So the bottom line is this: individuals can be selected, if the conditions are right, but they don’t evolve. This selection means that only some of the parents reproduce, so each successive generation looks as a group very different from the parental population as a group. The group, the population moving through generational time, is the entity that evolves. Sorry, you rugged individuals, but that’s the way the game of life is played.
Because you, as an individual, don’t evolve, passing on your genome—making babies—is the best you can hope to do in the evolutionary game of life. Individual life-forms can make babies in two ways. They can make multiple copies of themselves that have nearly identical genomes, a process that biologists call cloning, or asexual reproduction. Individual life-forms may take a second path, sexual reproduction, in which the individual produces eggs or sperm, known collectively as gametes, and engages in some process to put their gamete in close proximity to another gamete from the same kind of life-form. Most plants and animals reproduce sexually. Plants do this, as your parents told you, with flowers and pollen, sometimes with an animal, like a bee, acting as the intermediary. Animals reproduce sexually either by spawning or by depositing gametes in their partners.
Generally speaking, sexual reproduction brings together the genomes from two different individuals into one new individual; it is thought to be better than asexual reproduction in producing offspring that are variable (although some plants can fertilize themselves). Both asexual and sexual reproducers can also have mutations—changes in the genetic code—that can be passed on. To be passed on, the mutations have to occur in the cells that will make the offspring. For sexual reproducers that means mutations have to occur in the cells that create gametes. The making of gametes, a process known as gametogenesis, has several important features. One is that each gamete gets only half the parent’s genetic material, one from every pair of chromosomes (in humans, most cells have twenty-three pairs of homologous chromosomes, for a total of forty-six, and gametes have just twenty-three unpaired chromosomes). Another is that during gametogenesis a process known as crossing over, or recombination, occurs; essentially this means that the genetic material from one chromosome in a pair is shuffled to the other before the pair is split up and delivered to separate gametes. The result is both new (mutated and/or recombined) genes, and new combinations of genes in every gamete produced.
Sexual reproduction’s secret weapon is the final twist: bringing together sperm and egg. When sperm and egg meet, they create a single cell, called a zygote, which has half the genome of each parent. You can see right away why offspring from sexually reproducing parents are different and why sexual reproduction is such an excellent means of producing the variable populations required for evolution by natural selection to happen.
You’ve got enough information now to figure out how you can detect evolution in action. Think about measurement. What could you measure? Keep in mind that you’ve got to measure features of the population. You need to sample individuals and claim, usually with statistical reasoning, that the individuals you sampled represent the whole population. Or better yet, measure every individual in the population, as Rosemary and Peter Grant have done with ground finches on the island of Daphne Major in the Galapagos.
If you head out to Daphne Major with the Grants, you’ll see that they net finches, weigh them, and quickly measure the size and shape of their bodies with a pair of calipers, which is basically a high-resolution ruler.[5] They tag each individual with colored bands so that they can keep track of them. They spend hours and days observing males and females nesting together as the birds select and process food, lay eggs, and feed chicks. They measure and tag the chicks. The mountains of data, collected over years, are then analyzed for things like the average length of the bill in that generation of birds. The Grants can then look at how the average length of bills (and many other features) changes from generation to generation. They can also measure how the variability, what statisticians call variance, of the length of the bill changes over generational time.
When the average and/or variance of the length and thickness of the bill changes from one generation to the next, it is the first clue that natural selection and other evolutionary forces are at work in this particular population at this particular time and place. These visible physical and behavioral features of the birds are what biologists call “phenotypes.” Any phenotype may or may not have a genetic basis. If bill length has, at least in part, a genetic basis, then the change in the average bill length over generational time is evolution. The change in the average and variance of a phenotype within a population is one way to measure evolutionary change.
You can see, though, that we can run into trouble if we forget about our the conditions for evolution by natural selection. What if the phenotype doesn’t have a genetic basis? What if individuals learn some new trick that isn’t genetic? We can measure changes in the presence of the trick from generation to generation, so we think we are measuring evolutionary change, but upon closer inspection we find that the transmission of the behavior occurs by parents teaching their young how to do it. Orcas, for example, teach members of their pod how to specialize in hunting. Members of some pods eat otters. Members of other pods eat seals. Because we can observe the old teaching the young, we know that the tricks of the trade are learned rather than inherited genetically.
One way out of this problem is to focus, as many evolutionary biologists do, on the genotype. If the genes present in a population change, then we know that evolution has happened. We would do this by focusing on alleles. An allele is any particular version of a gene. If you have a gene that produces a protein, two different alleles of the gene may cause the protein to have a different shape or other properties. Every allele can be described as occurring in the population as a proportion, p, of all varieties of a specific gene. The change in p, where we indicate change using the Greek letter capital delta, Δ, is Δp (read out loud as “delta-p” or “change in allele frequency”). This gives a quick shorthand for measuring evolution: Δp 
0. If the proportion of an allele changes in a population over generational time, then we have evolution in action. Game on!
To be fair, here, we ought to use the same sexy mathematical notation for phenotypic change. The population’s average (what statisticians call “mean”) value of a trait is represented mathematically by 
(read out loud as “X bar” or “mean of the trait”). As long as this trait, like the length of a finch’s bill, has some genes that determine it, then we have another shorthand for measuring evolution: Δ 0. If the mean of a phenotypic trait changes in a population over generational time, then we may also have evolution in action.
To prepare you for the robotic world you’ll encounter in the upcoming chapters, I should mention at this point that one of the great things about creating your own evolutionary world is that you get to do things like predetermine how genes relate to phenotype. Rather than having to worry about how heritable a phenotype trait was, we just decided that genetics would control entirely every trait, X, and every variation of X. Thus, any phenotypic changes that we might see in a population would have a direct and proportional genetic underpinning: Δ
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You can take this journey by reading Jonathan Weiner’s book