Darwins Ideas

At about the same time in the mid-1800s, both Charles Darwin and the English naturalist Alfred Russel Wallace (1823-1913) formed a new theory to explain how evolution may take place. Both Darwin and Wallace had been on sea voyages around the world; Darwin's voyage is shown in Figure 15-3. In 1858, the ideas of Darwin and Wallace were presented to a prestigious group of scientists in London. The following year, Darwin published a book entitled On the Origin of Species by Means of Natural Selection.

Darwin had two goals in his book: first, he wanted to present the large amount of evidence that evolution occurs; and second, he wanted to explain the variety and distribution of organisms on Earth in terms of natural processes that are observable every day.

Descent with Modification

Darwin used the phrase descent with modification to describe the process of evolution. He carefully reviewed the evidence that every species—living or extinct—must have descended by reproduction from preexisting species and that species must be able to change over time. Darwin was not the first person to put forward the idea of descent with modification, but he was the first to argue that all species had descended from only one or a few original kinds of life.

Darwin saw the animals of the Galápagos Islands as evidence of descent with modification. For example, the islands are home to 13 similar species of finches. Each of these bird species has a beak that is best adapted for a certain kind of food. But Darwin suspected that all 13 species descended from and diverged from just a few ancestral finches. These ancestors could have flown to the Galápagos Islands from elsewhere sometime after the islands were formed.

figure 15-3

On his voyage, Darwin noticed the locations of similar organisms around the world. The rhea (a), ostrich (b), and emu (c), for example, are clearly related species that have similar body forms and occupy similar habitats. They occur on different continents, however. Darwin wondered what causes such distributions in nature.

www.scilinks.org Topic: Natural Selection Keyword: HM61016

www.scilinks.org Topic: Natural Selection Keyword: HM61016

figure 15-4

This diagram represents the process of natural selection explained by Charles Darwin as the mechanism for evolution.

Natural Selection

Darwin proposed the theory of natural selection as the mechanism for descent with modification. In forming his theory, Darwin thought carefully about the forces that could cause changes in organisms over time. The following summary explains the four main parts of Darwin's reasoning, as shown in Figure 15-4: O Overproduction More offspring can be produced than can survive to maturity. For example, each female deer has one or more offspring per year for many years in a lifetime. This multiplication could increase the total number of deer in a short time. But each new deer needs food and is vulnerable to predators and disease, so not all of the deer live for very long.

Darwin drew this part of his reasoning from a popular book about human social problems by English clergyman and economist Thomas Malthus (1766-1834). Malthus pointed out that human populations can increase more quickly than food supplies and that populations are often limited by conditions such as war, disease, or lack of food. Darwin realized that the environment limits the populations of all organisms by causing deaths or by limiting births. © Genetic Variation Within a population, individuals have different traits. For example, some deer may have thicker fur or longer legs than others. Also, some of this variation can be inherited. For example, deer that have thick fur tend to have offspring with thick fur. Occasionally, new traits may appear in a population. © Struggle to Survive Individuals must compete with each other in what Darwin called a "struggle for existence." Some variations improve an individual's chance to survive and reproduce, but some variations reduce this chance. For example, deer that have thick fur may survive in the cold better than deer that have thin fur. A trait that makes an individual successful in its environment, such as thick fur in cold climates, is called an adaptation.

Overproduction

Each species produces more individuals than can survive to maturity.

^^ Genetic Variation

The individuals of a population may differ in traits such as size, color, strength, speed, ability to find food, or resistance to certain diseases.

Overproduction

Each species produces more individuals than can survive to maturity.

^^ Differential Reproduction

Individuals that have certain traits are more likely to survive and reproduce than are individuals that lack those traits. Over time, those traits become more frequent in the population.

^^ Genetic Variation

The individuals of a population may differ in traits such as size, color, strength, speed, ability to find food, or resistance to certain diseases.

^^ Struggle to Survive

Individuals must compete with each other for limited resources. Also, some individua will be harmed by predation, disease, or unfavorable conditions

^^ Differential Reproduction

Individuals that have certain traits are more likely to survive and reproduce than are individuals that lack those traits. Over time, those traits become more frequent in the population.

© Differential Reproduction Darwin concluded that organisms with the best adaptations are most likely to survive and reproduce. And through inheritance, the adaptations will become more frequent in the population. So, populations may begin to differ as they become adapted to different environments, even if they descended from the same ancestors. This conclusion is the core idea of Darwin's theory.

Darwin explained that natural selection could account for descent with modification as species become better adapted to different environments. That is, the theory of natural selection proposes that nature changes species by selecting traits. The environment "selects" the traits that may increase in a population by selecting the parents for each new generation.

Darwin sometimes used the phrase "survival of the fittest" to describe natural selection. In evolutionary terms, fitness is a measure of an individual's hereditary contribution to the next generation. This kind of fitness is more than simply living a long time. A fit individual is one that has offspring that also live long enough to reproduce in a given environment. For example, if thick fur is an advantage for a deer living in the mountains, then deer that have thick fur are more likely to live long enough to reproduce and pass on the genes for thick fur. If a certain trait increases an individual's fitness, the proportion of individuals with that trait is likely to increase over time. So, adaptations are those traits that increase the fitness of individuals, and populations tend to be well adapted to survive the conditions in which they live.

In evolutionary theory, the term adaptation is also used to describe changes in traits in populations over time. This meaning is different from that of short-term adaptation by an individual to a temporary condition. Also, long-term adaptation in populations is not the same as acclimatization in individuals. Acclimatization is a short-term process in which physiological changes take place in a single being in its own lifetime. An example of acclimatization is an animal adjusting to a new climate by growing thicker fur.

^Eco ConnectionJ

Galápagos Islands

The unique ecosystems of the Galápagos Islands have been in danger since the discovery of the islands by Europeans in 1 535. For 400 years, the islands were a favorite stopping place for pirates and whalers. Sailors valued the large native tortoises as a meat source because the tortoises could live for long periods aboard a ship with little or no food or water. Over the years, the number of native tortoises was severely reduced. Three of the 14 original subspecies of tortoises are now extinct. However, since 1936, the government of Ecuador and scientists from many countries have taken steps to preserve and restore the islands' native species and habitats. In 1965, the Charles Darwin Research Station began to breed and reintroduce the tortoises and other species.

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Responses

  • reiss
    Why was Thomas Malthus’s book critical to Darwin’s thinking about descent with modification?
    2 years ago

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