Evolution and Heredity: How Does Life Change?

A. Evidence for Evolution by Common Descent

1. The Earth is old: James Hutton (1726-1797): Hutton was the first great british geologist. He compared Hadrian's wall - which looks new but was 1600 years old (122 AD) - with natural rock outcrops that were strongly weathered. Hutton concluded that the natural outcrops must be 100's of times older. He also examined an important formation at Siccar Point, where one series of nearly vertical strata is overlain by another series of horizontal strata. This is now called an 'unconformity', and Hutton explained it as follows. Based on Steno's laws of superposition, the bottom vertical sediments must have been laid down first, and they must have been laid down horizontally. Ages must have passed between each deposit, as each turned to rock. Then, uplifts must have occurred to bend them into a vertical aspect. Long periods of erosion must take place to wear that uplift flat, followed by the long intervals of time needed to deposit the second horizontal series. Also, if erosion and deposition acted slowly (as current observations show), then it must have taken a really long time to erode mountains or build up marine deposits (White Cliffs of Dover). He concluded that this slow, 'uniformitarian' cycle of deposition, uplift, erosion, and deposition meant that the Earth was unfathomably old. Indeed, the cycle may mean that it's age might not be discoverable. In short, Hutton concludes, the Earth has "no vestige of a beginning, no prospect of an end."

2. The patterns in the fossil record show change

Paleontology provided a variety of interesting patterns. First, there were extinct forms that were different from the species alive today. Although some earlier natural philosophers suggested that the creatures might still exist in some unexplored corner of the globe, that was a less satisfying hypothesis in the mid-1800's... most areas of the globe had been visited by Europeans. Also, the idea of extinction was repugnant to some people on theological grounds. If God had created a perfect world, then extinction renders that creation imperfect. Also, if species could go extinct since the creation, could species also come into existence since the creation? Just how dynamic was this system?

Darwin was impressed by two major patterns in the fossil record.

1. The major groups of animals accumulate in an orderly manner'. Everything is not represented at the beginning. In vertebrates, for instance, the fishes appear first, and exist throughout the rest of the record. Amphibians appear next, followed by reptiles, mammals, and birds. So it is not everything at the beginning, and it is not a replacement. Where did mammals come from? Spontaneous generation had been refuted, so Darwin knew that mammals had to come from other pre-existing animals. But the only completely terrestrial vertebrates before mammals were reptiles.

2. A second major pattern occurred within some lineages of similar organisms. Within some lineages, we seen orderly change in the size or characteristics of species in a geological sequence. For instance, consider the morphological patterns in a particular taxon (horses). Fossils in a stratigraphic sequence are similar, but often have traits that form a continuum...like the progressive loss of digits on the horse limb. And, with each innovation, there are often radiations - a "spurt" in the number of species that show this new trait. And finally, these species in recent strat are more similar to living ('extant') species than the species found in deeper, older strata. So, many of these transitional sequences terminate in living representatives.

3. Comparative anatomy shows patterns related to the environment:

Homologous Structures
    Although having a different outward "look" and although used for different purposes, they have an underlying similarity in structure - forelimbs of vertebrates all have one long upper arm bone, two lower arm bones, a bunch of wrist bones, and five digits. Darwin saw the similarity in structure as important. An engineer builds different things for different purposes - cars, boats, and airplanes are structurally DIFFERENT. Here, however, it seemed as if one basic structure was modified for different uses. Darwin knew why siblings in a family were similar - they had the same parents (ancestors). He reasoned that these structural similarities in different species might be due to the same principle - common ancestry. Also, he observed a correlation: Different uses correlated with different environments. Could this correlation be causal?

Analogous Structures
    Organisms in the same environment often have a similar outward structure or body plan. For example, flying animals all have an aerodynamic wing that is wider at the front than at the rear. However, the wings of differnt animals are differnt in underlying structure. Bats have fingers that support the membraneous wing, whereas birds lack fingers and the body of the wing consists of feathers. Insect wings don't involve the limbs at all (even though they have 6!). Again, Darwin observed this correlation with the environment: similar use (and outward structure) in similar environments. Could this correlation be causal?

Vestigial Organs
   These are organs that have no function in one organism (where they are 'vestigial') but they do function in other organisms. So, some whales have hip bones, but no legs. Why do they have these bones? Darwin was struck by the IMPERFECTIONS in nature, as much as the adaptations. Why do men have nipples? Why do we have muscles that wiggle our ears? Why do we have strong muscles in the front of our stomach, which are not "load-bearing", and weak muscles at the base of our abdomen (which rupture in a hernia)? This is a reasonable relationship in a quadraped, but not in a biped. Why do we have tail bones, but no external tail? Again, these are NOT well-designed features. In fact, attributing these imperfect designs to a perfect creator could be interpreted as heretical.  However, when we see them working in OTHER species, it suggests that maybe we inherited them from common ancestors where they DID serve a function. As a scientist, Darwin was trying to explain ALL the data (adaptations and imperfections), he was not simply bringing forward only the data that supported a preferred position (design).

4. Biogeography: Species distributions show patterns related to the environment:
    Under similar environmental conditions, we find different species filling similar ecological niches.  Outward 'form' correlates with ecological niche (role) across entire communities. So, in Australia, marsupials fill the role of dog-like predator, cat-like predator, burrowing animal, ant-eater, etc. These same roles are filled by outwardly similar placental mammals in South America. However, the similarity between a wolf (placental) and a Thylacine (marsupial - the 'tasmanian wolf') are strictly ANALOGIES. Their underlying structure shows them to be quite different - a wolf is more similar to a ground hog (both placentals) in underlying structure than to a thylacine.

    Islands often have fewer species than a mainland - even a patch of mainland the same size. As such, the patterns and interactions are often simpler to describe and understand. For both Darwin and Alfred Russel Wallace (the other independent author of the theory of evolution by natural selection), the study of islands was critical in to the development of their ideas.

1. Distance correlates with the uniqueness of the inhabitants: the animals on the Fauklands are the same species as on the mainland, but the Galapagos fauna is composed of unique species, found nowhere else:

"The natural history of these islands is eminently curious, and well deserves attention. Most of the organic productions are aboriginal creations, found nowhere else; there is even a difference between the inhabitants of the different islands; yet all show a marked relationship with those of America, though separated from that continent by an open space of ocean, between 500 and 600 miles in width. The archipelago is a little world within itself, or rather a satellite attached to America, whence it has derived a few stray colonists, and has received the general character of its indigenous productions. Considering the small size of the islands, we feel the more astonished at the number of their aboriginal beings, and at their confined range. Seeing every height crowned with its crater, and the boundaries of most of the lava- streams still distinct, we are led to believe that within a period geologically recent the unbroken ocean was here spread out. Hence, both in space and time, we seem to be brought somewhat near to that great fact -- that mystery of mysteries -- the first appearance of new beings on this earth."  The Voyage of the Beagle - Darwin (1839).

    2. The Galapagos fauna:
        - It was related to american fauna, yet different: the types of animals are new world animals.... there are iguanas like the green iguana of Central and South America, but the iguanas are different species. So, darwin describe it as " a world within itself, or rather, a satellite of the Americas" .... it was different, but more like the American fauna than any other...(no chameleons, for instance, which are old world lizards...)

        - It was dominated by dispersive forms. This is critical. The communities are dominated by reptiles, birds, and marine mammals. All of these organisms could MIGRATE to the islands from the mainland. (Terrestrial mammals don't migrate as well as terrestrial reptiles over open ocean. Throw a reptile in cold salty water, and: 1) its metabolism slows down (its cold), so 2) its demand for food and water decline; and 3) its scales protect it against water loss... which is why reptiles do well in the desert, too. Throw a mammal in cold salt water, and it's going to have a VERY tought time: 1) the temperature gradient between its warm body and the cold ocean is very large - in order to maintain its high body temperature against this gradient, it's metabolism has to INCREASE (to produce more heat to compensate for the heat lost to the environment). This increased metabolic demand will INCREASE the need for food and water... that's probably in pretty short supply in the open ocean; and 2) water is lost quickly from the skin to the salty ocean once the fur is wet... so, mammals are more likely to starve or die of exposure than reptiles.

       - So, the islands are dominated by dispersive forms, and this suggests they came from America. But if they came from America, WHY ARE THEY DIFFERENT SPECIES THAN THOSE IN AMERICA? They must have changed since their arrival.

- There are even differences between species on different islands. On the 14 species of finches - "Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends." The Voyage of the Beagle - Darwin (1839) VIDEO

2. Argument For Evolution as a Historical Fact:

Premise 1: Species that are alive today are different from those that have lived previously.
Premise 2: Spontaneous Generation is refuted, so organisms only come from other organisms.
Conclusion 1: Thus, the organisms alive today must have come from those pre-existing, yet different, species.
Conclusion 2: There must have been change through time (evolution).
Conclusion 3: The fossil record, vestigial organs, and homologies are all suggestive of descent from common ancestors.

Below, the figure from The Origin of Species that shows Darwin's idea of descent from common ancestors.

So, if species do change over time (evolve), the next question is "How?" How does this change occur?

B. How does change occur? Natural Selection

In 1838, Darwin wrote in his notebook that he read Thomas Malthus's "Essay on the Principle of Population" which said that populations would grow until they were limited by some resource... like food or shelter. hen, there would be a struggle for existence. Darwin wrote in his notebook that he immediately appreciated that, in this struggle, those individuals best suited to acquire those resources would be at an advantage, and the traits that gave them this advantage would be passed on when these individuals reproduced. In The Origin of Species, Darwin begins by analogy; he reminds readers of the great changes we have made in domesticated species by choosing who gets to breed. He called this 'artificial selection'. With this foundation, reader's could appreciate that nature could do this too, by limiting survial and reproduction to only those who could acquire limited resources.

(Know this.  Understand it.  You WILL be asked to outline NS in this very form.)

P1: Populations over-reproduce (Malthus)

P2: resources are finite (Malthus)

C1: Eventually, a population will grow until it becomes limited by its resources. At that time, their will be a "struggle for existence" and most offspring produced will die. (Malthus)

P3: Individuals in a population vary, and some of this variation is heritable (Darwin - observations and animal/plant breeding)

C2: Variations will not have the same probability of survival and reproduction in a particular environment; those well-suited to the environment will be more likely to survive and reproduce than others, passing on the genes for these adapted traits. There will be "Differential Reproductive Success" (Observations, breeding).

C3: Over time, adaptive traits will accumulate and the characteristics in a population will change. This is lineage evolution. (Like change in horse toes in a sequence of fossil species, or like the change in the chihuahua lineage from the ancestral wolves).

Corollary: Two sub-populations, separated in different environments, would be selected for different traits and may subsequently lose the capacity to interbreed. At this point, they are different biological species. This is Speciation and Radiational Evolution. (like the production of different Finches, mockingbirds, etc. on different islands in the galapagos, and like the radiation of St. Bernards AND chihuahua's, which diverged from one another over time).
 

Darwin here provides a natural explanation for why purposeful structures and behaviors occurs in nature. Through some process unknown to him, variation arises in natural populations. These varieties differ in terms of functional efficiency in a common environment; so some improving an organisms probability of surviving and mating than others. Organisms with these beneficial traits will leave more offspring, and the frequencies of these beneficial characteristics will increase through time - much as humans select for smaller and smaller dogs. He ends The Origin of Species (1859) like this:

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved". - The Origin of Species (Darwin 1859).

 Study Questions:

1. What observations did Hutton make, and what did he conclude from these observations?

2. What two patterns occur in the fossil record that impress Darwin regarding the hypothesis of evolution and common descent?

3. What are homologous structures?  What correlations occurs with the environment?

4. What are analogous structures?  What correlation occurs with the environment?

5. How did Darwin explain the existence of 'convergent communities"?

6. The Galapagos are dominated by many unique species of reptiles, birds, and marine mammals. What did this non-random assemblage suggest to Darwin about their origin, and how was evolution implied?

7. Why were the mockingbirds so critical to Darwin's ideas about the production of new species?

8. How did Darwin use the comparative method and observations of 'artificial selection' to produce the theory of 'natural selection'?

9. How were Malthus's observations and conclusion relevant to the development of Darwin's theory?

10. Outline the theory of natural selection as an argument, with three premises, 3 conclusions, and a corollary.