What is Evolution? Evolution involves inheritable changes in organisms through time Fundamental to biology and paleontology Paleontology is the study of life history as revealed by fossils Explains the development of life
Misconceptions of evolution Evolution proceeds strictly by chance Nothing less than fully developed structures, such as eyes, are of any use There are no transitional fossils 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 and by philosophers and theologians during the Middle Ages Nevertheless, the prevailing belief in the 1700s was that Genesis explained the origin of life. Contrary views were heresy!
Historical Perspective During the 18 th century, naturalists were discovering evidence that could not be reconciled with literal reading of Scripture Scientists gradually accepted a number of ideas: The principle of uniformitarianism, Earths great age Many types of plants and animals had become extinct change from one species to another occurred What was lacking, though, was a theoretical framework to explain evolution
Lamark Jean-Baptiste de Lamarck (1744-1829) is best remembered for his theory of Inheritance of Acquired Characteristics. According to this theory new traits arise in organisms because of their needs Once acquired new traits are somehow passed on to their descendants Lamarcks theory seemed logical at the time and was widely accepted
Darwin In 1859, Charles Robert Darwin (1809-1882) published On the Origin of Species details his ideas on evolution formulated 20 years earlier proposes a mechanism for evolution
What he noticed Plant and animal breeders practice artificial selection by selecting desirable traits and then breeding plants and animals with those traits to produce more usegul species dogs, cats, vegetables, flowers What if natural processes could do the same thing? Thomas Malthuss 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 (Key Points) Darwin proposes the idea of 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 some have 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.
Back to 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...
Limits on Natural Selection Darwins theory of Natural selection works on existing variation in a population. It could not account for the origin of new variations 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, Gregor Mendel, 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 or for example dog breedsBoxer+boxer =baby boxer Boxers are a true breeding strain
Mendels Work The parental generation consisted of true-breeding strains : One strain that produced red flowers and one strain that produced white flowers Mendel Cross-fertilized the two strains to yield a second generation all of which had red flower
Mendels Work Mendel then allowed the second generation to self fertilize and produced a third generation 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 is this important? The factors (genes) controlling traits do not blend during inheritance Although traits may not be expressed in each generation they are not lost Therefore, some variation in populations results from alternate expressions of genes (alleles) based on inheritance Variation can be maintained!
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 Thinking During the 1930s and 1940s, paleontologists, population biologists, geneticists, and others developed ideas that merged to form a modern synthesis or neo-Darwinian view of evolution Modern evolution incorporates chromosome theory of inheritance into Darwins theory of natural selection changes in genes (mutations) only one source of variation
Most Importantly Lamarcks idea of inheritance of acquired characteristics no longer accepted as a valid scientific theory Problems with politics Lisenkoism 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
Remember Evolution by natural selection works on variation in populations most of which is accounted for by the reshuffling of alleles from generation to generation during sexual reproduction The potential for variation is enormous with thousands of genes each with several alleles, and with offspring receiving 1/2 of their genes from each parent New variations arise by mutations change in the chromosomes or genes
Mutations Mutations result in a change in hereditary information ONLY mutations that take place in sex cells are inheritable, Can be chromosomal mutations (affecting a large segment of a chromosome) or point mutations ( individual changes in particular genes) Mutations: Random with respect to fitness May be beneficial, neutral, or harmful
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
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 During 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
Yeahbut how long? Although widespread agreement exists on allopatric speciation scientists disagree on how rapidly a new species might evolve Phyletic gradualism- the gradual accumulation of minor changes eventually bring about new species
Punctuated Equilibrium Holds that little or no change takes place in a species during most of its existence then evolution occurs rapidly Current thought is that evolution is most likely a mixture of these two ideas
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 dis