass extinctions or even?meteorite impacts. There have been major changes in the organization of the information structure of life.?

• The root's at about 3.7 billion years. What you see here are the three kingdoms of life, which are the bacteria, the archaea, and the eukaryotes; And at one point a purple bacterium got into the eukaryotes and became a mitochondrion, and at another point a cyanobacterium got into various plant lineages, three times, and became a chloroplast.

• For the?first two billion years of life most of the action is down in the basal radiation. So going on with bacteria, archaea and eukaryote ancestor; single-celled things. At that scale--we're just way up at a small twig on the?tip--and symbiotic events brought mitochondria and chloroplasts into?eukaryotic cells.?

• Multi-cellularity looks like it originated around 800 million to a billion years ago. And these are the fungi, these are the things we call the plants, multi-cellular plants, and then off in this direction we have got a fairly complicated series of branches that end up with us being up here.

• We?are not unitary?genome but?a community of genomes. We've got that mitochondria in us.

The main themes are basically that the speciation events that have occurred, particularly over the last billion years or so, have created a tree of life that describes the relationships of everything on the planet.?Systematic biology, phylogenetics,?tries to infer the history of life by studying those relationships?because the relationships define the history.?We work with hypotheses about history, and we test these hypotheses against each other and try to come up with the one that's most consistent with the data that we've got. And they give us a historical framework within which we can then interpret what's happened. There are major events that have happened. Briefly these are they.?

• Life originates about 3.6 to 3.9 billion years ago,?within probably about 100 million years?after water could exist on the surface of the planet in liquid form--so following the meteorite bombardment, when the surface of the planet cools down enough for water to be liquid--life probably originates pretty quickly. And arguably, within the first hundred generations, the first parasites were around.

• Then eukaryotes and meiosis, which is how a biologist refers to organized sex, happened about 1.5 to 2.5 billion years ago.

• Multi-cellularity, which gives us developmental biology, about a billion years ago.

• All the major body plans for animals appear to have, with the exception perhaps of the?jellyfish and a few of their relatives, they all seem to have originated about 550 million years ago.

• There was a near loss of life on the planet in the Permian mass extinction.?It seems to have occurred basically by a process of poisoning of the oceans.

• The flowers radiate about between 65 and 135 million years ago.

• Once language occurs, then we have an independent kind of information transmission from generation to generation; we get cultural transmission. That's probably about 60-100,000 years old; at least with syntax and complicated information storage.

• Writing is only about 6000 years old.

So this is a view of life that goes from bacteria to dinosaurs to rock and roll; and that all can be studied with evolutionary principles. How do the biological disciplines map onto this? There is a?temporal?assembly of?biology, as a discipline, as well as there is of life, on the planet.?

• Microbiology and biochemistry try to study things that are common to all life. ?That means that the same chemical reactions that go on in bacteria go on in the human liver, and that's about one-and-a-half to four billion years old. ?

• Genetics and cell biology study stuff that follows the evolutionary invention of meiosis; to a large degree.

• There is bacterial genetics, but eukaryotic genetics is something which is studying things that are about 1.5 billion years old.

• Developmental biology and general physiology, those are multi-cellular disciplines; they depend upon the existence of a multi-cellular organism. That thing didn't come along until about a billion years ago.

• Neurobiology, you need a complex--you need cephalization--you need to have a complex nervous system. That studies phenomena that are probably about 500 to 600 million years old. Same for behavior.

• Anthropologists probably originated along our branch of the tree, within the last 15 to 20 million years.

So the key concepts from this lecture are that there are two kinds of explanation in biology. One is the proximate or mechanical question, which is answered by studying how molecules and larger structures work. Those are basically physical and chemical explanations. And then there are the evolutionary questions, which is why does the thing exist; why did it get designed this way? And that could be answered either through selection or through history; or the best way to do it is to use both and combine those explanations.?

The thing that distinguishes biology from physics and chemistry is Natural Selection. This is not a principle that you can find in a physics textbook or in a chemistry textbook. This is something that is a general principle that actually applies to lots of things besides biology, but it's not contained within physics and chemistry. And there is a pattern in biology that unites biology with geology and astronomy, and that's history. So there is an important element of historical thought in evolutionary biology, as well as the more abstract action of natural selection on designing organisms for reproductive success and shaping changes and gene frequencies.?

We are continuous with non-life. Every step of the way there has been a parent. 3.9 billion years ago something extremely interesting happens. You pass through the origin of life, and there's no parent anymore. At that point you are connected to abiotic matter. ?Now this means that not only does the tree of life connect you to all the living things on the planet, but the origin of life connects you to the entire universe. That's a deep thought. Every element in your body, which is heavier than iron, and you need a number of them, was synthesized in a nova, uh, supernova. The planet that you're sitting on is a secondary recycling of supernova material, and your bodies are constructed of that stuff and they use it in some of their most important processes.?

So the vision that evolutionary biology gives you is not only the practical one of how to think about and analyze how and why questions in biology, it's also a more general statement about the human condition.