The Origin of Species Chapter 24 Both in space and time, we seem to be brought somewhere near to that great fact the mystery of mysteries-the first appearance of beings on Earth. Darwin from his diary kept aboard the HMS Beagle commenting on the life found on the Galapagos Islands 1
A common misconception about the process of evolution is that individual organisms change. It is true that natural selection acts upon the individual testing its relative fitness (the contribution an individual makes to the gene pool of the next generation, relative to the contributions of others - survival and reproduction), but the effects of natural selection are only noticed at the population level. Recall the levels of biological organization: Levels of biological organization: Cell Tissue Organ Organ System Organism Population Community Ecosystem Biosphere 2
Two levels of evolution Microevolution change in allele frequencies in a population over time (generations) Macroevolution broad pattern of evolution; includes to rise of new groups of animals or plants and considers the impacts of mass extinction events Genetic variation makes evolution possible. Where does genetic variation come from? Mutations in DNA Altering gene number or position; chromosomal changes Rapid reproductive rates i.e viruses Sexual reproduction 3
At the population level, changes in allele frequencies can cause evolutionary change There are three primary mechanisms that cause most evolutionary change Natural Selection described by Darwin. Differential reproductive success based on adaptations. Genetic drift random or chance events that can alter allele frequencies Founder effect small portion of a population becomes isolated Bottleneck effect random event that dramatically reduces population size Gene Flow change in allele frequencies by movement of individuals in or out of a population; prevents isolation of alleles. Of these; natural selection is the most viable and consistently causes adaptive evolution Looking at examples of natural selection modes 4
The environment puts selective pressures on populations and speciation may result. Speciation the origin of a new species; the focal point of evolutionary theory because the appearance of new species is the source of biological diversity. Species defined a group of living things with similar anatomical characteristics with the ability to interbreed. A group of actually, or potentially, naturally interbreeding organisms that share a common gene pool and are isolated from other such groups. This definition of species is the reason that it is the only taxon that exists in nature. Kingdom Phylum Class Order Family Genus Species How a species is defined Biologists must compare morphology (body forms) and Physiology, biochemistry, DNA sequences Why??? 5
The key to defining a species can be found in the definition of a species A group of actually, or potentially, naturally interbreeding organisms that share a common gene pool and are isolated from other such groups. This definition is known as the biological species concept coined by Ernst Mayr in 1942 Species are differentiated based upon reproductive isolation!! Barriers that impede members of two species from producing viable, fertile hybrids. These barriers can be classified according to whether they contribute to reproductive isolation before or after fertilization Prezygotic barriers before zygote Postzygotic barriers after zygote 6
Examples: aquatic vs. terrestrial Thamnophis puddle ducks donkey + horse = mule What causes speciation? We know that natural selection and the fitness of the individual drive changes within a population. There are two main ways that speciation can occur depending on how the gene flow between members of a population is interrupted 7
Allopatric speciation is fairly easily visualized as some type of geographic isolation separates a population into isolated subpopulations, resulting in an abrupt stop in gene flow between the entire gene pool; now there are two gene pools and selective pressures act upon each subpopulation. If this geographic isolation and gene pool divergence occurs over a long enough period of time reproductive isolation can occur Harris antelope squirrel (south rim) and white-tailed antelope squirrel (north rim) 8
Sympatric speciation may be a little more difficult to picture because the process of reproductive isolation does not coincide with a separation of the population by a visible barrier Polyploidy mutational change in the number of chromosomes (example autopolyploid two or more chromosome sets all derived from the same species; results in failure to reduce the chromosome number during cell division of gametes) Sympatric speciation is more common in plants than animals but it is not limited to the plant kingdom; animal examples do occur Behavioral mechanisms such as feeding on quickly ripening fruit in the apple maggot fly Non-random mating or mate selection This is usually based on coloration mutations 9
How can speciation produce biological diversity? Adaptive radiation evolution of many diversely adapted species from a common ancestor upon introduction to various new environmental challenges and opportunities. New habitats islands Mass extinction opens up new niches for survivors But we still must consider the tempo of speciation in order to explain the cumulative biological diversity of the Earth Gradualism slow change as descendants of a common ancestor gradually change (Darwin s view) Punctuated Equilibrium periods of little change followed by dramatic speciation events with pronounced differences in new species (Eldridge, Gould) What do you think?? 10