Biology 13.1 Theory of Natural Selection Theory of Natural Selection

Biology 13.1 Theory of Natural Selection

Theory of Natural Selection

Darwin’s Mechanism of Evolution

In 1859, in London, an English naturalist named Charles Darwin proposed a reasonable theory that explained that species evolved over time.

In 1831, Darwin travelled the globe as resident naturalist on board the ship the HMS Beagle. As a naturalist, he studied the people and creatures he encountered on the voyage, recording his findings as notes.

Darwin published his evidence from his decades of observations he made travelling the globe as a naturalist. Darwin’s book was called “The Origin of Species”

Darwin’s Voyage


Science before Darwin: Before Darwin, most people,

including scientists, held the view that each species was a divine creation of God that exists unchanged from it’s first creation.

Some scientists had begun to try to explain fossils by altering their traditional views of creation.

A handful of scientists proposed that organisms changed over time and tried to propose mechanisms of how this would happen.


In 1809, French scientist Jean Baptiste Lamarck proposed a hypothesis for how organisms change over generations.

Lamarck believed that , over the lifetime of an individual, physical features decrease in size because of disuse or increase because of use.

Lamarck believed these changes were than passed down to offspring.

While Lamarck’s hypothesis in now known to be incorrect, he did first identify one crucial truth: that change in a species is linked to the physical conditions of life.


Darwin’s Voyage and Observations:

During Darwin’s voyage around the globe, he found evidence that challenged the traditional beliefs that species wee unchanging.

As Darwin travelled and collected evidence, he found fossils that suggested that creatures were slowly changing over time.

Fossil’s from South America were similar to modern species but there were subtle differences showing that change had occurred over long periods of time.

Megatherium: The size of an African bull elephant, with enormous claws on its feet, twenty feet tall when standing:

Modern day sloth


Darwin’s key breakthroughs came when he visited the Galapagos islands off the coast of South America.

Darwin noticed that many animals and plants resembled those from the South American coast 1000 miles away.

Darwin suggested that the simplest explanation for this was that the Galapagos species were the ancestors of species from South America that had migrated to the island and changed after they arrived.

Darwin called this change “ descent with modification” or evolution.


It was many years after Darwin’s return from his voyage before he put together all his evidence. Darwin knew that evolution occurred but was puzzled about how it did so.

Darwin’s thinking about evolution came together when he read an essay by English economist Thomas Malthus.

Geometric progression doubles each step versus arithmeticProgression is steady evenincrease


Malthus wrote about human populations and pointed out that human populations are able to grow faster than their food supply. His work compared geometric population growth to arithmetic population growth.

Malthus pointed out that populations grow geometrically, faster than the food supply, which grows arithmetically.

He also suggested that human populations do not grow unchecked; that war, disease, and famine slow population growth.

Geometric progression doubles each step versus arithmeticProgression is steady evenincrease


A population consists of all individuals of a species that live in a specific geographical area and can interbreed.

A population may be all the species of tree frogs that live in the Amazon rainforest while another population may be all the species of penguins that live in Antarctica.


Darwin realized that Malthus’s hypothesis about human populations apply to all species.

Every organism has the potential to reproduce many offspring over it’s lifetime.

In most cases, however, only a limited number of those offspring survive to reproduce.

Those that have some type of advantage to survive are most likely to reproduce.


Combining Malthus’s ideas with his own observations, Darwin came to a key conclusion:

Individuals that have physical or behavioral traits that better suit their environment are more likely to survive and will reproduce more

successfully than those without the traits.


Darwin called this differing rate of survival-reproduction natural selection.

Natural selection is the process by which individuals that have favorable variations and are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do.


In time, the number of individuals that carry favorable characteristics that are inherited (passed along to them) will increase in population because of this adaptation.

This process is called evolution.

An adaptation is an inherited trait that has become common in a population because the trait provides a selective advantage.


Publication of Darwin’s Work:

In 1859, Darwin published his book “On the Origin of Species by Means of Natural Selection”.

Many people were deeply disturbed by Darwin’s theory, including the suggestion, that humans are related to apes.

Darwin’s theories to this day still draw criticism and attack from those that believe that a divine intervention created all life on Earth as it now appears.


While Darwin’s theories attracted criticism; they also attracted attention and convinced many biologists worldwide that he was correct.

Darwin’s Theory of Evolution by Natural Selection is supported by 4 major points:

1. Inherited variation exists within the genes of every population or species

2. In a particular environment, some individuals of a population or species are better suited to survive and thus have more offspring.

3. Over time, the traits that aid certain individuals of a population in survival and reproduction tend to spread in that population, being passed on to their offspring.

4. There is overwhelming evidence from fossils and many other sources that living species today evolved from organisms that are extinct.


Since the time Darwin’s work was first published, his ideas of Natural selection have been examined carefully by generations of biologists.

New discoveries, especially in the areas of genetics, have given scientists insight into how natural selection works.

Darwin’s key idea was based on the concept that in any population, individuals that are best suited to survive and do well in their environment will live longer and produce the most offspring.


Scientists know now that genes are responsible for inherited traits.

Certain forms of a trait become more common in a population because more individuals in a population carry the alleles for that trait.

Natural selection causes the frequency of certain alleles in a population to increase or decrease over time.

If an allele for better night vision gave a species an advantage in getting food; that allele will increase in the population because that species will eat better, survive longer, and breed more, passing along the allele to it’s offspring.


Environments differ from location to location. Thus, populations of the same species living in different locations tend to evolve in different ways.

They all evolve toward what gives them the best chance of survival in their unique location.

Reproductive isolation is the condition in which two populations of the same species do not breed with one another because of a geographical separation.


As two isolated populations of the same species become more different from each other over time, they eventually will become unable to breed with one another.

Generally, when two populations are no longer able to breed with one another, the populations become two different species.


Darwin discovered that varieties of finches on the Galapagos had been separated from each and developed differently depending on the food source available to them.

Darwin noticed that the beaks of finches on the islands near each other had developed over time to allow each variety of finch to best survive.

The most abundant food source for the finches on each island was different.

Over time, the finches with beaks best adapted to their local food source had increased chances for survival and thus breeding. Because they could get more food easier, they passed along this allele to their offspring.


The Tempo of Evolution:

For decades, biologists have understood evolution as a gradual process that continuously occurs.

Gradualism is the name we give to the model of evolution in which gradual change over time leads to a species formation.


The Tempo of Evolution:

American biologists Stephen Jay Gould and Niles Eldredge have suggested that successful species may remain unchanged for long periods of time .

They have hypothesized that major environmental changes in the past have caused evolution to occur in spurts.

This model of evolution, in which periods of rapid change are separated by periods of little or no change, is called punctuated equilibrium.


Gradualism model compared to punctuated equilibrium model.

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Biology 13.1 Theory of Natural Selection Theory of Natural Selection