11.1 Discovering How Populations Change

Evolution and Its Processes and diversified from a common ancestor is the foundation from which we understand all other questions in biology.

Some of the mechanisms for evolutionary change will be explained in this chapter.

The mechanism for species change is described in the theory of evolution by natural selection.

It was suggested and debated before Darwin.
The view that species were static and unchanging was based on the writings of Plato.

The ideas about the evolution of animals were brought back in the 18th century by a number of people.

It was also accepted that there were extinct species.
James Hutton proposed that geological change could be gradual by accumulating small changes from processes over a long period of time.

The view that the geology of the planet was a consequence of catastrophic events occurring during a relatively brief past was different.

In the 19th century, the view was popularized by the geologist.

His ideas were very influential on Darwin's thinking.

The greater age of Earth gave more time for gradual change in species, and the process provided an analogy for gradual change in species.

In Lamarck's theory, changes in an individual caused by its environment, or the use or disuse of a structure during its lifetime, could be passed on to its offspring and bring about change in a species.

Lamarck's ideas were an important influence on evolutionary thought, even though they were discredited.

In the mid-nineteenth century, Charles Darwin and Alfred Russell Wallace wrote about the mechanism for evolution.

Each spent time on expeditions to the tropics.
Wallace traveled to Brazil and the Malay Archipelago to collect insects.
Like Wallace's later journeys in the Malay Archipelago, Darwin's journey included stops at several island chains.
Darwin observed species of organisms on different islands that were very different.

He observed that the ground finches in the Galapagos Islands had different beak shapes and that the group of species in the archipelago formed a graded series of beak sizes and shapes.

The island species might be different from the mainland species.

The beak shape varies among finch species.

He believed that the beak of an ancestral species had evolved over time to give the finches different food sources.
The shapes of the ground finch's beak are shown in this illustration.

Wallace and Darwin both observed similar patterns in other organisms and came up with a mechanism to explain them.

The mechanism natural selection was called by Darwin.
The characteristics of organisms are passed on from parent to offspring.
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 a competition for resources in each generation.
The principle was discussed in relation to human populations in an essay by the economist Thomas Malthus.
Variations in offspring's characteristics are inherited.
Darwin and Wallace believed that offspring with inherited characteristics that 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.

The idea of natural selection was presented by Darwin and Wallace in 1858 before the Linnaean Society in London.

Evolution demonstrations can take a long time.

One of the best demonstrations has been in the birds that inspired the theory.
Every year since 1976, Peter and Rosemary Grant and their colleagues have studied the populations of the Galapagos finch.
The Grants looked at the beak shapes of medium ground finches on the Galapagos island and found changes from one generation to the next.
Some individuals have wide, deep bills while others have thinner bills.
Smaller billed birds feed more efficiently on small, soft seeds than large-billed birds.
The vegetation on the island was altered during the 1977 dry season.
The decline in small, soft seeds was greater than the decline in large, hard seeds after this period.
The large-billed birds were able to live longer than the small-billed birds.
The Grants found that the average bill size was larger when they measured beak sizes in the reduced population.
It was clear that the bill size was caused by the availability of seeds.
The Grants knew that the selection of offspring with larger bills would lead to the evolution of bill size.
Grants have shown selection on and evolution of bill size in this species in response to changing conditions on the island.
When large seeds became rare, the evolution occurred both to larger bills and to smaller bills.

Many of the small-beaked finches died in 1977 due to a shortage of small seeds on the Galapagos island.

Between 1976 and 1978 the average beak size of finches increased.

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

This is important because variation among individuals can be caused by non-genetic reasons, such as an individual being taller because of better nutrition.

There are two main sources of genetic diversity in a population.

New genetic variation is the result of a change in DNA.
A person with a genetic variation might have a different trait than other people in the population.
This is not always the case.
The OpenStax book is available for free at http://cnx.org/content/col11487/1.9

A beneficial effect on fitness may be produced by amutation.

There will be no effect on fitness.

There is a wide range of effect sizes on the fitness of the organisms that express them, from a small effect to a great effect.

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

An adaptation is a match between the organisms and the environment.

When a change in the range of genetic variation occurs over time that increases or maintains the match of the population with its environment, it is called adaptation.
The variation in finch beaks shifted from generation to generation to adapt to food availability.

The environment at the time can affect whether or not a trait is favorable.

Environmental conditions can change and the same traits can have different advantages and disadvantages.
Small bills were a disadvantage in one climate, while large bills were a benefit in another.

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

Flowering plants are related to one another.
The dense blazing star and purple coneflower have the same basic appearance.

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.
The wings of bats and insects are very different from the original structures.

They evolved their own way in the two lineages.

The wings of the hummingbird and the ostrich were descended from a common ancestor.

The mechanisms of inheritance, genetics, were not understood at the time Darwin and Wallace came up with their idea of natural selection.

A lack of understanding was a stumbling block to understanding evolution.

Blending inheritance was the main genetic theory of the time, which made it difficult to understand how natural selection might work.

In the early twentieth century, geneticists were coming to an understanding of the basics of inheritance after the rediscovery of Mendel's work.
The discovery of particulate nature of genes made it difficult for biologists to understand gradual evolution.

The relationship between natural selection and genetics that took shape in the 1940s is generally accepted today.

Evolutionary pressures, such as natural selection, can affect a population's genetic makeup and result in the gradual evolution of populations and species according to the modern synthesis.

It's important to remember that a character's genes may have several variations that code for different characteristics.

In humans, there are three alleles that determine the blood type on the surface of red blood cells.
Each individual in a population of diploid organisms can only carry two alleles for a particular gene, but more than two may be present in the individuals that make up the population.
As all genes were passed down from parent to offspring, Mendel followed them.

Evolution is defined as a change in the characteristics of a population of organisms, but behind that is genetic change.

In population genetic terms, evolution is defined as a change in the frequencies of alleles in a population.
A is the number of copies of that allele divided by the number of Abo genes in the population.
To be 26.1 percent.
All of the frequencies add up to 100 percent, with 0 alleles making up 13.4 percent and 60.5 percent of the alleles respectively.
Over time, a change in this Frequency would be considered evolution in the population.

There are many ways the frequencies of a population can change.

Natural selection is one of those ways.
If an individual has more offspring that survive and reproduce, that individual's allele will be more common in the next generation.
Since allele frequencies add up to 100 percent, an increase in the frequency of one allele always means a decrease in the other alleles.
Every individual of the population will carry the allele if it becomes "fixed" in this way.
Part of the study of population genetics is tracking how the allele frequencies in a population change over time, which can give scientists clues regarding the forces that may be operating on a given population.
There was more of the darker colored moths in the population because they were better camouflaged than the lighter colored ones.

The English mathematician and German physician provided an explanation for a somewhat counterintuitive concept in the early twentieth century.

There is a misunderstanding as to why a "dominant" allele, one that masks a recessive one, should not increase in frequency in a population until all the other alleles are eliminated.
The question resulted from a common confusion about what "dominant" means, but it forced Hardy, who was not even a biologist, to point out that if there are no factors that affect an allele frequencies, those frequencies will remain constant from one generation to the next.
The equilibrium principle is now known as the Hardy-Weinberg principle.
Unless some kind of evolutionary force is acting on the population, the population would carry the same alleles in the same proportions generation after generation, according to the theory.
Individuals would look the same and this would be 3.