A. Darwin's Hypothesis of Evolution
1. Influences on Darwin's thinking
a. Charles Darwin (1809-1882) was born in western England.
b. Darwin enrolled at Christ College at Cambridge University with the intent of becoming a clergyman.
1. At that time, most naturalists and scientists belonged to the clergy.
c. Darwin�s views were influenced by fossils, the relics or impressions of organisms from the past, mineralized in sedimentary rocks.
1. Sedimentary rocks form when mud and sand settle to the bottom of seas, lakes, and marshes.
2. New layers of sediment cover older ones, creating layers of rock called strata.
3. Fossils within layers show that a succession of organisms have populated Earth throughout time. (Figures 22.3, 22.2 and 22.4)
d. Darwin�s views were influenced by theories of geological processes
1. James Hutton, a Scottish geologist, proposed a theory of gradualism, that profound geological change results from slow, continuous processes.
2. Later, Charles Lyell proposed a theory of uniformitarianism, that geological processes had not changed in nature or rate throughout Earth�s history.
3. Hutton�s and Lyell�s observations and theories had a strong influence on Darwin.
a. First, if geologic changes result from slow, continuous processes, rather than sudden events, then the Earth must be far older than the 6,000 years assigned by theologians from biblical inference.
b. Second, slow and subtle processes persisting for long periods of time can add up to substantial change.
2. Field research helped Darwin frame his view of life
a. Darwin received his degree in 1831.
b. After graduation Darwin went for 5 years on the survey ship Beagle for a voyage around the world.
c. The main mission of the five-year voyage of the Beagle was to chart poorly known stretches of the South American coastline.
1. Darwin had the freedom to explore extensively on shore while the crew surveyed the coast.
2. He collected thousands of specimens of the exotic and diverse flora and fauna of South America.
3. Darwin explored the Brazilian jungles, the grasslands of the Argentine pampas, the desolation of Tiera del Fuego, and the heights of the Andes.
d. Five observations that Darwin made on his trip influenced his thinking.
1. Darwin noted that the plants and animals of South America were very distinct from those of Europe.
a. Organisms from temperate regions of South America were more similar to those from the tropics of South America than to those from temperate regions of Europe.
b. Example: rodents in South America look more like each other than they look like rodents of Europe.
2. Extinct species are most closely related to living organisms found in the same area. Why would this happen unless one evolved from the other?
a. Example: extinct glyptodont/living armadillo
3. Islands have many species found only in those islands and those species are closely related.
a. Example: Galapagos finches.
4. Island species resemble forms on closest mainland, not distant places.
a. Example: Darwin noted that while most of the animal species on the Galapagos lived nowhere else, they resembled species living on the South American mainland. (Galapagos are 600 miles west of South America.)
b. It seemed that the islands had been colonized by plants and animals from the mainland that had subsequently diversified on the different islands.
5. In rock layers there are progressive changes in characteristics in fossils from earlier and earlier layers.
a. Example: oyster shell shape (12 million years in the Jurassic).
e. By the early 1840s Darwin had developed the major features of his theory of natural selection as the mechanism for evolution.
1. Darwin made two main points:
a. Today�s organisms descended from ancestral species (descent with modification).
b. Natural selection provided a mechanism for evolutionary change in populations.
2. Central to Darwin�s view of the evolution of life is descent with modification.
a. In descent with modification, all present day organisms are related through descent from unknown ancestors in the past.
b. Descendents of these ancestors accumulated diverse modifications, or adaptations, that fit them to specific ways of life and habitats.
c. Viewed from the perspective of descent with modification, the history of life is like a tree with multiple branches from a common trunk.
d. Example: Evolutionary tree of the elephant family. (Fig. 22.7)
3. The other major point that Darwin pioneered is a unique mechanism of evolution - the theory of natural selection.
a. Ernst Mayr, an evolutionary biologist, has dissected the logic of Darwin�s theory into three inferences based on five observations.
1. Observation #1: All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully. (Fig. 22.8)
2. Observation #2: Populations tend to remain stable in size, except for seasonal fluctuations.
3. Observation #3: Environmental resources are limited.
a. Inference #1: Production of more individuals than the environment can support leads to a struggle for existence among the individuals of a population, with only a fraction of the offspring surviving each generation.
4. Observation #4: Individuals of a population vary extensively in their characteristics; no two individuals are exactly alike. (Fig. 22.9)
5. Observation #5: Much of this variation is heritable.
a. Inference #2: Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the individuals. Those individuals whose inherited characteristics best fit them to their environment are likely to leave more offspring than less fit individuals.
b. Inference #3: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over the generations.
b. Darwin�s main ideas can be summarized in three points.
1. Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce).
2. Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population.
3. The product of natural selection is the adaptation of populations of organisms to their environment.
c. Important points about natural selection
1. While natural selection involves interactions between individual organisms and their environment, it is not individuals, but populations that evolve.
a. Populations are defined as a group of interbreeding individuals of a single species that share a common geographic area.
b. Evolution is measured as the change in heritable variation in a population over a succession of generations.
2. Natural selection can only amplify or diminish heritable variations, not variations that an individual acquires during its life, even if these variations are adaptive.
3. Adaptations for one set of environmental conditions may be useless or even detrimental under other circumstances.
f. Publication of Darwin's hypothesis
1. In 1844, he wrote a long essay on the origin of species and natural selection, but he was reluctant to publish his theory and continued to compile evidence to support his theory.
2. In June 1858, Alfred Wallace, a naturalist working in the East Indies, sent Darwin a manuscript containing a theory of natural selection essentially identical to Darwin�s.
3. Later that year, both Wallace�s paper and extracts of Darwin�s essay were presented to the Linnaean Society of London.
4. On November 24, 1859, Charles Darwin published On the Origin of Species by Means of Natural Selection.
5. While both Darwin and Wallace developed similar ideas independently, the essence of evolution by natural selection is attributed to Darwin because he developed and supported the theory of natural selection earlier and much more extensively.
6. Darwin�s views were accepted by most intellectuals but not the general public.
B. Evidence for evolution
1. Natural selection
a. The evolution of resistance to insecticides in hundreds of insect species is a classic example of natural selection in action.
1. Insecticides are poisons that kill insects that are pests in crops, swamps, backyards, and homes.
2. The results of an application of a new insecticide are typically encouraging, killing 99% of the insects.
a. However, the insecticide becomes less effective in subsequent applications.
b. The few survivors from the early applications of the insecticide are those insects with genes that enable them to resist the chemical attack.
c. Only these resistant individuals reproduce, passing on their resistance to their offspring.
d. In each generation the percentage of insecticide-resistant individuals increases. (Fig. 22.12)
3. In general, natural selection operates not to create variation, but to edit existing variation.
a. For example, resistant insects are favored and non-resistant individuals are not when insecticides are applied.
b. Natural selection favors those characteristics in a variable population that fit the current, local environment.
b. While researchers have developed many drugs to combat the human immunodeficiency virus (HIV), drug-resistant strains evolve rapidly in the HIV population infecting each patient.
1. For patients treated with the drug 3TC, which interferes with genome replication in HIV, 3TC-resistant strains become 100% of the population of HIV in just a few weeks. (Fig. 22.13)
2. Artificial selection
a. Lab Experiments
1. Bristle number in fruit flies-investigators allowed those with the most bristles to mate with each other and those with the least bristles to mate with each other-populations had no overlap in bristle number after 35 generations.
2. Selection of rats resistant to tooth decay changed average time of onset of decay from 100 days to 500 days in 20 generations.
b. Agriculture
1. Selection in corn ear size
2. Other examples are milk production in cows, etc.
c. Cat and dog breeding-same species, different varieties
1. All modern dog breeds developed in the last 10,000 years since domestication of the wolf.
2. Natural selection has had millions of years to produce changes.
3. Homologous structures
a. Similarity in characteristics resulting from common ancestry is known as homology.
b. For example, the forelimbs of human, cats, whales, and bats share the same skeletal elements, but different functions because they diverged from the ancestral tetrapod forelimb. (Fig. 22.14)
1. They are homologous structures.
4. Vestigial structures
a. Vestigial structures have marginal, if any, importance to a current organism, but had important functions in ancestors.
b. For example, the skeletons of some snakes and of fossil whales retain vestiges of the pelvis and leg bones of walking ancestors.
c. Human appendix
5. Embryonic development
a. For example, all vertebrate embryos have structures called pharyngeal pouches in their throat at some stage in their development.
b. These embryonic structures develop into very different, but still homologous, adult structures, such as the gills of fish or the Eustachian tubes that connect the middle ear with the throat in mammals.
6. Molecular evidence
a. The concept of homology also applies at the molecular level (molecular homology) and allows links between organisms that have no macroscopic anatomy in common (e.g., plants and animals).
b. For example, all species of life have the same basic genetic machinery of RNA and DNA and the genetic code is essentially universal. (Table 22.1)
c. Evidently, the language of the genetic code has been passed along through all the branches of the tree of life ever since the code�s inception in an early life-form.
7. Geographical distribution evidence
a. Islands provide strong evidence of evolution.
1. Often islands have many species of plants and animals that are found nowhere else in the world.
2. Darwin observed that these species are typically related more closely to species living on the nearest mainland (despite different environments) than those from other island groups.
a. Example: Galapagos tortoise resembles those in South America most closely.
b. Example: Galapagos finches resemble those in South America most closely.
3. In island chains, individual islands may have different, but related, species.
a. The first mainland invaders reached one island and then evolved into several new species as they colonized other islands in the chain.
b. Example: finches on the Galapagos Islands.
c. Example: fruit flies (Drosophila) in the Hawaiian Islands.
1. All of the 500 species of Drosophila in the Hawaiian Islands descended from a common ancestor that reached Kauai over 5 million years ago. (Fig. 22.16)
8. Fossil evidence
a. The succession of fossil forms is compatible with what is known from other types of evidence about the major branches of descent in the tree of life.
1. For example, fossil fishes predate all other vertebrates, with amphibians next, followed by reptiles, then mammals and birds.
2. This is consistent with the history of vertebrate descent as revealed by many other types of evidence.
b. The Darwinian view predicts that evolutionary transitions should leave signs in the fossil record.
1. Recent discoveries include fossilized whales that link these aquatic mammals to their terrestrial ancestors.
2. Evolution of the horse in areas changing from forest to grassland in North America
a. Modern members of the Genus Equus include horses, zebras, and donkeys.
b. Earliest members in Genus Hyracotherium -size of cat
c. Horse evolution shows changes in 3 characteristics
1. Body size gets larger (to escape predators?).
2. Foot changed to hoof.
3. Ridges on molars changed so they could eat grass without wearing molar down.