Video transcript
(relaxing music snippet) - [Instructor] Naturalselection is Darwin's central and most brilliant insight underlying the mechanism for evolution. But selection of what? What is the raw materialupon which the selective forces in nature are acting? Although he had some ideas aboutit, what Darwin didn't know was that variation stems fromdifferences in the generic information contained ineach cell of every organism. It's a bit like an alphabetmade of only four letters. Letters that can be arranged into words of almost any length. The genes can be viewed aswords made of these letters. The sequences of nucleotidesspell out codes that give orders to the cellularmachinery to make the cell work. In fact, the genes helpto make cells themselves, ultimately providing the coded information that builds the entire organism. The sequences of nucleotidesare arranged in long molecules that have a long name. Deoxyribonucleic acid. I like to break down complexwords to their roots. So, deoxyribo is thelong molecule's backbone of special sugar molecules call deoxyribos that are joined together to make a pair of twisted long parallel chains. The nucleic part of the wordmeans we're talking about the nucleotides, thosefour special molecules that connect the twosugary backbone chains and represent the lettersof the genetic code. The specific order of thenucleotides makes the words, or genes, of the genetic code. Lastly, the term acid isthe chemical classification for the entire huge molecule,because this whole thing is actually, chemically speaking, an acid. So it's deoxyribonucleic acid. And that's how I spell DNA. You can imagine that, withthe complexity of organisms, there must be a lot ofDNA that needs to be read, a lot of words in thegenetic code that dictate those marching ordersto build the organism, and to make it do allthe things that it does. And there is! By some estimates, there areabout two to three meters of DNA in each cell of a human, and that's just in a single cell. If you add up DNA lengthsfrom all the cells in a human, that's roughly a long enough string of DNA to go back and forth, back and forth between the Earth and the Sun 70 times. So this is a very very long string, but it's a very very longthin string that, as I said, is one long molecule, andwith that much DNA packed into each cell, there'sactually a way of organizing that potential mess,of preventing tangles. The DNA in the cells ofeach type of organism is arranged neatly intoa species-specific number of packets or structuresknown as chromosomes. The number of chromosomesvaries from species to species. Humans have 46 chromosomes,but dolphins have 44. A platypus has 52. A dove has 78. A mosquito has six biteylittle DNA molecules. A slime mold has 12. Peas have 14. Rice has 24. The adder's-tongue fern has 1260. And there are some kindsof single-celled microbes that are said to have more than 15,000 tiny little chromosomes in each cell. You can see that chromosomeisn't directly related to the complexity of theorganism, and you can also see that species vary genetically,but it's important to note that individuals within a species do too. So why do individuals vary? Scientists studyinggenes know that changes of various types happen inthe genetic code itself, which introduces variationsamong individuals and among species. We call some of these changes mutations. Mutations are actual changesin the sequence of letters in the words that makeup the genetic code. Changes in the nucleotidesthat make up the genes, and therefore, changes in the instructions that come from the DNA. Mutations happen regularlythrough mistakes in replicating or reproducing the DNAduring cell division from chemicals that can interfere with the structure ofDNA, and from radiation. Though many of these factors are natural, they can be human-driven as well. The bottom line is that mutations can delete or change nucleotides. They can even changepieces of a chromosome, or even the whole chromosome. Mutations result in differentforms of the same gene. These different forms are called alleles. For example, eye color iscoded by different alleles of the same gene. When the DNA's instructions are read by the cell's machinery,these differing alleles can cause variations inthe traits of organisms. In their body shape, theirmetabolism, their behavior, and any other genetically-determinedfeature or process. Therefore, it's not surprisingthat every individual in a population is unique. Every individual iscomposed of a complex mix of many many traits,and behind those traits, there can be many many different alleles. But how do the alleles getdistributed to the offspring? That's what Darwin wondered too, and now we have to talk about sex. But unlike Darwin, ourdiscussion of sex can center on how variations in genetic information can get passed on to offspring. In the process of making thesperm cells and egg cells used in sexual reproduction, a huge amount of genetic recombination occurs. A kind of reshuffling of the genetic deck. This results in chromosomeswith new combinations of alleles, and whenthese genetically-varried sperm and eggs cometogether at fertilization, the result is a bunch of offspring that are genetically unique individuals. Even bacteria, which don'thave sex in the same way as organisms with males and females do, have similar processesgoing on that continuously reshuffle the geneticdeck during reproduction, allowing lots of variationin their offspring as well. And remember, it's thoseindividual differences that are the focus ofthe selection process, because some of theserecombinations are gonna make an individual more fit thanothers, more able to survive, and more able to haveoffspring off their own. And that's where naturalselection comes in. Removing, selecting against the non-viable and less fit individuals. Or, on the flip side,selecting for the individuals that are more viable, more fit. That's the idea behind thesurvival of the fitter. So we've seen how sexualreproduction can lead to tons of individual variationwithin a population, and how populations will change over time as a result of natural selection. But ironically, at the sametime, when there is widespread breeding among members ofa population, the resulting mixing of genetic informationwithin the whole population also means that individualswithin a population don't diverge too much from each-other in form or behavior or physiology. This will also mean that onepopulation in a species doesn't differ too much from anotherin a particular species. This mixing of geneticinformation among interbreeding members of a population orspecies is known as gene flow. It maintains enoughconsistency among individuals and populations of thespecies that members can still reproduce with one another. What happens if gene flow is slowed down or somehow prevented? Imagine a population inwhich sexual reproduction and variation is happening all the time, and then some barrieroccurs that separates this population into two parts? A really famous example iswhen the oceanic water levels dropped enough millionsof years ago to allow the Isthmus of Panama to becomea complete strip of land, separating the watersof the Eastern Pacific from the Caribbean Sea. Species of marine organismsthat had ranges extending to both sides of the isthmusnow encountered a barrier that kept some members frombeing able to breed with others. However, individuals oneither side of the barrier continued to reproduce among themselves and continued to have variable offspring that were selected for or against. But each of the sub-populationscontinued to do that without the influence of thegenes in the sub-population on the other side of the isthmus barrier. Therefore, we now have whatis called a restriction of gene flow between thetwo separated groups. What had once been a singleinterbreeding population has become two separatepopulations without gene flow between them because of the barrier. Scientists know of manyexamples of marine species on one side of the Isthmusof Panama that have, as their closest relatives,a second sister species on the other side ofthis important barrier. These related pairs of specieseven have a special name: Geminate species, from thesame ancient Latin root as in gemini, or twin. With time, and that's thecrucial ingredient here, time, enough time to makegenerations of reproduction, the two populationsdiverge in their traits. This can happen by randomchanges that occur on either side of the barrier, but sometimes, the environmentalconditions on either side of the barrier may be slightly different, creating different selectionforces for the two populations, serving to accentuatethe difference between the two populations overtime, and at some point, the two populations willhave diverged enough in their traits that they're recognizable as two different species. It's this divergence that's really crucial in understanding how evolution happens, and how new species are formed. That's what we're talking about here, speciation. This, for me, is the stuff of evolution. Speciation is not theaccumulation of changes within a single species, so that, at some point, you say that particularspecies has somehow transformed wholesale into a new species. Instead, it is the splittingof a single species into two descendant species. All of this happensrandomly by recombination and by mutation. Evolution has no goal,it has no direction. I like to tell my students, stuff happens. The stuff is just random events. Mutations and new combinations of alleles that produce variabilityin the genetic code, and therefore, in thetraits of individuals. Environmental circ*mstancesselect for or against the genetically-encoded traits. Traits that are selected for are passed on to succeeding generations. But the true wonder ofit all is the result. As populations diverge andcontinue to diverge over time, a branching tree full ofancestor and descendant species is formed and keeps growing. This is the tree of life,full of the diverging species that make up the endlessforms most beautiful that Darwin talked about. In other words, you endup with biodiversity.
