EVIDENCE FOR EVOLUTION

Evidence to support the theory of evolution comes from virtually all areas of science, particularly biology

I. Fossils = remains of ancient organisms

A. Fossils provide some of the most convincing evidence to support the theory of evolution

B. Fossils document the history of life on earth. The fossil record starts with simple unicellular prokaryotes and progresses to humans

1. Transitional forms reveal links between groups: Archaeopteryx is between reptiles and birds; Eusthenopteron is an amphibious fish; Seymouria is a reptilelike amphibian; therapsids were mammal-like reptiles, fossil whales with hind limbs

C. The fossil record shows major evolutionary trends over a period of time, e.g. transition from reptiles to mammals (recall extant Monotremes), origin and early radiation of flowering plants, and it documents phylogeny of many higher organisms

D. Fossil record shows progressive modification of forelimbs in vertebrates

E. Sedimentary fossils are the most common - sedimentation in oceans, lakes, and rivers protects dead organisms from scavengers, oxidation and fungal decay

F. Other types of preservation include: burial by volcanic activity, desert sands, petroleum springs (La Brea Tar Pits); entrapment in pine pitch - amber; or petrifaction - tissues replaced particle by particle by minerals in solution in local waters

G. It has been estimated that only 1% of organisms are potentially preservable as fossils. Only those with hard parts and living in areas where sedimentation was taking place are likely to be preserved 

H. The fossil record is relatively incomplete for several reasons:

1. Soft tissues are rarely preserved

2. Movement of the earth's crust has obliterated and/or covered many fossils

3. Fossilization takes place only in certain types of habitats and favorable environments

4. Paleontologists have not dug up every place on earth

NOVA series Chapt. 5 at 2:00

E. Methods to determine the age of rocks:

1. Relative dating by position of strata - William Smith, an English canal builder noticed that the different sets of strata he dug through were characterized by different fossils. He discovered that he could recognize the ages of rocks from many different places because they contained the same fossils. This led to the formulation of the geologic time scale 

 2. Radioactive dating - Isotopes are atoms which contain the same number of protons and electrons, but different numbers of neutrons. 

a. E.g. Carbon has three isotopes, each with a different number of neutrons. C¹² has 6 protons and 6 neutrons, C¹³ has 6 protons and 7 neutrons, C¹⁴ has 6 protons and 8 neutrons

b. Radioisotopes are unstable isotopes that emit electrons and energy, i.e. radioactivity. Over a period of time they change or decay into more stable forms

c. Rates of decay are measured in half-lives - the amount of time it takes for half of the unstable atom to change into the stable form

Common Radioisotopes
Radioisotope	More Stable Product	Half-Life (yr.)
Uranium 238	Lead 206	4.5 billion
Potassium 40	Argon 40	1.3 billion
Carbon 14	Nitrogen 14	5,730

d. Ages of rocks containing fossils are determined by measuring the proportion of an isotope relative to its more stable form. E.g. if rocks contained equal proportions of Potassium 40 and Argon 40, they would be 1.3 billion years old

e. Carbon dating is especially useful because both C¹⁴ and C¹² are fixed during photosynthesis and they eventually get into animals and plant products, e.g. wood and fibers. Once the organism dies the decay process begins. The reliability of Carbon dating has been cross checked on archaeological objects of known age such as ancient Egyptian remains

II. Comparisons of anatomical features of different organisms often provides evidence to support the theory of evolution. Note that much of paleontology is based on comparative anatomy between extinct and extant organisms 

A. Comparative study of anatomical structures within a particular group of organisms often reveals that they are based upon a common prototype or ground plan which varies with the organisms particular environment

1. Fossil record shows progressive modification of forelimbs in vertebrates

2. Vertebrate column demonstrates how, within a single phylum, an organ system seems to be based upon a prototype that is varied simply from class to class. Throughout the vertebrates the column has become modified within each group, however all share a common ancestor which had the prototypic column

B. The interpretation of anatomy requires an understanding of homologous versus analogous structures:

1. Homologous structures - similarity in structure due to common descent, irrespective of the diverse uses to which they may be put. E.g. vertebrate forearms

a. Adaptive radiation - the progressive modification of a structure to serve many different purposes. It is attained by mutual fitting of structure, function and environment. Readily apparent in the forelimbs of mammals which have been modified by changes of proportions, fusion of parts, or loss of parts., e.g. swimming, digging, running, flying

2. Analogous structures - similarity in structure based on adaptation for the same function, not common descent. E.g. wings have developed independently in insects, reptiles, birds, and bats (convergent evolution)

a. Analogous structures are the result of morphological convergence - structures "fitted" for a particular purpose tend to be similar, regardless of origin. E.g. body shape and flippers in dolphins, penguins and fish

b. All vertebrate eyes  are homologous - constructed of identical materials that are used in similar ways

c. In the Cephalopods (squids, octopi) eyes develop from skin whereas in vertebrates they develop from the brain, likewise in the former the lens is a crystalline secretion of skin cells whereas in the latter it is cellular. Eyes are analogous structures in vertebrates and cephalopods

3. Organisms derived from one common ancestor are monophyletic, e.g. all vertebrates are derived from one ancestor, all mammals from common ancestor

a. phylogeny = the evolutionary history of a group of organisms

4. A classification based on analogous structures, e.g. wings, would be polyphyletic, and incorrect

C. Vestigial organs are reduced and rudimentary compared to the same complex structure in other organisms. Vestigial characters, if functional, perform relatively simple, minor, or inessential functions using structures that were clearly designed for other complex purposes:

1. Human appendix useless yet in other mammals, including primates, it is necessary to aid in digestion of high cellulose diet

2. Human external ear muscles still present but useless

3. The coccyx (tailbone), is the remnant of a lost tail. All mammals have a tail at one point in their development; in humans, it is present for a period of 4 weeks, and most prominent at 31-35 days old.  Some babies occasionally have tails

4. Human wisdom teeth vestigial compared to other primates

5. Some snakes and dolphins have skeletal limbs, dolphins with fully developed hind limbs have been found

• Cetacean skeletal evolution. Cetacean embryos initiate hind-limb bud development but the bud arrests and degenerates around the fifth gestational week

6. Cave dwelling crayfish have eyestalks yet no eyes, or may be blind, or the eyes are much reduced and rudimentary

7. Sometimes vestigial organs may be adapted for new uses, e.g. penguin wings can't be used for flight yet adapted for swimming

8. Yellow flowers of dandelion resemble those of insect-pollinated plants, but seeds develop without pollinator (apomixis)

9. Kiwi, a flightless bird, has tiny stubby and useless wings

10. Other human vestigial features

III. Comparative biochemistry - The more closely related organisms are, the more similar is their biochemical makeup

A. Universality of genetic code supports theory of evolution

B. Similar chemistry and structure of chromosomes in Eukaryotes

1. Human vs. ape chromosome, centromeres and telomeres

a. Humans - 23 pr. chromosomes

b. Apes - 24 pr. chromosomes

c. Human chromosome # two = of fusion of two "ape" chromosomes, #'s 12 & 13

d. See video ID trial at 5:00

C. Chlorophyll is the same basic molecule in all photosynthetic organisms

D. Cytochrome C is respiratory enzyme common to all eukaryotic organisms

Consists of a central ring structure with an iron atom in the center, and a protein chain about 100 amino acids long. The more closely related organisms are, the more similar their amino acids are. Cytochrome C may be used to construct a "tree of life"

E. Nucleic Acid comparisons (DNA fingerprinting) - the more closely related two organisms are, the more similar is their DNA, e.g. identical twins

Comparison of mitochondrial DNA sequences in primates - chimpanzees are the most closely related to humans, lemurs are the first primates

Actual sequences of primate mitochondrial DNA

IV. Biogeography - the study of the distributions of organisms

A. The distributions of organisms had a major impact on both Darwin and Wallace. Darwin was impressed by the manner in which closely related animals replaced each other going southwards in South America

B. Darwin was also noted the diversity of organisms in different areas and the resemblance of the Galapagos tortoises and birds to those on the mainland, while still being distinct from each other on each island

C. Darwin postulated that the ancestors of these organisms had come from the mainland and that the species had become modified for life on each island. This is called adaptive radiation:

Adaptive radiation - the progressive modification of a structure in response to different environments. It is attained by mutual fitting of structure, function and environment. Common on islands all over the world. E.g. Galapagos finches, Hawaiian Honeycreepers, vertebrate forearms

E.g. there are 26 species of land birds on the Galapagos Islands, of which 23 are endemic (found nowhere else). Likewise, of the 436 species of plants on the Islands, 223 are endemic

Since the Galapagos Islands, like most islands all over the world, are very recent in origin, the endemics must have evolved after the islands were formed

D. Lack of native placental mammals in Australia

Australia has no native placental mammals (some recent immigrants)

They are replaced by marsupials - opossums, kangaroos, koalas and other mammals that complete their development in a marsupium (pouch)

Marsupials diverged from placental mammals several million years ago

Placental mammals arose in Laurasia (N. Hemisphere) and spread southward into Gondwana (S. Hemisphere). However, Australia had already drifted away from Gondwana before placental mammals got there

Monotremes - egg laying mammals, are the most primitive mammals and they are restricted to Australia. The Platypus shares features with reptiles, birds and mammals

A. Macroevolution refers to evolutionary change at or above the level of species. It means the splitting of a species into two. Any changes that occur at higher levels, such as the evolution of new families, phyla or genera, is also therefore macroevolution

B. Microevolution refers to evolutionary change below the level of species, and refers to changes in the frequency within a population or a species of its alleles and their effects on the form, or phenotype, of organisms that make up that population or species

C. Some creationists accept microevolution but reject macroevolution. However, based on your knowledge of meiosis, is it possible to accept microevolution yet reject macroevolution? In the next lecture you'll learn about the classification of genera, families, and phyla - classification is a human activity

VI. 29+ Evidences for Macroevolution the Scientific Case for Common Descent