18 Introduction to Darwinian Evolution

Lecture Outline

I. Ideas about evolution originated before Darwin

A. Aristotle believed that organisms were moving toward a more perfect state

B. Fossils had been known for centuries

C. Jean Baptiste de Lamarck proposed a theory of evolution based on inheritance of acquired characteristics in the 1800s

II. Darwin= s voyage was the basis for his theory of evolution

A. After receiving a degree in theology, he embarked on the HMS Beagle on a 5-year exploration of South America and the South Pacific

1. He was much influenced by the specimens and fossils found in South America, particularly the Galapagos Islands

B. Charles Lyell proposed that the earth was much older than commonly thought, in his book Principles of Geology, which Darwin read on his voyage

C. Principles of artificial selection also influenced Darwin= s theories

D. Malthus showed that populations can grow geometrically and food supplies increase arithmetically, therefore populations should outgrow the food supply

III. Darwin proposed that evolution occurs by natural selection

A. Inherited variations that are favorable to reproduction and survival result in the change of a population = adaptation

1. Darwin spent 20 years accumulating evidence by observation and experimentation

B. Alfred Russel Wallace came to the same conclusions while in the Malay Archipelago, and ultimately Darwin and Wallace presented this theory together at the London Linnaean Society in 1858

C. Darwin= s book, On the Origin of Species by Means of Natural Selection, was published in 1859

D. Darwin= s theory had 4 parts:

1. Members of a population vary, and some of these variations are heritable

2. Organisms produce more offspring than can survive to reproductive age

3. Resources available to a population are limited and therefore limit population growth

4. Organisms with the most favorable traits have differential reproductive success, and those traits are passed to the next generation

IV. The modern synthesis combines Darwin= s theory with genetics

A. Darwin and Mendel were contemporaries but unaware of each other= s work

B. The modern synthesis emphasizes the genetics of populations

1. This theory combines genetics of populations with the tangible evidence of evolution

C. Biologists debate the effects of chance on evolution

1. Biologists wonder: If we could repeat evolution, would the results be the same?

D. Evidence from field studies of various populations proves that natural selection has a greater influence than chance

E. Examples of transitional fossils found in the fossil record of whales seem to support this

V. Many types of scientific evidence support evolution

A. The fossil record provides strong evidence for evolution

1. Fossils provide direct evidence for change over time

a) Fossils range from bone, teeth, and shells, to actual body parts preserved in bogs, tar, amber, or ice

b) Fossils provide evidence of intermediates between extant and extinct forms

2. Fossils most commonly form in aquatic environments and typically form in sedimentary rocks

a) Fossils provide direct information about speciation

b) Fossils provide evidence concerning intermediates in evolution of species

c) The fossil record is biased toward animals with hard parts that live in aquatic or arid environments

(1) Terrestrial fossils are rare from tropical areas because of the rapid rate of decay

d) Various methods can be used to determine the age of fossils

(1) Most commonly, sedimentary layers nearer the surface are younger than those below

(2) Strata can also be dated by the presence of index fossils

e) Radioactive dating can be used to determine the age of fossils

(1) Radioisotopes decay at a characteristic and known rate

(2) The half-life is the amount of time required for 50% of the radioisotope to decay

(3) After 1 half-life has elapsed, 50% remains; after 4 half-lives have elapsed, 6.25% remains

(4) Potassium-40 is used to date very old fossils (hundreds of millions of years old)

(5) Carbon-14 is used to date organic remains that are relatively young (tens of thousands of years old)

B. Comparative anatomy of related species demonstrates similarities in their structures

1. Homologous structures are similar structures seen in evolutionarily related organisms

a) The arm of a human, the wing of a bat, and the flipper of a whale all have similar bones

b) Often overlooked, plants have homologous structures, such as a leaf of a tree, a spine of a cactus, and a tendril of a pea

2. Homoplastic structures are similar structures seen in organisms that are very distantly evolutionarily related but show convergent evolution, as they have evolved to meet similar environmental or anatomical demands (previously referred to as analogous)

a) The wing of a bird and the wing of a grasshopper are homoplastic

3. Vestigial organs are seemingly useless organs or structures

a) Humans exhibit over 100 vestigial structures, including our coccyx, wisdom teeth, and muscles that move our ears

b) Vestigial organs indicate that the organism evolved from ancestors in which the organ was functional

c) Vestigial hind limb bones seen in large snakes and whales indicate that they both evolved from tetrapod ancestors

d) Vestigial structures tend to persist as there is no selective pressure to eliminate them

C. Vertebrates have retained some developmental features of their ancestors

1. The embryos of vertebrates are very similar in morphology

2. Evolution is conservative, and as an example, there is no selective pressure to eliminate structures like gills and tails in mammalian embryos

D. The distribution of plants and animals supports evolution

1. Biogeography is the study of the distribution of living things

2. The study of biogeography gives clues to evolutionary biologists

a) A classic example is the distribution of marsupials

b) Darwin noted the distribution of species in the Galapagos

E. The Earth= s geological history is related to biogeography and evolution

1. In 1915, Alfred Wegner proposed the ideas we now refer to as continental drift

2. The original supercontinent, Pangaea, split into 7 large crustal plates

a) These plates of crust move on the more molten mantle

3. The study of plate tectonics explains the movement of these crustal plates

4. Biogeography supports the proposed crustal movements over the eons

VI. Developmental biology is increasingly being used to explain evolution

A. Molecular comparisons among organisms provide evidence for evolution

1. The analysis of nucleotide and amino acid sequences provides clues about evolution

2. The genetic code is nearly universal

a) Nearly all organisms utilize the same genetic code, indicating a common ancestor

3. Proteins contain a record of evolutionary change

a) Examination of proteins gauges evolutionary relatedness

4. DNA contains a record of evolutionary change

5. Phylogeny is the evolutionary history of a group of related species

a) DNA sequencing also provides evolutionary information

b) Human and chimpanzee DNA sequences are only about 2% different

c) Phylogenetic trees may be constructed based on differences in DNA sequences

6. DNA sequencing is used to estimate the time of divergence between 2 closely related species of taxonomic groups

a) Molecular clocks can be used to estimate the time since divergence between two species

b) Molecular clocks, fossils, and structural evidence can be used to reconstruct phylogeny

c) Because mutation rates vary, molecular clocks move at different rates

d) Fossil evidence and radioactive dating help to clarify evolutionary divergences

B. Evolutionary hypotheses are tested experimentally

1. Increasingly, biologists are designing experiments to test evolutionary hypotheses

2. Field experiments demonstrate evolutionary principles