XV. EVOLUTION: EVIDENCE AND THEORY
A. The Fossil Record
1. Nature Of Fossils
a. Fossil – a trace of a long-dead organism. They are often found
in layers of sedimentary rock.
1. Sediment such as dust, sand, or mud is deposited by
wind or water.
2. There are two principal forms of fossil:
i. Mold is an imprint in rock in the shape of an
organism.
ii. Cast is a rocklike model of an organism.
3. Robert Hooke concluded that fossils are the remains
of plants and animals.
2. Distribution Of Fossils
a. Nicolaus Steno made an important contribution toward a
modern understanding of Earth’s biological history.
1. Law of Superposition – stated that successive layers, or
stratum, of rock or soil were deposited by wind or
water.
2. Using Steno’s law, observers could establish:
i. The relative age of a fossil (one fossil is
younger/older than the other).
ii. The absolute age of a fossil (age in years),
which could involve radiological dating.
3. Fossil-bearing strata show that species appeared,
existed for a while, and then disappeared.
i. Extinction – the permanent disappearance of an
entire species from earth.
ii. Mass Extinctions – brief periods during which
large numbers of species vanish.
4. Biogeography – the study of the geographical
distribution of fossils and living organisms.
5. A comparison of fossil types shows that new organisms
arise in areas where similar ones lived.
B. Theories of Evolution
1. Lamarck’s Explanation
a. Jean Baptiste de Lamarck proposed that similar species
descended from a common ancestor.
1. He hypothesized that acquired traits were passed on to
offspring.
2. Acquired Trait – a trait that is not determined by genes.
It arises as a result of an organism’s experience or
behavior.
3. Ex. Lamarck believed that the webbed foot of shore
birds resulted from repeated stretching of membranes
between toes.
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i. Offspring of birds with webbed feet would
acquire that trait from their parents.
ii. If an organism did not use a certain part of its
body, a smaller version of that part would be
passed on.
4. His hypothesis was easily disproved, but laid out the
foundations for the eventual work of Charles Darwin.
2. The Beginning Of Evolutionary Thought
a. Natural Selection – organisms best suited for their environment
reproduce more successfully.
1. Population – a group of organisms of the same species.
2. Over time, the proportion of organisms with more
favorable traits increases.
b. Charles Darwin was born in 1809 to a wealthy British
physician.
1. In 1831, Darwin sailed aboard the H.M.S. Beagle as the
ship’s naturalist for five years.
i. Uniformitarianism – the geological structure of
Earth results from cycles. Detailed in Principles
of Geology by Charles Lyell.
ii. In Chile, Darwin observed fossil shells in rock
beds more than 4 km above ground. He
reasoned that the formation of mountains slowly
changed habitats.
c. Alfred Wallace approached Darwin with his own paper on
natural selection, prompting Darwin to publish his own work.
1. Both their hypotheses were presented to the Linnean
Society of London in 1858.
2. A year later, Darwin published On the Origin of
Species.
3. Darwin’s Theories
a. Darwin’s ideas about evolution and natural selection are
summed up in two theories:
1. Descent with modification – newer forms appearing in
fossil records are actually the modified descendants of
older species.
2. Modification by natural selection – states how
evolution occurs. Darwin was influenced by Thomas
Malthus.
b. Adapt – an organism becomes better suited to its habitat.
c. Fitness – a single organism’s genetic contribution.
C. Evolution in Process
1. Evidence Of Evolution
a. Homologous Features – similar features that originated in a
shared ancestor.
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1. Ex. beaks of birds, similar forelimbs of penguins,
alligators, bats, and humans.
b. Analogous Features – serve identical functions and look
somewhat alike.
1. They have very different embryological development
and may differ in anatomy.
2. Ex. A hummingbird and a humming moth both hover to
feed off of flower nectar, but do not share anatomy or
embryological structure.
c. Vestigial Features – features that seem to serve no useful
function in an organism.
1. A vestigial feature in a modern organism is evidence
that the structure was used in some ancestor.
2. Ex. humans tailbones, appendix, whale pelvis.
d. The early stages of different vertebrate embryos are strikingly
similar to each other.
e. Ernst Haeckel stated that embryological development repeats
evolutionary history.
1. His statement was over exaggerated; as during no stage
of development does a fish resemble a gorilla.
2. All vertebrate embryos are similar, but those
similarities fade as development continues.
f. Darwin hypothesized that more similar forms of organisms
have a more recent common ancestor than less-similar forms.
2. Patterns Of Evolution
a. Coevolution – the change of two or more species in close
association with each other. Ex. plants and pollinators.
b. Convergent Evolution – organisms that appear to be similar
are not related at all. Ex. sharks and dolphins.
c. Divergent Evolution – two or more related populations or
species become increasingly dissimilar.
1. Adaptive Radiation – many related species evolve from
a single ancestral species. Ex. Galápagos finches,
which diverged in response to food availability.
2. Artificial Selection – selection for reproductive success
in plants and animals that is directed by humans. Ex.
domestic dogs are all members of the same species but
come in multiple “breeds”.