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• 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
500 BC
322 BC
~2000 yrs old
1700s
1809
1830
1831-1836
• Charles Darwin: British naturalist born 1809– expanded on some existing ideas
• Geologists, naturalists, and scientists• Did not address origin of life, but rather focused on explaining the vast diversity of life
– 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
– 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?
• 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 matterexamples:
1.5 mya
5,500 ya40 mya
35 mya
375 mya
190 mya
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
– 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