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The Theory of Evolution
Ch. 13
Biology
Ms. Haut
Lamarck’s Theory of Acquired Inheritance
(early 1800s)
• Jean Baptiste Lamarck observed fossil records and the current diversity of life and formulated his theory
• Suggested giraffes acquired long necks because ancestors stretched higher and higher into the trees to reach leaves– Lengthened neck was passed to offspring
Charles Darwin• Compared South American fossils
with living species there and elsewhere
• Observed organisms and their distributions on Galápagos Islands
Darwin’s Theory of Natural Selection
• Observations:– Overproduction of offspring leads to competition
of limited resources (food, space, breeding partners)
– Individuals of a population vary in characteristics, and many such traits are passed on to offspring
• Conclusions:– Individuals with inherited characteristics make
them best adapted to survive in their environment and reproduce and leave more offspring than less fit individuals
Natural Selection
• Prominent force in nature
• Support in the results of artificial selection—selective breeding of domesticated plants/animals
• Populations tend to evolve in response to environmental conditions
Populations are the Units of Evolution
• Population=group of individual organisms living in the same place at the same time
• Evolution is measured as the change in frequency of a given characteristic within a population over a succession of generations
What is a species?
• Biological Species Concept– Species—group of organisms that have the
potential to interbreed and produce fertile offspring
– Reproductively isolated by various factors preventing mixing with other species
Geographic Isolation can lead to Speciation
Subspecies of Deer mice
Islands are Living Examples of Speciation
• Adaptive Radiation– Evolution of many diversely adapted species
from a common ancestor– Example: Darwin’s Finches
• Finches with different beak shapes fit different ecological niches
1. Seeds blown over from mainland and form small colony
2. Gene pool isolated—evolves into new species B
3. Storms/other agents blow seeds to nearby island and evolve into species C
4. Some of species C recolonize the first island and cohabit with species B and some populate a new island
5. Speciation continues between new areas and previously colonized areas
Overview: Theory of Evolution
1. Variation exists within the genes of every population or species (due to random mutation)
2. In a particular environmnet, some individuals of a population/species are better suited and have more offspring (natural selection)
3. Over time, the traits that make certain individuals of a population able to survive and reproduced tend to spread in that population
4. There is clear evidence from fossils and many other sources that living species evolved from organisms that are extinct
Evidence of Evolution
• Fossil Record
• Biogeography
• Comparative anatomy
• Comparative embryology
• Molecular Biology
Fossilization
• Most fossils are actually casts of animals or plants. • Animal dies and sinks to the sea floor. • Tissue begins to decay and is buried under layers of
sediment such as mud or sand. • These layers become rock. • The hard parts of the animal are
replaced with minerals such as iron pyrites or silica.
• These minerals form the fossil.
• Usually fossils show the hard parts of the animal or plant - such as shell or bones.
• Trace fossils—evidence of living plants or animals, such as worm burrows or dinosaur footprints.
• Most fossils are found in sedimentary rocks - rocks which were created when shells or small loose bits of rock are laid down in layers (limestone, sandstone, clay and chalk)
Fossil Record
Trilobite510 million years ago
Chancelloria eros543 million years ago
Keichousaurus hui250 million years ago
Knightia humilis, Diplomystus dentatus, Mioplosus
Plantanus wyomingensis (sycamore)
54 million years ago
Determining Age of Fossils
• Relative age—determined by position in sedimentary rock
• Absolute age—determined by radiometric dating (radioactive isotopes)– Based on half-life of an isotope—period it takes
for half the radioactive material to decay
Carbon-14 (C-14) Up to 50,000 yrs
Potassium-40 (K-40) Up to 1.28 billion yrs
Number of Half-Life C-14 Remaining
(atoms)
0 100
1 50
2 25
3 12.5
4 6.25
5 3.13
Half-Life of Carbon 14
0
20
40
60
80
100
120
1 2 3 4 5 6
Number of Half-Life
Nu
mb
er
of
Ato
ms o
f C
-14
Evidence: Biogeography
• Geographical distribution of species suggests organisms evolve from common ancestors
• Island forms are most similar to forms found on the closest mainland, rather than forms on ecologically similar, but more distant islands
Comparative Anatomy
• Comparison of body structures between different species– Similarities give signs of common descent
• Homologous structures—features that have similar structure but have different functions
Comparative Anatomy
• Comparison of body structures between different species– Similarities give signs of common descent
• Homologous structures—features that have similar structure but have different functions
• Vestigial structures—Small body structures that may have been functional in the ancestors of a species, but has no real function at the present time (appendix, tail bone)
Comparative embryology
• Different organisms go through similar embryonic stages
• All vertebrates have an embryonic stage in which gill pouches appear in the throat region—evidence of a common ancestor
Molecular Biology
• Study of molecular basis of genes and gene expression
• Universality of genetic code
• Conservation of amino acid sequences in proteins such as hemoglobin
Causes of Microevolution• Mutation—random change in organism’s DNA
that creates a new allele– Rare events– Ultimate source of the genetic variation that
initiates evolution
Insecticide-resistant
Populations
Modes of Natural Selection
• Original population demonstrates the continuum of shell color (light to dark)
Modes of Natural Selection
• Divergence– Accumulation of
differences between groups
– Leads to Speciation—process by which new species form
Reproductive Barriers Keep Species Separate
• Prezygotic barriers—prevent mating or fertilization between species– Habitat isolation—species live in same general
area but not the same places– Behavioral isolation—special signals
recognized– Temporal isolation—breeding occurs at
different times– Mechanical isolation—anatomically
incompatible– Gametic isolation—gamete recognition
Reproductive Barriers Keep Species Separate
• Postzygotic barriers—Prevent the hybrid zygote from developing into a viable, fertile adult
– Reduced hybrid viability—embryo aborted
– Reduced hybrid fertility—offspring sterile
– Hybrid breakdown—offspring of hybrids sterile
Overview: Natural Selection
1. All species have genetic variation2. The environment presents many different challenges to an
individual’s ability to reproduce3. Organisms tend to produce more offspring than their
environment can support; thus, individuals of a species often compete with one another to survive
4. Individuals within a population that are better able to cope with the challenges of their environment tend to leave more offspring than those less suited to the environment
5. The traits of the individuals best suited to a particular environment tend to increase in a population over time
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