Influences on Darwin
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.
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.
Lamarck believed that organisms improve traits through increased use and then pass the improved feature on to their offspring.
You can weight-train for years, but unless your children train as hard as you did, their muscles will never match yours! Darwin’s explanation for giraffes’ necks will be discussed later.
Darwin’s idea that individuals in a population compete for resources came from reading Thomas Malthus
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.
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.
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 initiated his study of barnacles to thoroughly understand at least one species, and their variations, before making the general statements needed for his theory of evolution by natural 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
Darwin learned how to stuff animals. His "teacher" was a freed slave from South America and through their interactions, Darwin became fascinated with this continent.
He read a very interesting book on natural history ("The Natural History of Selborne") and spent much of his time at the natural history museum in Edinburgh. This further developed Darwin's interest in the natural sciences. The curator of the museum taught Darwin about botany and animal anatomy. Darwin started his own field notebook on his observations of birds, which taught him how to make careful observations in nature. Darwin's skill in taking notes on his observations of nature was instrumental in his later work.
He joined the Plinian Society, a club at the University of Edinburgh for students interested in natural history. On 27 March 1827, at just 18 years old, Darwin gave his first speech before the Society on the marine biology of an estuary just north of Edinburgh. This society and their discussions were Darwin's first exposure to the possibilities of evolution.
He became a good friend of zoology Professor Robert Grant. They would hike together, and this furthered Darwin's skill in observing nature. Grant discussed the works of the French naturalist Lamarck with Darwin. (Lamarck's work will be discussed in an additional concept.)
In April of 1827, Darwin quit medical school for good and returned home to soon become the most highly respected naturalist of the 19th century. Before that, however, in January of 1828, Darwin started his first term at Cambridge University, at Christ's College, studying for the clergy. Just like his years at Edinburgh, Darwin's years at Cambridge furthered his interest in the natural sciences, but not in the clergy.
Darwin became an avid beetle collector. This taught Darwin skills important for a naturalist: how to identify species, the proper manner of cataloguing specimens, methods of comparative anatomy, and how to work efficiently in the field.
He befriended the Reverend John Stevens Henslow, who was was both a member of the clergy and a naturalist. Through his interactions with Henslow, Darwin's scientific knowledge was greatly expanded. He became familiar with diverse topics such as geology, mathematics, entomology, mineralogy, chemistry, and botany. It was through his interactions with Henslow that Darwin finally decided he wanted to be a naturalist.
He met geology Professor Adam Sedgwick. Darwin finally became interested in this field.
He read two books by Sir John Herschel (Preliminary Discourse on the Study of Natural Philosophy) and Alexander von Humboldt (Personal Narrative of the Travels to the Equinoctial Regions of America During the Years 1799-1804). These books greatly expanded Darwin's imagination. Darwin believed that there were no limits to what scientific investigation could uncover.
Charles Darwin’s Theory of Evolution represents a giant leap in human understanding. It explains and unifies all of biology – thousands of years of natural history from before Darwin’s time, as well as the 150 years of genetics, molecular biology, and even ecology since Darwin published the theory.
Apart from science, the Theory of Evolution has dramatically changed how we think about ourselves and how we relate to the world
Darwin's Theory of Evolution contains two major ideas. The first is evolution itself. Darwin postulated that present life has arisen gradually from past life forms.
Natural selection explains how the diversity of life has arisen through time.
Theory of Evolution
Darwin’s Theory of Evolution
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.
He predicted that individuals with traits which suited the environment would survive and reproduce to pass their favorable traits to offspring.
Thus, he called his explanation of how species naturally change over time 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.”
his observations of animal breeding and his detailed studies of both barnacles and other species convinced him that small, heritable variations in traits were common among individuals within a species and that these variations occur by chance.
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.
Malthus argued that human populations grow exponentially if unchecked, but that disease, starvation, or war will limit population growth eventually.
Darwin reasoned that populations of all species have the capacity to grow. Simply put, species produce more offspring than can survive.
The phrases "overproduction of offspring" and "struggle for existence" summarize this idea.
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.
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.
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.
Natural Selection vs. Inheritance of Acquired Characteristics
Heritable variation: In the past, some giraffes had shorter necks and some had longer 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 had more competition for food and 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.
Darwin showed that evolution is not goal-directed. Instead, the environment reinforces variations which occur by chance.
The Present Arises From The Past
Darwin studied the life found 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.
Darwin taught us how to see the relationships between them; he taught us to see that they developed from earlier, distinctly different species and that all of them - all of us - share common ancestors
Descent with Modification
Natural selection explains the story told by the fossil record – the long history of life on Earth.
Throughout the long history of life, variation has provided insurance that inevitable changes in the environmental will not mean the extinction of a species
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.
Causes of mutation may have pre-existed, but in a sense, life has embraced them. Sexual reproduction has evolved to add further variation and diversity.
Adaptations are logical because the environment imposes limits on organisms, selecting against those who are 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, and ability to learn.
You are born with your adaptations; they are not changes you make to fit yourself into an environment.
Darwin’s theory can be summarized in two statements:
All living species share common ancestors.
Natural selection explains how species change.