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 Biogeographical Evidence Figure 5: Biogeography of the Galapagos Finches Darwin observed that species in the same geographical regions (i.e. Galapagos finches) tended to resemble each other more than those living father apart. AR Wallace also made a similar observation while studying species in Indonesia. Biogeography: a) The most likely explanation for Darwin & Wallace s observations is that similar species living in the same geographic regions are more likely to share common ancestors. 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 6: 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 6.1: Transitional Fossils (Fish to Early Amphibians) Geologic Record: Anatomical Evidence: Homologous Structures Figure 7: Homologous Structures (Mammalian Forelimbs)
Figure 7.1: Homologous Structures via Common Ancestry Homologous Structures: Figure 7.2: 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 7.3: 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 8: DNA Hybridization Biochemical Evidence:
Embryological Evidence Figure 9: Comparative Embryology: Germ Layers Embryologists noticed that ALL animal embryos exhibit the same layers during embryonic development (ecto/endo/mesoderm). Moreover, each layer always gave rise to the SAME adult tissue types. These similarities can only be reasonably explained if we assume common ancestry among these groups. Figure 9.1: Comparative Embryology: Similar Embryos Embryologists noticed that the more closely related to species are, the more similar their embryos appear during the duration of development. For example, chimp & human embryos would appear similar longer than either of them would be to a chicken embryo. 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.