18.1 Understanding Evolution

The mechanism for how species change over time is described in evolution by natural selection.

Before Darwin began to explore this idea, scientists, philosophers, researchers, and others had made suggestions.
Plato emphasized in his writings that species were static and unchanging, yet there were also ancient Greeks who expressed evolutionary ideas.
The idea of the evolution of animals was reintroduced in the 18th century by a man named Georges-Louis Leclerc Comte de Buffon.
Some people at this time accepted that there were extinct species.

During the 18th century, a Scottish scientist proposed that geological change could be gradual by accumulating small changes from processes.

The view that the planet's geology was a consequence of catastrophic events occurring during a relatively brief past was different.
Charles Lyell was a pioneer in the field of geology.
The idea of the greater age of Earth gave more time for gradual change in species, and the process of change provided an analogy for this change.
The mechanism for evolutionary change was detailed in a book published in the early 19th century.
This mechanism is referred to as an inheritance of acquired characteristics by which the environment causes modifications in an individual, or offspring could use or disuse a structure during its lifetime, and thus bring about change in a species.
Lamarck's ideas were an important influence on evolutionary thought.

The actual mechanism for evolution was conceived and described by Charles Darwin and Alfred Russel Wallace in the mid-nineteenth century.

Each naturalist spent time on expeditions to the tropics.
Darwin traveled around the world on H.M.S.
There are stops in South America, Australia and the southern tip of Africa.
Wallace traveled to Brazil and the Malay Archipelago to collect insects.
Like Wallace's later journeys to the Malay Archipelago, Darwin's journey included stops at several island chains.
Darwin observed species of organisms on different islands that were very different.
There are several species of ground finches in the Galapagos Islands.
The beak sizes and shapes of the species on the islands were graded according to how similar they were.
The finches were very similar to another finch species on the South American mainland.
The island species might be different from the mainland species.
He realized that each finch's beak helped them acquire a specific type of food.
For example, finches with spear-like beaks broke seeds and insects with their beaks.

The beak shape varies among finch species.

He said that the finches had adapted to get different food sources.

Wallace and Darwin both observed the same patterns in other organisms and came up with the same explanation.

The mechanism natural selection was called by Darwin.
Evolutionary change is caused by this.

The population of giant tortoises in the Galapagos Archipelago had longer necks than those on other islands with dry lowlands, according to Darwin.

These tortoises were selected because they were able to reach more leaves and have more access to food.
Those that could reach more leaves had a better chance of survival than those that couldn't.
Long-necked tortoises are more likely to be reproductively successful and pass the long-necked trait to their offspring.
The population would only have long-necked tortoises over time.

Darwin believed that natural selection was the result of three principles.

Most characteristics of organisms are passed on from parent to offspring.
It was a common understanding that no one, including Darwin and Wallace, knew how this happened.
Resources for survival and reproduction are limited because more offspring are produced than are able to survive.
The capacity for reproduction in all organisms is greater than the availability of resources.
There is competition for resources in each generation.
Both Darwin and Wallace's understanding of this principle came from reading an essay by Thomas Malthus.
Variations in offspring's characteristics are inherited.
Darwin and Wallace believed that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete.
The next generation will better represent the characteristics that are inherited.
This will lead to population changes in a way that Darwin called descent with modification.
Natural selection leads to greater adaptation to the environment.
adaptive evolution has only one mechanism known for it.

The idea of natural selection was discussed by Darwin and Wallace at the Linnean Society in London in 1858.

Darwin's book, On the Origin of Species, was published the following year.
His argument for evolution is detailed in his book.

Both Charles Darwin and Alfred Wallace presented papers on natural selection at the Linnean Society in 1858.

It is difficult to present examples of evolution by natural selection.

The Galapagos finches are an excellent example.
Every year since 1976, Peter and Rosemary Grant and their colleagues have studied the populations of the Galapagos finch.
The Grants looked at the distribution of the medium ground finch on the Galapagos island and found that it had changed from one generation to the next.
Some birds have wide deep bills while others have thinner bills.
There was a lack of large hard seeds which the large-billed birds ate, but there was an abundance of small soft seeds which the small-billed birds ate.
The small-billed birds were able to reproduce.
The Grants measured beak sizes in the population and found that the average bill size was smaller.
Parents with smaller bills had more offspring and the bill evolved into a much smaller size.
The trend of smaller average bill size stopped as conditions improved in 1987.

Many people enjoy hiking, scuba diving, or climbing mountains.

People want to see wildlife.
Experiencing the outdoors can be great.
Any location outdoors, even under water, is referred to as a field.
A field Biologist focuses research on a group of organisms or a single habitat.

A field Biologist tranquilizes a polar bear.

Field biologists want to discover new species.

The findings expand our understanding of the natural world and lead to important innovations in fields such as medicine and agriculture.
New knowledge can be revealed by plant and microbial species.
If rare organisms need protection, other organisms can play a key role.
Researchers can use these important species as evidence when they are discovered.

The selection will not lead to change in the next generation if the differences have a genetic basis.

This is important because nongenetic reasons can cause variation in an individual's height because of better nutrition.

There are two main mechanisms for genetic diversity in a population.

New alleles, or new genetic variation in any population, are the result of a change in DNA.
One of three outcomes can be achieved by genetic changes.
There is a reduced likelihood of survival or fewer offspring because of a variation in the organism's phenotype.
A beneficial effect on fitness can be produced by amutation.

The phenotype's fitness won't be affected by many mutations.

These are called neutral mutations.
There is a wide range of effect sizes on the organism's fitness that expresses them in their phenotype, from a small effect to a great effect.

When two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each offspring, sexual reproduction also leads to genetic diversity.

Scientists describe groups of organisms adapting to their environment when a genetic variation occurs over time that increases or maintains the population's "fit" to its environment.

The webbed feet of a platypus are adapted for swimming.
The thick fur of a snow leopard is an adaptation for living in the cold.
The adaption of a cheetah's fast speed is to catch prey.

The current environment affects whether or not a trait is favorable.

Environmental conditions can change and the same traits are not always selected.
A plant species that grew in a moist climate did not need to conserve water.
Large leaves allow the plant to get more energy from the sun.
The moist environment provided favorable conditions to support large leaves.
The area no longer had excess water after the climate changed.
The direction of natural selection shifted so that plants with small leaves were selected because they were able to conserve water.

The variation in form and function is a result of the evolution of species.

Groups of organisms become vastly different from each other as a result of evolution.

The reproductive organs of flowering plants which share the same basic anatomies can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators.

Flowering plants are related to one another.

The dense blazing star (Liatrus spicata) and the purple coneflower (Echinacea purpurea) have the same basic structure.

Similar phenotypes can evolve independently in distantly related species.

Both bats and insects have structures we refer to as wings, which are adapted to flight.
Bat and insect wings are very different from the original structures.
Both species came to the same function, flying, but did so in different ways.

These physical changes occur over a long period of time.

Natural selection can shape an entire species.
Natural selection can take thousands or even millions of years for an entire species to evolve.
Life on earth has changed over the years and continues to change.

There is compelling evidence for evolution.

The signature of past and present evolution is seen by biologists when looking at every level of organization.
Since Darwin wrote On the Origin of Species, our understanding of evolution has become clearer and broader, and he dedicated a large portion of his book to identifying patterns in nature that were consistent with evolution.

Fossils show a progression of evolution and show that organisms from the past are not the same as those today.

To determine the age of fossils, scientists categorize them from all over the world.
The fossil record shows the evolution of form over millions of years.
Scientists have recovered highly detailed records showing the evolution of humans and horses.
The whale flipper is similar to bird and mammal appendages in appearance.

In this display, fossil hominids are arranged in chronological order.

The skull's shape changed as hominids evolved.
An artist's rendition of an extinct species of the genus Equus shows that they were similar to the modern horse but different in size.

The presence of structures in organisms that share the same basic form is a type of evidence for evolution.

The bones in human, dog, bird, and whale appendages all share the same overall construction, which is the result of their origin in a common ancestor's appendages.
The bones' shapes and sizes have changed over time, but the layout has remained the same.

The construction of these appendages suggests that these organisms are related.

Some structures in organisms have no apparent function at all, and may be remnants of a past common ancestor.

Wings on flightless birds, leaves on cacti, and hind leg bones in whales are examples of vestigial structures.
Not all similarities are related to the same structure.
When similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is an analogy.
The wing structure and origin of insects are completely different from birds and bats.
These structures are similar.

The video explores the bones in the human body.

The convergence of form in organisms that share the same environment is an example of evolution.

The ptarmigan and the arctic fox are two unrelated animals that have been selected for seasonal white phenotypes during winter to blend with the snow and ice.
The similarities are not due to common ancestry, but due to similar selection pressures.

The ptarmigan's plumage and the white winter coat are adapted to their environments.

The adult tends to conserve embryo formation by tweaking the embryo.
When structures are absent in some groups, they appear in their embryonic forms and disappear when they reach the adult or juvenile form.
The gill slits and tails are present in all vertebrates at some point in their early development.
Adult forms of aquatic groups such as fish and some Amphibians keep them.
The tail structure of great ape embryos is lost when they are born.

The distribution of organisms on the planet can be explained by evolution and plate movement.

The groups that evolved before Pangaea broke up are distributed around the world.
The flora and fauna of the north and south of the planet were formed from the supercontinent Laurasia and Gondwana, respectively.
Before the southern supercontinent Gondwana broke up, members of the plant family Proteaceae were most prevalent in Australia, southern Africa, and South America.

Australia's long isolation is reflected in the absence of other mammals.

Australia has an abundance of endemic species, which are found nowhere else in the world.
These species evolve into new species that look very different from their ancestors, which may exist on the mainland.

Islands are unique to their point of origin and have distant relationships to ancestral species on mainlands.

Evidence of a common ancestor for all of life is reflected in DNA's universality.

Major structural differences in otherwise conservative structures such as ribosome components and membrane structures are reflected in the fundamental divisions in life between the genetic code, DNA replication, and expression.
The relatedness of groups of organisms is reflected in their similarity to one another.

Some of the mechanisms of evolution have been shown to be possible through the use of DNA.

It is clear that the evolution of new functions for proteins occurs after gene duplication events that allow freely modifying one copy by the other, while the second copy continues to produce a functional protein.

The theory of evolution is not easy to understand.

The site addresses some of the main myths associated with the theory of evolution.

Critics of the theory of evolution dismiss it as unimportant because they don't use the word "theory" in the way scientists do.

A "theory" is a body of thoroughly tested and verified explanations for observations of the natural world.
The theories of the atom, gravity, and the theory of relativity describe the facts about the world.
Facts about the living world are described in the theory of evolution.
A theory in science has survived attempts to undermine it.
A "theory" is a word meaning a guess or suggested explanation.
Critics of evolution are implying that there is little evidence supporting it and that it is still being tested.
This is not a correct description.

The change in a population's genetic composition over time is called evolution.

Over their lifetime, individuals do change, but this is development and involves changes programmed by the set of genes the individual acquired at birth in coordination with their environment.
The change of the average value of the characteristic in the population over time is the best way to think about the evolution of a characteristic.
When natural selection leads to bill-size change in medium ground finches in the Galapagos, this does not mean that individual bills are changing.
As a result of evolution, the average bill size among all individuals in the population will be different than it is now.
Although some individuals may survive from the first time to the second, they will still have the same bill size; however, there will be many new individuals who contribute to the shift in average bill size.

An explanation of life's origins is included in evolution.

Some of the theory's critics think it can't explain the origin of life.
The theory doesn't try to explain the origin of life.
The theory of evolution shows how populations change over time.
The origins of the first cells, which define life, are not shed light on.
The biologists believe that the presence of life precludes the possibility that the events that led to life on Earth can be repeated.

Once a mechanism of inheritance was in place, these entities would be subject to the principle of natural selection.

The more effective reproducers would have a greater increase in frequencies.
Evolution doesn't explain the origin of life, but it may have something to do with some of the processes that existed before living entities acquired certain properties.

It's quite common for statements like "organisms evolve in response to a change in an environment" to be made, but they can lead to two different types of misunderstandings.

Do not interpret the statement to mean that organisms evolve.
Interpretation of the statement to mean that the evolution is somehow intentional may cause a second misunderstanding.
A changed environment results in some individuals in the population benefiting and therefore producing more offspring than others.
If the characteristics are determined, the population will change.

The variation that natural selection works on is already in a population and does not arise in response to an environmental change.

Over time, the population ofbacteria that are resistant to antibiotics will be selected.
The antibiotic did not cause the resistance.
The genes for resistance were present in the pool.