Chapter 15 Evolution I. Introduction: Charles Darwin’s book On the Origin of Species by Means of Natural Selection was the most significant scientific event in the 19th Century. In this chapter we will deal with organic evolution which is “the accumulation of genetic changes in populations of living organisms through many generations.”

II. Early development of evolutionary concepts: Aristotle (384-322 B.C.), arranged all organisms known to him from simplest to most complex (Scala Naturae). Leonardo da Vinci (1452-1519) was first to describe fossils and identify them as parts of pre-existing organisms. Count de Buffon (1707-1788) wrote a natural history of 44 volumes where he described all known plants and animals. Georges Cuviers (1769-1832) used comparative anatomy to describe animals. He believed that organisms did not change by time. Jean Baptiste Lamarak (1744-1829) believed that changes in populations over periods of time was due to inheritance of acquired traits.

III. Charles Darwin: Born in Shrewsbury, England (1809-1882). Attended Edinburgh Medical School, but dropped out after 2 years. Studied theology at Cambridge University, graduating 1831. Darwin was appointed as a naturalist and captain’s companion on the H.M.S. Beagle a sailing vessel commissioned to tour around the world and particularly the coastlines of South America. The voyage started lasted 5 years from 1831 to 1836 and visited South America, Galapagos Islands, and the coasts of Australia and New Zealand. Darwin collected many samples and investigated many aspects of natural history. Guided by Malthus’ ideas on human population, Darwin developed the idea of natural selection to explain changed in natural populations.

Alfred R. Wallace (1823-1913) independently arrived at idea of natural selection by studying animal populations. Darwin and Wallace jointly presented a paper on natural selection in 1958 Darwin published his classical book On the Origins of Species by Natural Selection in 1859

IV. Evidence of Evolution: Form and ecology of living organisms (e.g. homologies in the mustard family). Structural similarities between proteins, DNA and other molecules among organisms from different species (e.g. cytochrome c oxidase). Fossils. Geographical distribution of organisms. Physiology and chemistry of organisms.

V. Mechanisms of Evolution: A. Natural Selection 1. Overproduction - organisms produce large number of offspring 2. Struggle for Existence - competition for available resources - since resources are limited, many die 3. Inheritance and accumulation of favorable variations - variations with survival value are inherited from generation to generation - other variations are eliminated 4. Survival and reproduction of the fittest - the best adapted organisms will leave more offspring than less well adapted

B. Mutations: 1. Chromosomal alterations a. Deletion b. Translocation c. Inversion 2. Gene mutations a. Change in nucleotide pair(s) b. Mutator genes

C. Migration: gene flow occurs when individuals or gametes (pollen grains) migrate from one population to another. This leads in change of gene frequency in both populations. The smaller the population the higher the effect of migration on evolution. D. Genetic Drift: is a change in the genetic makeup (frequency of the different genes) of a population due to random events. The latter is due to random segregation of chromosomes and recombination or crossing over between homologous chromosomes during gamete formation. The smaller the population the higher the effect of migration on evolution.

E. Rates of Evolution: 1. Darwin - gradual, long-term change 2. Contemporary views - punctuated equilibrium theories

VI. Modern Approaches to Evolution: A. Reproductive Isolation 1. Geographic isolation - barrier prevents gene flow - genetic changes accumulate Examples: (a)The Canary Islands and introduction of rabbits from Portugal. In a matter of 600 years the rabbits on the islands became a new species and very different from their European ancestors. (b) In North America, the small shrubs called redbuds developed two new species one in the east coast and the other in the west coast. 2. Serpentine soils, have high magnesium content. Some new plant species have developed to tolerate this situation.

3. Seasonal isolation A species may be split into two new species if two population are exposed to two different seasons. Example is the flowering herb Dutchman’s breeches and the related squirrel corn, both evolved different in flowering times. 4. Mechanical Isolation In orchids, the pollinia (pollen sac is picked up by different body parts in bees. Each body part can fit the stigma of one specific species. Despite that a single bee can carry different pollinia, no cross hybridization occurs. The lack of cross hybridization created sympatric species (occupy the same territorial range without genes being exchanged.

B. The Role of Hybridization in Evolution 1. Introgression: backcross between hybrids of two related species and their parents. If the gene combination of the hybrid is more fit then continuous backcrossing to parents may eventually lead to disappearance of the parents gene combinations. 2. Polyploidy: by endoreduplication in mitosis, by doubling the chromosome number of a hybrid between two unrelated species (as in wheat) or the production of unreduced gametes. About 100,000 species of the existing flowering plants originated this way. 3. Apomixis (asexual reproduction): when new combinations of genes are combined by crosses and turn out to be very favorable for the plant, some plants adopt asexual means for reproduction (apomixis) to fix such useful combination of genes (e.g. dandelions and blackberries can reproduce both ways, sexually and by apomixis).