Chapter 23 (4)

Evolution by __ is a blend of chance and "__"- chance in the creation of new genetic variations (originally by mutation) and __ as natural selection favors some alleles over others. Because of this sorting effect, only __ consistently increases the frequencies of alleles that provide reproductive advantage and thus leads to __.

the phrases "__" and "__" are commonly used to describe natural selection, yet these expressions are misleading if taken to mean direct competitive contests among individuals. There are animal species in which individuals, usually the males, lock horns or otherwise do combat to determine mating privilege. But reproductive success is generally more subtle and depends on many factors besides outright battle.

__ is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

Although we often refer to the __ of a genotype, remember the entity that is subjected to natural selection is the whole organism, not the underlying genotype. Thus, selection acts more directly on the phenotype than on the genotype; it acts on the genotype indirectly, via how the genomtype affects the phenotype.

Furthermore, the __ conferred by a particular allele depends on the entire genetic and environmental context in which it is expressed. For ex., an allele that is slightly disadvantageous might increase infrequency by __, that is, as a result of being located close to an allele at another locus that is strongly favored by natural selection.

Natural selection can alter the frequency distribution of heritable traits in threee ways, epending on which pehnotypes in a population are favored. These three modes of selection are called __, __, and __.

__ occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, therby shifting the frequency curve for the phenotypic character in one direction or the other. __ is common when a population's environment changes or when members of a population migrate to anew (and different) habitat.

__ occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes.

__ acts against both extreme phenotypes and favors intermediate variants. This mode of selection reduces variation and tends to maintain the status quo for a particular phenotypic character.

True or False:
Regardless of hteh mode of selection, however, the basic mechanism remains the same. Selection favors individuals whose heritable phenotypic traits provide higher reproductive success than do the traits of other individuals.
True False

The __ of organisms include many striking examples. Such __ can arise gradually over time as natural selection increases the frequencies of alleles that enhance survival and reproduction. As the proportion of individuals have favorable traits increaes, the match between a species its environment improves: that is, __ occurs. However, the phsical and biological components of an organism's environment may change over tiem. As a result, what constitutes a __ between an organism and its environment can be a moving target, making __ a continuous, dynamic process.

__ and __ can both increase the frequencies of alleles tha timprove the match between organisms and their environment- but neither does so consistently. __ can cause the fequency of a slightly beneficial allele to increase, but it also can cause the frequency of such an allele to decrease. Similarly, __ may introduce alleles that are advantageous or ones that are disadvantageous. __ is the only evolutionary mechanism that consistently leads to adaptive evolution.

Darwin was the first to explore the implications of __, a form of natural selection in which individuals with certain inherited characteristics are more likely than othe rindividuals to obtan mates. __ can result in __, marked differences between the two sexes in secondary sexual characteristics, which are not directly associated with reproduction or survival. These distinction sinclude differences in size, color, ornamentaion, and behavior.

In __, meaning selection within the same sex, individuals of one sex compete directly for mates of the opposite sex. In many species, this occurs among males.

In __, also called __, individuals of one sex (usually females) are choosy in selection their mates from the other sex. In many cases, the female's choice depends on the showiness of the male's appearance or behavior. What intrigued Darwin about mate choice is that male showiness may not seem adaptive in any other wayand may in fact pose some risk.

How do femal epreferenes for certain male characteristics evolve in the first place?
One hypothesis is taht females prefer male traiits that are correlated with __. If th trait preerred by females is indicative of a male's overall genetic qualtily, both the male trait and female preference for it should increase in frequency.

Other researchers have shown that in several bird species, the traits preferred by females are related to overall male health. Here, too, female preference appears to be based on traits that relect __, in this ase alleles indicative of a robust immune system

Becuase most euk are __, a considerable amt of genetic variation is hidden from seleciton in the form of recessive alleles. Recessive alleles that are less favorable than their dominant counterparts, or even harmful in the current environment can persist by propagation in heterozygous individuals. This latent variation is exposed to __ only when both parents carry the same recessive allele and two copies end up in the same zygote. This happens only rarely if the frequency of the recessive allele is very low. Heterozygote protection maintains a huge pool of alleles that might not be favored under present conditions, but which could bring new benefits if the environment changes.

Selection itself may preserve variation at some loci. __ occurs when natural selection maintains two or more forms in a population. This type of selection includes __ and __.

If indiviuals whoa re heterozyogus at a part locus have greater fitness than do both kinds of homozyogtes, they exhibit __. In such a case, natural selection tends to maintain two or more alleles at that locus. __ is defined in terms of __, not __. Thus, whethere __ represents stabilizing or directional selection depends on the relationship between the __ and the __.

True or False
There are relatively few well documented examples of heterozygote advantage. (p 483)
True False

In __, the fitness of a phenotype declines if it becomes too common in the population. (483)

Much of the DNA variation in populations probably has little or no impact on reproductive success, and thus natural selection does not affect this DNA. In humans, many of the nucleotide differences in noncoding sequences appear to confer no selective advantage or disadvantage and therefore are considered __. Mutations that cause changes in proteins also can be __.
o Over time, the frequencies of alleles that are not affected by natural selection may increase or ecrease as a result of __.

though natural selection leads to adaptation, there are several reasons why nature abounds with examples of organisms that are less than ideally engineered for their lifestyles. What are the four ideas?

Which idea?
natural selection favors only the fittest phenotypes among those currently in thepopulation, which may not be the ideal traits. New advantageous alleles do not arise on demand.

Which idea?
each species has a legacy of descent with modificaion from ancestral forms. Evolution does not scrap the ancestral anatomy and build each new complex structure from scratch; rather, evolution co-opts existing structures and adapts them to new situations. We could imagine that if a terrestrial animal were to adapt to an environment which flight would be advantageous, it might be best just to grow an extra pair of limbs that would serve as wings. However, evolution does not work in this way- it operates on the traits an organism already has. Thus, in birds and bats, an existing pair of limbs took on new functions for flight as these organisms evolved from walking ancestors

Which idea?
eachorganism must do many different things. A seal spends part of its time on rocks; it could probably walk better if it had legs instead of flippers, but then it would not swim nearly as well. We humans owe much of our versatility and athleticism to our prehensile hands and flexible limbs. but these also make us prone to sprains, torn ligaments, and dislocations: Structural reinforcement has been compromised for agility.

Which idea?
Chance events can affect the subsequent evolutionary history of populations. Ex: When a storm blows insects or birds hundreds ofkm oveer an ocean to an island, the wind doesn't necessiarly transport those individuals that are best suited to the new environment. Thus, not all alleles present in the founding population's gene pool are better suited to the new environment than the alleles that are "left behind." In addition, the environment at a particular location may change unpredictably from year to year, again limiting the extent to which adaptive evoltuion results in a close match between the organism and current environmental conditions.

With these four constraitns, evolution cannot craft perfect organisms. __ operates on a "better than" basis. We can see evidence for evolution in the many imperfecttion of the organisms it produces.