The Living World Fifth Edition George B. Johnson Jonathan B. Losos Chapter 17 Evolution and Natural...

The Living WorldFifth EditionFifth Edition

George B. Johnson

Jonathan B. Losos

Chapter 17

Evolution and Natural Selection

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

17.1 Evolution: Getting from There to Here

• The idea of evolution by means of natural selection has played a central role in the science of biology proposed by Charles Darwin in 1859 with the

publication of On the Origin of Species may be summarized, as Darwin did, as

“descent with modification”• all species arise from other, preexisting species


Macroevolution is evolutionary change of a grand scale

• for example, changes that result in the creation of new species

Microevolution is evolutionary change at the level of a population

• these are changes that occur within a species that make that species different from its immediate ancestor

• adaptation results from microevolutionary changes that increase the likelihood of survival and reproduction of particular genetic traits in a population


• Darwin did not invent the idea of evolution

• Prior to Darwin there was no consensus among biologists about the mechanism causing evolution

• A predecessor to Darwin, Jean-Baptiste Lamarck proposed that evolution occurred by the inheritance of acquired characteristics


• According to Lamarck, individuals passed on to offspring body and behavior changes acquired during their lives

for example, giraffes evolved long necks because ancestral giraffes tended to stretch their necks and this neck extension was passed on to subsequent generations

Figure 17.1(a) How did long necks evolve in giraffes?


• According to Darwin, the variation is not created by experience but already exists when selection acts on it populations of ancestral giraffes contained

variation in neck length individuals who were able to feed higher up

on the trees had more food and so were able to survive and reproduce better than their shorter-necked relatives

Figure 17.1 (b) How did long necks evolve in giraffes?


• There are two views concerning the rate of evolutionary change

gradualism states that evolutionary change occurs extremely slowly

• such change would be nearly imperceptible from generation to generation, but would accumulate over the course of millions of years

punctuated equilibrium states that species

experience long periods of little or no evolutionary change (termed stasis), interrupted by bursts of evolutionary change

Figure 17.2 Two views of the pace of macroevolution

17.2 The Evidence for Evolution

• There are many lines of evidence supporting Darwin’s theory of evolution fossil record comprises the most direct

evidence of macroevolution fossils are the preserved remains, tracks, or

traces of once-living organisms• they are created when organisms become buried

in sediment• by dating the rocks in which the fossils occur, one

can get an accurate idea of how old the fossils are


• Fossils in rock represent a history of evolutionary change fossils are treated as samples of data and are

dated independently of what the samples are like

successive changes through time are a data statement

thus, the statement that macroevolution has occurred is a factual observation

Figure 17.3 Testing the theory of evolution with fossil titanotheres


• The anatomical record also reflects evolutionary history for example, all vertebrate embryos share a basic set

of developmental instructions

Figure 17.4 Embryos show our early evolutionary history


• Homologous structures are derived from the same body part present in an ancestor for example, the same bones might be put to different

uses in related species

• Analagous structures are similar-looking structures in unrelated lineages these are the result of parallel evolutionary

adaptations to similar environments• this form of evolutionary change is referred to as convergent

evolution

Homologous versus Analagous Structures

Figure 17.5 Homology among vertebrate limbs

Figure 17.6 Convergent evolution: many paths to one goal


• Traces of our evolutionary past are also evident at the molecular level organisms that are more distantly related

should have accumulated a greater number of evolutionary differences than two species that are more closely related

the same pattern of divergence can be seen at the protein level

Figure 17.7 Molecules reflect evolutionary divergence


• Evolutionary changes appear to accumulate at a constant rate this permits changes in an individual gene,

compared over a broad array of organisms, to be dated from the time of divergence

this dating is referred to as a molecular clock for example, changes that have accumulated

in the cytochrome c gene

Figure 17.8 The molecular clock of cytochrome c

17.3 Evolution’s Critics

• The theory of evolution by natural selection is the subject of often-bitter public controversy the controversy began soon after the publication of

The Origin of Species but, by the turn of the twentieth century, evolution was generally accepted by the world’s scientific community

more recent criticism has come from the following sources

• the Fundamentalist Movement• the Scientific Creationist Movement• Local Action• Intelligent Design

17.3 Evolution’s Critics• Critics have raised a variety of objections to Darwin’s theory of evolution by natural

selection

“Evolution is just a theory.”

“No one ever saw a fin on the way to becoming a leg.”

“The organs of living creatures are too complex for a random process to have produced.”

“A jumble of soda cans doesn’t by itself jump neatly into a stack—things become more disorganized due to random events, not more organized.”

“Hemoglobin has 141 amino acids. The probability that the first one would be leucine is 1/20, and that all 141 would be the ones they are by chance is (1/20)141, an impossibly rare event.”

“No scientist has come up with an experiment where fish evolve into frogs and leap away from predators.”

“Because the peptide bond does not form spontaneously in water, amino acids could never have spontaneously linked together to form proteins.”


• The previous quotations attack evolution on the following grounds, which have been refuted by biologists

evolution is not solidly demonstrated

there are no fossil intermediates

the intelligent design argument

evolution violates the second law of thermodynamics

natural selection does not imply evolution

life could not have evolved in water


• The irreducible-complexity fallacy refers to claims by proponents of intelligent design that the molecular machinery of the cell is irreducibly complex

• Yet natural selection acts on the systems and not the parts so that, at every stage of evolution, the system is functioning for example, the mammalian blood clotting system

has evolved in stages from much simpler systems

Figure 17.10 How blood clotting evolved

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule

• Population genetics is the study of the properties of genes in populations

• Gene pool is the sum of all of the genes in a population, including all alleles in all individuals

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• Variation within populations puzzled many scientists

why don’t dominant alleles drive recessive alleles out of populations?

• G.H. Hardy and W. Weinberg, in 1908, studied allele frequencies in a gene pool in a large population in which there is random mating, and in the

absence of forces that change allele frequencies, the original genotype proportions remain constant from generation to generation

because the proportions do not change, the genotypes are said to be in Hardy-Weinberg equilibrium

If the allele frequencies are not changing, the population is not evolving


• Hardy and Weinberg arrived at their conclusion by analyzing the frequencies of alleles in successive generations frequency is the proportion of individuals with

a certain characteristic compared to an entire population

knowing the frequency of the phenotype, one can calculate the frequency of the genotypes and alleles in the population


• By convention, the frequency of the more common of the two alleles is designated by the letter p and that of the less common allele by the letter q

• Because there are only two alleles, the sum of p and q must always equal 1


• In algebraic terms, the Hardy-Weinberg equilibrium is written as an equation

p2 + 2pq + q = 1

Figure 17.11 Hardy-Weinberg equilibrium

17.4 Genetic Change Within Populations: The Hardy-Weinberg Rule• The Hardy-Weinberg equilibrium only works if the

following five assumptions are met

1. The size of the population is very large or effectively infinite.

2. Individuals can mate with one another at random.

3. There is no mutation.

4. There is no immigration or emigration.

5. All alleles are replaced equally from generation to generation.


• Most human populations are large and randomly mating with respect to most traits and thus are similar to an ideal population envisioned by Hardy and Weinberg for example, the frequency of heterozygote

carriers for recessive genetic disorders can be estimated using the Hardy-Weinberg equilibrium

17.5 Agents of Evolution

• Five factors can alter the proportions of homozygotes and heterozygotes enough to produce significant deviations from Hardy-Weinberg predictions

1. mutation

1. migration

2. genetic drift

3. nonrandom mating

4. selection


• Mutation is a change in a nucleotide sequence in DNA mutation rates are generally too low to

significantly alter Hardy-Weinberg proportions mutations must affect the DNA of the germ

cells or the mutation will not be passed on to offspring

however, no matter how rare, mutation is the ultimate source of variation in a population


• Migration is the movement of individuals between populations the movement of individuals can be a

powerful force upsetting the genetic stability of natural populations

• the magnitude of the effects of migration is based on two factors

– the proportion of migrants in the population– the difference in allele frequencies between the migrants

and the original population


• Genetic drift describes random changes in allele frequencies in small populations, the frequencies of

particular alleles may be changed drastically by chance alone

in extreme cases, individual alleles of a given gene may be

• all represented in few individuals• accidentally lost if individuals fail to reproduce or

die

17.5 Agents of Evolution• A series of small populations that are isolated from one another

may come to differ strongly as the result of genetic drift

founder effect occurs when one of a few individuals migrate and become the founders of a new, isolated population at some distance from their place of origin

• the alleles that they carry will become a significant fraction of the new population’s genetic endowment

bottleneck effect occurs when a population is drastically reduced in size

• the surviving individuals constitute a random genetic sample of the original population


• Nonrandom mating occurs when individuals with certain genotypes mate with one another either more or less commonly than would be expected by chance sexual selection is choosing a mate based

on, often, physical characteristics nonrandom mating alters genotype

frequencies but not allele frequencies


• Selection, according to Darwin, occurs if some individuals leave behind more progeny than others the likelihood that they will do so is affected

by their individual characteristics• in artificial selection, a breeder selects for the

desired characteristics• in natural selection, conditions in nature

determine which kinds of individuals in a population are the most fit

Table 17.1 Agents of Evolution


• Stabilizing selection is a form of selection in which both extremes form an array of phenotypes are eliminated the result is an increase in the frequency of

the already common intermediate phenotype for example, human birthweight is under

stabilizing selection

Figure 17.14 (a) Forms of selection found in nature


• Disruptive selection is a form of selection in which the two extremes in an array of phenotypes become more common in the population selection acts to eliminate the intermediate

phenotypes for example, beak size in African blackbellied

seedcracker finches is under disruptive selection because the available seeds are only large or small

Figure 17.14 (b) Forms of selection found in nature


• Directional selection is a form of selection that occurs when selection acts to eliminate one extreme from an array of phenotypes for example, the enzyme lactate

dehydrogenase has a cold-adapted form that is more common in northern latitudes

Figure 17.14 (c) Forms of selection found in nature

Figure 17.13 Three kinds of natural selection

17.6 Sickle-Cell Anemia

• Sickle-cell anemia is a hereditary disease affecting hemoglobin molecules in the blood the disorder results from a single nucleotide

change in the gene encoding β-hemoglobin• this causes the sixth amino acid in the chain to

change from glutamic acid (very polar) to valine (nonpolar)

• as a result, the hemoglobin molecules clump together and deform the red blood cell into “sickle-shape”

Figure 17.16 Why the sickle-cell mutation causes hemoglobin to clump


• Persons homozygous for the sickle-cell genetic mutation frequently have a reduced lifespan the sickled form of hemoglobin does not carry oxygen

atoms the red blood cells that are sickled do not flow

smoothly through capillaries

• Heterozygous individuals make enough function hemoglobin to keep their red blood cells healthy


• The frequency of sickle-cell allele is about 0.12 in Central Africa one in 100 people is homozygous for the defective

allele and develops the fatal disorder sickle-cell anemia strikes roughly two African

Americans out of every thousand

• If natural selection drives evolution, why has natural selection not acted against the defective allele in Africa and eliminated it from the population here?


• The defective allele has not been eliminated from Central Africa because people who are heterozygous are much less susceptible to malaria the payoff in survival of heterozygotes makes

up for the price in death of homozygotes• this is called heterozygote advantage• stabilizing selection occurs because malarial

resistance counterbalances lethal anemia

Figure 17.17 How stabilizing selection maintains sickle-cell anemia

20% of individuals are heterozygous and survive malaria

1% of individuals are homozygous and die of sickle cell anemia

17.7 Selection on Color in Guppies

• Guppies are colorful fish that a popular for aquariums on the island of Trinidad, guppies are found in two

very different stream environments • in pools above waterfalls, the guppies are found along with

the killifish, a seldom predator of guppies• in pools below waterfalls, the guppies are found in pools

along with the pike cichlid, a voracious predator of guppies• guppies can move between pools by swimming upstream

during floods


• Guppy populations above and below waterfalls exhibit many differences guppies in high-predation pools are not as

colorful as guppies in low-predation pools guppies in high-predation pools tend to

reproduce at an earlier age and attain relatively smaller adult body sizes

these differences suggest the function of natural selection

Figure 17.18 The evolution of protective coloration in guppies


• John Endler conducted experiments on guppies to determine whether predation risk was really the driving selective force in this system

in a controlled laboratory setting, he created artificial pool environments in which he placed guppies in one of three conditions:

• with no predator present• with killifish present (low predation risk)• with cichlid present (high predation risk)

after 10 guppy generations, he found that the guppies from no or low predation risk pools were both larger and more colorful than the guppies from the high predation risk pool

he later found the same results in field experiments

Figure 17.19 Evolutionary change in spot number

17.8 The Biological Species Concept

• Speciation is the macroevolutionary process of forming new species from pre-existing species it involves successive change

• first, local populations become increasingly specialized

• then, if they become different enough, natural selection may act to keep them that way


• Ernst Mayr coined the biological species concept, which defines species as “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”

• Populations whose members do not mater with each other and cannot produce fertile offspring are said to be reproductively isolated and, thus, members of different species


• Barriers called reproductive isolating mechanisms cause reproductive isolation by preventing genetic exchange between species prezygotic isolating mechanisms

• prevent the formation of zygotes

postzygotic isolating mechanisms• prevent the proper functioning of zygotes once

they have formed


• There are six different prezygotic reproductive isolating mechanisms geographical isolation ecological isolation temporal isolation behavioral isolation mechanical isolation prevention of gamete fusion


• Geographical isolation occurs simply in cases when species exist in different areas and are not able to interbreed

• Ecological isolation results from two species who occur in the same area but utilize different portions of the environment and are unlikely to hybridize

Figure 17.20 Lions and tigers are ecologically isolated


• Temporal isolation results from two species having different reproductive periods, or breeding seasons, that preclude hybridization

• Behavioral isolation refers to the often elaborate courtship and mating rituals of some groups of animals, which tend to keep these species distinct in nature even if they inhabit the same places


• Mechanical isolation results from structural differences that prevent mating between related species of animals and plants

• Prevention of gamete fusion blocks the union of gametes even following successful mating


• If hybrid matings do occur, and zygotes are produced, many postzygotic factors may prevent those zygotes from developing into normal individuals in hybrids, the genetic complements of two species

may be so different that they cannot function together normally in embryonic development

even if hybrids survive the embryo stage, they may not develop normally

finally, many hybrids are sterile

Figure 17.21 Postzygotic isolation in leopard frogs.

Table 17.2 Isolating Mechanisms

17.10 Working with the Biological Species Concept

• Speciation is a two-part process first, initially identical populations must diverge second, reproductive isolation must evolve to

maintain these differences

• There are two mechanisms for speciation allopatric speciation

• geographically isolated populations become new species due to their evolving reproductive isolation

sympatric speciation• one species splits into two at a single locality


• Speciation is much more likely in geographically isolated populations for example, allopatric speciation can explain

how isolated populations of kingfishers in New Guinea are strikingly different from each other and from the mainland population

Figure 17.22 Populations can become geographically isolated for a variety of reasons


• Instantaneous sympatric speciation occurs when an individual is reproductively isolated from all other members of its species through polyploidy, a process common in plants polyploidy can arise in two ways

• autopolyploidy involves all sets of chromosomes come from the same individual

• allopolyploidy arises when two different species hybridize and the resulting offspring have a mixture of chromosomes

the polyploids can either self-reproduce or breed with other polyploids


• There are some problems associated with the biological species concept many recognized species of plants and some animals

can still hybridize• these observations suggest that reproductive isolation may

not be the only force maintaining species integrity

nothing is known about extinct species’ reproduction the concept is irrelevant to the many kinds of

organisms who do not reproduce sexually

Inquiry & Analysis

• At what latitude do fish populations exhibit the greatest variability in allele a frequency?

• Where along this latitudinal gradient in the frequency of allele a would you expect to find the highest frequency of heterozygous individuals?

Pie chart and graph on effect of latitude on allele frequency

The Living World Fifth Edition George B. Johnson Jonathan B. Losos Chapter 17 Evolution and Natural...

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