Chapter 13The Theory of Evolution

The Theory of Evolution by natural selection

Section 13.1

• The idea that life evolves was first proposed by a Roman named Lucretius who lived about 1,900 years before the modern theory of evolution.

• The theory of evolution has developed through decades of scientific observation and experimentation. The modern theory began to take shape as a result of the work of Charles Darwin.

• In 1831, Darwin was asked to voyage on the HMS Beagle as an unofficial naturalist. What he found would challenge the traditional belief that species are unchanging.

What Darwin Saw During His Voyage• Galapagos Islands – Darwin

found signs that species evolve. He was struck by the fact that the flora(plants) and fauna(animals) of the islands and those of the nearby coast of South America resembled each other.

• Darwin proposed that the ancestors of Galapagos species must have migrated to the islands from South America long ago and changed after they arrived.

Darwin Sought a Reasonable Explanation for Evolution

• The key that unlocked Darwin’s thinking was an essay written in 1798 by Thomas Malthus.

• Malthus pointed out that the human population grew faster than food supply (human pop. grew exponentially while food increased arithmetically) . If human reproduction was left unchecked, people would cover the world.

• We don’t because of things like disease, war, and famine that will slow population growth.

• Population – all the individuals of a species that live together in one place at one time. Ex. The population of deer in Vancleave, MS.

Natural Selection• Darwin realized Malthus’s ideas apply to all species. Every

organism has the potential to produce many offspring during its lifetime. In most cases, only a limited number of those offspring survive to reproduce.

• Individuals that have physical or behavioral traits that better suit their environment are more likely to survive and reproduce than those that do not have such traits.

–Charles Darwin• Natural selection – process by which populations change

in response to their environment.• Darwin suggested that organisms differ from place to

place because their habitats present different challenges to, and opportunities for survival and reproduction.

• Adaptation – the changing of a species that results in its being better suited to its environment

Darwin’s Ideas Have Been Updated• Best suited to survive and do

well in their environment = most offspring with the traits of the better suited parents. Certain forms of a trait become more common in a pop. because more individuals in the population carry the alleles for those forms. Natural selection causes the frequency of certain alleles to increase or decrease over time.

• Environments differ from place to place, so populations of the same species living in different places ten to evolve in different directions.

Isolation– condition in which two populations of the same species cannot breed with

one another due to changing.

Extinction Leads to Species Replacement

Over time, events such as climate changes and natural disasters result in some species becoming extinct, which

means that they disappear permanently. Species that are better

suited for the new conditions will replace the others.

Evidence of EvolutionFossils Provide an Objective Record of Evolution

Section 13.2

Darwin’s theory is almost universally accepted. Based on a large body of supporting evidence, most scientists agree on the following major points:

1.Earth is about 4.5 billion years old.2.Organisms have inhabited Earth for most of its history.3.All organisms living today evolved from earlier, simpler life-forms.

• However, the fossil record (and so the record of

evolution) is not complete. Many species have lived in environments where fossils do not form. Even if an organism does live in good fossil-making environments, the chances are slim that its dead body will be buried in sediments before it decays.

• Although the fossil record will never be complete, it presents strong evidence that evolution has taken place. Paleontologists, scientists who study fossils, can determine a fossil’s age fairly accurately using radiometric dating.

• Radiometric dating enables paleontologists to arrange fossils in sequence from oldest to youngest. When this is done, orderly patterns of evolution can be seen.

Biological Molecules Contain a Record of Evolution

• If species change over time, then the genes that determine their characteristics should have also changed (new taller giraffes, no stretched necks).

• Common Ancestor – species from which two or more species have diverged.

• Concerning Proteins: Species that share a recent common ancestor (ex. Humans and gorillas) have few amino acid sequences (order of amino acids to make proteins) where species that share a common ancestor more distantly (ex. Humans and frogs) have many amino acid sequence differences.

• Concerning Nucleic Acids: Scientists can directly estimate the # of nucleotide changes that have happened since two species diverged from a c. ancestor by comparing the exact nucleotide sequence of genes. Using this, they can make a phylogenetic tree to show how organisms are related through evolution (very strong evidence)

Anatomy and Development Suggest Common Ancestry

• Comparisons of the anatomy (structures) of different types of organisms often reveal basic similarities in body structures, even though their functions may be very different.

• Vestigial structures – some structures (like bones) are present in an organism, but reduced in size and either have no use or have a less important function (ex. Flightless wings on a cormorant or the appendix on humans)

• As different vertebrates evolved, particular sets of bones evolved differently, but similar bone structure can be seen. Homologous structures are structures that share a common ancestry – a similar structure in two organisms can be found in their commonancestor.

Does Evolution Occur in Spurts?• The model of evolution in

which gradual change over a long period of time leads to species formation is called gradualism.

• But Gould and Eldredge from America hypothesize that evolution occurs in spurts. This model of evolution, in which periods of rapid change in species are separated by periods of little or no change, is called punctuated equilibrium.

Examples of EvolutionSection 13.3

Industrial Melanism• A well studied ex. Of natural selection in action is

industrial melanism, the darkening of populations of organisms over time in response to industrial pollution.

• The best-known case involves the European peppered moth, Biston betularia. Among the members of this species, there are two color variations. Light and dark. The dark was very rare until about the 1850s when industrialization was occurring in major cities. 100 years later, almost all of the peppered moths near industrial centers were dark.

How? Think…Industry = factories

Factories = pollution or darkish grimeGrime covers tree trunks, so white moths get eaten and

dark moths survive. Survival of the fittest.

The Formation of New Species Begins with Small Changes

• Species formation occurs in a series of stages. Evolution continuously molds and shapes a species to improve its “fit” to its environment.

• The accumulation of differences between groups is called divergence.

• Within populations, divergence leads to the formation of new species. Biologists call the process by which new species form speciation.

Forming Ecological RacesA species often lives in several different kinds of environments. If their environment is different enough, local pops can become very different. Over time, populations of the same species that differ genetically because of adaptations to different living conditions become what biologists call ecological races. They are not different enough to be different species, but they may not be able to interbreed successfully.

Maintaining a New Species• Reproductive Isolation – inability of formerly

interbreeding groups to mate or produce fertile offspring. Could be geographically isolated or may reproduce at different times.