Chapter 3: Evolution by Natural Selection

Through their studies of medium ground finches, the Grants were able to demonstrate that, as a result of drought ___________.

-evolution occurred -natural selection occurred -adaptation occurred

What is exaptation?

A character that has been co-opted during evolution for a novel function.

What is natural selection?

A difference, on average, between the survival or fecundity of individuals with certain phenotypes compared with individuals with other phenotypes.

Which is the best definition of Darwinian fitness?

The ability of an individual to survive and reproduce in a certain environment, compared to other individuals.

An adaptation is best defined as _______________.

any trait that increases fitness

Based on their observation of the color preferences of the bumblebees visiting their snapdragons, Jones and Reithel predicted (and found) that the proportion of white-flowered plants _________ from the first to the second generation.

increased

The creationist "argument from design" is based on the idea that complex structures and organisms can't arise by chance. The argument fails because, although _________ is random, ___________ is not; it is directed in the sense of increasing adaptation.

mutation/selection

A key flaw of "intelligent design theory" is that it fails to recognize the importance of _____________.

preadaptation

By studying medium ground finch populations over the course of several decades, the Grants and their colleagues have demonstrated that, during drought years, natural selection favors birds with _______ beaks; during wet years, natural selection favors birds with ________ beaks.

deep/shallow

Successful artificial selection, such as that practiced by plant and animal breeders to modify their crops and livestock, depends on which of the following?

-Breeders must be able to selectively breed the individuals with the most desirable traits. -Some individuals must have more desirable traits than others. -When individuals with the most desirable traits are bred, their traits must be passed from parents to offspring.

Which of the following is a true statement regarding preadaptations? A trait that evolved accidentally (usually due to genetic drift) and has no function. A trait that is imperfect due to genetic or developmental constraints. A trait that is in the initial stages of evolving for a certain function, but has not yet been perfected. A trait that evolved for one function, but that coincidentally is also suited for a new, different function. A trait that is currently evolving for a function that will be needed in the future.

A trait that evolved for one function, but that coincidentally is also suited for a new, different function.

What is adaptive mean?

Describes a trait that increases the fitness of an individual relative to that of individuals lacking the trait.

What is blending inheritance?

The hypothesis that heritable factors blend to produce a phenotype and are passed on to offspring in this blended form.

What is an adaptation?

A trait that increases the ability of an individual to survive or reproduce compared with individuals without the trait.

What are secondary adaptations?

Additional adaptive evolution of a trait following co-option for a novel function.

What is blending inheritance, and why did it pose a problem for Darwin's theory? Why is it not considered a problem today?

Blending inheritance was the theory that offspring inherit traits by "blending" two components inherited from each parent, such that each original component is irrevocably altered. In the analogy of human skin color, if a black parent and a white parent had a brown-skinned child, the child was thought to have inherited a blended "brown" particle of inheritance. (The child was not thought to carry unaltered "black" and "white" particles of inheritance.) The problem for evolution is that, if this theory were true, populations would eventually arrive at a completely "blended" state, with no further variation for selection to work on. Blending inheritance was the leading theory of inheritance in Darwin's day, but we now know that it is false. Inheritance does not work this way. Instead, offspring inherit two intact genes from each parent, each of which is unaltered, and which can be passed on intact to succeeding generations.

Name the two scientists who independently discovered the principle of natural selection. Which one is more famous, and why?

Charles Darwin and Alfred Russel Wallace. Darwin's name is more famous because he had clearly thought of it first, and because he had compiled an enormous set of evidence that showed that his four postulates of selection were true for most populations.

Although the scientific community rapidly accepted that all organisms are related by descent, Darwin's mechanism for change over time - natural selection - wasn't fully accepted until nearly 100 years after the publication of "The Origin." Explain why.

Darwin did not know about the principles of genetics - how variation is created and how it is inherited. Kelvin's (inaccurate) estimate of the age of the Earth also presented a problem for evolutionary theory.

What is unusual about eel sperm, and why is it relevant for a recent United States district court case?

Eel spermatozoa have flagella that lack several features usually found in flagella and cilia - the central pair of microtubules, the inner row of dynein, and the radial spokes. The fact that eel sperm can swim perfectly well, despite lacking many standard flagella components, demonstrates that the standard flagellum design is not irreducibly complex. The flagellum was originally one of Behe's key examples of irreducible complexity, and was discussed in court at the 2005 Dover School District trial on intelligent design.

Why did Dawkins refer to evolution as a "blind watchmaker"? What famous objection to evolution was he arguing against? Do you think his analogy is a good one? Another analogy that has been proposed is evolution as a "short-order cook." Which do you think is a better analogy?

Evolutionary biologist Richard Dawkins was referring to cleric William Paley's classic argument that if one finds a watch lying on the ground, the logical conclusion is that the watch was created by an intelligent being, not that it arose by natural causes. Dawkins' analogy is meant to illustrate that natural selection, even though it is "blind" (not goal-directed, not purposeful, and not directed by an intelligent being) can produce highly complex structures. The "short-order cook" analogy emphasizes that natural selection is not goal-directed and does not have a long-term plan for the future - it simply responds to immediate environmental conditions. The choice of which analogy is better is a matter of personal preference; they emphasize different features of natural selection.

What is heritability?

In the broad sense, that fraction of the total phenotypic variation in a population that is caused by genetic differences among individuals; in the narrow sense, that fraction of the total variation that is due to the additive effects of genes.

Successful biology research often depends on choosing a good "model" - a particular species, population, or locality that is particularly amenable to study, and/or is a particularly clear case of the research question. Why were the finches of Daphne Major a good choice for an evolution study?

Many answers are possible. The fact that Daphne Major is an island made it unlikely that migration would have a great effect. The size of the island made it possible to catch and measure the entire population of medium ground finches, while still having a large enough population for genetic drift to not have a major effect. (See Chapter 7 for more about migration and drift.) With migration and drift minimized and the entire population measured, it was easier to detect any occurrence of selection. The simplicity of the ecosystem and the low diversity of the bird's diet made this a particularly clear case of evolution: a single environmental feature (rain) had a single pronounced effect on diet (disappearance of small seeds), which in turn was linked to a single, obvious, and easy-to-measure anatomical trait (beak depth). The choice to study a small bird was probably not an accident. Small birds tend to have a large population size per unit area, a short generation time, and they are small enough to catch and handle easily, while also being large enough to be easy to measure and to collect blood samples from. Birds also keep their young conveniently grouped in one location (the nest) for two weeks, making it feasible for researchers to find and measure all offspring, which is a key requirement for any evolution study. Finally, birds are active in the daytime (unlike most mammals) and are relatively easy to observe, and they can be identified permanently with colored leg bands.

Which of the following statements is true? Natural selection acts directly on phenotypes, and only indirectly on genotypes. Natural selection cannot be progressive because of its random nature. Natural selection usually results in an immediate increase in population variation. Natural selection acts on populations, but its consequences occur in individuals. Natural selection generally leads to perfection of existing traits.

Natural selection acts directly on phenotypes, and only indirectly on genotypes.

Ivory poachers (illegal hunters) in Africa kill elephants primarily for the ivory in their tusks, and big tusks are worth more money. Based just on this fact, make a prediction about how hunting pressure could cause (or might have already caused) evolution in elephants. Do you think elephant populations meet Darwin's four criteria for selection to occur? How could you test your ideas?

Poachers are likely to preferentially target elephants with large tusks (and, in fact, they clearly do). As for Darwin's four postulates: Any glance at an elephant herd shows that elephants have variation in tusk size. It is quite likely that this variation is heritable (though testing this would require data on parent and offspring tusk size). Not all elephants survive. Survival is nonrandom, since poachers prefer elephants with large tusks. Thus this population satisfies Darwin's four postulates, and selection should be occurring. As a matter of fact, it appears that evolution is already well underway. Large-tusked elephants are now extremely rare. An interesting recent development is the appearance of families of completely tuskless elephants. Tusklessness was formerly rare in African elephants, though not unheard of, but in some populations the majority of elephants are now tuskless. Thus, selection by human hunters may be causing the evolutionary loss of one of the most dramatic anatomical features of African elephants

Now consider another ocean example, but with a twist. Lobstermen in Maine measure every lobster they catch. They throw back all the small individuals, to allow young lobsters time to grow. This is a fairly common practice in fisheries. However, they also do two unusual things: they throw back the largest lobsters, keeping only the medium-sized ones; and they also throw back any female who has ever been found to be carrying eggs, even if she is not carrying eggs at the moment. ("Eggers," or egg-carrying females, are permanently marked by cutting a V-shaped notch in their tails.) Why do Maine lobstermen throw large lobsters and notched females back into the sea?

The Maine lobstermen are trying to avoid the mistakes of other fisheries (like southeast Asia) by deliberately selecting for large body size and reproduction. Large lobsters are thrown back to select for the alleles associated with large body size. Notched females are thrown back, even if they are not carrying eggs at the time, to select for the alleles associated with successful reproduction. (Males that have bred successfully cannot be identified as easily.) In this way, the Maine lobstermen hope to "breed" a lobster population that produces large numbers of good-sized lobsters. So far, the strategy may be working; record numbers of (medium-sized) lobsters have been landed in recent years, and the lobster fishery is currently the most productive fishery of the Atlantic coast of the United States. (For an entertaining and informative look at this story, see Trevor Corson's book The Secret Life of Lobsters, 2004, Harper Perennial.)

Describe the Second Law of Thermodynamics. Does natural selection violate the Second Law? Does the growth of an oak tree from an acorn, or a human being from a fertilized egg, violate the Second Law? Why or why not?

The Second Law states that in any closed system, entropy tends to increase, and cannot decrease. The Second Law is commonly invoked by creationists as an argument against natural selection, using the reasoning that development of a highly complex species from a less complex one would involve a decrease in entropy, which (they say) is not allowed by the Second Law. The same reasoning should apply to development of any large organism from a small seed or zygote. The logical flaw here is that organisms are not closed systems. They obtain their energy from outside, from eating or from photosynthesis, and ultimately from the sun. Another way of stating this is to recognize that the Second Law does allow entropy to decrease, if energy is received from an outside source. In such cases, entropy will always increase accordingly in that outside source, to make up for the decrease in the local subsystem. When the sun and the Earth are considered together, overall entropy is indeed increasing as the sun slowly burns out.

What is Darwinian fitness?

The extent to which an individual contributes genes to future generations, or an individual's score on a measure of performance expected to correlate with genetic contribution to future generations.

Most large-scale fishing is done with nets of a certain mesh size, which catch larger fish while allowing small, undesirable fish to pass through. In the seas of southeast Asia, fisherpeople have recently had to switch to smaller and smaller mesh sizes. Why? What is happening to the fish population? Can you think of two possible explanations?

The fishes of southeast Asia have become very small. This is because fisherpeople have been unintentionally exerting intense selection for small body size. There are at least two possible explanations for absence of large fishes. First, the small fishes may just all be young - i.e. young fish may all be caught before they have a chance to grow larger. (This is a well-known phenomenon called "growth overfishing.") Second, fishes may be evolving smaller body size and slower growth rate. The fish populations almost definitely satisfy Darwin's four postulates, and the selection pressure of overfishing in southeast Asia appears to be intense, so it is likely that evolution for small body size is occurring. Recent studies have demonstrated evolution of slow growth and small body size in other over-fished species (Atlantic cod, pink salmon). Interestingly, though evolution of smaller body size should be a very obvious consequence of overfishing, fisheries biologists have, until recently, not been aware of it because they are usually not trained to think like evolutionary biologists; they have instead concentrated on growth overfishing. The fisheries industry is only now becoming aware of the probably evolutionary consequences of overfishing.

What was the "Modern Synthesis" and why did it lead to widespread acceptance of the theory of evolution by natural selection?

The missing piece in Darwin's theory was a mechanism of variation and inheritance. The Modern Synthesis added this piece, by uniting the new discoveries of genetics with natural selection. This occurred in a series of landmark books published in the 1930s-1950s, showing that populations have abundant genetic variation, natural selection should act on this genetic variation to cause microevolution, and that these small changes could, in theory, accumulate over time to cause macroevolution. The Modern Synthesis showed that natural selection had a plausible genetic mechanism, and was in full agreement with the new information on how inheritance actually works. The clarity, logic, and elegance of this new synthetic theory immediately led to widespread acceptance of natural selection among the vast majority of scientists.

The panda's "thumb" is actually a wrist bone modified by natural selection to perform a new function. This is an example of a(n) _____________.

The panda thumb arose from a preadaptation - a pre-existing trait that was put to novel use. The process works in three stages: first, the trait evolves due to some other purpose. Second, the trait begins to be used for a novel purpose. Third, natural selection acts on variations to enhance the trait's adaptation for the new purpose.