Chapter 23 (1)

What is one common misconception of evolution?

It is true that __ acts on individuals: Each organism's combo of traits affects its survival and reproductive success compared to other individuals. But the evolutionary impact of __ is only apparent in the changes in a population of organisms over time.

Focusing on evolutionary change in populations, we can define evolution on its smallest scale, __, as change in allele frequencies in a population over generations. __ is not the only cause of this.
There are three main mechanisms that can cause allele frequency change: __, __, __

chance events that alter allele frequencies

transfer of alleles between populations

Each of these mechanisms has distinctie effects on the genetic composition of populations. However, ony __ consistently improves the match between organisms and their environment, thus bringing about the type of change we refer to as __.

In the Origin of Species, Darwin provided abundant evidence that life on earth has evolved over time, and he proposed __ as the primarry mechanism for that change. Darwin also emphasized the importance of heritable differences among individuals. He knew that __ could not cause evolutionary change unless individuals differed in their inherited characteristics. But he could not explain precisely how organisms pass heritable traits to their offspring.

A few years after The Origin of Species, __ wrote a groundbreaking paper on inheritance in pea plants, where he proposed a particulate model of inheritance, which stated that organsims transmit discrete heritable units (__) to their offspring. Although Darwin never learned about genes, __ paper set the stage for understanding genetic differences on which evoluition is based.

In addition to differences we can see or hear, species have extensive __ that can only be observed at the moleuclar level. Some __ variation is not heritable. __ is the product of an inherited __ and many environmental influences. Only the genetic part of variation can have evolutionary consequences.

Characters that vary within a populatio may be __ or __.
o __, such as purple or white flower color, can be classified on an either-or-basis. Many __ are determined by a single gene locus with different alleles that produce distinct phenotypes.
o However, most heritable variation involves __, which vary along a continuum within a population. Heritable __ usually results from the influence of two or more genes on a single phenotypic character.

Whether considering __ or __, biologists can measure genetic variation in a population at both the whoe-gene level (__) and the moleuclar level of DNA (__). Gene variability can be quantified as the __, the average percent of loci that are heterozygous.

__ is often estimated by surveying the protein products of genes using __. While useful, this approach doesnt detect silent mutations that alter the DNA sequence of a gene but not the amino acid sequence of the protein. To include silent mutatiosn in their estimates of __, researchers must use other approaches, like PCR- based methods and restriction fragment analyses.

_ is measured by comparing the DNA sequences of two individuals in a population and then averaging the data from many such comparisons.
o __ tends to be greater than __ because a gene consists of thousands of nucleotides. A difference at one of the nucleotides can be sufficient to make two alleles of that gene different and thereby increase __.

In addition to variation observed within a pop., species also exhibit __, differences in the genetic composition of separate populations.
o Variation between these populations appears to have resulted from chance events (__) rather than __.

Other ex. of geographic variaiton occur as a __, a graded change in a character along a geographic axis. Some __ are produed by a gradation in an environmental variable. __ probably result from natural selection- otherwise there would be no reason to expect a close association between the environmental variable and the frequency of the allele. But selection can only operate if multiple alleles exist for a given locus. Such variation in alleles is the product of __.

The ultimate soure of new alleles is __, a change in the nucelotide sequence of an organism's DNA. With a __, we cannot predict accurately which segments of DNA will be altered or in what way. In multicellular organisms, only mutations in cell lines that produce gametes can be passed to offspring. In plants and fungi, this is not as limiting, since many different cell lines can produce __. But in animals, most __ occur in somatic cells and are lost when the individual dies.

A change of as little as one base in a gene- a ___ can have a significant impact on phenotype. Organisms reflect thousands of generations of past selection, and hence their phenotypes generally provide a close match to their environment. As a result, its unlikely that a new mutation that alters a phenotype will improve it. In fact, most mutations are slightly harmufl. But much DNA of euk. genomes doesn't code for protein products, and __ in these noncoding regions are often harmless.

Also, because of the __ in the genetic code, even a __ in a gene that encodes a protein will have no effect on the protein's function if the amino acid comp is not changed. Moreover, even if there is a change in the amino acid, this may not affect the protein's shape and function. Some mutant alleles actually make its bearer better suited to the environment, enhancing reproductive success.

Chromosomal change sthat delete, disrupt, or rearrange many loci at once ar ealmost certain to be harmful. However, when such large-sclae mutations leave genes intact, their effects on organisms may be __. In rare cases, they may even be beneficial.

An important source of variation begins when genes are duplicated due to errors in meiosis (such as __), __ or the __.
o __ of large chromosomal segments, like other chromosomal aberrations, are often harmful but the dupications thtat do not have severe effects can persist over generations, allowing __ to accumulate. The result is an expanded genome with new loci that may take on new functions.

__ rates tend to be low in plants and animals, avging about one in every 100,000 gnes per generation, and tehy are often even lower in prokaryotes. But they typically have short generation spans, so __ can quickly generate genetic variation in populations of these organisms. The same is true of __. (HIV example on p. 471)

In organisms that reproduce sexually, most of the genetic variation in a pop results from the unique combo of alleles that each individual receives. Of course,at the __, all the differenes among these alleles have originated from past __. But it is the mechanism of __ that shuffles existing alleles and deals them at random to determine ind genotypes.

Three mechanisms contribute to shuffling: __, __, __.
o During meiosis, homologues, one from each parent, trade some of their alleles by __, which are then distribted randomly into gametes, which later results in __, providing much of the genetic variation that makes evolution possible.