Evolution By Natural Selection

The night of September 28, 1838 was important for Darwin: it was then that he realized the missing piece of the evolutionary puzzle - natural selection. He wrote in his autobiography (Darwin 1859) that,

I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favorable variations would tend to be preserved and unfavorable ones destroyed.

On that date he drew a simple branching evolutionary tree in his notebook, and a more elaborate version was the only illustration in the Origin (see Fig. 5.1b).

Darwin came to his flash of inspiration by a combination of thoughts and observations:

• He had seen the huge diversity of life during a 5-year long circumnavigation of the world on board the Beagle, a British surveying ship; he asked himself why life was so diverse - every island he visited had different plants and birds.

• He had seen evidence for relationships in time and space - in South America he saw the bones of giant extinct ground sloths and armadillos, obviously close relatives of living forms; and as he went from island to island in the Galápagos and elsewhere, he saw close similarities between species of plants, reptiles and birds.

• He was aware of the record of fossils in the rocks, and that fossils changed through time, and seemed to progress from simple forms in the oldest rocks towards modern forms in the Pliocene and Pleistocene.

• Thomas Malthus argued in his book An Essay on the Principle of Population (1798) that human populations tend to grow far faster than their food supply, and Darwin transferred this concept to the natural world, seeing that reproductive rates are higher than they need to be.

Thus, by September 1838, Darwin understood the concept of evolution, a view that had been expressed by many thinkers before, and that claimed that life had not been static forever, but that species changed and never stopped changing. He had a rich understanding of modern geographic variation. Why, he asked, does every island in the Galápagos archipelago have a different set of species of small birds when the same set would do perfectly well throughout? Further, why did the bird species on neighboring islands look more similar to each other than those on distant islands?

So, Darwin's first key insight was that life is more diverse than it ought to be if it had been created and his second was that all species living and extinct can be linked in a single great evolutionary tree that shows their relationships and that tracks back to a single ancestor. These are descriptive observations of pattern.

But Darwin is remembered most for his third insight, and this was the principle of natural selection, a process that explains the diversity of life and its branching history of relationships: only the organisms best adapted to their environment tend to survive and transmit their genetic characteristics in increasing numbers to succeeding generations while those less adapted tend to be eliminated. Darwin made the case with remorseless logic, and this can be dissected into a series of clear statements:

1 Nearly all species produce far more young than can survive to adulthood (Malthus' principle).

2 The young that survive tend to be those best adapted to survive (larger at birth, faster growing, noisier in the nest, faster to escape predation, less disease, etc.).

3 Characters are inherited from parent to offspring, so the characters that ensure survival (size, aggressiveness, speed, freedom from disease, etc.) will tend to be passed on.

4 These survival characteristics will increase generation by generation. The changes are not inexorable, so cheetahs run fast enough to catch their prey, not at 2000 km per hour, because they do not have to and their bodies would fall to bits if they tried.

Each of these observations can be supported by huge numbers of observations. For point 1, note that most plants and animals produce hundreds, thousands or millions of offspring; if every melon seed grew into a melon plant or every cod egg became an adult, melons and codfish would soon cover the surface of the Earth to a depth of hundreds of meters. For point 2, observe any litter of puppies or nest of fledgling birds and see how siblings compete with each other for their parents' attention. For point 3, observe your parents or children and see the evidence for inherited characters. For point 4, consider how this emerges from points 1-3. Evolution by natural selection is on the one hand rather simple, but also rather complex, and it is frequently misunderstood or misrepresented (Box 5.2).

Darwin's Origin (1859) said it all, and he said it so well. In conclusion of this section, Darwin described natural selection in action:

It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.

Box 5.2 The foolishness of intelligent design

Since the earliest days, philosophers have sought to understand the world and where it came from. At one time, many scholars argued that the Earth was flat, while others argued that the Earth was static in space and the sun and planets rotated around the Earth. These views were famously disproved and rejected some 500 years ago.

Most religions have also espoused so-called "creation myths" (see p. 184), often fanciful stories about how the Earth was created, and how it was populated with life. One of the most famous creation myths is the Bible story in Genesis of how God created the first man, Adam, and then the first woman, Eve. For some time, religious fundamentalists - people who believe in the literal truth of every word of the Bible, the Koran or any other religious text - have conducted a campaign against evolution, and often against science and the modern world in general. At present, we see a rise in Christian and Islamic fundamentalism in different parts of the world, and enthusiasts from both religions try to use the political system and the press, and sometimes even violence, to impose their view on others.

Creationism is a belief that the Earth and life were created perhaps 7000 years ago, and that all the areas of science that refer to long time scales (e.g. geology, astronomy, cosmology) and to evolution (all biological and medical sciences) are wrong, and has been particularly prevalent in the United States. After years of ridicule by scientists, creationism has been restyled as intelligent design (ID), the view that organisms are so complex that they must have been created by an intelligent being. Proponents of intelligent design range in their beliefs from the hard line (everything you see around you is exactly as it was created, and creation was only a few thousand years ago) to the liberal (the key large groups of plants and animals were created, but perhaps a very long time ago, and perhaps there is some evolution between species). The different branches of creationism, including ID, lack testable hypotheses and they lack evidence, so they are not credible alternatives to evolution.

As an example, many supporters of ID use the flagellum of bacteria as evidence. The flagellum (plural, flagella) is a thin structure that beats in a whip-like way to drive the bacterium through the fluid in which it lives. The flagellum is composed of several components, and it is normally driven by a proton pump, the flow of hydrogen ions across a concentration gradient. Supporters of ID have chosen the flagellum as a key piece of evidence that biological structures are so complex they could not have evolved, but must have been created whole. They argue that the flagellum is a good example of irreducible complexity, meaning that it can only function as a whole, and if any part is removed it fails to function. In fact this is not true, as has been shown repeatedly, and each component of the flagellum has other functions. So, irreducible complexity, the keystone of ID, has not been demonstrated, and it probably reflects a failure of imagination on the part of the investigator.

Read more about evolution in Darwin (1859), Ridley (1996), Futuyma (2005), Barton et al.

(2007) and at http://www.blackwellpublishing.com/paleobiology/, and National Academies Press

(2008) for a clear statement about evolution and the lack of evidence for intelligent design.

EVOLUTION AND THE FOSSIL RECORD Speciation_

Species consist of many highly variable individuals, often divided into geographically restricted populations and races. All human beings belong to a single species, Homo sapiens, and yet every person is different. The range of genetic and physical variation among humans is enormous, and much of it appears to be associated with geographic distribution. There has also been variation through time, with subspecies of Homo sapiens, like H. s. nean-derthalensis, the Neandertals, being stocky and heavily built, possible adaptations to the cold Ice Age conditions of Europe 30,000 years ago (see p. 473). All species show geographic variation and, where the fossil record is good enough, variation in time as well.

So what is a species? The commonest definition is the biological species concept that states, "a species consists of all individuals that naturally breed together and that produce viable offspring". So, all modern humans can breed together and produce fertile (viable) children, and they therefore all belong to one species. Wolves and domestic dogs are also highly variable in external appearance, and yet they interbreed successfully and so they all belong to the one species Canis lupus. Domestic dogs belong to the subspecies C. l. famil-iaris, the European wolf to C. l. lupus, and there are many other subspecies of wolves from other parts of the world. In other cases, the amount of physical variation may seem much less; there are certain species of frogs and birds, for example, that look identical but are differentiated by their songs and never interbreed with a frog or bird with a different song.

Local populations may be to a great extent autonomous, isolated from other populations of the same species, and with a subtly different gene pool, the overall array of genetic material in all the individuals within the population. The cohesion of a species is maintained over its natural range by processes of gene flow, the occasional wandering of individuals from one area to another, which interbreed with members of neighboring populations. These processes can be thought of as occurring on many different scales, ranging from the whole Earth for humans, to a tiny patch of forest for some insect species.

If species can show considerable, or little, physical variation, and they can be held together by gene flow, how do they split? The process of splitting of a population to form two species is speciation, and there are many models. The most convincing is the allopatric ("other homeland") or geographic model that was proposed in the 1940s by Ernst Mayr, based on the establishment of geographic barriers. He suggested that populations could be split and gene flow prevented by a barrier, such as a new strip of water, a new mountain chain or even the building of a major road -anything that stops free genetic mixing among populations. The separated populations would then diverge for two reasons:

1 Each population, or set of populations, would start out with a different gene pool, simply because part of the former genetic range of the intact species has now been separated off. 2 Selection pressures would be different, perhaps only subtly, on either side of the barrier.

The separation can cause a divergence in genotype, the genetic composition of an individual, population or species, and phenotype, the external appearance.

The allopatric model of speciation may take two main forms. The process may be symmetric (Fig. 5.2a), with the ancestral species being divided roughly down the middle of its geographic range, and the two daughter species starting out with similar-sized populations. More dramatic effects may be seen when the split is asymmetric (Fig. 5.2b). Here, a small population, perhaps isolated on an island, evolves independently of the parent species, which may continue roughly unchanged. The smaller population may show unusual and rapid evolution because of what Mayr called the founder effect, the fact that its gene pool is a small sample of the overall gene pool, and that new environmental pressures and opportunities may occur.

Speciation and evolution in the fossil record

Biologists generally assumed that speciation happens gradually, with new species branching off from their ancestors slowly. Up to 1970 this view was accepted by most paleontologists, but then everything changed.

Eldredge and Gould (1972) proposed an alternative to the gradual model of evolution, which they termed the punctuated equilibrium model. They argued that the fossil record does not show evolution occurring in species lineages: in fact, they argued, most species lineages show stasis ("standing still", i.e. no change) over long spans of time. Change occurs at the time of speciation. Eldredge and Gould contrasted the two evolutionary models in terms of the shape of a phylogeny:

1 In the phyletic gradualism model (Fig. 5.3a), with sloping branches, most evolution takes place within species lineages, and speciation events involve no special additional amount of evolution;

species B

geographic barrier t species C

species B

geographic barrier t species C

Figure 5.2 Allopatric speciation models, occurring either symmetrically (a), where the parent species is divided into two roughly equal halves by a geographic barrier, or asymmetrically (b), where a small peripheral population is isolated by a barrier. In the first case, two new species may arise; in the second, the parent species may continue unaltered, and the peripheral population may evolve rapidly into a new species.

species A

species B

species A

species B

0 0

Post a comment