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 18th 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, • Earth’s 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 • Lamarck’s 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 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 (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 benumerous variation in all inherited traitsFor 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 lostRed hair+blonde hair = strawberry blondeLong 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 breeds—Boxer+boxer =baby boxer • Boxers are a true breeding strain
Mendel’s 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
Mendel’s 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 synthesisor neo-Darwinian view of evolution • Modern evolution incorporates chromosome theory of inheritance into Darwin’s theory of natural selection • changes in genes (mutations) only one source of variation
Most Importantly • Lamarck’s idea of inheritance of acquired characteristics no longeraccepted as a valid scientific theory • Problems with politics • LisenkoismRussian 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
Yeah…but 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 evolutionoccurs 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 evolutioninvolves the development of similar characteristics in distantly related organisms • Parallel evolutioninvolves the development of similar characteristics in closely related organisms
Convergent Evolution Dolphin- mammal Icthyosaur-reptile
Misconceptions • One antievolution argument is “If humans evolved from monkeys, “why are there still monkeys?” • This involves two misconceptions • No scientist has ever claimed that humans evolved from monkeys. Humans and monkeys evolved from a common ancestor. • Even if they had, that would not preclude the possibility of monkeys still existing—dogs have been bread from wolves and wolves still exist
It wouldn’t be Geology without Death and Destruction….. • 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 in some survival sense than 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 • …so is the evolution of new species that usually quickly exploit the opportunities another species’ extinction creates • Mammals began a remarkable diversification when they began occupying niches the extinction of dinosaurs and their relatives left vacant
Extinction • The mass extinction of dinosaurs and other animals at the end of Mesozoic Era is well known…but tiny • The greatest mass extinction occurred at the end of the Paleozoic Era • More than 90% of all species died out • We will discuss these extinctions and their possible causes throughout the rest of the term