Download Unit IV: Chapter 22

I. Theories of Evolution
● Evolution:
●
Adaptation:
●
Jean Baptiste de Lamarck:
a) Use & Disuse:
b) Inheritance of Acquired Characteristics:
Figure 1: Lamarckian Evolution
III. Darwin & Evolution
The Voyage of the Beagle
● In 1831, Charles Darwin set sail on the survey ship the HMS Beagle. The Beagle spent 5 weeks at the Galapagos
Islands off the west coast of S. America. He compared the animals & plants of the Galapagos with those of the S.
American mainland. He was impressed by their similarities & wondered why the organisms of the Galapagos should
resemble those from the S. America more than those from islands elsewhere.
Figure 2: Voyage of the HMS Beagle
Shaping the Theory of Natural Selection
● Upon returning home, Darwin (& others) analyzed the specimens he collected during his voyage. It soon became
apparent to him that the current inhabitants of the Galapagos must have arrived from S. America & diversified into
new species due to differences in the surrounding environment (climate, resources, etc).
In considering a possible mechanism for how species evolved over time, Darwin drew upon the following sources &
experiences:
a) Geological Observations: during his voyage, Darwin read Charles Lyell’s Principles of Geology, which contends that
given the slow pace of many geological forces, Earth must be far older than the 6,000 years suggested by
theologians. Darwin’s observations in S. America led him to suspect the same, a revelation that provided Darwin
with a suitable time scale over which substantial biological evolution could occur.
b) Artificial Selection: practice by which breeders & farmers develop many varieties of domesticated animals in just a
few generations by choosing certain traits & breeding only those individuals that exhibited the desired traits.
Darwin hypothesized that a similar selective process occurred in nature.
●
Figure 3: Artificial Selection (Domestication)
*Many common vegetables have been derived through the selective breeding of the wild mustard plant. For instance, broccoli arose
by selecting only wild mustard plants having exceptionally large flower clusters to mate. Over time, this feature has become
exaggerated to form the plant we know as broccoli.
c) Thomas Malthus: an economist who noted that populations have the capacity to increase faster than the food
supply. In the case of humans, this may lead to famine, disease, & war, which serve as checks on population
growth. Darwin observed a parallel struggle for existence in nature.
Figure 3.1: Thomas Malthus: Human Population Growth & Food Supply
Evolutionary Mechanism
● The aforementioned sources helped Darwin construct a mechanism for evolutionary change, which he called Natural
Selection.
Figure 4: Natural Selection
●
Natural Selection:
a) Overproduction & Variation: overproduction within each population produces a wide range of variants (speed,
strength, camouflage, etc).
b) Struggle for Existence: overproduction results in competition among individuals. Factors of the environment
affecting the survival & reproductive success of individuals w/in populations (living space, predation, disease,
food, mates) are called Selecting Agents.
c) Survival of Fittest: in each subgroup, only those variants best equipped for survival (best able to cope with
existing selecting agents) will reach reproductive age, passing these variations to their offspring.
d) Divergence: the beneficial traits will increase in number within each population, causing them to Diverge (become
more dissimilar) from both each other & the original population. After many generations, as more & more
beneficial traits accumulate via natural selection, the populations will continue to diverge.
e) Speciation: ultimately, each population will acquire so many new traits that they will no longer resemble each
other or their original form. Even if reunited, they will no longer be able to successfully interbreed. At such a
point, Speciation is said to have occurred.
On the Origin of Species
●In 1859, Darwin published On the Origin of Species by Means of Natural Selection. In it, Darwin presented the first
coherent, logical theory as to the mechanism(s) by which evolution occurs.
V. Evidence for Evolution
Geologic Evidence: Fossil Sequences
●The geologic record illustrates a general progression from simple to more complex forms (each form modified toward
improvement).
At NO place do we observe the remains of a species before it could have evolved (i.e. fossil human remains in rocks of
the Jurassic period, when dinosaurs ruled & before the known appearance of humans).
●
Figure 5: Logical Fossil Sequences
Geologic Evidence: Transition Fossils
● Natural selection predicts that, within any group, intermediate forms between primitive & more modern forms should
exist within the fossil record. Such intermediates are called Transitional Fossils, which are so abundant within the
fossil record that the evolutionary history of most species can be accurately reconstructed.
Figure 5.1: Transitional Fossils (Fish to Early Amphibians)
●
Geologic Record:
Anatomical Evidence: Homologous Structures
Figure 6: Homologous Structures (Mammalian Forelimbs)
Figure 6.1: Homologous Structures via Common Ancestry
●
Homologous Structures:
Figure 6.2: Vestigial Structures (Whale Pelvis)
●
Vestigial Structures:
a) Vestigial structures provide evidence of evolution simply because we can observe their functional homologues in
other related groups. These groups must share a common ancestor in which the structure was fully functional.
Figure 6.3: Analogous Structures (Wings Structure across Groups)
●
Analogous Structures:
a) The presence of analogous structures across groups does NOT imply a common ancestry. They do however suggest
that each species evolved in a convergent manner –the non-related species adapted in similar ways to similar
environmental demands.
Molecular Evidence: DNA Hybridization
Figure 7: DNA Hybridization
●
Biochemical Evidence:
Biogeographical Evidence
Figure 8: Biogeography of the Galapagos Finches
●
Biogeography:
a) The most likely explanation for the observed distribution of species is that they reflect the environmental
conditions in which they evolved. Consequently, they may not be well suited for life in other regions (especially
where the environment is radically different) & are thus limited to regions defined by specific climates, soil types,
vegetation, etc.
Embryological Evidence
Figure 9: Comparative Embryology
Scientists noticed that features common to all animals (primitive features) appear early on in their embryonic
development while features unique to a particular species appear later on. For example, the differences in chimps
& humans observed late in their development represent features that were acquired AFTER they split from a
common ancestor about 5 million years ago.
●
●
Comparative Embryology:
V. Observed Cases of Natural Selection
Industrial Melanism
● The English peppered moth occurs in two varieties that differ in coloration. The form for which the moth is named is
light, with splotches of pigment. The other variety is uniformly dark.
Light-Colored Moth
Dark Colored Moth
Peppered moths rest on trees & rocks encrusted with light-colored lichens. Against this background, light moths are
camouflaged, but the more conspicuous dark moths are susceptible to predation. Thus light colored individuals were
more common prior to the industrial revolution.
●
In the mid-1800s, scientists recognized that dark-colored moths were becoming more common in industrialized areas.
Some suggested pollution might be causing the moths to turn gray or black, a phenomenon called Industrial Melanism.
Henry Kettlewell believed industrial melanism was due to NATURAL SELECTION & the color of individual moths must
have some effect on their survival to reproductive age.
●
To test his hypothesis, he conducted a mark-release-recapture experiment in a polluted & unpolluted forest. He first
did a set of experiments near a polluted area. When he recaptured the moths, he found that marked dark-colored
moths were twice as likely to avoid predation by birds in the unpolluted forests.
●
The next year, he conducted similar experiments in an unpolluted area. When he recaptured the moths, he found
that marked light-colored moths were twice as likely to avoid predation by birds in the unpolluted forests.
●
These results suggested that the body color of the moths indeed had an affect on their survival rate. Therefore, the
shift in color from light to black in industrialized regions was likely due to natural selection.
●
Antibiotic Resistance
● In a bacterial colony, a small percentage of cells may be naturally resistant to antibiotics. This ability may also be
acquired through chromosomal mutations or acquisition of plasmids that confers resistance.
Antibiotics act as a selecting agent by eliminating nonresistant cells which normally keep the number of resistant
varieties in check. Consequently, within hours a colony of 1 billion resistant bacteria may be produced.
●
Pesticide Resistance
● In a population of insect crop-pests, a small percentage of individuals may possess a mutant allele that results in a
low growth rate. Consequently, these individuals take longer to reach reproductive age & produce less offspring.
In the presence of a pesticide, such a gene also may allow the individual to tolerate the harmful chemical. Thus, in
the presence of such a selecting agent, these individuals will have a survival advantage & are more likely to reach
reproductive age. After a several generations, the entire population may become resistant to the pesticide.
●