Organic Evolution Chapter 6

Pre-Darwinian Ideas Lamark (1744 – 1829) – Inheritance of Acquired Characteristics - organisms, by striving to meet the demands of their environments, acquire adaptations and pass them by heredity to their offspring. - ex. Giraffe evolved its long neck by stretching Lyell (1797 – 1875) – Uniformitarianism - the laws of physics and chemistry have not changed throughout the history of the Earth - past geological events occurred by natural processes similar to those observed today

Darwin’s Voyage Charles Darwin sailed around the world for 5 years on the HMS Beagle. collected specimens and made observations in his journal Did research on the Galapagos Islands: - unique character of the plants and animals - animals and plants were related to those of the South American mainland - each island often contained a unique species related to forms on other Galapagos Islands After Beagle returned, Darwin worked on his Theory of Evolution for 20 years

Darwinian Theory of Evolution: The Evidence Perpetual Change: - The living world is always changing - Perpetual change can be seen most directly in the fossil record - Fossils are deposited in stratified layers - The fossil record allows us to view evolutionary change across the broadest scale of time - Fossil records have shown us trends (directional changes in the features or patterns of diversity in a group of organisms)

Evolutionary trends: - increased size - elaboration of molars - loss of toes

Common Descent - all plants and animals have descended from an ancestral form - Homology (major source of evidence) – the same organ in different organisms under every variety of form and function. - Embryology

Multiplication of Species - genetic variation present within a species, especially variation that occurs between geographically separated populations, provides the material from which new species are formed (speciation). - biological features that prevent different species from interbreeding are called reproductive barriers - Reproductive barriers between populations usually evolve gradually (must be kept separated for long periods of time)

- Allopatric speciation – the separated populations evolve independently and adapt to their respective environments, generating reproductive barriers between them as a result of their separate evolutionary paths. - allopatric speciation can happen two ways: vicariant speciation founder event

- Sympatric speciation – different individuals within a species become specialized for occupying different components of the environment. - Parapatric speciation – separate species form where diverging lineages are mostly nonover- lapping in geographic distribution but make contact along a narrow borderline. - Adaptive radiation – production of several diverse species from a common ancestral species. - especially when many species arise within a short period of time (few million years)

Gradualism: - small, continuous changes in phenotypes (can lead to major differences if accumulated over many thousands to millions of years). - Punctuated equilibrium – phenotypic evolution is concentrated in brief events, followed by long periods of evolutionary stasis.

Natural Selection: - the major process by which evolution occurs - Darwin developed his theory of natural selection as a series of five observations and three inferences: Observation 1 – organisms have great potential fertility. - organsisms produce more offspring than can survive Observation 2 – natural populations normally remain constant in size, except for minor fluctuations.

Observation 3 – natural resources are limited Inference 1 – a continuing struggle for existence exists among members of a population (food, shelter, space) Observation 4 – Populations show variations among organisms (no two individuals are exactly alike) Observation 5 – some variation is heritable (offspring tend to resemble their parents)

Inference 2 – varying organisms show differential survival and reproduction favoring advantageous traits (= natural selection) Inference 3 – over many generations, natural selection generates new adaptations and new species. - long-term “improvement” of populations

Neo-Darwinism The biggest weakness in Darwin’s theory was his failure to identify the mechanism of inheritance. - Darwin saw heredity as a blending phenomenon in which the hereditary factors of parents blended together in their offspring. - Darwin also used Lamark’s hypothesis that an organism could alter its heredity through use and disuse of body parts and through the direct influence of the environment

Neo-Darwinism August Weismann rejected Lamarkian inheritance by showing experimentally that modifications of an organism during its lifetime do not change its heredity. He revised Darwin’s theory We now use the term neo-Darwinism to identify Darwin’s theory that has been revised by Weismann

Microevolution Microevolution is the study of genetic change occurring within natural populations. Occurrence of different allelic forms of a gene in a population is called polymorphism. All alleles in a population form the gene pool. The frequency of a particular allelic form of a gene in a population is called its allelic frequency.

equilibrium unless disturbed by recurring mutations, Hardy-Weinberg Equilibrium: - a hypothetical population will remain in genetic equilibrium unless disturbed by recurring mutations, natural selection, migration, nonrandom mating, or genetic drift. - if Hardy-Weinberg is occurring, evolution is not!! - How genetic equilibrium is upset: Genetic drift – random change in allele frequencies (really affects small populations) Nonrandom mating – if mating is nonrandom, then genotypic frequencies will change. - sexual selection

Migration – individuals leaving or coming into a population will disrupt the allelic and genotypic frequencies Natural Selection – favorable traits are passed on to the next generation, thus changing the frequency of alleles and genotypes - three types of natural selection (stabilizing, directional, and disruptive)

Mutations – changes in DNA or genes can change allelic and genotypic frequencies.

Macroevolution Evolutionary change on a grand scale, encompassing the origin of novel designs, evolutionary trends, adaptive radiation, and mass extinction.