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Bio Crash Course
Evolution
Darwin
• Charles Robert Darwin
– English naturalist and geologist, best known for
his contributions to evolutionary theory
– Travelled on the HMS Beagle 1831-1836, to the
Galapagos Island and observed species that lead
to his theory of evolution
– On the Origin of Species was a joint publication
with Alfred Russel Wallace
The Theory
– Darwin's Theory of Evolution is the widely held notion that
all life is related and has descended from a common
ancestor
– descent with modification: complex creatures evolve from
more simplistic ancestors naturally over time.
– random genetic mutations occur within an organism's
genetic code, the beneficial mutations are preserved
because they aid survival -- a process known as "natural
selection."
– These beneficial mutations are passed on to the next
generation.
– Over time, beneficial mutations accumulate and the result
is an entirely different organism (not just a variation of the
original, but an entirely different creature).
The Theory
– Natural selection acts to preserve and accumulate minor
advantageous genetic mutations. Suppose a member of a
species developed a functional advantage (it grew wings
and learned to fly). Its offspring would inherit that
advantage and pass it on to their offspring. The inferior
(disadvantaged) members of the same species would
gradually die out, leaving only the superior (advantaged)
members of the species.
– Natural selection acts only by taking advantage of slight
successive variations; she can never take a great and
sudden leap, but must advance by short and sure, though
slow steps." "If it could be demonstrated that any complex
organ existed, which could not possibly have been formed
by numerous, successive, slight modifications
Evidence of evolution
• Overview
– Evolution encompasses common descent and
adaptation
• Common descent
– All organisms are composed of cells
– All take chemicals and energy from the environment
– All reproduce, respond to stimuli, and evolve
– Earth is approximately 4.5 billion years old
• Prokaryotes arose about 3.5 billion years ago
• Eukaryotes about 2.1 billion years ago, but multicellularity
came much later at 700 million years ago
– Most evolutionary events occurred in less than 20% of the history
of life!
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Evidence of evolution cont’d.
• Fossil evidence
– Hard body parts are preserved in most cases
– Often embedded in sedimentary rock
• layers represent eras in geological time
• Each layer is older than the one above and younger than the one below
– Transitional fossils
• Especially significant- represent evolutionary links ex. Tiktalik
• Geological time scale
– History of Earth is divided into eras
• Based on dating of fossil evidence
– Relative dating method-noting which layer a group of fossils comes from
– Absolute method-radioactive carbon dating
– The geological time scale is shown on the following slide
• Note the examples of principal plant and animal life during each era
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Evidence of evolution cont’d.
• Biogeographical evidence
– Study of distribution of plants and animals
– Earth has 6 biogeographical regions
• Each has its own distinctive mix of species
– Barriers prevented evolving species from migrating to other
regions
– Continental drift-positions of continents and
oceans has shifted through time
• 225 million years ago continents were one land mass
• Distribution of fossils and existing species allows us to
determine approximate timeline
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Evidence of evolution cont’d.
• Anatomical evidence
– Common descent offers explanation for anatomical
similarities
• Homologous structures- have same function and same basic
structure, indicating a common ancestor
– Ex: human arm and whale forelimb
• Analogous structures- same basic function but different origins
– Ex: wing of bird and wing of bee
• Vestigial structures-anatomical structures fully functional in one
group and reduced, nonfunctional in another
– Humans have a tailbone (coccyx) but no tail
– Homology extends to embryonic structure
• Gill slits, notocord, pharyngeal pouches
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Convergent and Divergent
• Divergent Evolution:
- When a species diverges over time into two different species,
resulting in a species becoming less like the original one
- Live in different ways than the common ancestor
- Caused by migration or nearby extinction of a nearby environment
- Prime example are the dinosaurs
• Convergent Evolution:
- When unrelated organisms evolve similarities when adapting to
similar environments
- Live in the relatively same way as each other
- Emus, rheas, and ostriches are prime examples of convergent
evolution
Bones of the vertebrate forelimb
• Fig. 27.4
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Evidence of evolution cont’d.
• Biochemical evidence
– All organisms use same basic biochemical molecules
• DNA
• ATP
• Identical or nearly identical enzymes
– Many developmental genes are shared
– Degree of similarity between DNA base sequences
and amino acid sequences indicates the degree of
relatedness
• Evolution is one of the great unifying theories of
biology
27-11
27.3 Process of evolution
• Microevolution- a change in gene frequencies within a
population
– Population genetics
• Population- all members of a species occupying a particular area at
the same time
– Mating is purely random
– Genes are passed on according to Mendel’s laws
• Gene pool- the sum total of all alleles of all genes in a population
– Hardy and Weinberg used the binomial equation
p2+2pq+q2 to calculate the genotype and allele frequencies
in a population
– Predicts that gene frequencies will remain constant from
generation to generation
– This is illustrated in the following slide of Fig. 27.11
27-12
Process of evolution cont’d.
• The Hardy-Weinberg law
– Equilibrium of allele frequencies in a gene pool will remain
constant in each generation of a large sexually reproducing
population as long as the following 5 conditions are met
•
•
•
•
•
No mutations occur
No genetic drift occurs-random changes in gene frequency
No gene flow
Mating is random
No selection is occurring
– In real life these conditions are virtually never met
– Hardy-Weinberg law gives us a baseline by which to access
whether or not evolution has occurred
• Any change in allele frequencies indicates evolution
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Process of evolution cont’d.
• Five agents of evolutionary change
– Mutations
• Only source of new alleles in a population
• Can be an adaptive variation
– Genetic drift
• Change in allele frequencies due to chance
• 2 main mechanisms
– Founder effect-a few individuals found a colony and their
collective genes represent only a fraction of the original gene
pool
– Bottleneck effect-population is subjected to near extinction by a
disaster and so only a few genotypes contribute to next
generation
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Process of evolution cont’d.
• Five agents of evolutionary change cont’d.
– Gene flow
• Movement of alleles between populations
• Keeps the gene pools of 2 or more populations similar
– Nonrandom mating
• Occurs when individuals pair up according to
phenotype or genotype
• Inbreeding is an example-increases frequency of
recessive abnormalities
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Process of evolution cont’d.
• Five agents of evolutionary change cont’d.
– Natural selection
• Process by which populations adapt to their environment
• Charles Darwin explained evolution through natural
selection
• Evolution by natural selection requires the following
–
–
–
–
Variation-members of a population differ
Inheritance-differences are inheritable
Differential adaptedness-some differences have a survival benefit
Differential reproduction-better adapted individuals survive to
reproduce more offspring
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Process of evolution cont’d.
• Natural selection cont’d.
– Fitness- measured by the number of fertile
offspring produced by an individual
• Variations that can contribute to fitness can arise from
– Mutation
– Crossing over
– Independent assortment
– Most traits on which natural selection acts are
controlled by polygenic inheritance
– Range of phenotypes which follows a bell-shaped curve
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Speciation cont’d.
• Process of speciation
– Occurs when on species give rise to two species
• Occurs when reproductive isolation develops
– Allopatric speciation- geographical barriers separate a
population into 2 groups
• Premating and then postmating isolating mechanisms occur
– Sympatric speciation-occurs without geographical
barriers
• 2 subgroups of a population become reproductively isolated
• Best illustrated in plants- multiplication of chromosome
number in one individual may lead to asexual reproduction
and offspring with the same multiple chromosome numberisolates them from others
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Speciation cont’d.
• Adaptive radiation
– A specific type of speciation which gives rise to
many new species
– Galapagos Islands finches- studied by Darwin
• Example of adaptive radiation
• Mainland finches migrated to one of the islands
– Reproduced and eventually spread to all the islands
– Subjected to different environmental selection pressures
• Gave rise to many species of finches which differ
primarily in beak shape
– Adapted to allow use of different food sources
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Cladogram
• Fig. 27.23
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