Chapter 15 Darwin’s Theory of Evolution

15-1 The Puzzle of Life’s Diversity Biological Diversity- variety of living things. Evolutionary Theory- a collection of scientific facts, observations & hypotheses; it can explain the diversity of life on Earth. Evolution- change over time; the process by which modern organisms have descended from ancient organisms. Scientific Theory- a well supported testable explanation of natural events; supported by evidence & can be tested with new evidence.

Charles Darwin Contributed more than any other scientist to our understanding of evolution. Voyage of the Beagle- sailed around the world stopping at the Galapagos Islands. He made observations & collected evidence. He noticed many plants & animals were well suited to their environment. He collected fossils- preserved remains of ancient organisms. He collected many different species he had never seen before.

Galapagos Islands Islands west of South America that are close together but have very different climates. Darwin studied shell shape of the many different tortoises. He discovered many similar but different birds that turned out to be finches. Each species of finch had a different shaped beak which was a structural adaptation. Darwin felt the animals on all the different islands once came from the same species or common ancestry.

15-2 Ideas that shaped Darwin’s Thinking He felt that many would find his ideas too shocking to accept. He began to put all of his work on paper. Several scientists influenced Darwin: Hutton- earth is shaped by geological forces Lyell- earth is shaped by geological forces Lamarck- in inherited acquired traits; organisms change over time. Malthus-predicted that human pop. Would grow faster than space and food would provide.

15-3 Darwin’s Case He was reluctant to publish his ideas because they were so radical. He was disturbed by his own findings which challenged fundamental scientific beliefs. When he heard a scientist name Wallace had the same idea as his, Darwin published his work.

Darwin Published his Book On the Origin of Species – 1859. The book provided evidence that evolution has occurred. His theory on how evolution comes about was based on artificial selection. He believed that natural selection occurs in nature.

Artificial & Natural Selection Artificial Selection Animal breeders select for breeding only animals with the desired traits. Ex. Crops, livestock & horses. Natural Selection Individuals differ & some of the differences can be passed on to their offspring. More offspring are produced than can survive or reproduce. Individuals best suited to their environment survive & reproduce most successfully. Ex. run fast, outsmart with behavioral tactics.

Survival of the Fittest Animals prey on other animals. Some die and some survive. Fitness- the ability to survive & reproduce in a specific environment. Only the fittest organisms pass on their traits, thus the species change over time. Adaptation- an inherited characteristic that can be physical or behavioral.

Principle of Common Descent All species come from common ancestors. It links all organisms on Earth into a single tree of life. Ex. Tigers-panthers-cheetahs, felines-horses-dogs-bats, mammals-birds-alligators-fish.

Types of Evidence to Support Evolution 1. Fossil record- comparing fossils from older & younger rock layers documents the fact that evolution has occurred; it is a detailed record of evolution. Pg. 382,15-13 2. Geographical distribution of living species- the presence of similar environments suggests natural selection. Ex.-the finches could have descended from common ancestors on the mainland. Pg. 383, 15-14

4 Types of Evidence to Support Evolution 3. Homologous structures of living organisms- they have different mature forms but develop from the same embryonic tissues. Bone structure is the same but they look different. All are adapted to survive in different environments. Ex. Legs, wings, fins. Pg 384, 15-15 Vestigial organs- some homologous structures no longer serve important functions in descendants. Their size is reduces.

4 Types of Evidence to Support Evolution 4. Similarities in early development, or embryology- early stages, or embryos, of many animals are very similar. Pg. 385, 15-17 5. DNA evidence

Chapter 16 Evolution of Populations

16-1 Genes & Variations What we already know: Traits are controlled by genes. Many genes have at least 2 forms or alleles. Individuals of all species are heterozygous for many genes.

Genetic Variation Studied in populations Population- a group of individuals of the same species that interbreed; share a common group of genes. Gene Pool- results from interbreeding in populations; it’s all the genes that are present in a population; contains 2 or more alleles for each inheritable trait. Relative Frequency- the # of times the allele occurs in a gene pool, compared with the # of times other alleles for the same gene occur; it’s often a %. Pg. 394, 16-2 Ex. An allele makes up ¼ of population’s allele for a given trait. The relative frequency is 25%. In genetic terms, evolution is any change in the relative frequency of alleles in a population.

2 Main Sources of Genetic Variation Mutations Gene Shuffling Occurs during gamete production; results in many different combinations. Single gene trait controlled by 1 gene. 2-3 different genotypes are possible. Compare populations gene pool to a deck of cards. Shuffle & you get a different hand each time but it never changes the # of kings, aces, etc.

Polygenic Trait Can have many possible genotypes, producing many possible phenotypes. Controlled by 2 or more genes, each may have more than one allele. Ex. Height. Bell Shaped Curve Pg. 396, 16-4 How phenotypes are expressed. Most people are in the middle of the curve

16-2 Evolution as Genetic Change Natural Selection acts directly on phenotype. Ex. Lizard color. Pg. 397, 16-5 Natural selection on Polygenic Traits are more complicated. Directional Stabilizing Disruptive

Directional Selection When individuals at one end of the bell-shaped curve have higher fitness than individuals near the middle or the other end of the curve. The curve shifts toward the higher fitness end. Ex.- birds with larger beaks will have higher fitness if a small-medium seed shortage occurs. Pg. 398, 16-6

Stabilizing Selection When individuals near the middle of the curve have higher fitness than those on either end of the curve. This is where the average form of trait is. The curve narrows around the middle Ex.- a babies birth weight. Less wt., lower survival rate. Higher wt., difficult birth. Pg. 399, 16-7

Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than the individuals near the middle. The curve develops a peak at each end and a low point in the middle. Ex.- Medium bird seed has a shortage. Small & large bird seed are abundant. Birds with small & large beaks will have greater fitness. Pg. 399, 16-8

Genetic Drift Allele frequencies change. Occurs in small populations. Individuals with a particular allele leave more descendants than other individuals. Alleles become more or less common by chance. The only similarity to natural selection is that they involve a change in a population’s allele frequencies.

Genetic Equilibrium A situation in which allele frequencies of a population remain constant. 5 conditions required to maintain genetic equilibrium: Random mating. Population must be very large. There can be no movement into or out of the population (migration). No mutations. No natural selection When all 5 are met, evolution will not occur.

16-3 Process of Speciation Speciation- formation of new species For 1 species to evolve into 2 new species: Gene pools of 2 populations must become separated. They must be reproductively isolated. Ex. – finches on the Galapagos islands.

Reproductive Isolation When members of 2 populations cannot interbreed & produce fertile offspring. Behavioral- different courtship rituals. Ex. Songs. Geographical- populations are separated by geographic barriers. Ex. Mountains, rivers. Temporal- populations reproduce at different times. Ex. Pollination.

Speciation of Darwin’s Finches The species evolved on the Islands from a single species of founding birds. They traveled from mainland South America to one island to found a new population. They survived & reproduced then crossed to a 2nd island. The became geographically isolated & gene pool was no longer shared. Seed size on the island favored large beak birds. Large beak birds reproductively isolated & evolved into a new species.