READ: Macro Evolution

Lesson Objectives

• Students will understand the differences between macroevolution and microevolution.
• Students will understand that speciation is the formation of new species.
• Students will understand the mechanisms of speciation.

Vocabulary

allopatric speciation

Speciation that occurs when groups from the same species are geographically isolated physically for long periods.

artificial selection

Occurs when humans select which plants or animals to breed to pass specific traits on to the next generation.

behavioral isolation

The separation of a population from the rest of its species due to some behavioral barrier, such as having different mating seasons.

evolutionary tree

Diagram used to represent the relationships between different species and their common ancestors.

genotype

The genes that make up an individual.

geographic isolation

The separation of a population from the rest of its species due to some physical barrier, such as a mountain range, an ocean, or great distance.

macroevolution

Big evolutionary changes that result in new species.

microevolution

Small changes in inherited traits; does not lead to the creation of a new species.

natural selection

Causes beneficial heritable traits to become more common in a population, and unfavorable heritable traits become less common.

primate

A group of related mammal species that have binocular vision, specialized hands and feet for grasping, and enlarged and differentiated brains; includes humans, chimpanzees, the apes, monkeys, and lemurs.

reproductive isolation

allopatric and sympatric speciation; isolation due to geography or behavior, resulting in the inability to reproduce.

speciation

The creation of a new species; either by natural or artificial selection.

sympatric speciation

Speciation that occurs when groups from the same species stop interbreeding, because of something other than physical separation, such as behavior.

temporal isolation

Isolation due to different mating seasons.

Introduction

Small changes or large changes, how does evolution occur? It is easy to think that many small changes, as they accumulate over time, may gradually lead to a new species. Or is it possible that due to severe changes in the environment, large changes are needed to allow species to adapt to the new surroundings? Or are both probable methods of evolution?

Microevolution and Macroevolution

Microevolution

You already know that evolution is the change in species over time, due to the change of how often an inherited trait occurs in a population over many generations. Most evolutionary changes are small and do not lead to the creation of a new species. These small changes are called microevolution.

An example of microevolution is the evolution of pesticide resistance in mosquitoes. Imagine that you have a pesticide that kills most of the mosquitoes in your state one year. As a result, the only remaining mosquitoes are the pesticide resistant mosquitoes. When these mosquitoes reproduce the next year, they produce more mosquitoes with the pesticide resistant trait. This is an example of microevolution because the number of mosquitoes with this trait changed. However, this evolutionary change did not create a new species of mosquito, because the pesticide resistant mosquitoes can still reproduce with other mosquitoes if they were put together.

Macroevolution

Macroevolution refers to much bigger evolutionary changes that result in new species. Macroevolution may happen:

1. when many microevolution steps lead to the creation of a new species,
2. as a result of a major environmental change, such as volcanic eruptions, earthquakes or an asteroid hitting Earth, which changes the environment so much that natural selection leads to large changes in the traits of a species.

After thousands of years of isolation from each other, some of Darwin’s finch population, which was discussed in the Evolution by Natural Selection lesson, will not or cannot breed with other finch populations when they are brought together. Since they do not breed together, they are classified as separate species.

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Genotype or Phenotype?

Natural selection acts on the phenotype - the traits or characteristics - of an individual, not on the underlying genotype. For many traits, the homozygous genotype, AA for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot choose a homozygous individual over a heterozygous individual. If homozygous recessive aa individuals are selected against, that is they are not well adapted to their environment, acting on the phenotype allows the a allele to be maintained in the population through heterozygous Aa individuals.

Carriers

Because natural selection acts on the phenotype, if an allele is lethal in a homozygous individual, aa for example, it will not be lethal in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers of the a allele. This allele is then maintained in the population's gene pool. The gene pool is the complete set of alleles within a population.

Tay-Sachs disease is an autosomal recessive genetic disorder. It is caused by a genetic defect in a single gene with one defective copy of that gene inherited from each parent, rr for example. Affected individuals usually die from complications of the disease in early childhood. Affected individuals must have unaffected parents, each being a carrier of the defective allele, so the parents are heterozygous Rr. This lethal allele is maintained in the gene pool through these unsuspecting heterozygous individuals; they do not show any symptoms of the disease, so most individuals do not get tested to see if they are carriers.

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Hardy-Weinberg Equilibrium

The Hardy-Weinberg model (sometimes called a law) states that a population will remain at genetic equilibrium - with constant (unchanging) allele and genotype frequencies and no evolution - as long as five conditions are met:

1. No mutation (no change in the DNA sequence)
2. No migration (no moving into or out of a population)
3. Very large population size
4. Random mating (mating not based on preference)
5. No natural selection.

These five conditions rarely occur in nature. For example, it is highly unlikely that new mutations are not constantly generated. If these five conditions are met, the frequencies of genotypes within a population can be determined given the phenotypic frequencies.

Genetic Drift

Recall that the third requirement for Hardy-Weinberg equilibrium is a very large population size. This is because variations in allele frequencies that occur by chance are minimal in large populations. In small populations, random variations in allele frequencies can significantly influence the "survival" of any allele. Random changes in allele frequencies in small populations is known as genetic drift. As the population (and therefore the gene pool) is small, genetic drift could have substantial effects on the traits and diversity of a population. Many biologists think that genetic drift is a major cause of microevolution.

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The Origin of Species

The creation of a new species is called speciation. Most new species develop naturally, but humans have also artificially created new subspecies, breeds, and species for thousands of years.

Natural selection causes beneficial heritable traits to become more common in a population, and unfavorable heritable traits become less common. For example, a giraffe’s neck is beneficial because it allows the giraffe to reach leaves high in trees. Natural selection caused this beneficial trait to become more common than short necks.

As new mutations (changes in the DNA sequence) are constantly being generated in a population's gene pool, some of these mutations will be beneficial and result in traits that allow adaptation and survival. Natural selection causes evolution through the genetic change of a species as the beneficial traits become more common within a population.

Artificial selection is when humans select which plants or animals to breed to pass specific traits on to the next generation. A farmer may choose to breed only the cows that produce the best milk (the favored traits) and not breed cows that do not produce much milk (a less desirable trait). Humans have also artificially bred dogs to create new breeds (Figure below).

Reproductive Isolation

There are two main ways that speciation happens naturally. Both processes create new species by isolating groups (populations) of the same species from each other. Organisms can be reproductively isolated from each other either geographically or by some behavior. Over a long period of time (usually thousands of years), each population evolves in a different direction. One way scientists test whether two populations are separate species is to bring them together again. If the two populations do not interbreed and produce fertile offspring, they are separate species.

Geographic Isolation

Allopatric speciation happens when groups from the same species are geographically isolated physically for long periods. Imagine all the ways that plants or animals could be isolated from each other:

• a mountain range
• a canyon
• water such as rivers, streams, or an ocean
• a desert

Charles Darwin recognized that speciation could happen when some members of a species were isolated from the others for hundreds or thousands of years. Darwin had observed thirteen distinct finch species on the Galápagos Islands that had evolved from the same ancestor. Several of the finch populations evolved into separate species while they were isolated on separate islands. Scientists were able to determine which finches had evolved into distinct species by bringing members of each population together. The birds that would not or could not interbreed were regarded as separate species.

A classic example of geographic isolation is the Abert squirrel, shown in Figures below) and below). When the Grand Canyon in Arizona formed, squirrels from one species were separated by the giant canyon that they could not cross. After thousands of years of isolation from each other, the squirrel populations on the northern wall of the canyon looked and behaved differently from those on the southern wall. North rim squirrels have white tails and black bellies. Squirrels on the south rim have white bellies and dark tails.

Isolation without Physical Separation

Sympatric speciation happens when groups from the same species stop interbreeding, because of something other than physical separation, such as behavior. The separation may be due to different mating seasons, for example. Sympatric speciation is more difficult to identify.

Some scientists suspect that two groups of orcas (killer whales) live in the same part of the Pacific Ocean part of the year, but do not interbreed. The two groups hunt different prey species, eat different foods, sing different songs, and have different social structures.

Different behaviors may have also led to the emergence of two Galápagos finch species that live in the same space. The two species are separated by behavioral barriers such as mating signals. In this case, members of each group select mates according to different beak structures and bird calls. They do not need physical barriers, because behavioral differences do enough to keep the groups separated.

Allopatric speciation and sympatric speciation are both forms of reproductive isolation. Allopatric speciation is due to geographic isolation. Sympatric speciation is due to behavioral isolation, or isolation due to different mating seasons, which is also known as temporal isolation.

Rates of Evolution

How fast is evolution? How long did it take for the giraffe to develop a long neck? How long did it take for the Galápagos finches to evolve? How long did it take for whales to evolve from land mammals? These and other questions about the rate of evolution are difficult to answer, but evidence does exist in the fossil record.

The rate of evolution is a measurement of the speed of evolution. Genetically speaking, evolution is how much an organism’s genotype (the genes that make up an individual) changes over a set period of time. Evolution is usually so gradual that we do not see the change for many, many generations. Humans took millions of years to evolve from a mammal that is now extinct.

Not all organisms evolve at the same rate. It would be difficult to measure evolution in your family because you are only looking at a small population over a few generations. However there are organisms that are evolving so fast that you may be able to observe evolution! Many scientists use bacteria or other species that reproduce frequently to study evolution. Species with short life cycles and that reproduce frequently evolve much faster than others. Bacteria evolve hundreds (or thousands or more) times faster than humans do. Bacteria go through many generations in a few days, so that we can actually witness evolution. A human takes about 22 years to go through one generation. But some bacteria go through over a thousand generations in less than two months.

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Evolutionary Trees

Charles Darwin came up with the idea of an evolutionary tree to represent the relationships between different species and their common ancestors. The base of the tree represents the ancient ancestors of all life. The separation into large branches shows where these original species evolved into increasingly different populations that would not come back together again. The branches keep splitting into smaller and smaller branches as species continue to evolve into more and more species. Some species are represented by short twigs spurting out of the tree, then stopping. These are species that went extinct before evolving into new species. Other “Trees of Life” have been created by other scientists.

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Theory?

Darwin's Theory of Evolution by Natural Selection is supported by well over 150 years of scientific evidence, ranging from fossil evidence to DNA evidence. By definition, this is a well tested scientific theory. An abundance of scientific evidence supports this theory. The world is very old and has undergone some dramatic changes. Life has been on the planet for most of that time. As you will see in the next lesson, life started as single celled organisms and has evolved over billions of years into complex plants and animals. But this journey has not been easy. Most species that have ever lived are now extinct. There have been a number of mass extinctions, where many species vanished all at once. The tremendous diversity of species has allowed some to adapt to whatever changes nature throws in its path, from small changes to major environmental disturbances. So it is nature that selects - hence Natural Selection - which species adapts, survives and evolves.

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Lesson Summary

• Microevolution results from evolutionary changes that are small and do not lead to the creation of a new species.
• Macroevolution refers to large evolutionary changes that result in new species.
• Macroevolution may happen when many microevolution steps lead to the creation of a new species.
• Macroevolution may happen as a result of a major environmental change, such as volcanic eruptions, earthquakes or an asteroid hitting Earth, which changes the environment so much that natural selection leads to large changes in the traits of a species
• The creation of a new species is called speciation.
• Natural selection causes beneficial heritable traits to become more common in a population, and unfavorable heritable traits become less common.
• Artificial selection is when humans select which plants or animals to breed to pass specific traits on to the next generation.
• Allopatric speciation occurs when groups from the same species are geographically isolated physically for long periods.
• Sympatric speciation occurs when groups from the same species stop interbreeding, because of something other than physical separation, such as behavior.
• Allopatric speciation and sympatric speciation are both forms of reproductive isolation.
• The rate of evolution is a measurement of the speed of evolution. Genetically speaking, evolution is how much an organism’s genotype changes over a set period of time.
• Not all organisms evolve at the same rate.
• Evolutionary trees are used to represent the relationships between different species and their common ancestors.