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introduction to darwin: lecture 2

This lecture considers the evidence for descent with modification from a common ancestor

descent with modification from a common ancestor

Charles Darwin received a state funeral and was buried in Westminster Abbey because he had persuaded most scientists that the theory of evolution as descent with modification from a common ancestor was true.

Darwin’s position was that all life descended from one original primordial form.

Evidence of evolution, available to Darwin, comes from four sources

• Embryonic development
• Specialization and adaptation in the world around us
• Bones and carcasses of extinct species
• Fossils

In addition, from the middle of last century there has been the evidence of

• Common genetic code

embryonic development

When a human sperm fertilizes a human egg a single celled entity with 23 pairs of chromosomes is the result, and a predictable chain of events ensues. If all goes well some nine months later a human baby is born. That baby consists of some 10 trillion (ten million million) cells, most of which are specialized as liver cells, skin cells, brain cells, muscle cells, bone cells, blood cells, and so on. Ten trillion is not that big a number. It corresponds to about 47 divisions of the original cell with 80-90% of the daughter cells dying or not dividing or each cell dividing every five or six days on average.

During the first few weeks of development, all chordate embryos proceed through a very similar sequence of events and the embryos have a very similar external appearance. For example human embryos go through a stage when they have structures which become gills in fish. These structures, in us, go on to form our head and face. Our circulatory system goes through stages where it is similar to the circulatory system of an embryonic fish, then that of an embryonic amphibian, then reptile, before finally settling on the mammalian circulatory structure.

During embryonic development, humans form three different sets of kidneys. The first two (resembling the embryonic kidney of jawless fish and reptiles) are discarded before the third appears.

Darwin regarded this recapitulation of an evolutionary sequence in the development of embryos as the strongest evidence for evolution available to him.

Specialization and adaptation in the world around us

High school text books are full of bizarre examples of specializations and peculiar adaptations to local environments.

In so far as these show change in response to environmental niches these are examples of evolution, but only in a restricted sense where evolution is “specialization” (so called "adaptive evolution"). Often the specializations are so extreme that habitat change results in extinction. They are evolutionary dead ends and show natural selection as a pruning force – first pruning for optimal fit then pruning out failure.

Bones and carcasses of extinct species

Frozen mammoths, frozen cavemen, skeletons of animals that died 10,000 or 20,000 years ago and are now extinct.

These often provide good evidence of Darwin’s theory of evolution by the gradual accumulation of changes as the similarities to modern animals and plants are striking.

It is intuitively acceptable that a mammoth is related to an elephant, and a saber-toothed tiger to a tiger or cat. The similarities between the preserved, extinct species and the modern “descendant” are sufficient to persuade observers that these remains may be of species that shared a common ancestor with, if they weren’t the direct ancestor of, modern species.

Fossils

Most fossils are casts of the hard parts of organisms which have fallen to a sea or lake floor shortly after death and been covered with sediment. The soft parts have rotted away while the hard parts have been replaced by minerals. The sediment has then been compressed by more sediment, eventually forming rock. Over geological time (millions or tens of millions of years) the sea bed has been lifted to finish up higher than the current water level and the fossils have then been exposed by erosion or rain.

Fossils of this type are overwhelmingly of marine animals.

It is possible to date the rock formations in which fossils are found (although the time scale is millions rather than thousands of years).

The fossil record is incomplete, but that is not surprising as fossilization requires a sequence of events, each of which is improbable.

In general terms

• fossils from younger rocks resemble modern life more closely than those from older rocks
• fossils from one area resemble modern species living in that area
• the first evidence of life, photsynthetic bacteria, appeared about 3.6 billion years ago. Multicellular life such as jellyfish, worms and sponges appeared shortly before the Cambrian explosion
• some transitional fossils have been discovered. One is the Tiktaalik a link between lobe-finned fish and amphibians.
• The Cambrian explosion yields at least as many body plans (disparity) as are present today

common genetic code

It is remarkable that all life on Earth is based on a common genetic code.

The common genetic code is consistent with descent from a single common ancestor.

However, we are not far enough into the course to discuss this topic with any rigor. It is deferred to the section 'comparative genomics' in lecture seven.

reading

Richard Dawkins' popular book The Greatest Show on Earth is sub-titled "The Evidence for Evolution"