Module 3 Part B Evolution

Module 3 Part B Evolution

Objectives:

1. Define the following: natural selection, rock strata, convergent evolution, vestigial structure, homologous structures, analogous structures, speciation, adaptation, normal distribution, microevolution, Hardy Weinberg equilibrium, gene frequencies, biotic factors, abiotic factors, genetic drift, bottleneck effect, gene pool, random mating, gene flow, founder effect, speciation, polyploidy, plate tectonics, Pangaea.

2. Compare and contrast Darwin and Lamark's views on evolution.

3. Describe how sedimentary rock is formed and how geologists can determine which fossils are older based on their positions in the rock.

4. Relate mutation, variation and natural selection to evolution. Describe the sources of variation.

5. List and describe the five conditions necessary for Hardy Weinberg equilibrium.

6. Compare and contrast directional, stabilizing and diversifying selection and give examples of each type.

7. Compare and contrast microevolution and macroevolution and give examples of each type.

8. List and describe the various prezygotic and postzygotic isolating mechanisms between species. Identify the simplest mechanism.

9. Compare and contrast allopatric and sympatric speciation and give examples.

10. Relate plate tectonics to the movement of modern day continents, earthquakes and mountain building.

11. Describe how a meteor impact could have led to the extinction of the dinosaurs.

Evolution- a quick preview

Definition of evolution= changes that occur over time.  The changes are called adaptations

The changes occur because of natural selection (survival of the fittest)

Controversy

What are some examples that are well agreed upon by both scientists and creationists?

 

Creationism and Evolution

Creationism deals with the supernatural and is therefore outside of the field of science

Here's where creationism and pure evolution agree:

 

The difference between creationism and Darwin's theory is the amount of evolution that is agreed upon


14.1  Darwin's Theory of Evolution

Charles Darwin's derived his theory of evolution in part from observations he made while serving as the naturalist aboard a ship named the HMS Beagle.

Recall from Module 1 Part A that theories are not called theories because they are "in doubt". A theory is a well supported idea that explains "why".

Before Darwin

Prior to Darwin's theories, there were different views of the world and living organisms. 

Jean-Baptiste de Lamark offered explanations for the changes in fossil organisms between strata.

Evolution occurred as the environment forced organisms to adapt.

These adaptations caused less complex organisms to evolve into more complex organisms.

Adaptation occurs because of the use or disuse of a structure, an element of the theory of acquired characteristics.   

Darwin's ideas were similar to those of Lamarck, but with important differences.

Darwin believed that living things share common characteristics because they have a common ancestor.

Darwin also believed that the organisms adapt to the environment, but through a process called natural selection.

Mechanisms of evolution

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Darwin's Conclusions

Darwin's conclusions were based upon his study of geology, fossils, and biogeography.

Darwin's Study of Geology and Fossils

Geological theories contributed to Darwin's efforts.

James Hutton proposed that the Earth undergoes slow, continuous cycles of erosion and uplifting. 

Erosion deposits thick layers of sediment, which eventually form sedimentary rocks.

The uplifting of sedimentary rock forms new land and can expose fossils.

formation of rock

Darwin observed similar geological changes and collected fossils during his time on the Beagle.

Rock formation

These activities caused Darwin to accept the fact that the Earth was very old. 

From this, Darwin proposed that modern organisms may have descended from now extinct organisms.

 

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Darwin's Study of Biogeography

Darwin made extensive comparisons between similar animals from around the world to understand evolution. 

He proposed that similar environments may have caused convergent evolution, or the development of similar adaptations.

When Darwin's ship reached the Galapagos Islands, he noticed that the finches had greatly diversified. These different species must have had common ancestors that originally come from South America.

The finches had different beak structures as well as different feeding habits. 

Three species of finches found on the Galapagos Islands

From his observation of the Galapagos finches, Darwin proposed that speciation had occurred.

Speciation is the process by which different groups of an organism evolve independently from one another, ultimately becoming a different species. 

Natural Selection and Adaptation

Darwin suggested that natural selection was the process that caused adaptation.

The process of natural selection has several preconditions that must be met.

1. The members of a population show variation.

2. More individuals are born in a population than die.

3. Some individuals inherit adaptive characteristics that favor their survival and reproduction.  

If the preconditions are met, natural selection has consequences.

In each generation, an increasing number of individuals have the adaptive characteristics.

Natural selection adapts a population to its local environment.

Natural selection relies on the variations produced by genetic changes (mutations). 

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More on the unique organisms on the Galopagos islands

 

 

 

Organisms Have Variations

variations in shell color

Variation in one species of snail

Prior to Darwin, variation within a population was ignored.

Darwin argued that the random occurrence of these variations was essential to natural selection. After all, there could be no "survival of the fittest" unless there were differences between the members of a population. If all the members were exactly the same, none would have an advantage over the others. 

These variations that allow for adaptation to the environment must also be heritable.

Organisms Struggle to Exist

There is a constant struggle to obtain the resources needed to survive and reproduce. 

Organisms Differ in Fitness

Those organisms best capable of obtaining the resources necessary to survive and reproduce are those with the greatest fitness.

The trait that determines whether an organism is fit varies from population to population.

When humans carry out artificial selection, breeders select specific traits that are favorable. 

The concepts of natural selection and fitness argue that interaction with the environment and random variation are responsible for evolution.

This differs from the theory of acquired characteristics proposed by Lamarck. 

Organisms Become Adapted

The consequence of natural selection and fitness is that populations to adapt to their environment. Individual organisms do not evolve, populations evolve over many generations.

The adaptations that make organisms more suited to the environment can occur simultaneously in different species.

Convergent evolution occurs when different organisms acquire similar adaptations. Example- rose bushes and thistles aren't cousins to each other, but they both have sharp, pointed structures that help keep plant eating animals from consuming them. 

 

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14.2 Evidence for Evolution

According to the theory of evolution, all species descend from a common ancestor but have become adapted to particular environments.

There are several lines of evidence that support the theory of evolution. Click here to view a website that discusses evidence that supports evolution. The website will open in a new window. 

1. Fossil Evidence

The fossil record contains rich information about the life on Earth before recorded human history.

One piece of evidence provided by the fossil record is that the pattern of evolution is typically from simple to more complex. Click here to view a website that shows many pictures of fossils arranged by how old the fossils are. The website will open in a new window.

The fossil record also reveals transitional links between different organismal groups.

 

large and small armadillo like species

The modern armadillo (right) and the extinct armadillo like animal above (left) are likely cousins to each other, but differ greatly in size.

2. Biogeographical Evidence

Biogeography is the study of the distribution of plants and animals throughout the world.

While there are many similar environments around the world, the plants and animals that live there are often unique.

The different organisms must have arisen from different evolutionary events.

Example: there are 13 species of finches on the Galapagos islands. These species must have originated from a few birds of the same species that made their way from south America over 500 miles of ocean.

Click here to view a website that lists some examples of biogeographical evidence that supports evolution. The website will open in a new window. 

 

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3. Anatomical Evidence

Darwin and others have also showed that different species share vestigial structures that reveal their common descent.

Whales and snakes all show vestigial bones related to four-legged animals.

Humans have a tailbone but not tail.

Organisms may also have homologous structures, which have very different shapes and functions. These are anatomically similar structures that suggest common ancestry.

Homologous structures

In contrast, organisms may have analogous structures, which have a similar function but are derived differently evolutionarily

Example: human and panda "thumb", rose thorns and thistle stickers.

 

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4. Biochemical Evidence

The study of biochemistry and genetics has shown that many organisms use similar chemicals and genes.

The diversity in living organisms is due to slight differences in this set of genes.

Ex. Beagles and poodles will have more similar genes than beagles and a cats because they are close cousins of each other.

 

 

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15.1 Microevolution

A population is defined as all the members of a single species occupying a particular area and reproducing with one another.

Microevolution involves the evolutionary changes within a population.

While variation within a population is important to evolution, it is not the only factor. 

Evolution in a Genetic Context

Evolution at the population level can be studied using population genetics.

In population genetics, the various alleles for a particular trait in all the individuals make up the gene pool of that population.

The gene pool of a population can be described in terms of gene frequencies. 

Sexual reproduction alone will not result in the change of allele frequencies in a population.

This equilibrium illustrates the Hardy-Weinberg principle. 

Microevolution occurs when events other than sexual reproduction disrupt Hardy-Weinberg equilibrium and cause the allele frequencies within a population to change.

Hardy-Weinberg equilibrium is expressed as a simple equation.

p2 + 2 pq + q2

The letters p and q are used to represent the frequency of the two alleles in the population. 

Hardy-Weinberg equilibrium is maintained in a population of sexual reproducing individuals if five conditions are met.

1. No net change in frequency due to mutations

2. No gene flow (migration of alleles in or out of the population)

3. Random mating must occur

4. No genetic drift

5. No natural selection

These conditions are rarely if ever met in the real world.

Thus allele frequencies continually change and microevolution occurs.

The value of the Hardy-Weinberg principle is that it describes the factors that cause evolution.

In order for natural selection to act on allele frequencies, the change must affect the phenotype associated with the gene.

A classic example of microevolution is industrial melanism and peppered moths. There are two coloration patterns, dark and light. In England, prior to the Industrial Revolution, light colored lichens (picture a below) covered many tree trunks and branches. As you can see below, the light colored moths were more camouflaged under these conditions and since fewer were eaten by predators, studies revealed that a higher percentage of the population of moths had this trait. During the Industrial Revolution, as coal was burned, a lot of soot filled the air and coated the tree trunks and branches killing many of the light colored lichens. Looking at picture b below, you can see that the dark moths had an advantage under these conditions and scientists at that time noted that the dark colored moths outnumbered the light colored moths. So, in this case, natural selection caused the light and dark coloration gene frequencies to change over time.

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light and dark colored peppered moths

 

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Causes of Microevolution

If any of the conditions of Hardy-Weinberg equilibrium are not met, the allele frequencies will change- this is called microevolution.

 

Situations that will cause microevolution are:

Mutations

Gene flow

Nonrandom mating

Genetic drift

Natural selection

 

Genetic Mutations

Mutations, changes in the DNA sequence, are the raw material of evolutionary change.

Mutation introduces new variation into a population. Most mutations are harmful, but a small percentage can be beneficial.

This variation is adaptive (good) if it helps members of a population adjust to specific environmental conditions. 

Gene Flow

Gene flow, or gene migration, occurs when breeding members of a population leave a population or new members enter.

Gene migration can introduce new alleles into populations.

However continual gene flow between populations decreases differences in allele frequencies, preventing speciation.   

Nonrandom Mating

When males and females reproduce together strictly by chance it is called random mating.

Any behavioral activity that fosters the selection of specific mates is nonrandom mating. 

Assortive mating occurs when organisms select mates with a similar phenotype.

Sexual selection favors traits that increase the likelihood of securing a mate.   

Genetic Drift

Chance events that cause the allele frequency to change, this is called genetic drift.

genetic drift

The effect of genetic drift becomes increasingly important as the size of the population decreases.

Another example of genetic drift is the bottleneck effect.

A bottleneck occurs when an event or a catastrophe drastically reduces the number of organisms in a population.

bottleneck effect

The variation in that population may also be reduced, changing the allele frequencies within the population. 

The founder effect is another example of genetic drift.

The founder effect occurs when combinations of alleles occur at a higher frequency in a population that has been isolated from a larger population. In other words, imagine a pair of animals leave one population and make their way to a new island and begin a new population. The new population will look more similar to the "founders" than to the original population.

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A simulation that inverstigates the bright coloration of guppies

 

 

 

 

15.2  Natural Selection

Natural selection is the process that adapts populations to the environment.

Some aspects of the environment can involve biotic (living) components.

Some aspects of the environment can involve abiotic (nonliving) components.

 

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Types of Selection

The variation within a population can create different phenotypes for a highly variable trait

Ex. Size (tall, medium, short and everything in between) or color  (light, dark and everything in between)

The distribution of those phenotypes typically forms a normal distribution. 

The effect of the three types of natural selection have different effects on this normal distribution.

Population of plants with different flower colors

1. Directional Selection

When one extreme phenotype is favored by natural selection, the distribution of the phenotype shifts in that direction.

This type of selection is therefore called directional selection.

This is responsible for the increase in antibiotic resistant bacteria because the bacteria with the most resistance are most likely to survive when antibiotics are used

directional selection of horse like animals

 

2. Stabilizing Selection

Stabilizing selection occurs when the intermediate, or most common, phenotype is favored. 

This type of selection tends to narrow the variation in the phenotype over time. 

This is the most common type of selection because it is associated with the adaptation of an organism to the environment.

stabilizing selection

3. Disruptive Selection

In disruptive selection, natural selection acts upon both extremes of the phenotype.

This creates a increasing division within the population which may ultimately lead to two different phenotypes.

Disruptive selection is the process that leads to speciation.

disruptive selection in a species of snail

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Maintenance of Variations

The preservation of variation in a population is important because it provides a foundation on which natural selection can act.

Variation is preserved by a variety of processes:

Mutations and genetic recombination

Gene flow

Natural selection

Diploidy and the heterozygotes

Diploidy and the Heterozygote

Natural selection can only cause evolution if the different alleles produce different phenotypes. 

Because many organisms are diploid, heterozygotes are carriers of recessive alleles, preserving them in the population.

Imagine that G= good trait and g=bad trait. If G is dominant over g, the genotypes GG and Gg would have an equal chance of survival. This keeps the g trait in the population even though it is not the good trait.

In the case of the sickle cell allele, heterozygotes can be the most fit when malaria is prevalent because malaria is less severe in people who are carriers of the sickle cell allele.

 

 

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16.1  Macroevolution

Microevolution involves changes on the small scale at the level of gene pool alleles.

In contrast, macroevolution involves evolution at the large scale as species originate, adapt to their environment, and possibly become extinct.

Defining Species

Speciation is an evolutionary event that gives rise to new species.

The biological species concept provides one definition of a species:

A group of organisms that naturally interbreed with each other and share the same gene pool and which produce fertile offspring.

Each species is reproductively isolated from every other species. 

Reproductive Barriers

In order for species to be reproductively isolated, they must be separated by barriers which prevent gene flow. 

Reproductive barriers are also called isolating mechanisms.

Prezygotic isolating mechanisms prevent reproduction and make fertilization unlikely.

Habitat isolation occurs when organisms cannot reproduce because they are in different habitats. 

Temporal isolation occurs if the reproductive cycles of organisms occurs at different times.

The unique courtship patterns displayed by organisms can create behavioral isolation. This is the simplest type of isolation.

Mechanical isolation occurs when the genitalia are structurally incompatible. 

Genetic isolation occurs when the fertilization does not occur, even when sperm and egg are brought together.

Postzygotic isolating mechanisms prevent hybrid organisms from developing (zygote mortality) or reproducing (hybrid sterility). Mules are good examples of hybrid sterility.

hybrid sterility

In the case of F2 fitness, even a hybrid organism develops and reproduces, but the offspring of the hybrid are sterile.

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Models of Speciation

There are different ways in which the process of speciation can occur.

In allopatric speciation, an ancestral population is geographically isolated, resulting in the evolution of separate species largely due to genetic drift. The additive effect of differences due to genetic drift can eventually result in behavioral isolation (refusal to mate) if the two groups were to meet again in the future. If they refused to interbreed, they would be considered separate species.

allopatric speciation

 

 

Sympatric speciation involves speciation without a geographic barrier. 

One example of sympatric speciation is polyploidy, found more often in plants. 

Polyploidy occurs when a failure of meiosis increases the number of chromosome sets to 3N or more. At this point, the two "cousin" species can no longer mate with each other.

sympatric speciation

 

 

 

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16.2  The History of Species

The evolutionary history of a species, such as is origin and extinction is reflected in the fossil record. 

The study of fossils is called paleontology.

The Geological Timescale

The geological timescale of the earth has been constructed by studying the fossils in the various strata of rock.

Mass Extinction of Species

Most species exist for a limited period of geological time and then become extinct.

Within the fossil record there are also instances of mass extinctions. 

Evidence of six mass extinctions can be seen in the fossil record. 

There are two primary events that are believed to have contributed to these mass extinctions. 

The movement of the Earth's surface via continental drift is one such event. 

Plate tectonics provides the explanation for why continental drift occurs.

The earth's crust is made up of many different plates that move around independently of each other. Where the plates collide with each other, mountains can be formed and there are earthquakes. Mount Everest is getting measurably taller each year because of this activity.

movement of plates

continental drift  

The current direction of the drift of the plates has been determined using GPS data from satellites. By assuming that the plates have always been moving in the directions they are moving now, all the plates would have been connected together as a huge supercontinent called Pangaea.

The separation of the supercontinent Pangaea created dramatic changes, including allopatric speciation.

A meteorite impact was another event that contributed to mass extinctions.

The impact of a meteor in Central America is thought to have caused the Cretaceous extinction of the dinosaurs. There is a large meteor crater off the east coast of the Yucatan peninsula of Mexico. This meteor impact would have knocked megatons of dirt and dust high into the atmosphere which would have greatly cooled the climate of earth. This cooling would have lasted for decades and could have caused frosts that would have killed the plants that supported large plant eating dinosaurs. As the plant eating dinosaurs starved, meat eating dinosaurs would have ran out of food.

 

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