Transcript Evolution – change over time

Evolution – change over time
• Populations evolve
not individuals
• Adaptations
– Inherited traits that
enhance survival and
thus reproduction in a
particular environment
• Charles Darwin
– Theory of Evolution
• Based on many
observations
Path to Darwin’s Theory
500 BC
322 BC
~2000 yrs old
1700s
1809
1830
1831-1836
• Similar idea: simpler forms of life preceded more
complex forms from ancient Greeks (2500 years
ago)
• Aristotle believed that species are fixed
– Judeo-Christian thought (book of genesis)
• Divine creator; earth to be 6,000 years old
• Early fossil studies; extinct spp.; earth may be
older
• Lamarck: inheritance of acquired characteristics
• Lyell: Geologist; gradual change by nat. forces
• Darwin’s voyage
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Charles Darwin: British naturalist born 1809
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expanded on some existing ideas
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Did not address origin of life, but rather focused on explaining the vast diversity of life
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Geologists, naturalists, and scientists
provided supportive evidence; 5 year voyage around the world 1831-1836
Galapagos Islands
Wrote essay on evolution based on his observations/experiences 1844
Published “On the Origin of Species” 1859; under competition by Wallace
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Descent with modification: ancestral species could diversify into many descendent species
by accumulation of adaptations to environment
Hypothesized natural selection as driving force
What is natural selection?
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Over production of offspring
Limited natural resources
Heritable variations
Differential or unequal reproductive success
Offspring w/in a varied population, whose
characteristics best adapt them to the environment
are most likely to survive and reproduce
• more fit individuals leave more offspring than less fit
individuals
• with so much diversity in just a few
thousand years…
• even more diversity over thousands of
generations
– With natural selection over vast time allows for
changes to accumulate
Evidence for evolution
• Fossils: preservation of dead organic matter
35 mya
examples:
1.5 mya
375 mya
190 mya
40 mya
5,500 ya
Fossil record
• Layers of sediments
• Deposits pile up
over millions of
years forming strata
• Young on top; older
on bottom
– Read the layers
– compare preserved
fossils
– track changes over
time
Is Earth 6,000 years old or 4.6 billion?
• Radiometric Dating
– Dating geologic structures
by rate of radioactive decay
• Atomic elements decay at a fixed rate
– Half-life = time it takes for half of an element to
decay
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C14
or radiocarbon dating (plants and animals)
• unstable carbon isotope, 5730 years to decay
half of a sample
• C14 : C12 ratio is half in fossil than atm = 5730
• If C14 : C12 ratio is ¼ of atm = 11,460
– K40 has half life of 1.3 billion years
• Famous tests:
– Iceman (Italy, 1991); dated to 5500 years
old
– Shroud of Turin; evidence shows to be
about 700 years old rather than 2000
years old
Dr. Willard Libby – Atomic
physicist – Nobel Prize for
C14 dating work. Before
that, was a key researcher
in the development of the
atomic bomb.
Comparative anatomy & embryology
• Homology
– Similarities in form and
structure from
common ancestory
• E.g. mammal forelimbs
with different functions
• Similar embryological
stages
– Common structures
during similar early
development (e.g., gill
slits)
Molecular Biology
• Hereditary background
and proteins encoded
in DNA
– Compare gene
sequences
• Similar sequences – more
recent ancestor
• More dissimilar – more
distant ancestor
Why is evolution the best explanation to the
vast natural diversity?
• Mountains of evidence of various types
– e.g. fossils, radiometric dating, comparative anatomy
& embryology, molecular biology
• each agrees with the other
• provides tremendous support of evolution
theory…
– disagreement would be falsifiable evidence
• Theory still challenged
– Theory  guess or based on any belief
– Theory = falsifiable idea supported by extensive
evidence
• Theory of Gravitation, Theory of Relativity…principles based
on facts (e.g. earth is round)
Populations evolve
• Although natural
selection acts on
individuals, which
affects survival &
chances to reproduce
w/in an env., a
population changes
over time
• Population genetics
– Darwin’s and Mendel’s ideas
together
– Populations change
genetically over time
• Gene pool
– All alleles in a population
• Microevolution
– Change in allele frequencies
over time
– e.g. pesticide resistant allele
will increase while its alternate
decreases frequency
Agents of potential changes in allele frequency
• Natural Selection – pesticide example
• Non-random mating
– Plants closer to each other may get fertilized
– People sometimes choose similar mates (short
couples…)
• Mutation - creates new alleles
• Gene flow – gain or loss of alleles in a popln.
– Immigration or emmigration
• Genetic drift – change in gene pool due to
chance
– Founder effect – colonization of small group
– Bottleneck effect – reduction of population
Genetic drift –
bottleneck effect
Drastic reduction of
popln. size
– Earthquakes, floods,
fires, etc
– Surviving popln has
underrepresented
alleles
e.g. elephant seals were hunted down to 20;
restored now to 30k; found only 1 allele in
ea. of 24 genes; no variation
Selection pressures
• A particular phenotype selected for or against
depending on the environment
• Guppy example:
– 2 forces of natural selection working against each
other: mate preference and predator vulnerability
– Balance where females are attracted to males with
brighter colored tails; risk of attracting predators
♀
♂
– How could you test these selection pressures?
• Observe many generations…
• In predator-free environment?
– More brightly colored males with large tails evolved
• Re-introduced predators
– Less flashy males became more “fit”
Natural selection affects
populations
• Affects the distribution
of phenotypes
– Normal distribution of
varied fur frequencies
• Stabilizing selection
– Reduces extremes;
favors intermediates
– Most common
• Directional selection
– Acting against one
extreme or
environment favors
one extreme
– e.g. darker landscape
or, insects exposed to
pesticides
• Disruptive selection
– Environment is varied to favor both extremes
– e.g. patchy landscape with light soil and dark
rocks