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introduction to darwin: lecture 1

Darwin proposed

• 1.That all the various forms of life on Earth have evolved from a single common ancestor.
• 2.That there is a source of copious, small, undirected changes in inherited characteristics
• 3.That natural selection acts to increase the proportion of the population which possesses “beneficial” changes in inherited characteristics.

Darwin’s theory of evolution (descent with gradual modification) from a common ancestor by natural selection acting on new inherited characteristics is simple and elegant. It has considerable explanatory power. Unfortunately, it has little or no predictive power.

This is not a course in zoology or taxonomy, but we do need some idea of how life on Earth is classified

At the broadest level, life on Earth is classified into three domains

• Archaea
• Bacteria
• Eukaryota (cells have a nucleus) Animals and plants are in here

The classification system goes domain > kingdom > phylum > class > order > family >genus > species

Within the domain Eukaryota, animals and plants are two of the kingdoms. Others are protozoa, chromista, and fungi.

The animal kingdom contains 35 phyla (depending on which system one prefers).Humans are in phylum chordata. The largest phylum is arthropoda (segmented bodies, jointed limbs, chitin exoskeleton)

The vertebrates are often classified as a sub-phylum of the chordata

The tetrapods (four limbs) are a "superclass" within the (jawed) vertebrates.
Tetrapod classes include Amphibia, Reptiles, Birds (Aves), and mammals.

Primates are one of the orders within class mammalia

Then there is ongoing debate about whether humans and chimpanzees form a distinct family or whether the family should include the other great apes, the gorilla and orang-utan.

It is important to note that this classification is not an evolutionary tree. If you are interested the timetree web-site has a search function which provides "best evidence" estimates of the time at which various species, taxa, or whatever diverged.

This classification system ends at the level of "species". For our purposes a species is a gene pool reproductively isolated from other gene pools. That is, a species is the largest group whose members can reproduce with each other. So, humans of whatever race belong in one species. Elephants and cows are in different species. This is called the 'biological species concept' (BSC). The definition was originally provided by Mayr in 1942 : a species is a group of interbreeding natural populations that are reproductively isolated from other such groups.

The definition is not perfect. For lots of reasons lions and tigers should be classed as separate species. However, lions and tigers can reproduce to produce hybrid offspring.

Also, there are examples where population A can breed with population B and population B can breed with population C but population A can't breed with population C.

evolution: descent with modification from a common ancestor

Evolution began, in the Darwinian narrative, with a chemical molecule or complex that could reproduce, but imperfectly. This was the original material upon which natural selection operated

With the passage of time the common ancestor of Archaea, Bacteria, and Eukaryotes appeared. Other forms of primordial life may well have appeared but they had become extinct before there were any fossils.

That first common ancestor, with further variation upon which natural selection operated , diverged and gave rise to the first Eukaryotes.

Those first eukaryotes became the common ancestor of animals and plants, as well as protozoa, chromista, and fungi

The pattern of introduced variation plus natural selection led to a common ancestor of all animals, a common ancestor of chordata and arthropoda, a common ancestor of tetrapods, a common ancestor of birds and mammals, and finally, about six million years ago a common ancestor of chimpanzees and humans.

The above paragraph does not say that chimpanzees, or humans, walked this Earth six million years ago. It says that our last common ancestor walked this Earth then and that at some later time speciation occurred. One species evolved into chimpanzees. The other evolved into humans.

Darwin was firmly committed to the concept of evolution by the accumulation over long time periods of small changes (gradualism).

Richard Dawkins explains the concepts of gradualism and common descent well in the chapter "Dogs, Cows, and Cabbages" of The Greatest Show on Earth. He imagines following a rabbit lineage backwards in time. Each generation of parents is imperceptibly different from their children, but as one goes further and further back in time, while the difference between successive generations is tiny, if present at all, the 'ancestor' rabbits begin to look quite different from modern rabbits

Eventually, in this thought experiment, one arrives at an ancestor animal, the common ancestor of rabbits and, say, leopards. Dawkins calls this the 'hairpin' animal. If one follows the other limb of the hairpin forward, once again the differences between successive generations are tiny but after a large number of generations the animals are 'leopard like' and finally we arrive at modern leopards.

A source of copious, small, undirected changes in inherited characteristics

In the current view this is a source of errors when DNA is copied. Many such errors are know to occur - mutations, deletions, insertions, crossovers, and so on. They will be discussed in the lectures dealing with molecular biology.

natural selection

The theory of evolution as descent with modification from a common ancestor explains the “what” of evolution. According to this theory, a primordial life form appeared about 3.9 billion years ago, and, since then, by a series of gradual changes, has evolved into the diversity of life forms found on Earth today.

Darwin believed that natural selection was the main mechanism, the “how”, of evolution.

Natural selection refers to the accumulation of small inherited changes where each small change has arisen by chance and where each small change confers a reproductive advantage on its possessor. “Reproductive advantage” means that that organism has more offspring that survive to reproduce themselves than average for the species.

The key idea of natural selection is that when a small inherited change confers a reproductive advantage on an individual the proportion of the population with that change increases over time.

There are three central features of Darwin’s theory of evolution by natural selection

• 1.Natural selection occurs.
• 2.Natural selection occurs at the level of the organism. What counts is the reproductive success of the organism.
• 3.Natural selection is sufficient, over the life of planet Earth to explain the diversity of all life on the planet.

There is a fourth claim - that all life on Earth has arisen as the result of natural selection. In the absence of a time machine this is a difficult claim to prove.

There is little real dispute that natural selection occurs.

The classic example is that of the peppered moth. The following quote is lifted from the Wikipedia entry 'peppered moth evolution' (http://en.wikipedia.org/wiki/Peppered_moth_evolution).

The evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light colouration, which effectively camouflaged them against the light-coloured trees and lichens which they rested upon. However, because of widespread pollution during the Industrial Revolution in England, many of the lichens died out, and the trees that peppered moths rested on became blackened by soot, causing most of the light-coloured moths, or typica, to die off from predation. At the same time, the dark-coloured, or melanic, moths, carbonaria, flourished because of their ability to hide on the darkened trees.

Since then, with improved environmental standards, light-coloured peppered moths have again become common

The claim that natural selection occurs only at the level of the organism is now, probably, a minority viewpoint

It seems obvious today that species compete with each other. One example is competition for food. Rabbits and sheep compete for pasture. Humans compete with many species through habitat destruction.

Darwin's contemporaries did not accept that natural selection was sufficient to explain the diversity of life on this planet. In Darwin’s time most biologists were prepared to concede that natural selection occurred, but it was seen as a pruning force only, capable of refining new changes once they had appeared and capable of eliminating changes that were detrimental in a given environment. It is probably fair to say that the sufficiency claim is accepted today, if only because there are no alternatives consistent with 'descent with modification.' (but see later lectures in this course)

reading

This is a perfectly standard Evolution 101 course
http://evolution.berkeley.edu/evosite/evo101/IIntro.shtml