Measuring the Strength of Selection
Evolution by natural selection relies on heritable variation and the strength of selection. For that reason, examining those two different factors is important. Scientists can easily measure how phenotypes vary in a population. What they don't know as easily is how much of this variation is heritable. But they can find out. Figure 7-1 shows the variation of a particular quantitative continuous pheno-typic trait in a population in this case, height. The measured heights fall along the x-axis....
Species Concept for Bacteria
The biological-species concept, explained in an earlier section, classifies species based on their ability to mate and reproduce. That system is all well and good for organisms that mate and reproduce, but it leaves lots of other organisms out in the cold. Bacteria are perhaps the best examples of organisms that reproduce without mating. They simply divide into two daughter bacteria, each of which goes happily on its way, dividing further. Mutations can occur in this process, just as in any...
Reconstructing the history of hominid evolution
With all these hominid species, scientists are still trying to figure out which species may have given rise to which other species along the lineage that led to humans. Because no one can say with certainty that a fossil represents a common ancestor of two other species (the fossil may represent a closely related dead end, for example), this task is fairly challenging. Figure 16-2 shows a hypothetical evolutionary pathway from an ancestral species similar to the extant apes and leading to...
Drawing the Tree of Life Branching Patterns and Speciation
Scientists show phylogenetic relationships by drawing phylogenetic trees. If you trace the process of speciation on paper, you end up with a branching pattern that's referred to as the tree of life. Each branch in the tree represents a speciation event, when one species evolved into another. The easiest way to explain phylogenetic trees is to start with the simplest tree possible one with three species, which I'll call species A, B, and C. Imagine that you know that species B and C are more...
The age of the Earth
Although earlier scientists didn't have the tools to date the age of the Earth as we do today, they understood that the lower bands (and the fossils in them) were older than the higher bands and their fossils. Still, Darwin had no idea how immensely old the Earth was or how long the evolutionary process had been going on. Even when people began to understand that the world was quite a bit older than previously thought, they couldn't give an exact age to it. Dating the age of the Earth...
Understanding the Importance of Phylogenetic Classification
Scientists in general like to classify things, and they've been doing it for centuries. Why Not because it makes them feel good (although it does), but because it helps keep things neat while at the same time providing a wealth of information. The simple process of sorting reveals patterns and relationships, and gives clues to past events revelations that are absolutely fundamental to the study of evolution. This information is so important to evolutionary biologists, in fact, that they've come...
Selecting for nicer chickens Applications of group selection
Although group selection can seem awfully technical, it has practical applications. Take, for example, egg production. A great deal of effort has gone into producing chickens that lay lots of eggs. Historically, farmers have tackled the problem at the level of the individual chicken. The chicken breeder would found the next population with the descendants of the most prodigious egg-layers. The strategy was successful Chickens can lay 100 eggs a year. Here's a little bit of detail that you...
Influenza One Flu Two Flu Your Flu Bird
Before the mid-20th century, people gave this disease the respect it deserved. Now we call it the flu act fairly cavalier about the symptoms unless we need a break from work and poo-poo the vaccine unless we're in a high-risk group populated by babies, octogenarians, and hypochondriacs. Periodically, though, we get reminded that the flu can be serious business a fact that epidemiologists and virologists have been trying to beat into our thick skulls forever and then we fly into a...
Going in Circles Ring Species
Through the existence of ring species, scientists can say with 100 percent certainty that small differences can accumulate in nature to the point that two populations of the same species can become reproductively isolated. They can actually go out and see it. Variation exists among different populations of the same species. The following terms are used to describe the different types of variation I Subspecies A group within a particular species that shares genetic characters with other group...
Reconstructing Trees A How To Guide
Reconstructing a phylogenetic tree involves searching for the clues that tell you about the relationships among different species and then applying rigorous, and often quite complicated, analytical techniques to turn your pile of clues into your best hypothesis about the actual tree. This process boils down to essentially three steps 1. Identify and analyze characters shared by the species for which you're constructing the tree. 2. Use outgroup analysis to determine whether each character state...
Your fitness your relatives fitness inclusive fitness
Inclusive fitness is simply the sum of an individual's fitness plus the additional benefits accrued through increasing the fitness of related individuals. In English You're more fit not only if you reproduce, but also if relatives who share your genes reproduce, too. Here's a simple example of inclusive fitness in action Helping your identical twin sister have a baby that she would not have been able to have without your help is just as good a way to pass on your genes to the next generation as...
Sexual Selection The Art of Picking a Mate
For sexually reproducing species, it's not enough just to find food and dodge predators. You also have to get a mate, because the prime directive, evolution-arily speaking, is to pass on your genes. How you go about choosing the best candidate to help you do just that is the topic of sexual selection. Sexual selection refers to choosing certain characteristics over others when looking for a mate, and it's a subcategory of natural selection the process by which heritable traits that promote...
Being sneaky Alternative male strategies
In the evolutionary process, there is often more than one way to accomplish the same task. As biologists continue to study systems in which mating success seems to be determined by the outcome of male-male competition, they're discovering some interesting alternative male strategies. One example is the sneaky strategy. Scientists speculate that the sneaky strategy is a way for younger males to have some chance of reproductive success before they get old enough to bark with the big dogs. Males...
Infanticide
When male lions take over a pride of female lions, they kill or attempt to kill the cubs. They don't indiscriminately kill baby lions, however. They leave their own offspring alone but kill cubs fathered by the previous dominant male. Killing the cubs sired by the previous dominant male increases the new male's fitness, because it gives him a better shot of producing his own offspring before he gets dethroned by some other male lion. The violent male-male competition to control a pride of...
Idea Sex speeds up adaptation by combining rare beneficial mutations
After all, randomly changing some piece of an organism's DNA is far more likely to mess up something that was working just fine than it is to improve upon something that wasn't. Every once in a while, however, some random change is actually beneficial and makes an organism better at doing whatever it is that particular organism does. One possible benefit of sexual reproduction is the ability to combine these rare but beneficial mutations more rapidly. Imagine a couple of...
The development of queen vs worker bees
A beehive consists of at least one queen bee who lays the eggs, a larger number of worker bees who tend the eggs, and developing larvae. The queen and worker bees look very different. The most noticeable difference is the size of the queen she's much larger than the workers. Yet the size difference isn't the result of different genes. Whether an egg develops into a queen or into a worker bee depends on its environment within the hive, specifically whether it's fed exclusively royal jelly for...
Phylogenetic evidence Hangin round on the Tree of Life
As I explain in Chapter 9, phylogenetics takes data about existing species and reconstructs the evolutionary branching pattern that led to those species. Not surprisingly, no small amount of effort has been devoted to reconstructing the parts of the tree of life where humans reside. Our particular branch includes the apes gibbons, orangutans, gorillas, the two types of chimpanzees the standard one that you're familiar with and the bonobo, which used to be called the pygmy chimp but which turns...
Kin selection in bees ants and wasps
Bees, wasps, and ants live in highly structured colonies, with each individual performing particular tasks. Each colony, for example, contains a single reproducing female and many non-reproducing females that gather food, feed the young, and defend the nest but don't lay eggs of their own. So how does helping the group help that is, increase the fitness of the individuals, especially those that forgo reproduction themselves 4jtJABBi You could say that all these insects are working toward the...
The Thrifty Phenotype hypothesis Genes for flexibility
For mammals, the embryo develops within the mother therefore, the maternal environment influences development. Because one major component of the maternal environment is how well nourished Mom is, embryonic development may respond to changes in maternal nutrition a position that medical evidence seems to support. Medical evidence indicates that a fetus deprived of nutrition during key parts of development will develop into a baby with a greater degree of metabolic thriftiness, a group of...
Preventing bad mutations
Mutations tend to be bad, so not surprisingly mechanisms exist within cells to reduce the probability of mutation. Biochemical mechanisms repair damaged DNA proofreading mechanisms catch errors that occur during DNA replication. Yes, each and every one of us has spell check built in So if mutations can be fixed, why do they exist For a few reasons. No repair or proofreading system is perfect. I hope, for example, that this book has no typos it's gone through several editing and proofreading...
Like father like son The sexysons hypothesis
Beyond looking good as a couple, how does picking a male with a showy tail increase the female's fitness, particularly if the showy tail makes survival more difficult You'd think that if she wants to get her genes into the future, the female would be better off choosing males with less-showy tails so that her offspring have a better chance of survival. As it turns out, that's not quite how it works. Instead, specific cases have been found in which female fitness is reduced when females mate...
Size isnt everything Sizing up the genome
Different organisms have radically different genome sizes, but not necessarily in the way you might expect. You have a much bigger genome than a bacterium or a mushroom does, and at first, that seems to make a lot of sense. After all, humans certainly appear to be more complex than bacteria or mushrooms. We have a lot more parts arms, eyes, complicated nervous systems, and so on so it seems reasonable that our genome would be bigger. Right Well . . . maybe, and maybe not. The range of genome...
Genome sizes at a glance
Haploid organisms have only one copy of their DNA diploid organisms have two copies. Humans are diploid A person's genome consists of two DNA copies, one from Mom and one from Dad. Both copies contain the same type of genes eye-color genes, for example , whose specifics blue eyes versus brown eyes, for example may or may not be different. To standardize across all organisms, when scientists talk about genome size, they talk about the size of a haploid genome. For diploid organisms, genome size...
Poison semen
Because of the promiscuous mating system of fruit flies, each fly will mate with many other flies over its lifetime. Neither partner has any interest in increasing the fitness of any particular mate, only in increasing its own fitness i Male fruit flies have toxic chemicals in their seminal fluid that inhibit the sperm of other males this is an example of sperm competition, explained in the earlier section Sperm competition in this chapter . This trait is advantageous for the male because it...
Sex Its Expensive So Why Bother
Lots of organisms reproduce sexually, but plenty don't. Many organisms reproduce asexually, without sex. Given that sex is expensive and some organisms reproduce just fine without it, why do it Evolutionary biologists ponder this question because at first glance, sex doesn't seem like a very fit thing to do i A sexual organism is effectively throwing away half its genes when it reproduces. When an organism reproduces sexually, only half its genes get passed to its offspring the other half come...
The case for preexisting preferences
The brain, as you may have noticed, can be a strange thing. It allows us to do a lot of things that are obviously adaptive find food, avoid bears, and so on. At the same time, it is responsible for traits that don't have obvious advantages, such as dreaming or a propensity to enjoy skydiving. As Chapter 5 explains, not all traits are adaptive. Some traits that aren't adaptive can get dragged along by the traits that are. Pre-existing preferences could fall into this category in this scenario....
Viral reproduction Dna Rna or retro
The only way a virus can reproduce is to infect a host cell, hijack the cell's machinery to make copies of itself, and then move on to infect other cells. But not all viruses do things the same way, and these differences can have consequences for everything from viral evolution to treatment. Different viruses use different nucleic acids DNA or RNA as the genetic material. Some, like herpes, use DNA. Some, like the flu, use RNA. And some like HIV use both, alternating back and forth. This third...
Genotype and phenotype
Genotype refers to the alleles that a particular organism has the actual sequences of DNA in its genome, such as a gene for growth hormone. Phenotype refers to the physical characteristics of the organism, such as the organism's height. Genotype and phenotype are often connected, but the important thing to remember is that the connection is not always absolute. Organisms with the same phenotypes may have different genotypes similarly, organizations with the same genotypes may have different...
But Wait not all traits are adaptations
Some traits are obviously adaptive, such as antibiotic resistance in bacteria. Because scientists understand the selective force humans added the antibiotics they know that the evolutionary change they observe is a response to that selection. The increase in frequency in the population of antibiotic-resistant bacteria is an adaptation to the presence of antibiotics. Other traits are obviously not adaptations. An example is a change at the level of the DNA that doesn't result in a change in the...
Flour beetles A group selection example
Michael Wade set up a laboratory experiment to investigate group selection with the goal of showing that it occurs. In his experiment, Wade used flour beetles little insects that are happy to grow and reproduce in a vial of flour. They eat flour, but they're also cannibalistic Adult beetles eat larvae and eggs, and the larvae eat eggs. Wade set up four experimental treatments, each consisting of 48 populations iff of beetles. For each population, he allowed 16 individual beetles to grow and...
Analyzing the Heritability of Quantitative Traits
As stated earlier in this chapter, quantitative traits are traits that result from complex interactions between multiple genes and that may be influenced by environmental factors. To understand how these traits evolve, evolutionary biologists analyze the heritability of quantitative traits. As you can imagine, the first task is to determine what proportion of the trait or phenotype is due to genetic factors the heritable bits and what portion is due to the environment the non-heritable bits ....
