L>Readings for Core 8.1Click here toan e-textbook forIntroductory BiologyEvolution in Actionfor the e-textbook Exploring Life by Professor John Blamire.Required readings forEvolution in ActionQuick Menugene poolsHardy-Weinberg principlegenetic equilibriumnormal distributiondirectional selectionstabilizing selectiondisruptive selectionbottleneck effectgenetic driftfounder effectspeciationisolationorigin of speciesadaptive radiationdivergent evolutionconvergent evolutionextinctionAmbling peacefully across the dusty sands of southern Texas, Arizona, and New Mexico are numerous small herds of peccaries, animals that at first glance are often mistaken for pigs. The mistake is a common one, because peccaries look like wild pigs and both species are similar in many ways.Peccaries, however, are not pigs despite their appearance. As near as we can tell, their lineage diverged from that of modern pigs about 40 million yeas ago in the Oligocene epoch.Nonetheless, the mistake is an interesting one that illustrates several important questions in the study of evolution. For example, how do we know that peccaries are a different species of organism from pigs?It is fairly obvious that elephants are a different species from pigs, but peccaries and pigs are similar in so many ways, so why are we justified in placing them in a different group?How different must two groups be before they constitute separate species?The question about peccaries and pigs are all part of two larger fundamental problems in biology: What is a species and what are the origins of new species?Charles Darwin wrestled with these questions in his day and came up with the original framework of the answer. Since then others have added to his formative ideas and produced a modern synthesis of Darwinism, molecular genetics and population genetics.Measuring EvolutionIt is at the level of the population of organisms that evolution is measured. Individual peccaries cannot evolve. But a little herd, or population of peccaries is subject to all the forces of variation and natural selection, and can undergo all the changes in genotypes and phenotypes associated with evolutionary change.A population is the smallest unit of living organisms that can undergo evolution.Within similar organisms are combinations of different genes and different gene types. A peccary population, therefore, can be considered as a “pool” of genes and gene types that reflect the genetic diversity of all the individuals within the group. This “pool-of-genes” is what changes when evolution takes place.G.
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H. Hardy, a Cambridge Professor, G. Weinberg in Germany and W.E. Castle in America demonstrated mathematically that populations are genetically stable over time.Every population has a pool of gene variants that acts as the raw material for evolutionary change. Armed with the measuring stick of the Hardy-Weinberg principle and genetic equilibrium, it is possible to study real populations of organisms as they interact.Quantitative and qualitative observations help define what types of agents influence gene pools and what type of changes they bring about.Even under conditions where nothing is changing, there is a range of variation for any phenotypic trait in any population. This can be quantified and represented in graphical form; a normal distribution.As pressures from predators or the environment begin to act on the range of variation of phenotypes within a population, several types of changes can be seen; directional selection, stabilizing selection or disruptive selection.In small populations, other forces are at work. When a population is small, the presence or absence of a single individual can have a profound effect on the population gene pool. A sudden reduction in population size can also alter the remaining gene pool. This is the bottleneck effect.A change in the gene pool brought about by chance is a genetic drift.An extreme form of genetic drift, combined with the bottleneck effect is called the founder effect, which depends on a small group becoming isolated from the larger group, and can rapidly lead to the creation of a new species.What is a species?What are the origins of new species?When any species finds itself in a brand new environment, an explosion of evolution is possible as creatures radiate out and adapt to new possibilities.Some species rapidly diverge in shape, form or adaptations, while others acquire lots of superficial similarities, and yet others vanish.BIOdotEDU© 2002, Professor John Blamire