Download Chapter 22: Descent with Modification: A Darwinian View of Life

Chapter 22: Descent with Modification: A Darwinian View of Life
Evolution by natural selection (by Charles Darwin) was 1st proposed (1859) before classical genetics was
known (1899)
 Darwin made 2 important points:
1. modern species were produced by gradual descent by different ancient species
2. natural selection provided the mechanism for the evolutionary change
 evolution occurs at the level of the population on the gene pool
 evolution can now be defined as the change in the frequency of alleles of the gene pool
 natural selection is only one mechanism by which evolution proceeds
 evolution is a foundational concept of biology that provides a fabric of understanding that no other theory
provides
History of the Theory of Evolution
 up to 1750, the Book of Genesis provided the explanation for the origin of life
 even Aristotle (384-322 BC) believed that species were fixed and unchanging
o species could be arranged on a ladder from simple to complex
 Carolus Linnaeus, 1735, published a taxonomy of all known life
o Linnaeus’ classification put species into categories based on similarities that did not imply kinship
but instead a creator’s genius
 Paleontology began with Georges Cuvier (1769-1832)
o fossils, remains or traces of past organisms, were uncovered in sedimentary rocks
o theories grew on how sedimentary rock was created, through erosion, deposition, accumulation,
and then more erosion - to expose the fossils
o Cuvier noted that the lower the layer, the more dissimilar the fossils were from present life
forms
o Cuvier theorized that major catastrophes had occurred at transitions between layers causing
extinction of forms
 ~1750, evidence began to grow that the earth was old and organisms had evolved
o James Hutton (1726-1797) theorized that huge geologic changes took place through extremely
slow processes of erosion, deposition, and uplift
o Charles Lyell (1797-1875) proposed that present geologic processes were common throughout
Earth’s history
o both ideas presented a slow and gradual change in the landscape of earth, and a much older
planet than the bible indicated
 Jean Baptiste de Lamarck (1744-1829) - French, developed a theory of evolution based on the
inheritance of acquired characteristics
o in 1809, Lamarck proposed a theory of evolution
o organisms could acquire traits that made them better adapted to their environment
o as an organism acquired a trait, the new trait would pass information to its germ cells so that the
next generation could benefit from the trait
o the process occurred slowly so that only over many generations would visible changes occur
o organisms had an innate drive to become increasingly more complex
o T. D. Lysenko, a Soviet Agriculturalist, proposed Lamarck’s theory would work on agriculture
o if humans were given good growth conditions, they should become healthier and more
intelligent over time (1930’s)
Darwin and Wallace
 Charles Darwin (1809-1882) and Alfred Russel Wallace both presented natural selection at the Linnaean
Society of London in 1858
o both came to the same realization of natural selection through similar experiences

Darwin at the age of 16 was sent to study medicine at the University of Edinburgh
He left Edinburgh without degree and enrolled at Cambridge, receiving a B. A. in 1831
Darwin was appointed to the H.M.S. Beagle, a British naval survey ship embarking on a 5 year
mapping and collecting expedition (left 1831, returned 1837)
 he was the ship’s naturalist and companion to Captain Robert FitzRoy
 he had enormous amounts of time for exploration and collection, from rainforest, to
pampas, to desert, and mountains
 he noted that South American temperate regions had flora and fauna dissimilar to
Europe, but more similar to the Brazilian tropics
 South American fossils were more related to modern South American species that to
Europe’s
o Darwin’s most striking exploration was that of the Galapagos Islands, with species that were
distinct and found nowhere else but similar to South American forms
 in particular, the 13 species of finches he found there resembled South American finch
species but each differed in beak shape and size, adaptations for feeding on different
food supplies
o while sailing, he read Charles Lyell’s book Principles of Geology
 the book explained the earth was very old, animals and plants have appeared and
disappeared over time, giving Darwin a different-than-Biblical view of Earth’s history
 Darwin’s observations of South American flora and fauna supported this idea
o in 1838 Darwin had read Thomas Malthus essay, Essay on the Principle of Population (published
in 1879)
 the essay stated that populations grow geometrically and outgrow their food supplies
which grow arithmetically
 the essay made the phrase “struggle for existence”
 this planted the idea of “over-reproduction” for many species in Darwin’s mind
 in 1858, Darwin received Wallace’s manuscript which stated a similar theory to his own
 Darwin presented his theory and Wallace proposed his at the 1858 Linnaean
Society meeting
 in 1859, Darwin published The Origin of Species by Means of Natural Selection
 Darwin was painstaking about detail and logic and his ideas were VERY clearly
stated, he had a mountain of supporting evidence
 oddly, blending inheritance was believed at the time the book was published, yet
Darwin’s theory required modern genetics
 within a decade, most biologists believed the theory
Origin of Species
 all organisms descended from a common ancestor through “descent with modification”

“adaptations” described the changes that descendants develop in new habitats
 the theory tied all organisms together by lineage on a “giant tree of life” with closely related organisms
as separate twigs and distantly related organisms on separate branches
 taxonomy was suddenly explained by natural selection - all organisms actually shared a heritage
Ernst Mayr analysis of Natural Selection
 Observation 1: organisms reproduce exponentially
 Observation 2: despite exponential reproduction, populations remain stable
 Observation 3: resources are in finite supply
 inference 1: a struggle for existence occurs because more organisms are produced than
resources can support
 Observation 4: organisms vary in their characteristics
 Observation 5: variations are inherited
o
o
o
inference 2: the struggle for existence is not random; winners have adaptations that
made them winners
 inference 3: differential survival leads to gradual change with favorable adaptations
developing over unfavorable ones
Darwin’s mountain of evidence included artificial selection - the practice of picking the best of one generation
to produce the next generation
o if breeders can produce such profound changes in species over short periods of time, nature
could do much more in much longer periods of time
Natural Selection Important Points:
 individuals cannot evolve, only populations
 natural selection cannot work on acquired traits, only inherited ones
 adaptations that are favorable in one environment may be lethal on the same species in a different
environment
Natural Selection Theory Explains...
 the guppy Poecilia reticulata, exists in the Aripo River system in Trinidad
o John Endler and David Reznick have studied it in different microhabitats
o some pools have different predators than others - killifish or pike-cichlid
o killifish eat small guppies, whereas pike-cichlids eat large killifish
o guppies reproduce at younger ages in pike-cichlid pools whereas they reproduce at older ages in
killifish pools
o extensive testing has shown that even when guppies are moved from one habitat to another,
their reproductive behavior gradually changes to adapt to their predation pressure
o in pike-cichlid pools, guppies that reproduce early have more success than those that don’t get
to reproduce at all because they were eaten
o in killifish pools, if the guppies survive to adulthood, a long reproductive span is more
advantageous
 HIV has become drug resistant to drugs such as 3TC
o 3TC is a cytosine mimic and is used by reverse transcriptase to produce cDNA in host cells
o usage of 3TC terminates elongation of the DNA and prevents HIV reproduction
o 3TC resistant strains have no selective advantage normally, but in a patient taking 3TC, the drug acts
as a powerful selective agent and creates an entire population of 3TC-resistant HIV, which can then
be transmitted to the next host
In summary:
 natural selection “edits” the population of vulnerable adaptations and is specific to the habitat where it
works; what is beneficial in one area may not be beneficial in others
Evidence of Evolution
 Comparative Anatomy
o similar organisms have similar structures
o mammals and other animals share similarities - homologies
o homologous structures - structures with the same origin but in different species
o analogous structures - structures with similar functions but which have different origins
o analogous structures exist because different organisms evolved through different lineages and
solved their adaptations with different structures; ex: bat and bird wing
o vestigial structures - structures useless to an organism but which are homologous to the
functional structure of another organism
 ex: pelvic girdle of a Boa, ear muscles of a human, human appendix
 Embryology
o animal embryology reveals that dissimilar organisms have embryos that possess common
structures, mRNA, homeoboxes, and similar mechanisms of pattern formation

this suggests that the animal may have evolved from an ancestor where the adult does possess
the structure
o embryology also reveals that derived characters evolved by the reshaping of ancestral structures
 Biochemistry
o the sequencing of genomes and proteomics shows that dissimilar organisms have important
similarities, indicating common ancestry
 The Evidence from Biogeography
o biogeography is the study of the geographic distribution of organisms
o Creationism offers no rational explanation why unusual species are present on isolated islands
o adaptive radiation - the production of many new species adapted to unique habitats and niches
 ex: marsupials dominate Australia
o evolution will proceed independently on different isolated land masses
o biogeographical regions are large land masses characterized by the animal plants they contain
 ex: Ethiopian Region has African elephant, chimpanzee, gorilla, giraffe, rhinoceros, and
antelope
 ex: Oriental Region has Malay tapir, Indian elephant, and tiger
o there are often not clear cut distinction where one region ends an another begins; intermediate
regions exist with species of both regions
o isolated islands often have unique endemic species found nowhere else
 ex: 13 species of Galapagos finches, the Galapagos tortoise, and Galapagos iguanas
Chapter 23 - The Evolution of Populations
Population Genetics
 Darwin had no knowledge of Mendelian genetics, but held that when the secrets of genetics were finally
revealed, they would support his theory
 blending inheritance would eliminate the variations that natural selection requires
 Mendel published only a few years later, but the Austrian audience that received his paper (incredibly)
did not understand or appreciate it
 almost 50 years would pass before Mendelian inheritance would apply to natural selection
Modern Evolutionary Synthesis
 at first study, Mendelian inheritance was thought to contradict natural selection theory
 Darwin’s theory described polygenic traits that varied along a continuum while Mendel described simple
“either-or” traits
 then it was discovered that polygenic traits are based on loci that are each inherited in a Mendelian
fashion, and population genetics was born
 1940s - the modern synthesis - was developed
o integrating paleontology, biogeography, population genetics, and taxonomy
o stresses populations as units of evolution and the importance of natural selection as the force
that changes them through gradual accumulation of mutations over time
Population and Gene Pool
 population = all members of a given species in a defined area
 species = all organisms that can naturally interbreed to produce fertile offspring
 gene pool = all genes present within all members of a population
 individuals can be heterozygous or homozygous for a gene; if there is only one allele for the gene then
the allele is “fixed” at the locus
 allele frequencies can be calculated:
o 500 wildflowers with two alleles, CR (red) and CW (white); the heterozygotes are pink--flowered
o 4% (20) are CWCW; 64% (320) are CRCR; 32% (160) are CRCW
o with 1000 alleles in 500 plants:
o
frequency of CW = (20*2 + 160)/1000 = .2 = 20%
frequency of CR = (320*2 + 160) = .8 = 80%
 Hardy-Weinberg Genetics
o the Hardy-Weinberg theorem states that allele frequencies will not change (evolution will not
occur) despite meiosis and reproduction provided 5 criteria are met:
1. Extremely large population size.
2. No gene flow (no migration).
3. No mutations (new genes).
4. All mating is random (mates are not picked based on any criteria).
5. No natural selection.
o from the previous example, the chance of each genotype in the next generation is:
 CRCR: .8 * .8 = 0.64 = 64%
 CRCW: .8 * .2 = .16 = 16%; CWCR: .8 * .2 = .16 = 16%; total = 32%
 CWCW: .2 * .2 = .04 = 4%
o thus, meiosis and gene shuffling cannot alter gene frequency in a non-evolving population, they
only maintain it
o the general Hardy-Weinberg equation starts with alleles of frequency p and q
 p + q = 1 (the sum of their gene frequencies must be 1)
 in our previous example, p = frequency of the red allele, q = frequency of the white allele
 thus p * p = frequency of CRCR = .8 * .8 = .64 = 64%
 2pq = frequency of CRCW = 2 * .8 * .2 = .32 = 32%
 q * q = frequency of CWCW = .2 * .2 = .04 = 4%
 the Hardy Weinberg equation is written: p2 + 2pq + q2 = 1
 .64 + .32 = .04 = 1
o the Hardy-Weinberg equation is not exactly valid for any real population because the 5
conditions cannot be met in reality, but, since most equations evolve VERY slowly, it is a great
way to ESTIMATE the allele frequencies of heterozygotes in a population
o ex: PKU occurs in 1/10,000 newborns in the US, in aa individuals; Aa are carriers and AA are
homozygous normal
o the US population is very large, although migration does occur the migrants have approximately
the same gene frequencies, mutation rates for PKU are negligible, marriages are not made with
regard to PKU, and natural selection operates only against aa individuals
o thus: q2 = 1/10000; q = square root of 1/10000 = .01
o p = 1 - q = 1 - .01 = .99
o 2pq = 2 * .01 * .99 = .0198 = .02 = 2%
o thus, 2% of the US population are carriers, who can only be identified by pedigree and genetic
testing
Mutation and Sexual Recombination
 mutation = change in a nucleotide sequence in a cell’s DNA
o somatic mutations die with an organism while germ cell mutations may get passed along to the
next generation
o point mutations are generally harmless because of codon redundancy and the quantity of
noncoding DNA
o some point mutations change the amino acid composition of the resulting protein and thus have
an effect on the phenotype, altering its selection possibilities
o chromosomal mutations are possible, including duplications
o tansposons cause gene shuffling and gene duplication, possibly playing a large role in evolution
by creating a large copy base of genes that can mutate and carry different functions
 sexual recombination


is more important in the short term than mutation
it produces novel combinations of genes that are often very important under changing
conditions
o although bacteria and viruses lack regular mechanisms for recombination, they can pas DNA
between species
Natural Selection
 recombination causes shuffling and variety but cannot change gene frequency
 natural selection can alter gene frequency
o differential reproductive success occurs because individuals vary in reproductive success
o successful alleles become more common at the expense of their counterparts
 genetic drift - random chance fluctuations in allele frequencies
o is caused by sampling errors, such as those that occur in small groups
o the bottleneck effect occurs when a large population is reduced by disaster
o with the massive loss of individuals, some genes are completely lost and others are overrepresented in the small remaining population
o the founder effect is when a new population is originated from a small number of individuals
o again, some alleles are lost, some are underrepresented, and others are over-represented
 gene flow - migration of alleles to new populations
o tends to reduce the differences between populations
o new alleles can have large effects but the same alleles may migrate and alter the existing gene
frequencies rather than introduce new ones
 Natural Selection is the important mechanism of generating adapted organisms
o while gene flow and genetic drift alter gene frequency, they don’t adapt organisms
o while not all variation is heritable, natural selection maintains favorable adaptations in the
population
 Genetic Variation - the Building Block of Evolution
o genetic variation occurs within and between species
o quantitative characters vary along a continuum within a population, usually caused by polygenic
inheritance; ex: human height
o discrete characters are caused by an individual locus with a short list of alleles that cause
different phenotypes; ex: Mendel’s pea plant characters
 phenotypic polymorphism occurs when two ore more phenotypes are represented and
maintained in a population not by mutation alone
 ex: human blood type; not human height (quantitative character)
o average heterozygosity measures gene variability, the average percent of gene loci that are
heterozygous
 fruit flies - 14% of their loci are heterozygous
o nucleotide variability measures the mean level of difference in nucleotide sequences among
individuals of a population
 fruit flies - about 1% nucleotide variability
 humans - 0.1% nucleotide variability
o geographic variation
 occurs when phenotype/genotype frequencies differ in populations inhabiting different
areas, because of natural selection differential or genetic drift
 it can occur within an area with subpopulations where the environment is patchy (spatial
variation)
 cline - graded change in a trait along a geographic area, possibly caused by natural
selection
 ex: North American bird body size rises with increasing latitude (conservation of heat)
o
o
Natural Selection - A Closer Look
 natural selection depends on reproductive success, not physical prowess
 fitness = contribution of an organism to the next generation gene pool relative to its peers
 relative fitness compares reproductive success of an individual to the most fit individual in the
population
o if the most productive organism makes 100 offspring, its fitness is 1; an organism that produces
only 40 offspring has a fitness of 0.4
o fitness is 0 if an organism leaves no offspring, even if it survives longer than its counterparts,
because it is sterile
 The 3 modes of selection:
1. directional - a phenotype at one extreme is more fit than the other phenotypes; common during
periods of environmental change
2. disruptive selection - extreme phenotypes on either side of the average phenotype are more fit
than the average phenotype; has been documented in bird bill size, with large bills crushing large
seeds and small bills eating small seeds while the average bill size has no favored seeds;
important to early speciation by driving a “wedge” into the population
3. stabilizing selection - where the average phenotype is most fit; both extremes are selected
against - ex: human birth weights
Diploidy and Balancing Selection
 diploidy and balanced polymorphism counteract the tendency for natural selection to reduce genetic
variation
 diploidy “hides” recessive alleles in heterozygotes and maintains their frequency in the gene pool; only
the homozygous recessive is selected against
o the recessive alleles may be favorable in a different future environment
 balanced polymorphism occurs when natural selection maintains 2 or more phenotypes in a gene pool
o can be caused by heterozygote advantage where heterozygotes are more fit than homozygotes
o SCA is an example of heterozygote advantage; the maintained balanced polymorphism consists
of homozygous normal individuals which are fit in a malaria-free environment, heterozygous
individuals which are fit in a malaria environment; and homozygous disease individuals who are
selected against
o up to 32% of some African populations are heterozygotes (q = .2, p = .8)
 frequency-dependent selection occurs when the fitness of one phenotype changes with its frequency in
the population
o ex: predators develop search images of prey, so as a particular morph of prey becomes more
common, predators become more successful hunting and killing them
o neutral variations have little/no impact on fitness and are not affected by natural selection
o ex: human fingerprints; pseudogene inheritance
 Sexual Selection and Dimorphism
o sexual selection produces sexual dimorphism where male and females have different physical
phenotypes
o intrasexual selection is direct competition among members of the same sex whereas intersexual
selection is competition of one sex for the other sex
o the selection plays out in males competing amongst one another for access to females for
reproduction, and females are choosy about what males they will choose to breed
o in most vertebrates, males are larger and showier than females, as as result
o males put themselves at considerable risk during times of competition, by injury from opponents
and predators
o female choice perpetuates specific male phenotypes that were chosen for reproduction
o it has been shown that female choosiness may prevent the selection of a diseased mate
o

Sex is an Evolutionary Enigma
o asexual reproduction is much more efficient and rapid than sexual reproduction
o if two identical species compete with only mode of reproduction different, the asexual type wins
by sheer numbers of offspring
 in sexual organisms, half of the offspring must be male and cannot bear young, whereas
all members of the asexual offspring bear young
o disease is believed to play a major reason why sexual reproduction is so common
 cell surface markers are used by parasites to gain entry to organisms and cells
 meiosis and gene shuffling ensure that all offspring will have different cell-surface
markers and thus not all can be parasitized by pathogens
 humans have hundreds of alleles for cell-surface markers, an unusually high number for
any loci
 despite the rapid evolution of pathogens, sex produces a massive distribution of cell
marker combinations and thus successful offspring
o Natural Selection - Perfection?
 natural selection is limited by the historical context of an adaptation
 old features cannot be scrapped to build new ones “from scratch”
 adaptations are often compromises of structure and function
 ex: human knee is amazing in function, but often weak in structure
 natural selection is often affected by chance events
 ex: pregnant female turtle washing ashore a remote island
 it can only work with the alleles present in the gene pool
 new alleles cannot be made in response to new environments