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Genetic variation, gene flow, and new species

Video transcript

(relaxing music snippet) - [Instructor] Natural selection is Darwin's central and most brilliant insight underlying the mechanism for evolution. But selection of what? What is the raw material upon which the selective forces in nature are acting? Although he had some ideas about it, what Darwin didn't know was that variation stems from differences in the generic information contained in each cell of every organism. It's a bit like an alphabet made of only four letters. Letters that can be arranged into words of almost any length. The genes can be viewed as words made of these letters. The sequences of nucleotides spell out codes that give orders to the cellular machinery to make the cell work. In fact, the genes help to make cells themselves, ultimately providing the coded information that builds the entire organism. The sequences of nucleotides are arranged in long molecules that have a long name. Deoxyribonucleic acid. I like to break down complex words to their roots. So, deoxyribo is the long 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 word means we're talking about the nucleotides, those four special molecules that connect the two sugary backbone chains and represent the letters of the genetic code. The specific order of the nucleotides makes the words, or genes, of the genetic code. Lastly, the term acid is the 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, with the complexity of organisms, there must be a lot of DNA that needs to be read, a lot of words in the genetic code that dictate those marching orders to build the organism, and to make it do all the things that it does. And there is! By some estimates, there are about 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 lengths from 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 long thin string that, as I said, is one long molecule, and with that much DNA packed into each cell, there's actually a way of organizing that potential mess, of preventing tangles. The DNA in the cells of each type of organism is arranged neatly into a species-specific number of packets or structures known as chromosomes. The number of chromosomes varies from species to species. Humans have 46 chromosomes, but dolphins have 44. A platypus has 52. A dove has 78. A mosquito has six bitey little DNA molecules. A slime mold has 12. Peas have 14. Rice has 24. The adder's-tongue fern has 1260. And there are some kinds of single-celled microbes that are said to have more than 15,000 tiny little chromosomes in each cell. You can see that chromosome isn't directly related to the complexity of the organism, 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 studying genes know that changes of various types happen in the genetic code itself, which introduces variations among individuals and among species. We call some of these changes mutations. Mutations are actual changes in the sequence of letters in the words that make up the genetic code. Changes in the nucleotides that make up the genes, and therefore, changes in the instructions that come from the DNA. Mutations happen regularly through mistakes in replicating or reproducing the DNA during cell division from chemicals that can interfere with the structure of DNA, 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 change pieces of a chromosome, or even the whole chromosome. Mutations result in different forms of the same gene. These different forms are called alleles. For example, eye color is coded 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 in the traits of organisms. In their body shape, their metabolism, their behavior, and any other genetically-determined feature or process. Therefore, it's not surprising that every individual in a population is unique. Every individual is composed of a complex mix of many many traits, and behind those traits, there can be many many different alleles. But how do the alleles get distributed to the offspring? That's what Darwin wondered too, and now we have to talk about sex. But unlike Darwin, our discussion of sex can center on how variations in genetic information can get passed on to offspring. In the process of making the sperm 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 chromosomes with new combinations of alleles, and when these genetically-varried sperm and eggs come together at fertilization, the result is a bunch of offspring that are genetically unique individuals. Even bacteria, which don't have sex in the same way as organisms with males and females do, have similar processes going on that continuously reshuffle the genetic deck during reproduction, allowing lots of variation in their offspring as well. And remember, it's those individual differences that are the focus of the selection process, because some of these recombinations are gonna make an individual more fit than others, more able to survive, and more able to have offspring off their own. And that's where natural selection 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 the survival of the fitter. So we've seen how sexual reproduction can lead to tons of individual variation within a population, and how populations will change over time as a result of natural selection. But ironically, at the same time, when there is widespread breeding among members of a population, the resulting mixing of genetic information within the whole population also means that individuals within a population don't diverge too much from each-other in form or behavior or physiology. This will also mean that one population in a species doesn't differ too much from another in a particular species. This mixing of genetic information among interbreeding members of a population or species is known as gene flow. It maintains enough consistency among individuals and populations of the species that members can still reproduce with one another. What happens if gene flow is slowed down or somehow prevented? Imagine a population in which sexual reproduction and variation is happening all the time, and then some barrier occurs that separates this population into two parts? A really famous example is when the oceanic water levels dropped enough millions of years ago to allow the Isthmus of Panama to become a complete strip of land, separating the waters of the Eastern Pacific from the Caribbean Sea. Species of marine organisms that had ranges extending to both sides of the isthmus now encountered a barrier that kept some members from being able to breed with others. However, individuals on either 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-populations continued to do that without the influence of the genes in the sub-population on the other side of the isthmus barrier. Therefore, we now have what is called a restriction of gene flow between the two separated groups. What had once been a single interbreeding population has become two separate populations without gene flow between them because of the barrier. Scientists know of many examples of marine species on one side of the Isthmus of Panama that have, as their closest relatives, a second sister species on the other side of this important barrier. These related pairs of species even have a special name: Geminate species, from the same ancient Latin root as in gemini, or twin. With time, and that's the crucial ingredient here, time, enough time to make generations of reproduction, the two populations diverge in their traits. This can happen by random changes that occur on either side of the barrier, but sometimes, the environmental conditions on either side of the barrier may be slightly different, creating different selection forces for the two populations, serving to accentuate the difference between the two populations over time, and at some point, the two populations will have 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 the accumulation of changes within a single species, so that, at some point, you say that particular species has somehow transformed wholesale into a new species. Instead, it is the splitting of a single species into two descendant species. All of this happens randomly 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 variability in the genetic code, and therefore, in the traits of individuals. Environmental circumstances select for or against the genetically-encoded traits. Traits that are selected for are passed on to succeeding generations. But the true wonder of it all is the result. As populations diverge and continue to diverge over time, a branching tree full of ancestor and descendant species is formed and keeps growing. This is the tree of life, full of the diverging species that make up the endless forms most beautiful that Darwin talked about. In other words, you end up with biodiversity.
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