Earth History
GEOL 2110
Lecture 6
EVOLUTION
Part II
Major Concepts
• In the late 1700/early 1800’s, a number of natural scientists
had proposed that evolution of life was likely, however,
without a mechanism to explain evolution, the idea was
generally dismissed.
• Darwin’s Origin of Species (1859) not only provided
multiple lines of evidence for evolution, but moreover,
proposed a plausible cause (natural selection).
• With the discovery of genetic theory in the early 1900’s
providing a mechanism for evolution, it became
overwhelmingly accepted as a “fact” of science.
• The genetics within isolated, small populations provided
the triggers for the “origin” of new species
• The mechanisms of evolution are still being figured out, but
not the concept of evolution is not in doubt and has
achieved the status of a paradigm of science.
Hypothesis/Theory/Paradigm
The Scientific Method - Seeking understanding
• Observations of phenomenon lead to the
development of multiple possible interpretations or
explanations - hypotheses
• Physical, chemical, mathematical or thought
experiments are devised that attempt to prove the
various hypothesis wrong
• A set of related hypotheses that withstand repeated
testing (are well corroborated) and explain a wide
range of observations develop the status of theory
• Theories that continue to stand up to scrutiny evolve
into paradigm status - largely unquestioned until
too many anomalies develop
Charactersitics of a Scientific Theory
• Based on facts, but not a statement of
certainty or truth
• Testable – holds up to repeated testing
• Predictive – forward and backward in time
• Explains many related natural phenomenon
• Deemed by a majority of scientists to have a
high probability of being correct
• Questioning and skepticism are evidence of
healthy science, not evidence of doubt and
uncertainty
Genetic Theory
The Mechanism behind Evolution
• By the mid-1800, the high probability that
evolution happens was generally accepted
• Based on Darwin’s (1859) evidence and well
reasoned arguments, the idea that natural
selection of favorable characteristics within
populations of organisms drove evolution was
also generally accepted.
• However, Darwin did not explain how new
characteristics (and thus new species)
originated
Genetic Theory
The Mechanism behind Evolution
Gregor Mendel (1822-1884)
• Austrian monk, plant breeder
• Devised experiments cross-breeding pea
plants with purple and white flowers,
examining about 28,000 plants in the
course of his experiments.
• He chose peas for his experiments because
he could grow them easily, could control
their pollination, and they had simple
patterns of heredity.
• Mendel’s math-rich results were published
Pisum
in 1865 in an obscure journal, but was not
sativum
recognized until ~1900.
Genetic Theory
The Mechanism behind Evolution
• Mendel discovered that when crossbreeding red and white flower colors, a
blended color did not result, but rather
colors remained distinct with the red
color being more common - discrete
inheritance.
• Morever, the first generation would
produce only red colors and in the
second generation 25% of the flowers
would be white, 75% red
• Devised the notion of dominant and
recessive hereditary traits. Each flower
had 2 possible genetic factors (alleles).
The recessive trait would only appear
when two recessive alleles would merge
.
Genetic Theory
The Mechanism behind Evolution
• Mendel’s laws of heredity formed the
observational foundation of genetic
theory, but the biochemical basis for
heredity was still a mystery
• In mid-century, the carrier of genetic
information was discovered in the form
of DNA, which occurs in the nucleus of
all cells
• Variations in genetic code (genotype)
stored in DNA gives rise to variations in
physical appearance (phenotype )
• Variations could form by mutation of
the genes, alteration of the DNA
strands, the way that daughter DNA
Double Helix model for DeoxyriboNucleic Acid
recombine in the offspring
Watson and Crick (1953)
The Genetic Code
Universal to ALL Life
20 Amino
Acids are
coded by
sequences of
3 Nucleotides
Uracil
Cytosine
Adenine
Guanine
Revisiting Lamarck’s Giraffes
Neo-Darwinism
An Explanation for the Origin of Species
• Natural Selection could explain gradual changes in the
phenotype of an organism (phyletic gradualism), but
what gave rise to abrupt changes and new species?
• The development of population genetics in the 1930’s
& 40’s began looking at evolution by natural selection
in a mathematical and statistical manner was called Synthetic Theory of Evolution or Neo-Darwinism
• 1938 – Genetics and the Origin of Species (Dobzhansky)
integrated genetic theory with evolutionary biology
• 1942 – Systematics and the Origin of Species (Mayr) applied
Natural Selection Theory to current species
• 1944 – Tempo and Mode of Evolution (Simpson) showed
evidence evolution in the fossil record
Neo-Darwinism
An Explanation for the Origin of Species
Ernst Mayr (1904-2005)
Systematics and the Origin of Species (1942)
Population Genetics shows:
• The genetics of large populations of
are stable because new character
traits are muted by the large gene
pool; little chance of genetic
differentiation into new species
• In small, isolated populations, genetic anomalies can become
more prominent quickly (founder effect)
• Genetic isolation of a subpopulation that is separated from the
main population, by migration, geographic barriers, or lack of
mobility, causes genetic divergence - allopatric speciation
Neo-Darwinism
An Explanation for the Origin of Species
Punctuated Equilibrium
The theory proposes that most
evolution is marked by long periods
of evolutionary stability, which is
punctuated by rare instances of
branching evolution. This would be
expected from periodic events of
genetic isolation producing allotropic
speciation.
The theory was contrasted against
Stephen J. Gould (1941-2002) phyletic gradualism, the popular
Punctuated equilibria: an
(Darwinian) idea that evolutionary
alternative to phyletic
change is marked by a pattern of
gradualism, with Niles
smooth and continuous change in the
Eldredge (1972).
fossil record.
Phyletic Gradualism?
Later 12 species of Horse were discovered to coexist at this time!
Traditional Representation of Horse Evolution (Matthew, 1926)
Or Punctuated Equilibrium?
Current
Representation of
Horse Evolution
Evolutionary Changes in the
Fossil Record
• Some species in the fossil record persist for millions of years
relatively unchanged despite significant environmental changes
• Differentiation of species (divergence) is typically subtle
• At certain times when many new ecological niches were
abruptly created (e.g., K-T extinction of the dinosaurs),
divergence resulted in rapid and extreme speciation – adaptive
radiation
• Availability of new niches, can also lead to distinct species
converging on a common body form and dietary preference
(e.g. marsupial evolution in Australia)
• In some cases, certain species can re-acquire traits that were
lost in previous manifestations (iterative evolution), and in
other cases, different species can acquire a similar trait (parallel
evolution)
Parallel Evolution
Reappearance of Features in Different Species
Oligocene
Hoplophoneus
(relative of dogs
and bears)
Pleistocene
Smilodon
(cat family)
Pliocene
Machairodus
(cat family)
Pliocene (SA)
Thylacosmilus
(opossum
family)
Recent Challenges to Neo-Darwinism
Neutralism
• Apparently, organisms have more genetic variability
than they need
• Some of the extra traits may be unaffected by natural
selection, i.e., they are neutral
• Consequently, many mutations are simply accidents
of nature that have no advantage or disadvantage
African Rhino
Indian Rhino
Recent Challenges to Neo-Darwinism
Inheritance of Acquired Characteristics (revisited)
• Natural selection implies that rapid responses
to environmental change is not likely
• However, some evolutionary change are
remarkably rapid – immune system
Exposure to disease triggers the immune system
to create (acquire) antibodies
However, some experiments suggest that
immunity can be passed on to offspring.
Recent Challenges to Neo-Darwinism
MacroEvolution
• Natural selection is good at explaining
incremental changes (microevolution), but can it
explain dramatic changes, like making an eye or a
wing?
• Possibly not as abrupt as they appear in the fossil
record; many intermediate stages
• It is clear that some organisms simply die out
because oaf their inability to rapidly adapt, but
some do adapt quickly
• Pre-adaptation – organs adapted for one function
change for a new function
Challenges to NeoDarwinism
MacroEvolution
Squid
Evolutionary stages of the mollusk’s eye
Challenges to NeoDarwinism
MacroEvolution – Pre-Adaptation
Transform
-ation of
gill arches
into jaw
bones
The Mexican salamander
(Ambystoma) prefers to stay in its
embryonic state submerged and
breathing with juvenile gills. If
stressed, however, it will emerge
and breath with adult lungs
Evolution and Creation “Science”
A False Equivalence
Evolution is a scientific theory because it is:
• Testable
• Predictable
• Self-correcting
• Open to further refinement
Creationism/Intelligent Design is not a scientific
theory because it is none of the above
Next Lecture
Fundamentals of Stratigraphy
Chapter 4