CK-12 Biology I - Honors

by CK-12 Foundation

  Figure 12.8

  Darwins writings on geology and the collections of plants, animals, and fossils he sent back to England established his reputation as a naturalist even before he returned from his voyage. After his return, his father supported him as a gentleman scientist, allowing him to further develop the ideas inspired by his travels.

  Standing on the Shoulders of Giants

  Science, like evolution, builds on the past. Darwin’s theory was a product not only of his own intellect, but also of the times in which he lived and the ideas of earlier great thinkers. Some of these ideas colored Darwin’s perspective during his five years on the Beagle; many contributed to his thinking after the voyage. Not until 23 years after he returned to England did Darwin crystallize his thoughts and evidence sufficiently to publish his theory.

  Before Darwin, most people believed that all species were created at the same time and remained unchanged throughout history. History, they thought, reached back just 6,000 years.

  One of the first scientists to explore change in species was Jean Baptiste Lamarck. Lamarck believed that organisms improve traits through increased use, and then pass the improved feature on to their offspring. According to this idea of inheritance of acquired characteristics, giraffes have long necks because early giraffes stretched their necks to reach tall trees and then passed the longer necks on to their calves, as shown in Figure below. This attempt to explain adaptation was popular during the 19th century, and undoubtedly influenced Darwin’s thinking. Although Lamarck advanced the proposal that species change, evidence does not support inheritance of acquired characteristics. You can weight-train for years, but unless your children train as hard as you did, their muscles will never match yours! We will look later at Darwin’s explanation for giraffes’ necks.

  Figure 12.9

  According to Lamarcks idea, inheritance of acquired characteristics, giraffes have long necks because earlier giraffes stretched their necks to reach tall trees, and then passed their lengthened necks down to their calves. Evidence does not support this hypothesis, but many credit Lamarck for advancing the idea that species develop and change.

  Much as Lamarck questioned the dogma that species do not change, Charles Lyell challenged the belief that the earth was young. In Principles of Geology, he recorded detailed observations of rocks and fossils, and used present patterns and processes as keys to past events. He concluded that many small changes over long periods of time built today’s landscapes, and that the earth must be far older than most people believed. Captain FitzRoy gave Darwin a copy of Principles of Geology just before the Beagle left England, and Darwin “saw through [Lyell’s] eyes” during the voyage. Darwin’s theory that present species developed gradually over long periods of time reflects Lyell’s influence.

  The idea that natural laws, rather than miracles, govern life as well as geology grew during the early 19th century. Charles Babbage wrote that God had the power to make laws, which in time produce species. His close friend, John Herschel, called for a search for natural laws underlying the "mystery of mysteries" of how species formed. Later, Darwin cited Herschel as “one of our greatest philosophers” and then said he intended "to throw some light on the origin of species — that mystery of mysteries.”

  Darwin’s idea that individuals in a population compete for resources came from reading Thomas Malthus. Malthus described a human “struggle for existence” due to exponential population growth and limited food. Darwin thought that animal and plant populations might have similarly limited resources. If so, offspring suited to their environment would be more likely to survive, while those less “fit” would perish.

  Breeders of pigeons, dogs, and cattle inspired Darwin’s ideas about selection. By choosing which animals reproduced, breeders could achieve remarkable changes and diversity in a relatively short time. Variations in traits were clearly abundant and heritable. Darwin referred to selective breeding as artificial selection. His observations of how artificial selection worked helped him to develop his concept of natural selection (Figure below).

  Figure 12.10

  The way in which animal breeding artificially selects desirable variations influenced Darwins ideas of natural selection. The English Carrier Pigeon (left), the English Fantail (center), and the Fiary Swallow (right) have all descended from the common rock pigeon (), with the help of human breeders.

  One of the last individuals to influence Darwin’s theory was Alfred Russel Wallace, a naturalist whose work in Malaysia led him to conclusions similar to Darwin’s. In 1858 - over 20 years since the Beagle returned to England - Wallace sent Darwin a paper which described concepts nearly identical to Darwin’s ideas about evolution and natural selection. Lyell helped arrange a joint presentation to the Linnean society two weeks later. Darwin, shocked by the sudden competition, worked quickly to complete his book by the following year. Although both naturalists had independently come to the same conclusions, the extensive evidence and careful logic Darwin presented in The Origin of the Species earned him the greater share of recognition for the theory of evolution by natural selection.

  Standing on the shoulders of the giants who went before him, Darwin was able to see past the countless details of his beloved work in natural history to formulate a unifying theory to explain the diversity of life.

  Darwin’s Theory of Evolution

  Darwin lived in an increasingly scientific society which had begun to accept the idea that universal "laws" governed processes in nature - perhaps including life itself. Like Lamarck, Darwin understood that species change. With Lyell, he saw that the history of Earth and its life covered a vast amount of time. From his observations of animal breeding, he recognized that even within species, individuals showed variation in traits, and that the variations could be passed to offspring. Recalling Malthus, he knew that populations could produce far more offspring than the environment could support. He predicted that individuals with traits which suited the environment would survive and reproduce to pass their favorable traits to offspring, as shown in Figure below. Those whose traits were less suited to the environment would die. Just as humans select for breeding those cattle which produce more milk, he reasoned, nature (the limited environment) selects individuals which use resources most efficiently. Thus, he called his explanation of how species change natural selection.

  Darwin defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved," and he later regretted that he had not named it “natural preservation.” Today it is often defined as the process by which a certain trait becomes more common within a population. Let’s look once more at the parts of this process, and then we will consider its consequences.

  Figure 12.11

  Natural selection involves heritable variation, overproduction of offspring, preferential survival of individuals having variations favorable for the environment, and reproduction by survivors. This diagram shows two selection events, with reproduction after each one.

  By chance, heritable variations exist within a species.

  Darwin did not know that genes made of DNA determine traits. Much later, scientists learned that mutations in DNA can change genes and produce variations in traits. However, his observations of animal breeding and his detailed studies of barnacles and orchids convinced him that small, heritable variations in traits were common among individuals within a species. Darwin probably recognized that sexual reproduction increased variety in offspring. He expressed considerable concern that his own health problems might be heritable, especially when his beloved daughter Annie grew ill and died. He believed that his marriage to his cousin may have contributed to his children's weaknesses.

  Species produce more offspring than can survive.

  Malthus argued that human populations grow exponentially if unchecked, but that disease, starvation, or war will limit population growth eventually. High birth rates and high death rates were characteristic of human history. Darwin himself had ten childre
n; three died before maturity. Darwin reasoned that all species had the capacity to grow. However, his observations showed that most populations remained stable due to environmental limits. He concluded that many offspring must die. The phrases overproduction of offspring and struggle for existence summarize this idea.

  Offspring with favorable variations are more likely to survive to reproduce.

  Although heritable variations appeared to be random, death, Darwin reasoned, was not. Offspring which, by chance, had variations which “fit” or adapted them to their environment would have a greater chance to survive to maturity and a greater chance to reproduce. Offspring without such adaptations were more likely to die. Thus, well-adapted individuals produce more offspring. Differential survival and reproduction is a cornerstone of natural selection.

  Gradually, individuals with favorable variations make up more of the population.

  Can an individual organism evolve? No. The accomodation of an individual organism to its environment is not evolution. Though an individual organism can be better adapted to its environment, it still must mate with others of its species, so by definition, it is not a new species. It is just an individual with a better chance of survival in its environment. It is the gradual accumulation of many adaptations that, over many generations within one lineage of organisms, results in a new species. These adaptations occur through genetic change.

  Through chance variation, overproduction of offspring, and differential survival and reproduction, the proportion of individuals with a favorable trait (or favorable phenotype) will increase. The result is a population of individuals adapted to their environment. It is the variation within a species that increases the likelihood that at least some members of a species will be adapted to their environment and survive under changed conditions.

  It is important to note that natural selection is not directed or intentional. It depends on chance variations - due to genetic variations - and can work only with the “raw material” of existing species. Occasionally, variations which have no particular adaptive logic may survive. However, the limits set by resources and environment usually mean an increase in traits which help survival or reproduction, and the loss of traits which harm them. Gradually, species change. Eventually, changes accumulate and a new species is formed.

  Let’s compare natural selection to inheritance of acquired characteristics (Lamarck’s idea mentioned above). How would Darwin’s mechanism explain the long necks of giraffes?

  Heritable variation: In the past, some giraffes had short necks, and some had long necks.

  Overproduction of offspring: Giraffes produced more young than the trees in their environment could support.

  Differential survival and reproduction: Because the long-necked giraffes could feed from taller trees, they were more likely to survive and produce more offspring. Short-necked giraffes were more likely to starve before they could reproduce.

  Species change: The long-necked giraffes passed their long necks on to their calves, so that each generation, the population contained more long-necked giraffes.

  Recall that Lamarck believed that giraffes could stretch their necks to reach tall trees, and pass their stretched necks on to offspring. If this were true, evolution would reward effort toward a goal. Darwin showed that evolution is not goal-directed. Instead, the environment reinforces variations which occur by chance.

  Lyell studied the geology which surrounded him and saw that the environment had changed many times over a vast amount of time. Darwin studied the life across continents and saw, in addition to tremendous variation, that species had changed – in response to the changes in their environment – over that vast amount of time. Both proved, with careful observations and well-reasoned inferences, that the present arises from the past. Limited to our brief lifespans, we see today’s species as fixed. Darwin taught us how to see the relationships between them; to see that they developed from earlier, distinctly different species; to see that all of them - all of us - share common ancestors (Figure below). The cartoons which showed Darwin as an ape (an example is shown in the next lesson) did a great disservice to his theory of evolution. Far too many people limit their understanding of evolution to the simple phrase that “we came from apes.” We humans share common ancestors not only with the great apes, but with ALL of life – blue whales, gazelles, redwood trees, saguaros, fireflies, mosquitoes, puffballs, amebas, and bacteria. As Darwin said in closing the Origin, “There is grandeur in this view of life.”

  Figure 12.12

  A sketch from Darwins this "Tree of Life" depicts his ideas of how todays species (top row, XIV) have descended with modification from common ancestors. The theory implies that all species living today have a universal common ancestor that we humans are related to all of Earths plants, animals, and microorganisms.

  Darwin delighted in the great diversity of life, but also saw unity within that diversity. He saw striking patterns in the similarities and differences. Seeking an explanation for those patterns, he developed the concept of natural selection. Natural selection explains how today’s organisms could be related – through “descent with modification” from common ancestors. Natural selection explains the story told by the fossil record – the long history of life on Earth. Natural selection is a scientific answer (if only partial) to the old questions: Who are we? How did we come to be?

  In the light of natural selection, it is easy to see that variation – differences among individuals within a population – increases the chance that at least some individuals will survive if the environment changes. Here is a strong argument against cloning humans: if we were all genetically identical – if variation (or genetic variation) did not exist – a virus which previously could kill just some of us would either kill all of us, or none of us. Throughout the long history of life, variation has provided insurance that inevitable changes in the environmental will not mean the extinction of a species. Similarly, the diversity of species ensures that environmental change will not mean the extinction of life. Life has evolved (or, the Earth’s changing environment has selected) variation and diversity because they ensure survival. Causes of mutation may have pre-existed, but in a sense, life has embraced them. And sexual reproduction has evolved to add further to variation and diversity (as discussed in the Cell Division and Reproduction chapter).

  Adaptations are logical because the environment imposes limits on organisms, selecting against those who do not “fit.” Adaptations arise through gradual accumulation of chance variations, so they cannot be predicted, despite the fact that they appear to be goal-directed or intentional. Adaptations relate to every aspect of life: food, water, oxygen, nutrients, shelter, growth, response, reproduction, movement, behavior, ability to learn. Adaptations connect organisms to the resources in their environments. You are born with your adaptations; they are not changes you make to fit yourself into an environment. If the environment changes, the adaptive value of some of your inherited characteristics may also change. Our human appetites for salt and fat, for example, may remain from our past, when fat and salt were rare in our environment; now that they are easily available, we consume more than is good for us. Biologist E.O. Wilson believes adaptations reach every aspect of human life - that social, political, and even religious behaviors are rooted in our genes. Of course, we can learn – and learning allows us to adapt within our lifetimes to environmental change. The ability to learn is itself an adaptation – perhaps our greatest gift. But more and more, we are discovering that much of our behavior – including learning - is genetically programmed – a gift from our ancestors similar to vision and hearing, or breathing and digestion.

  Natural selection explains how species change.

  Darwin’s theory can be summarized in two statements

  All living species share common ancestors, and

  In this lesson, we have explored Darwin’s reasoning. In the next lesson, we will consider the abundant evidence which supports his ideas.

  Lesson
Summary

  The Theory of Evolution has changed how we see ourselves and how we relate to our world.

  The theory has two basic ideas: the common ancestry of all life, and natural selection.

  Darwin studied medicine and theology, but he first worked as ship’s naturalist on the HMS Beagle.

  During the 5-year voyage, Darwin spent over 3 years on land exploring new rocks, fossils, and species.

  From his observations, Darwin developed new ideas which later formed the foundation of his theory.

  Rock and fossil formations suggested that continents and oceans had changed dramatically.

  Tropical rain forests encouraged Darwin to reconsider the source of the vast diversity of life.

  Native cultures raised questions about the relationship between humans and animals.

  Sedimentary rocks implied gradual, as opposed to catastrophic, changes in the earth and in life.

  The distribution of life on island chains challenged the dogma of the immutability of species.

  After he returned, his reputation as a naturalist and his father’s financial support allowed him to become a “gentleman scientist,” free to analyze his collections, formulate his theory, and write about both.

  Like all scientific theories, Darwin’s was a product of both his own work and the work of other scientists.

  Before Darwin, most people believed that all species were created and unchanging about 6000 years ago.

  Jean-Baptiste Lamarck proposed that acquired characteristics could be inherited. Evidence did not support his mechanism for change, but Darwin shared his ideas of change in species.