EVOLUTION Evolution - changes in allele frequency in populations over generations. Sources of genetic variation: genetic recombination by sexual reproduction (produces new combinations of genes) mutation (source of new genes) Principles of evolution: 1. Overproduction. More offspring are produced in a generation than can be supported by limited resources. Each offspring represents a unique genetic combination of traits. 2. Struggle for existence. Offspring must compete for limited resources (food, space, moisture, light, etc.), but not all can be supported and most will die young. 3. Survival and reproduction of the fittest. Some genetic combinations will result in individuals that are more likely to survive to reproductive age, by conferring a competitive edge. 4. Inheritance and accumulation of favorable traits. Successful combinations will be passed on to the next generation. Less favorable combinations will be removed because the individual does not live to reproduce and pass those traits along. WHAT DRIVES EVOLUTION? 1. Changes in population size. If the population becomes small or some individuals do not reproduce, then some alleles may be lost from the population, thus leading to changes in allele frequency in subsequent generations. Effects of small populations: genetic drift - changes in allelic frequency due to chance. bottleneck effect - population is drastically reduced in size, remaining population is not genetically representative of the orginal population. founder effect - small population gets isolated and starts new population in new environment - has new set of selective pressures and may evolve in a different manner 2. Migration. If groups or individuals come into or leave the population, the proportion of alleles may change.
3. Mutation. If mutation produces a new gene, unless the gene mutates back at the same rate at which it is produced, the gene frequency will be altered through successive generations. 4. Selection. Nonrandom reproduction of genotypes. a) Natural selection - removal of less well-adapted individuals from the population by the effects of the environment. b) Artificial selection - alterations of characteristics over successive generations by selecting which individuals are allowed to breed. The changes are seen only in subsequent generations. Evolution can lead to speciation, the development of new species. Species - a group of actually or potentially interbreeding populations that is reproductively isolated from other such groups. Sources of Reproductive Isolation: 1. Geographic Isolation - If physically distant, can t get together to reproduce. 2. Behavior Isolation - don t recognize mating signals, or may live in different strata of common environment. 3. Mating Isolation - anatomically incompatible, e.g. complexities of insect genitalia. 4. Genetic Isolation - can get fertilization, but offspring is non-viable (mismatched chromosome numbers, etc.) 5. Hybrid Isolation - production of sterile offspring, can t breed back into population (e.g. horse X donkey = mule) Mechanism of Speciation: Allopatric speciation: When a population exists over a large and geographically diverse habitat, there will be different selective pressures on different portions of the populations. This may result in the formation of a cline - continuous and progressive trait variation over the population range. When so many changes have occurred in one group of organisms that they will no longer interbreed with a group descended from common ancestors, speciation has occurred. Of course they will still share many of the same genes because they descended from the same ancestors.
Sympatric Speciation: Speciation can also occur by hybridization between existing species. If the hybrid has an unbalanced chromosome complement, the organism will be sterile, but may reproduce asexually. Alternatively, if the organism becomes polyploid, it will attain a balanced number of chromosomes that could meiotically segregate. For example, wheats used for making bread and pasta are polyploid hybrids. DIVERGENT EVOLUTION The result of the populations divergence and formation of new species is called adaptive radiation. CONVERGENT EVOLUTION If similar habitats arise in different locations (e.g. same latitude, different hemispheres), different species may evolve to exhibit similar adaptations, thus filling similar niches. This accumulation of similar characteristics by different organisms is know as convergent evolution. PUNCTUATED EQUILIBRIUM Evolution does not always result in speciation. Evolution may occur slowly and gradually over many generations, or it may occur rapidly if selection pressure is intense. Niles Eldredge and Stephen Jay Gould in 1972 proposed that natural selection may produce rapid changes in species through the process of punctuated equilibrium. This theory states that speciation can occur rapidly in small groups of a population on the edge of the range of the species. The newly evolved population may then outcompete the descendants of the ancestral population so that the old species quickly disappears. EVIDENCE OF EVOLUTION Natural selection has been documented in historical times. Examples: changes in peppered moth populations, penicillin resistant bacteria, herbicide resistant Brassica weeds. Mutations have been documented by DNA sequencing techniques. Evolution is a scientifically observable and testable process that is occurring in contemporary times. FOSSIL RECORD Fossil evidence indicates that more primitive life forms on this planet were simpler than more contemporary life forms.
Fossil remains may be dated by relative dating based on geologic stratigraphy in which older remains are found in lower strata. Fossil remains may be dated by radioactive dating in which isotopic ratios of radioactive elements and their decay products are used to calculate the approximate number of years that the remains have been in existence. COMPARATIVE ANATOMY All living organisms share certain cellular structures and processes in common. Many organisms share particular anatomical features, physiological processes, and genes that allow them to be placed in a common group. Organisms in common groups show many similar characteristics during development. Some organisms have characteristics that are vestigial, that is apparent remnants from ancestral times. For example, pythons have pectoral and pelvic girdles, but no arms or legs attached to them. Birds have genes for teeth, but do not express those genes. ARTIFICIAL SELECTION Humans controlling which alleles get passed to next generation. Has led to many evolutionary changes in domesticated plants and animals. BIOGEOGRAPHY Similar environments provide similar evolutionary opportunities - convergent evolution COMPARATIVE MOLECULAR BIOLOGY Protein sequence homology DNA sequence homology THINKING POINT: How do we explain all these variations and similarities? How do we explain the unity and diversity of life on earth in terms of scientifically testable processes? Of the various attempts to explain diversity and unity, the process of evolution provides the best explanation. The application of the currently observable process of evolution to explain the diversity of the past forms the basis of the theory of evolution.
Extinction: There have been five periods of mass extinction, according to indications in the fossil record. The first came about 500 million years ago at the end of the Cambrian period. Photosynthesis had produced enough oxygen to change the composition of the earth s atmosphere. Oxygen content reached present levels some 400 million years ago and coincided with the second great extinction at the end of the Ordovician period. Another mass extinction occurred around 345 million years ago at the end of the Devonian period. The largest mass extinction occurred at the end of the Permian period, around 225 million years ago, when about 90% of marine life went extinct. The last major extinction was only 65 million years ago at the end of the Cretaceous period. This is when the dinosaurs went extinct. Alvarez hypothesis: asteroid hit earth. Created steam/dust/smoke cloud that obscured the sun and cooled the planet. Also inhibited photosynthesis, and many animals died of starvation and cold. North America hit especially hard in extinction because of landing zone of asteroid.