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Before presenting some of the major
lines of evidence for the fact that evolution has taken place, it might be
useful to partition the scale of evolution into two categories: 1) the changes readily observed by population biologists
today (microevolution), and 2) the profound changes which have
accumulated from these population level processes (macroevolution).
Both scales of evolution provide evidence that evolution has
occurred. Microevolution
– the following three statements are meant to express the same basic
concept and define
microevolution as: 1) change in the genetic makeup of a population, 2)
change in a population’s allele or genotype frequencies, 3) change
within the gene pool of a population.
Gene pool is simply a term created to express the concept of
the collective total of all the genes in a population.
Basically, microevolution is evolution within a species such
that gene frequencies change over time. Given an accumulation of population
level changes, it is possible to look backward into history and see the
larger scale of change, including the formation of new species, genera,
families, etc. (macroevolution).
Evidence for microevolution comes from studies in population
genetics. Evidence for the
larger scale of change is presented below.
Macroevolution
- origin of groups at and above the species level.
Macroevolution is said to be the “Grand View of Evolution” and
includes the origin of new designs. New designs are often reflected in the taxonomic hierarchy,
e.g. the notochord is a new design unique to the phylum Chordata.
New designs are also reflected in phylogenies, e.g. a shared,
derived character common to all deuterostomes is the new formation of the
mouth from a 2nd embryonic derived opening into the animal
body. Evidence of Macroevolution: 1.
Fossils -- Paleontology- the study of fossils. Fossils are the visual
remains of once living organisms preserved by the earth. Fossils are
formed when sediment deposition buries dead bodies and the body remains
undergo preservation via mineralization or an impression of body remains
is preserved. Fossils show:
1) change over time; & 2) continuity of change.
ex.
Archaeopteryx
- has teeth and long bony tail like reptiles but has feathers
like birds. Archaeopteryx is
one of many known "missing links" between reptiles & birds
(see page p. 259, "Interpreting and misinterpreting the past"). 2.
Biogeography - study of the geographical distribution of
plants and animals; it includes both the historical distributions as
revealed by paleontology and present day distributions.
Mammal Distribution From Their origin 150 Million Years Ago To
Today. The concentration
of diverse marsupial (pouched) mammals in Australia and the restriction to
Australia of the only remaining monotremes (egg laying mammals) is thought
to be the result of evolution within these groups in the absence of
placental mammals on this island continent.
Placental mammals, having evolved on northern continental masses,
did not reach what is now Australia, thus leaving the marsupials to
diversify without placental mammal competition.
(Bats are an exception, they are indigenous to Australia, other
placental mammals did not reach the continent until the recent
introductions by man—the rabbit, a placental mammal, is especially
successful in Australia today as an exotic pest).
Presumably, placental mammals would have lead to the mass
extinction of marsupial species. At
least in South America, the introduction of placental mammals from the
north coincides with the extinction of many marsupial species known from
the S. A. fossil record. 3.
Comparative morphology - study of body parts, esp. internal
body parts. The evolutionary
view that mammals and birds are descendents Homologous structures – structures in different species that are similar because of common ancestry; structures in different species that have undergone morphological divergence becoming modified for different functions; Homologous structures have the same basic structure but may appear somewhat different depending on the degree of modification produced by evolution, ex. Whale flipper, bat wing, & human arm are all homologous (have same bones, are descended from common ancestor). But a bat’s wing & a fly’s wing are analogous-similar in function but not origin or structure. Analogous structures result from morphological convergence where dissimilar body parts become similar through evolution (ex. exoskeleton forms fly wing, feathers form bird wing, and skin forms bat wing--each wing type is analogous to the others). Comparative
anatomy reveals Vestigial organs
– structures reduced in size & of little (or modified) function,
e.g., in some snakes, such as pythons, evidence of their shared ancestry
with legged creatures is found in their vestigial pelvic girdle and leg
bones; some modern-day whales also have vestigial hind leg bones. 4.
Comparative Embryology -
ex. Humans, like all vertebrates, have a notochord, post anal tail
and gill pouches as embryos. Such
evidence does support the view that all vertebrates share a common
ancestor and that the differences between vertebrates today is the result
of “descent with modification” from this ancestral type.
A replay of the changes from the ancestral type was once thought to
exist in the changes during embryonic development.
The well-worn phrase "ontogeny
recapitulates phylogeny" (or stated more clearly, embryonic
development recapitulates evolutionary history) is a gross overstatement.
The point is, that useful features to unraveling the evolutionary
relationships among animals can be learned from comparative embryology. 5.
Molecular Biology – The genetic code is universal.
Also, degrees of relatedness are reflected in nucleotide base
sequence (DNA) similarity and in amino acid sequence similarity in
proteins. For example,
horse hemoglobin amino acid sequence is more similar to the sequence in
hedgehog hemoglobin (both the horse and hedgehog are mammals) than to the
sequence in hummingbird hemoglobin.
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