HOW BIOLOGICAL DIVERSITY EVOLVES
E
volution provides the unifying concepts for the following chapters that deal with the diversity of organisms and the progressive development of derived anatomy and physiology. This section discusses the history and mechanisms of evolution, the history of life, and the most recent developments in classifying organisms.Chapter 13/14 Darwin and Evolution
Discussion of Darwin’s formulation of a mechanism of evolution by natural selection is followed by description of the extensive and varied evidence for evolution.
|
The
following is the first of our notes covering the principles of
evolution. Additional notes
will follow. The
modern theory of evolution explains the diversity of life from the
perspective of science. It
provides a scientific explanation of how the multitude of species was
created. Advances in
science, especially genetics, have strengthened this view of life in
part by explaining the basis of variation and inheritance.
As you will see, variation and inheritance are both very
important to the theory. All
aspects of modern biology are affected by the theory of evolution by
means of natural selection. It
has been said that ‘nothing makes sense in biology except in light of
the theory of evolution.’ Note
the often used phrase, “The Darwinian Revolution”—a scientific
revolution that has changed the focus of biology. Theories
of Evolution I. The Theory of Inheritance of Acquired Characteristics. (1809) by Jean-Baptiste Lamarck Modifications
acquired during one’s lifetime are inherited by the next generation,
ex. giraffes acquired a long neck slowly over time as each generation of
giraffe stretched its neck slightly longer in trying to reach leaves
high in trees. At fist
glance this theory is deceptively close to Darwin’s theory (both
include the concept that evolution produces life forms adapted to their
environments) but the inheritance of acquired characteristics
implies that the organism itself can control the direction of change.
Unfortunately, there have been no discoveries of any such
mechanism of change. II.
The Theory of Evolution by means of Natural Selection A.
The founders:
Both
men were excellent observers and prolific collectors and both worked
as taxonomists. B.
Seven Influential Factors for both Wallace and Darwin 1).
Geology
– study of the earth’s structure, origin, and history.
The book, Principles of Geology by Charles Lyell
(1797-1875), includes the “theory
of uniformity” or “uniformitarianism"
- Geological processes have been
uniform through time. Geological
changes of the past were caused by same, observable processes of today.
The process of change has been uniform through time.
Canyons and thick layers of sedimentary rock are the profound
results of accumulated gradual change over vast stretches of time.
The significance to biology is this, slight
changes over a long time have large-scale impact and one can look at the
present to see the processes of the past.
Three concepts from geology are important in shaping the theory
of evolution by means of natural selection: 2).
Fossils -
extinct forms similar yet distinct from extant forms.
Perhaps a history of change (evolution) connects the living with
the extinct. 3).
Island Life. Isolated
island populations
differ slightly from nearby mainland populations.
e.g. Galapagos Islands (2 MYO, 600 mi. off W coast of S.A)
the birds, lizards, huge
tortoises, similar to ‘nearby’ mainland species yet slightly
different. Therefore
there must have been Descent
with modification (i.e., the island life evolved from mainland
ancestors). 4).
Overproduction of individuals
- Thomas Malthus (British Economist) in his “Essay on the
Principle of Populations”(1798) attempts to explain that much of
human suffering, hunger, sickness, homelessness, and war are due to
competition between an ever increasing number of individuals or groups
of individuals for limited resources.
The number of individuals (population size) tends to increase
exponentially. This
exponential capacity can’t be sustained (if it were, earth would be
covered over many times with individual life forms).
Eventually, more individuals are born than can live to reproduce,
leading to a struggle for existence.
Darwin and Wallace saw the concept relative to all species not
just humans. Only some
individuals live to reproduce, many (in fact, most) will die before
reaching reproductive age. What
determines, in nature [not human society] who lives and who dies?
An answer lies in part in variations among individuals.
[Social Darwinism is
a distortion of the Theory of Evolution in the Darwinian sense, Social
Darwinism was used by Hitler to justify the extirpation of Jews and his
quest to destroy other countries; Antievolutionists use Social Darwinism
as an attack on the Theory of Evolution.
Social Darwinism is the erroneous application of “survival of
the fittest” to human societies.] 5).
Individuals within a species vary extensively.
How
does one know variations exist? Taxonomy!
A taxonomist observes and describes the variation within a species. You
don’t have to be a taxonomist to know that individual people vary.
People vary not only in morphological traits such as hair texture
and body shape but also in physiological traits as seen in the
susceptibility of American Indians to European diseases [measles,
mumps]. A knowledge of the
variation in species of wild animal be they bird, beetle, or barracuda,
requires the careful and detailed observation of many specimens.
Only a scrutinizing taxonomist, whether professional or amateur,
sees the variation between individuals of wild species. 6).
Many of the variations are inheritable
and some variations may impart greater reproductive success, as in
artificial selection below. 7).
Artificial Selection
– “selective breeding by humans of another species” (Harcourt
Dictionary). Variation
among crops & livestock may be favored (selected) or disfavored
(selected against), or neutral, i.e., neither favored nor disfavored, by
man's control over which individuals are allowed to reproduce.
Man selects the best (according to his taste) and breeds these
for future generations. [see
fig. 18.11-12, p. 290-291]
|
Charles Darwin and The Origin of Species
Charles Darwin's _______________________________________ was published on November 24, 1859.
Darwin argued that contemporary species arose from ancestors through a process of ____________________________ with ____________________ as the mechanism.
The basic idea of natural selection is that:
1. _______________________________________________
2. _______________________________________________
The result of natural selection is _________________________________________.
I. Darwin's Cultural and Scientific Context
The Origin of Species challenged the notion that the Earth was relatively _______________ and populated by _______________ species.
A. The Idea of Fixed Species
The Greek philosopher Aristotle held the belief that species are ____________ and do not ____________________.
The ___________________________ culture fortified this idea and suggested that the Earth is only 6000 years old.
B. Lamarck and Evolutionary Adaptations
In the mid-1700s, the study of ______________ began to form as a branch of science.
Naturalist Georges Buffon:
1. _________________________________________________
2. _________________________________________________
Jean-Baptiste Lamarck:
1. _________________________________________________
2. _________________________________________________
C. The Voyage of the Beagle
In December 1831, Darwin left _____________________ on the ______________________ to explore the world.
On his journey on the Beagle, Darwin:
1. _____________________________________________
2. _____________________________________________
Darwin was intrigued by:
1. _____________________________________________
2. _____________________________________________
D. The New Geology
Darwin was strongly influenced by the writings of geologist _______________________________
Darwin would apply Lyell's principle of _______________________________ to the evolution of Earth's life.
II. Descent with Modification
Darwin made two main points in The Origin of Species:
1. ____________________________________
2. ____________________________________
Evidence of Evolution
Biological evolution has left ________________________.
I. The Fossil Record
Fossils are preserved _________________ or _______________________ left by ___________________ that lived in the past.
They are often found in _____________________________.
The fossil record:
1. ____________________________________________________
2. ____________________________________________________
3. ____________________________________________________
____________________ are scientists that study fossils. They have discovered many ___________________ forms that link past and present.
II. Biogeography
Biogeography is the study of ___________________________________. Biogeography first suggested to Darwin that today's organisms evolved from___________________________.
Many examples of biogeography support _____________________________.
III. Comparative Anatomy
Comparative anatomy is the ________________________________________________.
Confirms that evolution is a _________________________________________________.
Homology is the _________________________________________________________.
_____________________________ are remnants of structures that served important functions in the organism's ancestors.
IV. Comparative Embryology
Comparative embryology is the study of _______________ that appear during the development of different organisms.
Comparative embryology of vertebrates supports ________________________________.
V. Molecular Biology
Evolutionary relationships among species:
1. ___________________________________________
2. ___________________________________________
Natural Selection
Darwin's __________________ are an excellent example of ____________________ and __________________________.
I. Darwin's Theory of Natural Selection
Darwin based his theory of natural selection on two key observations:
Observation # 1: _________________________
1. _________________________________________
2. _________________________________________
Observation # 2: ______________________________
1. _________________________________________
2. _________________________________________
Inference: _____________________________________________________________________
Those individuals with traits best suited for the local ______________________ leave more fertile offspring.
II. Natural Selection in Action
Examples of natural selection include:
1. ____________________________________
2. ____________________________________
3. ____________________________________
A. Does Predation Drive the Evolution of Lizard Horn Length?
Scientists concluded that horn length in these lizards was driven by ______________________.
Modern Synthesis: Darwinism Meets Genetics
The modern synthesis is the fusion of __________________ with ___________________________.
I. Populations as the Units of Evolution
A population is a group of individuals of the same _________________ living in the same _____________ at the same _______________.
A population is the smallest ________________________ that can evolve.
Population genetics focuses on populations as the ________________________ units. It tracks the _____________________ of populations over time.
II. Genetic Variation in Populations
Individual ________________ abounds in populations. Not all of this variation is ___________________. Only the ____________________ of variation is relevant to natural selection.
A population is said to be _______________________ for a characteristic if two or more morphs, or forms, are present in noticeable numbers.
A. Sources of Genetic Variation
______________________ and ___________________ produce genetic variation.
Mutations: __________________________________________________
Sexual recombination: _________________________________________
III. Analyzing Gene Pools
The gene pool consists of all ____________________ of all individuals making up a population.
Alleles in a gene pool:
1. ______________________________________________________________
2. ______________________________________________________________
Genotypic frequencies:
1. ______________________________________________________________
2. ______________________________________________________________
The Hardy-Weinberg Formula:
1. ______________________________________________________________
2. ______________________________________________________________
II. Population Genetics and Health Science
The Hardy-Weinberg formula can be used to calculate the percentage of a ______________________ that carries the __________________ for a particular inherited disease.
III. Microevolution as Change in a Gene Pool
Hardy-Weinberg equilibrium describes a __________________________ population that is in genetic equilibrium.
Microevolution is defined as a ___________________________________ change in a population's frequencies of _______________.
Mechanisms of Microevolution
The main causes of microevolution are:
1. ___________________________
2. ___________________________
3. ___________________________
4. ___________________________
I. Genetic Drift
Genetic drift is a change in the gene pool of a small population due to _______________________.
A. The Bottleneck Effect
The bottleneck effect is an example of ____________IV. ___________. Results from a ____________________ in population size.
Bottlenecking in a population usually reduces ___________________________.
B. The Founder Effect
The founder effect is genetic drift in a new ___________________.
C. Genetic Drift and Hereditary Disorders in Human Populations
The founder effect explains the relatively high frequency of certain ____________________ among populations.
II. Gene Flow
Gene flow is _____________________ with another population. It tends to reduce ___________________ between populations.
III. Mutations
Mutations:
1. _________________________________________
2. _________________________________________
3. _________________________________________
IV. Natural Selection: A Closer Look
Of all causes of microevolution, only natural selection promotes ______________________.
A. Darwinian Fitness
Darwinian fitness is the contribution an individual makes to the ________________ of the next generation relative to the _________________ of other individuals.
B. Three General Outcomes of Natural Selection
Directional selection:
1. ___________________________________________
2. ___________________________________________
Disruptive selection:
1. ___________________________________________
Stabilizing selection:
1. ___________________________________________
Evolution Genetics of the Sickle-Cell Allele
Sickle-cell disease:
1. _____________________________________
2. _____________________________________
The sickle-cell allele confers resistance to the disease _____________________.
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- 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.
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 mammals in Australia (see p. 295) is
thought to be the result of evolution within this group 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 anatomy - study of body parts, esp. internal
body parts. The
evolutionary view that all mammals share a common ancestor is supported
by the anatomy of vertebrate forelimbs (see fig. 18.16).
Closely related species (e.g. species of mammals) show
modification of shared structures, i.e., homologous structures.
Homologous
structures – “structures in different species that are similar
because of common ancestry”; 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.
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 (for images see the following web link: Docent
Webpage); 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 (see p. 296 fig. 18.17).
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 (see fig. 18.18
for similar example).
|
Natural
Selection
– the mechanism of evolution as told by Wallace and Darwin **Natural
Selection - differential reproductive success that results from
the interaction of organisms with their environment.**
I.
Some Background Remember
three important subtle points: 1)
Natural selection can amplify or diminish only heritable
variations. Only heritable
variations are affected; acquired characteristics play no role in
evolution.2) Populations
evolve, not individuals.3) Natural
selection is situational (the direction it takes depends on the
environment, i.e., the selective forces). To
illustrate these subtle points using the case of the peppered moth… 1)
moth color is inherited perhaps under control of a single gene; 2) the
changes in nature over time involve whole populations, that is, in
pre-industrial times the dark moth form was quite rare and perhaps in
the distant past the dark mutation had not yet appeared.
With time and natural selection at work, the dark moth became the
most common form within the population.
3) the 3rd subtle point is quite important, for what
is advantageous in one environment may not be in another.
In the saga of the peppered moth, under polluted tree bark
conditions following the industrial revolution the dark from became most
common, yet a return to a predominance of light colored forms follows
restoration of healthy, non-polluted tree bark.
(The story of the peppered moth has additional complexities but
we will ignore them). A.
About heritable variations B.
About Populations Population
-
a) a group of individuals of the same species, b) live together in same
area at same time, c) exchange genes (reproduce sexually). Sources
of Variation w/in populations: C.
About Situations or Circumstances Expounding on the situational aspect in which
evolution operates, Stephen Jay Gould, Harvard paleontologist and
prolific writer popularizing evolutionary concepts, puts fourth an
interesting idea that he calls the “contingency of evolutionary
history” in his 1989 book, Wonderful Life: The Burgess Shale and
the Nature of History. The
title was inspired by the 1946 movie It’s a Wonderful Life in
which George Bailey learns how his own life was a contingency for many
other events. One such
contingency that became a prerequisite for human evolution was the
extinction of the dinosaurs that made way for mammal diversification.
Apparently, dinosaur extinction was contingent on an asteroid
that impacted Earth 65 million years ago.
Had these events not occurred, humans may never have evolved.
(The question of why humans evolved (to what purpose) is
not a question science can address; the question of how humans
evolved is explored through the process of science.
See chapter 21 if you are interested in human evolution.) II.
Types and examples of Natural Selection Genetic variation within a population creates, in part, a diversity of phenotypes, phenotypes that have a strong genetic component (as opposed to environmentally induced phenotypic differences). Depending upon the situation/environment or circumstances affecting survival, certain variant phenotypes may enjoy greater reproductive success. Those aspects of the phenotype that correlate with greater success we think of as adaptations, e.g. the shell of reptilian eggs is an adaptation to life on land. Adaptations result from natural selection. The
following examples illustrate differential reproductive success (i.e.,
natural selection) as driven by different scenarios of selection
pressures. 1.
Stabilizing selection
- favors intermediate phenotype 2.
Directional selection
- important when environmental change occurs The
origin of the hard, reptilian shelled egg can also be thought of in
terms of directional selection coupled with genetic mutations leading to
ever more protective egg coverings evolving from jelly-like frog egg-ish
beginnings. Selection pressures would favor those eggs that
survive greater desiccation as experienced in the ever more terrestrial
lifestyles of the early reptiles. Sexual
selection provides an explanation for sexual dimorphism - male
& female differ beyond
gonads and genitalia. Recall
the extreme case of the dwarf male angler fish.
Based on a morphological species concept alone, male
angler fish may have been named as a separate species when originally
captured from the depths of the ocean.
Only after the realization that the small angler fish were
simply males which spawned with the much larger female angler fish
would a single species rather than two be recognized.
The knowledge of the capacity for interbreeding is of course the
basis for the biological species concept.
|