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