© 2007-2015 by Zack Smith. All rights reserved.
What is evolution?
It is two things: The fact of evolution and the hypotheses surrounding how evolution works. The hypotheses are collectively known as the theory of evolution.
The fact of evolution
Evolution is defined simply as genetic change over time, meaning more precisely between generations. Examples of genetic change over time are extremely easy to identify. Every year we observe new flu viruses and new bacterial strains. This is evolution. Every human has slightly different genes from his or her parents. That is evolution, too.
The theory of evolution
There are numerous hypotheses about how evolution occurs.
These hypotheses are generally supported by enormous amounts of evidence, both observational evidence and experimental evidence.
In science, when a set of hypotheses are related and supported by evidence, we call this collection of evidence and hypothese a scientific theory, or simply
theory for short.
Note how term
theory in science means something different than in everyday English.
A scientific theory is not the same thing as a theory in the common dictionary sense.
One word can have multiple different or related meanings.
Does belief in religion means the same as belief about what color your car is? No.
Origins of Life
The Earth is 4.5 billion years old. The Earth's crust ws stable at 3.9 billion years ago.
Profile of the Early Earth
It was a very different world than what we know today. There was:
- No oxygen (O2)
- Much ultraviolet light (UV) because there was no ozone layer (O3) for protection
- Strong winds
- Hot weather
- Much lightning
- Plus gases from volcanoes
The atmosphere at the time was comprised of methane, ammonia, water, and carbon dioxide (CO2).
The origin of life can be summarized by saying that basically, simple molecules combined with UV and lightning to create simple organic compounds that are the constituents of life. This combination has been simulated in the lab, proving that it is a plausible hypothesis.
Aleksandr I. Oparin (Russian) and Haldane (English) in 1920 proposed that life could arise from prebiotic soup i.e. oceans.
Oparin argued that new life doesn't arise today because O2 attacks chemical bonds and the ozone layer prevents sufficient heat, lightning, and UV.
But back then, basic chemicals, subjected to the harsh early Earth environment, formed into the basic molecules of life.
Four stages of chemical evolution (proven in the lab), called the Oparin-Haldane model:
- Abiotic (nonliving) molecules e.g. ammonia, methane, H2, water form into monomers i.e. amino acids, sugars, lipids, purine and pyrimidine bases, even ATP (the energy of life).
- Monomers form into polymers, specifically proteins and RNA, establishing the
- Eventually DNA evolves, providing more stable inheritance, less mutation.
- Phospholipid bilayers formed creating protobionts i.e. pre-cells with enzymes inside that did osmosis and had selective permeability and did basic self-replication.
Stanley Miller, who was grad student of Harold Urey at U. Chicago in the 1950's, put this model to the test for the first time in the lab, simulating early Earth in a special-built apparatus.
The two proved that abiotic synthesis occurs to create monomers, i.e. the building-blocks of complex molecules.
Since then, experiments using their apparatus have produced all 20 amino acids, all bases, lipids, and some sugars.
Oparin shook a container of these molecules, and he got protobionts.
Sidney W. Fox in the 1960's discovered proteinoids, which are abiotically synthesized polypeptides that are 3-dimensional and catalytic, in other words, they're like enzymes. These proteinoids automatically form from amino acids when placed on hot rocks or hot clay.
Thomas Cech in 1986 discovered that some RNA are self-splicing: They catalyze their own replication because they can act as an enzyme. An RNA that is enzymatic is called a ribozyme.
Prokaryotes (single cell, loose genetic material without nucleus) evolved 3.5 billion years ago, i.e. becoming Bacteria and Archea.
Eukaryotes (cells with nuclei) evolved from bacteria 1.7 billion years ago, leading to plants and animals.
The Endosymbiotic Theory says eukaryotes arose when bacteria engulfed other bacteria which became its mitochondria and chloroplasts. Evidence for this: Both of these have DNA and divide by fission. Mitochondrial DNA (mtDNA) has been proven to be just like bacterial DNA.
A protobiont (Oparin's term) would be the earliest cell-like entity, an aggregation of simple organic molecules into a unit that
- Does osmosis
- Has a selectively permeable membrane
- Can do basic self-replication
Life started with RNA (Gilbert's
- RNA self-replicates from bases.
- It acts as an enzyme i.e. is catalytic.
- Translatable into proteins.
- However RNA mutates a lot and stability was achieved when DNA evolved.
Whittaker's Five-Kingdom Taxonomy:
- Monera = bacteria
- Protista = single celled organisms with nuclei
Domains of life:
- Archaea = single-celled organisms without nuclei
- Bacteria = single-celled organisms without nuclei that came later
- Eukarya = organisms whose cells have nuclei
Mechanisms of Evolution
Natural Selection is differential success in reproduction due to environmental factors.
Artificial Selection is differential success in reproduction due to decisions by intelligent mammals.
Sexual Selection is differential success in reproduction due to sexual attractiveness e.g. the peahen selects a peacock because of its plumage.
Kin Selection is differential success in reproduction due to a parent acting to save its offspring from perishing.
Lamarck had claimed that traits respond to greater use, e.g. giraffes' stretching of neck led to longer neck in offspring.
Lamarck was wrong.
Charles Darwin publishes Origin of Species Nov 24, 1859.
- He presents evidence for many species of heredity from ancient times.
- He proposes Natural Selection as the mechanism of this evolution.
He had traveled around the world on the HMS Beagle as an unpaid naturalist just collecting data. He especially looked at Galapagos finches. Finch beaks differed by food type e.g. cactus.
Alfred Wallace had the same theory at same time as Darwin but less data and he published later.
Carolinus Linnaeus, inventor of taxonomy, rejected Natural Selection. He had religion.
Evidence for evolution
- Georges Cuvier proved that species went extinct; started vertebrate paleontology.
- Comparative anatomy shows that related species have homologous structures.
- Vestigial traits were important for ancestors: Coccyx was tail. Goosebumps were erect hair.
- Comparative embryology shows embryos are strikingly similar but get expressed differently.
- Comparative molecular biology looks at proteins etc.; for instance differences in hemoglobin protein Cytochrome C of Electron Transport Chain.
- Universal Codon of Life.