Download Chapter 11: Evolution and Natural Selection

Chapter 11: Evolution and Natural Selection
11.1 Evolution: Getting from There to Here
The word “evolution” refers to how an entity changes through time
Darwin initially used the phrase “descent with modification” to explain the concept of evolution
The concept of evolution helps explain the great paradox of biology:
In life there exists both unity and diversity
Natural selection, the process that leads to evolution, occurs in steps
1. Gene variation exists among individuals in a population
2. This variation is often passed to offspring
3. All populations overproduce offspring
4. Individuals with traits that aid survival and reproduction have a better chance of contributing to
the next generation
5. Over time, the population changes such that the traits of the more successful reproducers are
more prevalent
Macroevolution
Evolutionary change on a grand scale
Encompasses the origins of new species and major episodes of extinction
•Microevolution
Evolutionary change on a small scale
Encompasses the genetic changes that occur within a species over time
These changes are the result of changes in gene frequencies
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Jean-Baptiste Lamarck proposed that evolution occurred by inheritance of acquired characteristics
Individuals passed on to offspring body and behavior changes acquired during their lives
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In Darwin’s theory, by contrast, variation is not created by experience
It already exists when selection acts on it
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The Rate of Evolution
Different kinds of organisms evolve at different rates
Bacteria evolve much faster than eukaryotes
The rate of evolution also differs within the same group of species
In punctuated equilibrium, evolution occurs in spurts
In gradualism, evolution occurs in a gradual, uniform way
11.2 The Evidence For Evolution
Evidence for evolution comes from the following
Fossil record
Molecular record
Anatomical record
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Fossil Record
Provides the most direct evidence for macroevolution
Fossils are the preserved remains, tracks, or traces of once-living organisms
They form when organisms become buried in sediment and calcium in hard surfaces
mineralizes
•Arraying fossils according to age often provides evidence of successive evolutionary change
Fossils have been found linking all the major groups
•The forms linking mammals to reptiles are particularly well known
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Molecular Record
•New alleles arise by mutations and they come to predominance through favorable selection
Thus, evolutionary changes involve a continual accumulation of genetic changes
Distantly-related organisms accumulate a greater number of evolutionary differences than closelyrelated ones
This divergence is seen among vertebrates in the 146-
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This same pattern of divergence is seen with DNA sequences, such as that of the cytochrome c gene
•The changes appear to accumulate at a constant rate
This phenomenon is referred to as a molecular clock
Note: Different proteins evolve at different rates
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Anatomical Record
•All vertebrates share a basic set of developmental instructions
•Homologous structures
Have different structure and function but are all derived from the same part of a common ancestor
•Analogous structures
Resemble each other as a result of parallel evolutionary adaptations to similar environments
They are the result of convergent evolution
Different animals often adapt in similar fashion when challenged by similar opportunities
•Vestigial organs
Structures that are no longer in use
The human appendix
Apes have a much larger appendix that is involved in digestion
11.3 Evolution’s Critics
•Critics of evolution raise seven principal objections
1. Evolution is not solidly demonstrated
2. There are no fossil intermediates
3. The intelligent design argument
4. Evolution violates the 2nd law of thermodynamics
5. Proteins are too improbable
6. Natural selection does not imply evolution
7. The irreducible complexity argument
All of these objections are without scientific merit.
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11.4 Genetic Change Within Populations: The Hardy-Weinberg Rule
Population genetics is the study of the properties of genes in a population
Genetic variation in populations puzzled scientists
Dominant alleles were believed to drive recessive alleles out of populations
In 1908, G. Hardy and W. Weinberg pointed out that in large populations with random mating,
allele frequencies remain constant
Dominant alleles do not, in fact, replace recessive ones
A population that is in Hardy-Weinberg equilibrium is not evolving
Hardy and Weinberg came to their conclusion by analyzing allele frequencies in successive
generations
Number of individuals falling within a category
Frequency = Total number of individuals being considered
If a population of 100 cats has 84 black and 16 white
Then the frequencies of black and white phenotypes are 0.84 and 0.16, respectively
B allele  Black color b allele  White color
By convention
The more common allele (B) is designated p
The less common allele (b) is designated q
p+q=1
The Hardy-Weinberg equilibrium can be written as an equation
(p + q)2 = p2 + 2pq + q2
The equation allows calculation of allele frequencies
Frequency of white (bb) cats = 16/100 = 0.16
=> q2 = 0.16
=> q = √0.16 = 0.4
p + q =1 => p = 1 – q =1 – 0.4 = 0.6
What about genotype frequencies?
Frequency of the homozygous dominant genotype is
p2= (0.6)2 = 0.36 (36 out of 100 cats are black (BB)
Frequency of the heterozygous genotype is
2pq = 2(0.6)(0.4) = 0.48 (48 out of 100 cats are black (Bb)
Hardy-Weinberg Assumptions
•The Hardy-Weinberg equation is true only if the following five assumptions are met
1. Large population size
2. Random mating
3. No mutation
4. No migration
5. No natural selection
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11.5 Why Allele Frequencies Change
Five evolutionary forces can significantly alter the allele frequencies of a population
1. Mutation
2. Migration
3. Genetic drift
4. Nonrandom mating
5. Selection
Mutation
Errors in DNA replication
The ultimate source of new variation
Mutation rates are too low to significantly alter allele frequencies on their own
Migration
Movement of individuals from one population to another
Immigration: movement into a population
Emigration: movement out of a population
A very potent agent of change
Genetic Drift
Random loss of alleles - More likely to occur in smaller population
Founder effect - Small group of individuals establishes a population in a new location
Bottleneck effect - A sudden decrease in population size to natural forces
Nonrandom Mating
Mating that occurs more or less frequently than expected by chance
Inbreeding: Mating with relatives, Increases homozygosity
Outbreeding: Mating with non-relatives, Increases heterozygosity
Selection
Some individuals leave behind more offspring than others
Artificial selection - Breeder selects for desired characteristics
Natural selection - Environment selects for adapted characteristics
11.6 Forms of Selection
Selection is a statistical concept
One cannot predict the fate of any single individual
But it is possible to predict which kind of individual will tend to become more common in
a population
Three types of natural selection have been identified
Stabilizing selection
Acts to eliminate both extreme phenotypes
Disruptive selection
Acts to eliminate intermediate phenotypes
Directional selection
Acts to eliminate a single extreme phenotype
Stabilizing Selection - In humans, infants with intermediate weight at birth have the highest
survival rate
In chicken, eggs of intermediate weight have the highest hatching success
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Disruptive Selection - In the African seed-cracker finch, large- and small-beaked birds predominate
Intermediate-beaked birds are at a disadvantage
Unable to open large seeds
Too clumsy to open small seeds
Directional Selection - Drosophila flies that flew toward light were eliminated from the population
The remaining flies were mated and the experiment repeated for 20 generations
11.7 Sickle-Cell Anemia
Sickle-cell anemia is a hereditary disease affecting hemoglobin molecules in the blood
It was first detected on December 31st, 1904
The sickle-cell mutation changes the 6th
-hemoglobin chain from glutamic acid to
valine
This causes hemoglobin molecules to clump - The result is sickled red blood cells
In normal RBCs, the hemoglobin chains do not clump
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Sickle-cell homozygosity leads to a reduced life span
Heterozygosity produces enough hemoglobin to keep RBCs healthy
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The disease originated in Central Africa
It affects 1 in 500 African Americans
But it is almost unknown in other racial groups
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Why is the defective allele still around?
People who are heterozygous for the sickle-cell allele have less susceptibility to malaria
This is an example of heterozygote advantage
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Stabilizing selection is thus acting on the sickle-cell allele
It occurs because malarial resistance counterbalances lethal anemia
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11.8 Selection on Color in Guppies
Poecilia reticulata (guppy) is a popular aquarium fish
In nature, it is found in small streams in NE South America and in mountainous streams in
Trinidad
Due to dispersal barriers, guppies can be found in two different pool environments
Below waterfalls, where risk of predation is high
Above waterfalls, where risk of predation is low
In high-predation pools, males
Exhibit drab coloration
Reproduce younger
Attain relatively small adult sizes
The evolution of these differences in guppies was experimentally tested in laboratory greenhouses
Website for simulation: http://www.pbs.org/wgbh/evolution/educators/lessons/lesson4/act2.html
11.9 The Biological Species Concept
•Speciation is the species-forming process - It involves progressive change
1. Local populations become increasingly specialized
2. Natural selection acts to keep them different enough
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Ernst Mayr coined the biological species concept
“Species are groups of actually or potentially interbreeding natural populations, which are
reproductively isolated from other such groups”
•Reproductively isolated populations
Populations whose members do not mate with each other or who cannot produce fertile offspring
11.10 Isolating Mechanisms
Reproductive isolating mechanisms are the barriers that prevent genetic exchange between species
Prezygotic isolating mechanisms
Prevent the formation of zygotes
Postzygotic isolating mechanisms
Prevent the proper functioning of zygotes after they have formed
Prezygotic Isolating Mechanisms
Geographic isolation
Ecological isolation
Hybridization has been successful in captivity
But it does not occur in the wild
Temporal isolation
Behavioral isolation
Mechanical isolation
Prevention of gamete fusion
Postzygotic Isolating Mechanisms
Examples:
Hybridization between sheep and goats produces embryos that die in the earliest
embryonic stages
Leopard frogs of the United States are a group of similar species, and NOT a single species
as was long assumed
Hybrids between them produced defective embryos in the lab
11.11 Working with the Biological Species Concept
•Speciation is a two-part process
–1. Identical populations must diverge
–2. Reproductive isolation must evolve to maintain these differences
Speciation occurs much more readily in the absence of gene flow
–This much more likely in geographically isolated populations
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Populations can become geographically isolated for several reasons
Allopatric speciation
The differentiation of geographically isolated populations into distinct species
•Sympatric speciation
The differentiation of populations within a common geographical area into distinct species
Instantaneous sympatric speciation may occur through polyploidy
More than two sets of chromosomes
Autopolyploidy  All chromosomes from one species
Allopolyploidy  Chromosomes derived from two species, via hybridization
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Much more common in plants than animals
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Problems with the Biological Species Concept
The biological species concept has been criticized for several reasons
1. The extent to which all species are truly are reproductively isolated
It is becoming increasingly evident that hybridization is not that uncommon in plants
and animals
2. It can be difficult to apply the concept to populations that do not occur together in nature
It is not possible to observe whether they would interbreed naturally
3. The concept is more limited than its name would imply
Many organisms are asexual and reproduce without mating
For these reasons, other concepts have been proposed to define a species
However, none has universal applicability
Indeed, because of the diversity of organisms, it may be that there is no single definition of a species
11.12 The Pace of Evolution
•The standard view since Darwin was that evolutionary change occurred extremely slowly
Imperceptible changes accumulate such that, over thousands or millions of years, major changes
could occur
This is termed gradualism
In 1972, Niles Eldredge and Stephen Jay Gould proposed the Punctuated Equilibrium hypothesis
Evolutionary change occurs in bursts separated by long periods of little or no evolutionary change
(termed stasis)
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Examples of both gradualism and punctuated equilibrium exist
So speciation clearly occurs in different ways
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However, the idea that speciation is necessarily linked to phenotypic change has not been supported
Speciation can occur without phenotypic change
Phenotypic change can occur within a species in the absence of speciation
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