Variation and Selection in Populations II

Field studies show that natural selection occurs for phenotypic traits in all natural populations. A straightforward example is the fur coloration of pocket mice living in the deserts of New Mexico (2-story)

In this population, the genotype frequencies AA, Aa, and aa are p², 2pq, and q², respectively. Now suppose that the viability, that is, the probability of survivng from zygote to adult, depends on _______, while success at productive mating-the second component of fitness-is in this case independent of _______. If we define the ______ ______ of each of the three genotypes as W_AA, W_Aa, and W_aa, respectively frequencies of the three genotypes at adulthood are ____, _____, and _____

The fitness-modified HW equation is most useful when the relative fitnesses are normalized (explain 2-story) 
Each term in this normalized equation represents the actual _____ that each ______ will assume in the next generation following the one used for the original calculation

A key feature of Eq 20.6 (pg 675) when W_dd is less than 1 is its prediction that the rate at which q ______ over time diminishes as q becomes ______. Why does this prediction emerge?

To understand this effect, consider the special case of a lethal recessive disease for which W_dd = 0. The dotted line in the fig shows the decrease in ______ ______ predicted by Eq 20.6 (pg 675), starting from an initial allele frequency of ____. The decrease in ______ _______ is rapid at first and then slows 

After 10 generations, the predicted frequency of the recessive disease allele is still nearly ____% even though the homozygous recessive genotype is ______. The solid line in fig plots actual data for the decrease in frequency of an _____ _____ _____ allele in a large experimental population of Drosophila; the predicted and observed changes in allele frequency match _______

Why does selection become less effective as the frequency of a recessive lethal allele moves closer to zero?

Modifying the HW equation with relative fitnesses overcomes one limitation of the original equation (what is that limitation?). But the analytical solution of this modified  equation to determine Δq still suffers from a dependence on the assumption of an _______ population. However, we can use the modified HW equation to develop ______ ______ simulations that explore the impact of ______ ______ on finite populations. Explain an example

Figure 20.13 shows the results of six simulations of this population model. Notice first that in three of these, the new B allele _______ takes off, going ______ within 65 generations. But in the populations where the B allele _______ in frequency to about 0.10, it inevitably moves toward ________

This example illustrates two important points concerning the impact that a new mutant allele with a small yet realistic, fitness advantage can have on a population (explain to the two step concept)

When people migrated out of the East African region in which H. sapiens originated, beginning about ______ to ______ years ago, ______ populations encountered ________ conditions in Europe and Asia that were distinct from those in Africa. As a result, the ______ ______ of alternative alleles at a number of genes became ________. Among the most obvious changes were differences in ______ frequencies at genes that determine ______ ________

The ______ _____ of the sun provide people with benefits as well as harm. One benefit is the catalysis of ______ ____ production; the harm is in the _______ of mutations in our skin that can lead to _____ _____. Closer to the equator, the sun's rays are most ______; alleles that cause a _______ of the skin are advantageous in tropical regions (why?). At higher latitudes, where the sun's rays are less intense, skin cancer is less of a problem, and alleles that lighten the skin allow enough ____ penetration for sufficient ______ _____ production

The KITLG gene is among the small number with a prominent role in ______ ______. In the fig., Europeans and Asians share a common SNP (single nucleotide polymorphism) variant of KITLG responsible for a ________ in pigmentation, suggesting that they derived it from a ______ ______ who lived after humans migrated from Africa into the Arabian Peninsula and prior to the separation of populations heading northwest and northeast.

In contrast, Europeans and Asians independently accumulated variants with roles in skin pigmentation at ____ other loci (MC1R and SLC24A5). Thus, although the same selective pressure of reduced sunlight existed in both populations, the selection acted on ______ ______ that occurred at different times in human history

Sickle-cell anemia, which includes episodes of severe pain, serious anemia, and a probablility of early death, is a _______ condition resulting from two copies of the ______ _____ allele at the β-globin gene (Hbβ). It is thus surprising that the disease allele has not ______ from several African populations, where it seems to have existed for a very long time

One clue to the maintenance of the sickle-cell allele Hbβ^S in human populations lies in the observation that its allele frequency is _______ in regions of Africa in which _______ is endemic. A second clue is that heterozygotes for the normal and sickle cell alleles (Hbβ^A Hbβ^S) are _______ to malaria

This resistance to malaria is due, in part, to the fact that red blood cells containing a sickle-cell allele _____ _____ after being infected by the malaria parasite, _______ the parasite as well as the red blood cell itself

By contrast, in red blood cells with two normal hemoglobin alleles, the malaria parasite ______. Thus, individuals of genotype Hbβ^A Hbβ^S have a _____ ______ in malaria-infested regions over either type of homozygote: The carriers are less susceptible to _______ than are either Hbβ^A or Hbβ^S homozygotes, and less susceptible to ________ than are Hbβ^S Hbβ^S homozygotes. Heterozygote advantage is one of several processes leading to _____ ______ that actively maintains genetic polymorphisms

To understand heterozygote advantage mathematically, assume that Hbβ^A Hbβ^S heterozygotes have the maximum relative fitness of ___, while the relative fitness for the Hbβ^A Hbβ^A homozygotes is _____, and the relative fitness for Hbβ^S Hbβ^S homozygotes is ____. (To simplify the following equations we are temporarily renaming the HbβA alleles as A whose frequency is p, and renaming the HbβS allele as a whose frequency is q.) Selection will maintain both alleles in the population only if Δq = ___ for some value of q between ____ and ____

pg  678 cover until equation 20.9

Why is the allele frequency is stabilized at equilibrium?

Like infectious bacteria, many insects that threaten human health and agriculture spawn large populations because of their _____ generations times and ______ rates of reproduction. Via selection for _______-conferring mutations, these large, ______ reproducing populations of ______ insects evolve ______ to the chemical pesticides used to control them

Since the 1950s, resistance to every known insecticide has evolved within ____ years of its commercial introduction. Why did insecticide resistance likely develop many separate times in many insect species since the introduction of insecticides?

A field study of the use of DDT in Bangkok, Thailand to control Aedes aegypti mosquitoes, the carriers of ______ ______, illustrates the rapid evolution of resistance. Spraying of the insecticide began in 1964 and was very effective in controlling the mosquitoes. Within a year, however, DDT resistant ______ _______ (R) emerged and rapidly ________ in frequency. By mid-1967, the frequency of resistant RR homozygotes was was nearly ___%

Because DDT became _______ in reducing mosquito populations in Bangkok due to the near ______ of the DDT-resistant allele, the insecticide spraying program was stopped. What was the response of the mosquito population to the cessation of spraying?

The precipitous decline of the R allele suggests that in the absence of DDT, the RR genotype produces a _____ _____ than the rr genotype (2-story explain)

Human populations in different regions of the globe vary _____ in the frequency of alleles dictating lactose tolerance or intolerance. These current-day variations in ______ frequency reflect many processes that occurred during the course of human history. The earliest humans almost certainly were all ______ ______, as are other primates

At least twice in human history, and in different geographical locations, mutations occurred that allowed expression of the ______ enzyme to continue through adulthood. The frequency of the mutant alleles increased rapidly in populations that raised dairy animals (why?). ______ _____ in small populations of dairy herders may also have contributed to the spread of these alleles. The mutant alleles were then introduced into other populations by _______ of individuals carrying the alleles

Individual people have two kinds of ancestors: _______ ancestors and _______ ancestors. Biological ancestry is simply a description of ____ ____ ___: You have two biological parents, four grandparents, eight great grandparents, and so forth. Any individual alive today could potentially have ____ biological ancestors ____ generations ago, assuming that the ancestors in any one generation were ______

Thus ___ generation ago (about 400 years) you could have had over 1 million biological ancestors, and ___ generations ago (about  600 years) more than 1 billion. This latter number is much higher than the number of humans thought to have been on the earth at that point history (during the Middle Ages) (why?)

What does genetic ancestry refer to? Comparison of part b and c shows that for diploid regions of the human genome, we have many more ______ ancestors than ______ ancestors (why? 2-story)

Explain MRCA
Because of recombination, different regions of the _____ of two relatives may have different ______. In fact, across their genomes, two relatives have many different ______ in the past, reflecting the fact that they have inherited bits and pieces of their genome from many (but not all) of their many ______ ancestors

In the fig, for example, the individual carrying the most recent common ancestor for an allele of a particular autosomal region in the five people alive today highlighted at the bottom of the figure existed ______ generations ago

For any specific region of the genome, the MRCA for all humans is the most recent allele from which all current day people have obtained DNA sequences in an ______ line of descent. The MRCA for the particular locus was carried by an individual who co-existed with other people who also contributed _____ of their genes only to ______ current day humans

In contrast to the recombining diploid portion of our genome, mitochondrial DNA (mtDNA) is passed directly from _______ to their offspring with no contribution from the ______. Similarly, excluding the small _____ regions shared by the X and Y chromosomes, the DNA on Y chromosomes is passed directly from ______ to _____: _______ do not inherit a Y chromosome. Each of us thus has just one _______ ancestor each generation for mtDNA or Y-chromosomal DNA. Because of a lack of ________, the entire mtDNA and almost the entire Y chromosome each have only ______ MRCAs for all humans

Looking at the history of human populations through the lens of genetic ancestry is a powerful method for interpreting DNA sequence variation in present-day people. The different allele lineages coalesce to the MRCA as we go ________ in time. The MRCA thus provides a starting point for analysis, in the form of an ancestral allele whose descendant sequences are found in _____ people now on the earth

Even though an unbroken line of descent connects the MRCA with sequences found in all modern day people, this does not mean that all MRCA-related sequences are ______ (Why?)

Researchers determine these lines of descent by analyzing mutations shared by present day individuals. Because _______ accumulate over time, the ______ the ancestry branch length between alleles in two individuals alive today, the more different are their _____ ______.

The DNA sequence of that MRCA was AAAAA. As that sequence was passed down over generations, ____ ____ mutations could occur in it, creating ______ ddescendant alleles. Note in the fig. hwo individuals closely linked in the family history (such as the siblings at the bottom left) are _______ in sequence (AATAA), but they _____ at four of the five nucleotide positions from the individual at the bottom right (TGAAC), who comes from a _______ family lineage that is nonetheless still derived from the MRCA at the to the top

If a SNP allele is found both in some present day humans and in some present day members of other primate species such as chimpanzees, the allele is ______ and must have been inherited directly from a _____ _____ of humans and chimps without ______ modification.

By contrast, for a derived SNP allele found in some human populations but not in other human groups nor in chimps, the mutational event must have occurred at some generation after the MRCA in a specific ______ ______. Human populations who share more alleles must ahve separated from each other more _______ than populations whose DNA sequences are more divergent

Tracing variatinos in _______ and ________ sequences is particularly valuable because these sequences provide clear estimates of _______ and ______ genetic ancestries, respectively. Sequence variation assayed from your autosomes provides a more complex picture of your ancestry due to the presence of _______ each generation. Our autosomes also provide ancestry info, but the data require more complex analysis because of _____ _____ and ______

The first insights came from the study of the matrilineally inherited _____ molecule. These investigations established that people representing populations in sub-Saharan Africa, southeast Asia, Europe, aboriginal Australians, and aboriginal New Guineans all shared a most recent common ancestral mtDNA from a female who lived no more than _______ years ago

The estimate of time came from a calibration of the rate of accumulation of _______ in mtDNA from samples with independent archeological or geoligical estimates of times since _______. This rate has been found to be remarkably ______ for many lineages and genomes, and it thus constitutes a ______ ______

Scientists further concluded that a female carrying MRCA for human mtDNA lived in ______. The evidence for this statement is that African populations show much greater _____ ____ ____ than do populations in other parts of the world. (explain)

Studies of the patrinlineally inherited Y chromosome reached a very similar conclusion (What was that conclusion?)
What was the conclusion for biparentally inherited autosomes?

Modern humans all originated from a sub-Saharan African population that was established roughly _______ years ago. This population subsequently dispersed throughout _______. Then, no later than _______ years ago, a subgroup of Africans left the continent and dispersed along a southern ______ route, followed by a more recent dispersal from Africa that settled in the ______ ______. From these initial groups in ________ and the ______ _______, people then spread out further to colonize the globe in several waves of _______.

What is the reason that populations outside of Africa have less genetic diversity than do African populations?

One long-running issue in anthropology has been the relationshipp between anatomically modern humans and fossil remains that are clearly humanlike _______ but also quite different in key elements of _________. Of particular interest are the Neanderthals, whose fossils, dating to as recently as _______ years ago, have been found in caves across southern Europe and into central Asia

Recently, scientists have been able to address questions concerning the relationship between Homo sapiens and Neanderthals by comparing genome sequences of current day humans with those obtained from particularly well preserved ______ _______ such as that shown in the fig. 

For example, researchers remarkably have been able to ______ the full nuclear genome from a fragment of a Neanderthal femur found in a cave in Croatia and dated to approximately 38,000 yrs ago. Several lines of evidence, including the finding that the ______ ______ between Neanderthal DNA and that of any present day human is several fold _____ than that between any two present day humans, resulted in the estimate that the hominin lineages leading to Neanderthals and Homo sapiens _______ between 500,000 and 800,000 years ago

The analysis of DNA isolated from a small bone fragment from the tip of a finger of a girl who lived and died in a cave in Siberia just north of Mongolia revealed an unexpected result (What was that result?). Individuals in this lineage are called _______; the fossil record indicates that the _______ died out roughly 30,000 years ago, during a period in which modern humans had already spread out through much of Asia

These insights into past interbreeding as well as inferences of the relatedness of DNA sequences of contemporary and ancient human lineages suggests a hypothetical _______ for the evolution of modern humans. _____ exchange between Neanderthals and the lineage of modern humans occurred in the Middle East and in Eurrope 85,000-30,000 yrs ago, soon after Homo sapiens emerged from ______. From there, Neanderthal DNA variants accompanied human migrants as they spread into the rest of the world outside of _______

_______ also occurred during roughly the same timeframe in Asia between the ancestors of modern humans and the _______ already indigenous there. DNA variants obtained from _______ then spread with migrant groups of modern humans to Southeast Asia and Oceania, where these variants are still found in current day populations