Download Bio1B Evolution section (Thomson) Fall 2003 lecture 1: Unity and

Bio1B
Evolution section (Thomson)
Fall 2003
lecture 1: Unity and diversity of life (ch. 1: 1-15; ch. 21: 395-397)
• The unity and diversity of life (summary of topics)
• Evolution explains the dual faces of life on earth—unity and diversity
• Examples of evidence of evolution and the unitary origin of life (morphological, developmental,
fossil, and molecular)
• The unitary origin of life allows us to draw a "tree of life" with a common ancestor and
branching and evolution along branches, and extinctions
• Groupings of organisms are based on their sharing of a recent common ancestor with a new
trait shared by no other groups
• Description of the broad features of the history of life on earth
Theodosius Dobzhansky: "Nothing in biology makes sense except in the light of evolution"
Burton Guttman 1999: "Biology only makes sense if we recognize that organisms have acquired their
forms and functions through evolution, and evolution only makes sense if we view it ecologically,
understanding that each type of organism is suited for a particular environment.
A brief summary of features that influenced Darwin in his development of the theory of evolution
by natural selection:
• structural and developmental homologies that indicate close genetic relationships, e.g., see
Figure 1.2 of the textbook
• the fact that organisms are well adapted to their environments, but also that they are not
perfectly adapted
• the existence of vestigial traits which are no longer of use to the organism but show their
evolutionary origins, e.g., pelvic bones in whales
• the resemblance of some species in an island group to each other, e.g., the finches in the
Galapagos islands with divergence to occupy different ecological niches
• the fact that fossil and living species in an area appear to be related, e.g., extinct ground sloths
and living sloths in South America, also the extinct glyptodont and living armadillos
• artificial selection (our ability to selectively breed favored types in fruits and vegetables,
agricultural livestock, dogs, pigeons, etc.)
• The diversity of organisms living today and in the past have evolved from a common ancestor
by cladogenesis (branching of lineages) and anagenesis (evolution of a feature over time
along a branch), combined with extinctions to give—the "tree of life"
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Darwin closed The Origin of Species with the following: "There is grandeur in this view of life, with
its several powers, having been originally breathed into a few forms or into one; and that, whilst
this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning
endless forms most beautiful and wonderful have been, and are being evolved."
Additional molecular features that support evolution:
• Common molecular and biochemical features of all life forms
• Universal use of DNA as the genetic material, rules of genetic transmission, and genetic code
• Universal processes of gene expression, protein synthesis, and protein function
• DNA shows evidence of variation (diversity), the continuity of life, and the unitary origin of life
• Evolutionary trees based on DNA are strikingly similar to those based on anatomical,
developmental, and fossil evidence
• The genes and their functions are strikingly similar in different organisms
• The more closely related two species are to each other, the more similar their DNA, and vice
versa, the more distantly related two species are to each other, the more they will differ in
their DNA
The fossil record since Darwin: a tremendous number of new fossils have been and continue to be
found, and all support the evolutionary process. Some have clarified relationships between
groups, others have helped date the the first appearance of a group of animals, etc.
1858 Darwin and Wallace independently proposed the mechanism of biological evolution, natural
selection.
The idea of evolution, that living organisms have evolved from other organisms no longer alive
on earth, had been proposed by a number of researchers before 1858, but Darwin and Wallace
were the first to propose the method of natural selection. (Darwin defined evolution as descent
with modification. The modern definition is change over time in the genetic composition of a
population.)
Darwin realized that evolutionary change is a two-step process: the first step consists of the
production of inherited variation, and the second of the sorting of this variability by natural
selection.
Since there is high mortality amongst all species, traits that increase the probability that their
bearers will survive and reproduce are more likely to be passed on to their offspring, and to their
offspring’s offspring.
In the Darwinian view of the world, organisms evolved their particular features because
individuals with those features survived and reproduced better than individuals with different
features.
So, the living world is constantly changing, and evolutionary change occurs without any “goals.”
(We now know that chance (genetic drift) also plays a large role in the genetic differentiation of
populations, selectively equivalent (neutral) mutations can become established in a population.)
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unity and diversity: are the dual faces of life on earth.
Biological evolution accounts for the relatedness among organisms by explaining that the millions
of different species of plants, animals, and microorganisms that live on earth today are related by
descent from a common ancestor. We can draw a so-called "tree of life" to depict the evolutionary
relationships of all living organisms; groups of organisms are based on their sharing of a recent
common ancestor with a new (derived) trait shared by all members of the group, and by no other
groups.
Evolution also explains the great diversity of modern species. Populations of organisms with
adaptations enabling them to occupy ecological niches not occupied by similar organisms have a
greater chance of surviving. Species have diversified and have occupied more and more
ecological niches to take advantage of new resources.
unitary origin of life: modern animals and plants developed from "simpler" ancestors, which in turn
evolved from yet "simpler" types, and so on, back to the first appearance of an incredibly simple
microscopic ancestor, at some time in the distant past. While complexity has increased, all living
species are equally distant from this original common ancestor. No species per se is more
advanced than any other living species.
species: a set of interbreeding organisms. There are approximately 1.5 million living species that have
been named, with estimates of the total number of species currently living ranging from 5-100
million (the book says 30 million).
adaptation: a feature or characteristic, e.g., particular structure, physiology, or behavior, of an
organism enabling it to survive and reproduce in its natural environment better than if it lacked the
feature.
Adaptation to its environment means an organism possesses characteristics that enhance its
survival and reproductive success in that particular environment (Form fits Function).
Examples of adaptations: the wings of birds are adapted for various types of flight; many animals
are cryptically colored—that is, they blend into their surroundings so well as to be nearly
undetectable and are difficult for visually hunting predators to locate; other animals may resemble
inanimate objects commonly found in their habitat, such as leaves, twigs or sticks, etc.; animals
which are particularly distasteful or poisonous may advertise this by being very brightly colored
(warning coloration).
coevolution: evolution in two or more species in which the evolutionary changes in each species
influence the evolution of the other species.
Specific examples of coevolution are: flowering plants and the insects, birds, bats, etc. that pollinate
them, host parasite interactions, and predator prey interactions.
The 3 Domain system: the deepest divisions in the evolutionary history of life: bacteria, archae, and
eukaryotes is based primarily on molecular evidence (see Figure 1.10 of the text). The bacteria and
archae are prokaryotes—single cells that lack a nucleus. Eukaryotic cells contain nuclei, and are
divided into four groups—the protists and the classical Kingdoms of plants, fungi, and animals.
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Brief overview of history of life on earth:
billion years (= 109 years)
~15 Big Bang, formation of the universe
4.5 earth forms
3.8 origin of life?
3.5 oldest known definitive evidence of life on earth (prokaryotes)
1.75 oldest known definitive fossils of eukaryotes (well defined nucleus)
1.5 eukaryotes proliferate
million years (Myr)
700 origin of first animals, soft bodied marine invertebrates
543 (Paleozoic era): beginning of an abundant fossil record. Marine invertebrates with exoskeletons,
vertebrates, plants
245 (Mesozoic era): dinosaurs, mammals, birds and flowering plants appear
65 (Cenozoic era): diverse forms of mammals, birds, flowering plants, pollinating insects
5-7 Myr: humans in their most primitive form appear
100,000 - 200,000 years: modern humans appear
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