APBIO- Evolution Review

APBIO- EVOLUTION REVIEW

WARM-UP: DESCRIBE HOW DESCENT IS RELATED TO EVOLUTION. ANALYZE WHY POPULATIONS, NOT INDIVIDUALS,

EVOLVE.

OBJECTIVES(1) Students will be able to describe the concept of evolution and the

four major modes in which it occurs.

(2) Students will be able to describe natural selection and the works of Lamarck and Darwin.

(3) Students will be able to analyze types and importance of adaptations.

(4) Students will be able to describe the five major types of selection: directional, stabilizing, disruptive, sexual, and artificial selection.

(5) Students will be able to identify sources of variation within populations.

(6) Students will be able to list two main types of speciation.

(7) Students will be able to identify the Hardy-Weinberg equation and list conditions necessary for its existence.

DEFINITION OF EVOLUTIONBiological definition of evolution: descent with modification. • Descent can only happen when one group of organisms

gives rise to another. • Evolution occurs in populations, not individuals. • It describe changes in allele frequencies in populations

over time. • Over many generations, the species can change so much

that it becomes different from the ancestral species, or part of the population can branch off and become a new species, called speciation.

QUESTION TO CONSIDER…

Q: Describe reasons why we see this change in allele frequencies with time.


Q: Describe reasons why we see this change in allele frequencies with time.A: Allele frequencies may change because of random factors or natural selection.

FOUR MODES OF EVOLUTION(1) Genetic drift- change in allele frequencies due to chance events.

When genetic drift dramatically reduces population size it’s called bottleneck.

(2) Gene flow- the change in allele frequencies as genes from one population are incorporated into another. It’s also known as migration when individuals are actively relocating. Gene flow is random regarding which organisms succeed.

(3) Mutation- always a random event regarding which genes are affected although the changes in allele frequencies may not be.

(4) Natural selection- explains why organisms look and behave the way they do. It’s based on three conditions: (1) variation, (2) hereditability, and (3) differentiated reproductive success.

FOUR MODES OF EVOLUTION(4) Natural selection is based on three conditions:

• (1) Variation- for natural selection to occur, a population must exhibit phenotypic variance.

• (2) Hereditability- parents must be able to pass on the traits that are under natural selection.

• (3) Differentiated reproductive success- Reproductive success measures how many offspring you produce that survive relative to how many the other individuals in your population produce.


Q: Since natural selection increases the frequencies of advantageous alleles, why don’t we get to a point where all individuals have all the best alleles?


Q: Since natural selection increases the frequencies of advantageous alleles, why don’t we get to a point where all individuals have all the best alleles?

A: For one, different alleles confer different advantages in different environments. Remember that the environment- which includes everything from habitat, to climate, to competitors, to predators, to food resources- is constantly changing. Species are therefore also constantly changing as the traits that give them an advantage also change. If a trait becomes unconditionally advantageous fixed alleles could occur. But where there are heritable characteristics that both vary and allow fitness advantage (or disadvantages) on their host organisms, natural selection can occur.

REFLECTION1. Describe the concept of evolution and the

four major modes in which it occurs.

LAMARCKJean Baptiste Lamarck proposed the idea that evolution occurs by the inheritance of acquired characteristics. • Classic example: giraffe necks (Changes happened

within the organism during their lifetimes and then the changes in the trait was passed on).


Q: Describe what is wrong with Lamarck’s theory. Try explain to yourself how the change could have been passed on to the offspring.


Q: Describe what is wrong with Lamarck’s theory. Try explain to yourself how the changed could have been passed on to the offspring.

A: The answer is that it couldn’t- the instructions in the sex chromosomes that direct the production of offspring cannot be changed after they are created at the birth of an organism. Lamarck confused genetic and environmental (postconceptive) change, but at this time period it’s not surprising because no one had discovered genes yet.

DARWINCharles Darwin suggested the idea of natural selection and coined the term “survival of the fittest.” • Although he didn’t call them genes, he proposed a

hypothetical unit of heredity that passed from parent to offspring.

• At the same time period a man named Wallace also came up with the idea of natural selection. But Darwin published his work first and has become famous as a result.

REFLECTION2. Describe natural selection and the works of Lamarck and Darwin.

ADAPTATIONSAdaptation- trait that if altered, affects the fitness of the organism. They are the result of natural selection and include not only physical traits (i.e., eyes, fingernails) but also the intangible traits (i.e., lifespan length) of organisms.• Mating behavior is also an adaptation- it has been selected

by natural selection because it is an effective strategy. • Behavioral adaptations (i.e., reproductive maturity) can

evolve.

REFLECTION3. Analyze types and importance of adaptations.

TYPES OF SELECTIONNatural selection can change the frequencies of alleles in populations through various processes. Three of the most common types include: (1) directional selection, (2) stabilizing selection, and (3) disruptive selection.

1. Directional selection- occurs when members of a population at one end of the spectrum are selected against, while those at the other end are selected for.

TYPES OF SELECTION2. Stabilizing selection- this describe selection for the mean of a population for a given allele.

TYPES OF SELECTION3. Disruptive selection- also known as diversifying selection, it’s the opposite of stabilizing selection.

TYPES OF SELECTIONThese three processes describe the way in which allele frequencies can change as a result of the forces of natural selection.

Two other types of selection also compliment natural selection: (1) sexual selection and (2) artificial selection.1. Sexual selection- occurs because individuals differ in mating

success. It occurs by two primary processes: within-sex competition and choice.

1. Within-sex competition leads to the evolutionary characteristics that are designed for two main functions: (1) as weaponry or other tools for male competition (i.e., large testes for sperm competition) and (2) as traits that increase mating opportunities because females prefer to mate with males who have them (i.e., colorful feathers in many birds).

TYPES OF SELECTIONTwo primary processes of sexual selection (cont’d):

2. Choice- female mate choice for certain characters is not random. Sexually selected traits that are the result of female choice are called honest indicators. Selecting a mate for particular features does not have to involve conscious thought.

QUESTION TO CONSIDER

Q: Describe how natural selection differs from sexual selection.

QUESTION TO CONSIDER

Q: Describe how natural selection differs from sexual selection.

A: Natural selection includes both reproduction and survival. Sexual selection is purely about access to mating opportunities.

TYPES OF SELECTION2. Artificial selection- involves humans becoming the agents of natural selection. We specifically select certain individuals to breed while restraining others from doing so. It has resulted in the domestication of a wide range of plant and animal species and the selection of certain traits (i.e., cattle with lean meat, flowers with particular color combinations).

REFLECTION4. Describe the five major types of selection: directional, stabilizing, disruptive, sexual, and artificial selection.

EVOLUTION PATTERNSThere are four basic patterns of evolution:

(1) Coevolution- mutual evolution between two species, which is exemplified by predator-prey relationships (i.e., moth-orchid).

(2) Convergent evolution- two unrelated species evolve in a way that makes them more similar. Two characters are called convergent characters if they are similar in two species, even thought the species do not share a common ancestor (i.e., insect-bird wing).

EVOLUTION PATTERNSFour basic patterns of evolution (cont’d):

(3) Divergent evolution- Two related species evolve in a way that makes them less similar. It can lead to speciation (allopatric or sympatric).

(4) Parallel evolution- Similar evolutionary changes occurring in two species that can be related or unrelated. They are responding in a similar manner to a similar environmental condition (i.e., Thylacosmilus in South America-Smilodon in North America).

SOURCES OF VARIATIONOne of the conditions for natural selection is variation. This variation within populations comes from the following:

(1) Mutation- random changes in the DNA of an individual can introduce new alleles into a population.

(2) Sexual reproduction- related to why offspring are not identical to their parents (crossover, independent assortment of homologous pairs, and the fact that sperm and ova are unique and create unique individuals when joined).

(3) Balanced polymorphism- Some characters are fixed, meaning all individuals in a species or population have them (i.e., tulips develop from bulbs). However, other characters are polymorphic, meaning there are two or more phenotypic variants (i..e, tulips come in a variety of colors).

HOW BALANCED POLYMORPHISM IS MAINTAINED

MECHANISM DESCRIPTION EXAMPLE

Heterozygote advantage

The heterozygous condition has an advantage over either homozygote, so both alleles are maintained (AA is worse off than Aa).

Sickle cell trait, a heterozygous condition, gives people in malarial environments an advantage because they are resistant to this disease.

Hybrid vigor and outbreeding

Two unrelated individuals are less likely to have the same recessive, deleterious allele than are relatives; therefore, their offspring are less likely to be homozygous for that allele; in addition, outbreeding increases the number of heterozygous alleles, increasing heterozygote advantage.

Artificially selected plants are carefully outbred in order to increase hybrid vigor; mating two inbred strains of potato will increase the number of heterozygous loci and increase the species’ resistance to disease.

Frequency-dependent selection

The least common phenotype is selected for, while common phenotypes have a disadvantage.

In some fruit flies, females choose to mate with makes that have the rarer phenotype, resulting in selection against the more common variants.

REFLECTION

5. Identify sources of variation within populations.

SPECIATIONA species is a group of interbreeding (or potentially interbreeding) organisms. Speciation, the process by which new species evolve, can take one of several forms. Two main forms of speciation include:

(1) Allopatric speciation- interbreeding ceases because a barrier separates a single population into two (area with no food, a mountain, etc.). The two populations evolve independently and if they change enough, even thought he barrier is removed, they cannot interbreed.

(2) Sympatric speciation- interbreeding ceases even though no physical barrier prevent it. This may take several forms: (1) polyploidy and (2) balanced polymorphism.

SPECIATION(2) Sympatric speciation (cont’d)-

• (A) Polyploidy- condition in which an individual has more than the normal number of sets of chromosomes. This is unusual in animals (i.e., goldfish), but common in some plants (i.e., ferns), and can result in new species.

• (B) Balanced polymorphism- condition can also lead to speciation if two variants diverge enough to no longer be able to interbreed (i.e., if potential mates no longer recognize each other as possible partners).

• Sickle Cell disease is an autosomal recessive disorder that causes anemia, joint pain, a swollen spleen, and frequent, severe infections. It illustrates balanced polymorphism because carriers are resistant to malaria, an infection by the parasite Plasmodium falciparum that causes cycles of chills and fever. The parasite spends the first stage of its life cycle in the salivary glands of the mosquito Anopheles gambiae. When an infected mosquito bites a human, the malaria parasite enters the red blood cells, which transport it to the liver. The red blood cells burst, releasing the parasite throughout the body.

REFLECTION6. List two main types of speciation.

SPECIATIONAdaptive radiation- the rapid series of speciation events that occur when one or more ancestral species invades a new environment. If there are many ecological niches several species sill evolve because each can fill a different niche.

WHEN EVOLUTION IS NOT OCCURRING: HARDY-WEINBERG EQUILIBRIUMBiologists use a theoretical concept called the Hardy-Weinberg equilibrium to describe those special cases where a population is in stasis, or not evolving. There are five conditions that must be met for a population to be in Hardy-Weinberg equilibrium and they include:

1. No mutations

2. No gene flow

3. No genetic drift

4. No natural selection

5. Random mating

WHEN EVOLUTION IS NOT OCCURRING: HARDY-WEINBERG EQUILIBRIUMThe equation used to determine if a population is in Hardy-Weinberg equilibrium: p + q = 1• p is the frequency of allele 1 (often the dominant

allele)• q is the frequency of allele 2 (often the recessive

allele)• The frequency of two alleles always adds up to 1 if

the population is in Hardy-Weinberg equilibrium.

WHEN EVOLUTION IS NOT OCCURRING: HARDY-WEINBERG EQUILIBRIUM

There is a second equation that goes along with this theory: p2 + 2pq + q2 = 1• p2 and q2 represent the frequency of the two homozygous

conditions (AA and aa).• The frequency of the heterozygous is pq plus qp or 2pq

(Aa and aA).• Since p represents the dominant allele, it makes sense

that p2 represents the homozygous dominant condition. By the same logic, it makes sense that q2 represents homozygous recessive condition.

WHEN EVOLUTION IS NOT OCCURRING: HARDY-WEINBERG EQUILIBRIUMExample: You are told a population of acacis trees is 16% short (which is a, recessive) and 84% tall (which is A, dominant). What are the frequencies of the two alleles? Remember that this is not 0.16 and 0.84 because there are also the heterozygotes to consider!

WHEN EVOLUTION IS NOT OCCURRING: HARDY-WEINBERG EQUILIBRIUM

Hint: In a problem like this, it’s important to determine the value of q first because we know that all individuals with the recessive phenotype must be aa (q2). You cannot begin by calculating the value of p because it is not true that all the individuals with the dominant phenotype can be lumped into p2. Some displaying the dominant phenotype are heterozygous Aa (pq).

ANSWER TO PRACTICE EXAMPLEWe know that q2 = 0.16, so we find q by calculating the square of 0.16 = 0.400.

What about p? Since p + q is 1, and we know q = 0.40, then p must equal 1 - 0.40 or 0.600.

If asked to go a step further and give the percentages of the homozygous dominant and heterozygous conditions (remember we know the recessive condition is 16%- all these individuals must be aa in order to express the recessive trait). Just plug in what you know about p and q:

2pq = (2) (0.6) (0.4) = 0.48 or 48%

p2 = (0.6) (0.6) = 0.36 or 36%

Now check your math: do the frequencies add up to 100%?

16 + 48 + 36 = 100


Q: Why do we ever use the Hardy-Weinberg equation if it’s rarely ever applied to real populations?

QUESTION TO CONSIDER…Q: Why do we ever use the Hardy-Weinberg equation if it’s rarely ever applied to real populations?

A: This can be an excellent tool to determine if a population is evolving or not; if we find that the allele frequencies do not add up to one, then we need to look for the reasons for this (perhaps the population is too small and genetic drift is a factor, or perhaps one of the alleles is advantageous and is therefore being selected for and increasing in the population). Although it is theoretical, the Hardy-Weinberg equilibrium does have some important uses in evolutionary biology.

REFLECTION7. Identify the Hardy-Weinberg equation and list conditions necessary for its existence.

THE EVIDENCE FOR EVOLUTIONSupport for the theory of evolution includes:

1. Homologous characters- traits are homologous if they are similar because their host organisms came from a common ancestor.

THE EVIDENCE FOR EVOLUTIONSupport for the theory of evolution includes (cont’d):

2. Embryology- similarities between organisms at early life stages, although in adulthood they look completely different. • Darwin used embryology as an

important piece of evidence for process of evolution.

• In 1866 Ernst Haeckel said, “Ontogeny recapitulates phylogeny.” Ontogeny is an individual’s development; phylogeny is a species’ evolutionary history.

THE EVIDENCE FOR EVOLUTIONSupport for the theory of evolution includes (cont’d):

3. Vestigial characters- Most organisms have characters that are no longer useful, although they once were. The environment may have changed so much that it is no longer needed, but yet not selected against and eliminated. • Darwin used vestigial characters as evidence in his original

formulation of the process of evolution (i.e., human appendix).

THE EVIDENCE FOR EVOLUTION• Fossil record- the

physical manifestation of a species that have gone extinct that includes evidence of homologous characters, embryology, and vestigial characters.

• Adaptations are the result of natural selection.

MACROEVOLUTIONMicroevolution- evolution at the level of the species and populations.

Macroevolution- the “big picture,” which includes the study of evolution by looking at groups of species over very long periods of time. The pattern of macroevolution is a hot debate because the fossil record is incomplete.

1. Gradualism- the belief that evolutionary change is a steady, slow process

2. Punctuated equilibria model- believe that change occurs in rapid bursts separated by large periods of stasis (no change).

HOW LIFE PROBABLY EMERGEDLife on Earth began about 3.5 billion years ago. As opposed to the current atmosphere, which is mostly nitrogen and oxygen, the early Earth atmosphere contained mostly hydrogen, water, ammonia, and methane.

In experiments, scientists have showed that the electrical discharges of lightning, radioactivity, and ultraviolet light caused the elements in the early Earth atmosphere to form the basic molecules of biological chemistry, such as nucleotides, simple proteins, and ATP (i.e., Stanley Miller & Harold Urey experiments). It seems likely, then, that the Earth was covered in a hot, thin soup of water and organic materials. Over time, the molecules became more complex and began to collaborate to run metabolic processes.

Heterotroph theory- first organisms were heterotrophs, which cannot make their own food. They are thought to have fed on the organic material from the primordial soup.

REFLECTION1. Describe the concept of evolution and the four major

modes in which it occurs.

2. Describe natural selection and the works of Lamarck and Darwin.

3. Analyze types and importance of adaptations.

4. Describe the five major types of selection: directional, stabilizing, disruptive, sexual, and artificial selection.

5. Identify sources of variation within populations.

6. List two main types of speciation.

7. Identify the Hardy-Weinberg equation and list conditions necessary for its existence.

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APBIO- Evolution Review