Evolution
What is Evolution? Evolution involves inheritable changes in a population of organisms through time Fundamental to biology and paleontology Paleontology is the study of life history as revealed by fossils Explains the development of life as seen in the fossil record
Misconceptions regarding Evolution Evolution proceeds strictly by chance Nothing less than fully developed structures, such as eyes, are of any use There are no transitional fossils (the so-called missing links connecting ancestors and descendants) humans evolved from monkeys so monkeys should no longer exist
Historical Perspective Evolution is usually attributed solely to Charles Darwin, but actually considered long before he was born. ancient Greeks, philosophers, theologians, and all others who tried to explain fossils during the Middle Ages Nevertheless, the prevailing belief in the 1700s was that Genesis explained the origin of all life.
Historical Perspective Evolution is usually attributed solely to Charles Darwin, but actually considered long before he was born. ancient Greeks, philosophers, theologians, and all others who tried to explain fossils during the Middle Ages Nevertheless, the prevailing belief in the 1700s was that Genesis explained the origin of all life.
Darwin (1809-1882) On the Origin of Species, published in 1889 details his ideas on evolution formulated 20 years earlier proposes a mechanism for evolution called natural selection
What Darwin notices By selecting desirable traits and then breeding plants and animals with those traits we produce more useful species dogs, cats, vegetables, flowers What if natural processes could do the same thing? Thomas Malthus s essay on population suggested that competition for resources and high infant mortality limited population size What was different about the animals that survived?
Natural Selection Organisms in all populations posses heritable variations. size, speed, agility, visual acuity, digestive enzymes, color, and so forth Some variations are more favorable than others and give an organism a competitive edge in acquiring resources and/or avoiding predators (i.e. baby birds and rabbits) Not all young survive to reproductive maturity, however, Those with favorable variations are more likely to survive and pass on their favorable variations.
What about the giraffes? In any population there is bound to be numerous variation in all inherited traits For example giraffe with all different neck lengths (some long some a bit shorter) As environments changed and trees grew taller and taller those giraffe with longer necks had distinct advantage over those with shorter necks. These giraffe were more likely to survive and therefore pass on there characteristics
Survival of the fittest In colloquial usage, natural selection is sometimes expressed as survival of the fittest This is misleading because natural selection is not simply a matter of being the strongest; it involves differential rates of survival and reproduction One characteristic might provide an advantage to the individual in a specific circumstance but nature may favor the something else the smallest if resources are limited the most easily concealed those that adapt most readily to a new food source those having the ability to detoxify some substance and so on...
Apparent limits to darwin s theory Darwin s theory of Natural selection works on existing variation in a population. It could not account for the origin of new variations within a population Some critics also reasoned that traits would blend with other traits and be lost Red hair+blonde hair = strawberry blonde Long neck+ Short neck= medium neck The answer to these criticisms existed but remained hidden until 1900
Gregor Mendel During the 1860s, performed a series of controlled experiments with true-breeding strains of garden peas strains that when self-fertilized always display the same trait, such as flower color Labs, pugs, boxers, shepherds are examples of true breeding strains of dog
Mendel s work Medel first cross pollinated to different strains of garden peas; one strain that produced red flowers and one strain that produced white flowers The resulting seeds produced plants that only produced red flowers
Mendel s experiment Mendel then allows the second generation to self fertilize and produced a third generation. The third generation seed produces the following result. From his experiments Gregor determined that traits are controlled by a pair of factors now called genes Genes occur in alternate forms, called alleles One allele may be dominant over another Offspring receive one allele of each pair from each parent
Why this is important His work showed that genes controlling traits do not blend during inheritance Although traits may not be expressed in each generation or each individual they are not lost Therefore, some variation in populations results from alternate expressions of genes (alleles) based on inheritance
Modern Genetics Complex, double-stranded helical molecules of deoxyribonucleic acid (DNA) called chromosomes are found in cells of all organisms Specific segments of DNA are the basic units of heredity (genes) The number of chromosomes varies from one species to another fruit flies 8; humans 46; horses 64
Modern Genetics Modern evolution incorporates chromosome theory of inheritance into Darwin s theory of natural selection along with alternate expression of variation described by Mendel and the environmental forces described by Darwin genes may also mutate. mutation occurs constantly in all animals Most mutations have no visible effect only mutations that occur in sex cells (sperm and egg) can be passed on.
Politics and Science Lamarck s idea of inheritance of acquired characteristics no longer accepted as a valid scientific theory Russian agronomist Lisenko believed Lamarcks ideas fit much more closely with communist ideology (no gene could be better than another). Eventually put in charge of Russian Science and purges all evolutionary scientist Responsible for massive wheat famine
The Species Species is a biological term for a population of similar individuals that in nature interbreed and produce fertile offspring Species are reproductively isolated from one another Goats and sheep do not interbreed in nature, so they are separate species
Genetic Drift The randomness with which alleles are transferred through a population as a result of sexual reproduction
Recipe for a species Speciation is the process by which a new species arises from an ancestral species It involves change in the genetic makeup of a population, which also may bring about changes in form and structure Allopatric speciation species arise when a small part of a population becomes isolated from its parent population The peripheral isolates evolve as a result of genetic constriction and new environmental factors
Process of change Phyletic gradualism the gradual accumulation of minor changes eventually bring about new species
process of change Punctuated Equilibrium Holds that little or no change takes place in a species during most of its existence then evolution occurs rapidly
styles of evolution Divergent evolution occurs when an ancestral species giving rise to diverse descendants adapts to various aspects of the environment Divergent evolution leads to descendants that differ markedly from their ancestors Convergent evolution involves the development of similar characteristics in distantly related organisms Parallel evolution involves the development of similar characteristics in closely related organisms
divergent evolution
convergent evolution Icthyosaur-reptile
Parallel evolution
Death the driving force of evolution Perhaps as many as 99% of all species that ever existed are now extinct Organisms do not always evolve toward some kind of higher order of perfection or greater complexity Vertebrates are more complex but not necessarily superior to bacteria after all, bacteria have persisted for at least 3.5 billion years Natural selection yields organisms adapted to a specific set of circumstances at a particular time
Extinction The continual extinction of species is referred to as background extinction It is clearly different from mass extinction during which accelerated extinction rates sharply reduce Earth s biotic diversity Extinction is a continual occurrence As species go extinct new species usually quickly exploit the opportunities the other species extinction creates Mammals began a remarkable diversification when they began occupying niches the extinction of dinosaurs and their relatives left vacant