Evolution

Evolution

A change in the genetic makeup (gene frequency) of a population

Equilibrium is achieved by meeting ALL of these qualifications:

• Not having ANY mutations.• All mating is RANDOM. Selecting a mate

causes… selection.• The population can have no immigration or

emigration. This causes a change in gene frequency.

• There are no environmental factors which cause selection to occur. This includes short and long-term factors.

• The population is large.

Evolution vs. Equilibrium

If a species is changing,

it is evolving.

If it is not changing,

we say it is in equilibrium.

Natural Selection

1. There is variation

in traits.

For example, let’s say in a beetle population, some beetles are green and some are brown.

2. There is often a struggle for survival. Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential.

In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.

3. Survivors pass on their traits to their offspring.The surviving brown beetles have brown baby beetles because this trait has a genetic basis.

4. The traits that are better-suited for an environment will become more common. These traits are called adaptations.In our example, brown coloration allows the beetle to have more offspring, so it becomes more common in the population. If this process continues, eventually, all individuals in the population could be brown.

• If the brown beetles outnumber the green beetles, are they the “best fit” for the environment?

• For ALL environments?

• What happens if the environment changes?

• Do organisms create mutations because they HAVE to change in order to survive?

What is Genetic Drift??

• Random change in allele frequency, based on the laws of probability. Example Founder Effect

–Founder Effect: change in allele frequencies as a result of the migration of a small subgroup of population. Gene Flow is cut off from the original population.

• Another example of genetic drift is called the Bottleneck Effect

• Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

The role of Isolation in Evolution

• Geographic isolation – groups of the same species are physically separated, so gene pools don’t mix.

• Ecological Isolation – groups of the same species are in the same area, but end up occupying separate habitats.

• Temporal Isolation – reproduction occurs at different times of the day or year.

• Behavioral Isolation – they aren’t understanding each other, so mating doesn’t occur.

• Mechanical/Reproductive Isolation – reproduction doesn’t occur either because the parts aren’t working together, or matings aren’t producing fertile offspring.

• Grasses around the mine have all experienced selection – they can tolerate the heavy metal content of the soil.The plants are in close proximity, so in theory they could cross-pollinate.However, it has been noted that the plants that tolerate the mine waste also flower at a different time.

What type(s) of isolation are represented?

“Darwin’s Finches”

What type of isolation is represented?

The mating rituals between male individuals may vary, and the females may vary in what they find attractive. This may lead to two different groups of birds, even though they live in the same general location.

What type of isolation would this represent?

+ =

Horses and donkeys are able to reproduce, but the offspring – a mule - will be sterile. In the wild, these matings rarely occur because the female will often reject the male of the other species.

What type(s) of isolation are represented?

Microevolution is a change that occurs within a species.

A “species” refers to a group of organisms that can mate and produce fertile offspring.

Macroevolution refers to changes among species, for example when one species goes extinct and another takes over its role.

“Speciation” is when evolution leads to the formation of new species.

Five Evidences used for studying evolution

1. The fossil record

provides a ‘picture’ of the transitional stages of many species. It also supports the idea that simple organisms came before more complex organisms.

2. Anatomical similarities

• Homologous structures are those that share a similar basic framework (even if they are used for a different purpose)

• Vestigial structures – are reduced in size or function from a previous ancestor. (note: they are not “useless”)

Ex: human tailbone: In most people it is not even noticeable, and serves no known function.

• human appendix: Does serve some function as a part of your lymphatic system, but is much reduced from the digestive purposes served by it’s counterpart (the cecum) in herbivores.

by the way…

• Analagous structures are NOT evidence for evolution

These serve a similar function, but have different underlying structures.

3. Genetics and molecular similarities

All organisms have DNA and share a great similarity in proteins, carbohydrates and lipids that make them up. Metabolic processes are also very similar in many ways.

4. Geographic distribution of related species

• The more similar two organisms are, the more likely they are to be found near each other. Australia had nearly all marsupial species, yet no land mammals such as dogs, bears, or horses.

• Isolated islands generally have unique organisms found nowhere else, and lack common organisms found elsewhere. For example, there are no snakes in Hawaii.

5. Experience with artificial selection …

… and observed natural selection

What are the five categories of evidence for evolution?

• 1. Fossil Record

• 2. Anatomical features• Homologous structures• Vestigial structures

• 3. DNA and molecular sequences

• 4. Geography

• 5. Artificial selection and observed natural

selection.

Cladogram

How to read a Cladogram

• This diagram shows a relationship between 4 species. These relatives share a common ancestor at the root of the tree.

• Cladograms are also a rough timeline. The organism at the bottom of the tree has been around the longest.

• The four descendents at the top of the tree are DIFFERENT species. This is called SPECIATION.

Image courtesy of http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_05

• Branches on the tree represent SPECIATION, the formation of a new species.

• The event that causes the speciation is shown as the fork of the “V”.


• Species B and C each have characteristics that are unique only to them.

• But they also share some part of their history with species A. This shared history is the common ancestor.


Write a few sentences that summarize the relationship between organisms A, B, and C.

Does “B” have to be extinct in order for “C” to exist? Does the common ancestor have to be extinct?


• Look at the cladogram at the right. What conclusions can be drawn about the relationship between humans and chimps?


• If we made a cladogram of ALL living things, the oldest organism would be the common ancestor of all living things.

• What would this organism be like? What characteristics would it have? What characteristics would it NOT have? What living organism would it most closely resemble? Explain…

3 Types of Selection

Directional selection

Low mortality, high fitness

High mortality, low fitness

Key

Food becomes scarce.

Directional Selection

Form of natural selection by which the entire curve moves.

Occurs when individuals at one end of the distribution curve have higher fitness than individuals in the middle or at the other end of the curve.

Stabilizing Selection• Form of natural selection by which the center of the curve remains

in its current position.

• Occurs when individuals near the center of distribution curve have higher fitness than individuals at either end.

Selection against both

extremes keep curve narrow and

in same place.



Key

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Birth Weight

Stabilizing Selection

Disruptive Selection• Form of natural selection in which a single curve splits

into two.

• Occurs when individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle.

Population splits into two subgroups specializing in different seeds.

Disruptive Selection



Key

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Beak Size

Largest and smallest seeds become more common.

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Beak Size

• When speciation results in a “split” it is called divergent evolution.

• If there are a lot of “splits” (like the finches) it is often called adaptive radiation.

• Convergent evolution is when two different species become more similar.

• Why would convergent evolution occur?

• Would two different species ever become one species?

Does evolution happen gradually, or in short bursts?

• GRADUALISM is the hypothesis that changes in species occur slowly and only small changes at a time.

• PUNCTUATED EQUILIBRIUM This hypothesis was published in 1972 by Niles Eldredge and Stephen J. Gould.They suggested that major changes can occur relatively suddenly, and that they "punctuate" long periods of relatively little change.

Which hypothesis do you think is better at explaining changes in species?

Could both hypotheses coexist?

Endosymbiotic Theory• In 1968, Lynn Margulis

proposed the endosymbiotic theory.

• Based on similarities between prokaryotes & mitochondria and chloroplasts, she proposed that these organelles used to be prokaryotes.

A very brief comparison:

Prokaryotes Eukaryotes Mitochondria Chloroplasts

DNA 1 single, circular chromosome

Many linear chromosomes

1 single, circular chromosome

1 single, circular chromosome

Replication Binary Fission Mitosis Binary Fission Binary Fission

Electron Transport Chain

In the plasma membrane of the cell

Found only in mitochondria and chloroplasts

In the plasma membrane around the mitochondria

In the plasma membrane around the chloroplast

Size 1-10 µm 50-100 µm 1-10 µm 1-10 µm

Ribosomes 70 S 80 S 70 S 80 S

Education

Evolution