Darwinian Evolution

Darwinian Evolution

Darwin's theory of evolution due to natural selection is based on the following assumptions:

There are inheritable variations among the members of a population;
Many more individuals are produced each generation than can survive and reproduce. This statement is based on Malthus' observation that populations can increase geometrically (1-2-4-8-16) while the food supply can increase only arithmetically (1-2-3-4-5);
Individuals with adaptive characteristics are more likely to be selected to reproduce by the environment;
Gradually, over long periods of time, a population can become well adapted to a particular environment;
The end result of organic evolution is many different species, each adapted to specific environments

Support for Darwin's theory of evolution came from tortoises and finches on the Galapagos Islands, an archipelago 600 miles west of the coast of Ecuador.

All are "variations on a theme"
Each island has its own unique species

Darwin could not account for either mechanism of trait transmission or the basis of variability among individuals

Was a contemporary of Mendel, but apparently did not know of his work.
Weakness is compensated by neo-Darwinism or synthetic theory of evolution, developed in the 1930's and 1940's.

Scientific evidence supports evolution

Fossils

Types include actual preservation, petrification, imprints, molds, casts, footprints & trails, borings, and coprolites. (You gotta be a special person to do research on coprolites.)

Can be dated by radioactive isotopes in fossil or in geological formation in which fossils are found

Requires long periods of time and unusual conditions for fossil preservation

Some are still alive!!

Homologous features

Comparing the structural details of features found in different but related organisms reveals a basic similarity.

Quintessential example is the forelimb of mammals - human arm, cat forelimb, whale front flipper, bat wing.

Although function is quite different, "there is no over-riding mechanical reason for them to be so similar structurally."

Homoplastic features

Reflect responses of dissimilar organisms to identical environmental challenge, e.g. flight led to evolution of wings in arthropods and birds.
Similarity of desert euphorbs and cacti reflect responses to hot, dry environments.

Developmental features are retrained in vertebrate embryos

All vertebrate embryos have segmented muscles, gill pouches, a tubular heart without left & right sides.

"Evolution is a conservative process, and natural selection builds on what has come before rather than starting from scratch."

The Genetic Code is Universal

Proteins indicate degree of relatedness. Differences in Amino Acid Composition of the Protein Cytochrome C obtained from a Variety of Organisms on the Evolutionary Scale. (From Atlas of Protein Sequence and Structure, 1967-68 by Margaret O. Dayhoff and Richard V. Eck. Washington D.C.: National Biomedical Research Foundation.

Number of different amino acids found in human cytochrome C as opposed to selected organisms
Organism	# of amino acids different	Where they are compared to humans in the phylogenetic tree.
Human	0	Self (Family Hominidae, Order Primates)
Monkey	1	Different family (Pongidae), same order (Primates)
Pig, bovine, sheep	10	Different order (Carnivora), same class (Mammalia)
Horse	12
Dog	11
Rabbit	9
Chicken, Turkey	13	Different class (Aves), same phylum (Chordata) - homeothermic
Duck	11
Rattlesnake	14	Different class (Reptilia), same phylum (Chordata) - poikilothermic
Turtle	15
Tuna	21	Different class (Ostheichthys), same phylum (Chordata) - poikilothermic
Moth	31	Different phylum (Arthropoda), same Kingdom (Animalia)
Candida fungus	51	Different Kingdom (Fungi)

Evolutionary hypotheses can be tested experimentally

For guppies, size, color, and time required to reach sexual maturity are genetically determined
When populations of guppies are transferred to "high predation areas", they become smaller, and reach sexual maturity sooner.