Unit 7: Evolution and Natural Selection

Factors Separating Species

-Prezygotic isolating mechanisms: Fertilization never occurs (egg & sperm never meet). -Postzygotic isolating mechanisms: If an offspring is born, the offspring demonstrates reduced viability.

What are the requirements for Hardy Weinberg equilibrium to occur?

1. Large population 🡪 Population is large in size, not easily affected by any events such as a storm or hurricane 2. Random mating 🡪 Organisms randomly reproduce, thus there is no sexual selection (no competition to reproduce) 3. No mutations 🡪 No new alleles are generated through random mutations, thus genetic variation is constant. 4. No gene flow 🡪 Organisms do not move into our away from the population, thus genetic variation is constant. 5. No selection 🡪 All alleles have equal fitness

How does evolution occur?

1. Mutations 🡪 Source of raw genetic variation for evolution to be possible 2. Natural Selection 🡪 Changing environment is selective pressure 3. Artificial Selection 🡪 Human choice is the "selective pressure" 4. Sexual Selection 🡪 Mate choice is the "selective pressure" 5. Gene Flow 🡪 Movement of organisms into & out of populations 6. Genetic Drift 🡪 Random events alter frequency of traits/alleles

What is a Gene Pool?

A gene pool is the collection of alleles that make up a population.

What is a Population?

A population is a group of organisms of the same species located in the same general area.

What is Adaptive Radiation?

Adaptive radiation is a process when an ancestral species diversifies into many descendant species because of the different environmental niches that become available to be occupied. (Also observed with Darwin's finches) New species formed fitting different ecological niches. Each species can fit into a specific ecomorph grouping.

Evidence #5: Molecular Biology

All organisms share the same genetic code (DNA and RNA). We can compare nucleotide sequences (by looking at the sequences of A, T, C and G) in order to determine how similar or different species genomes are to one another. More nucleotides differences indicate the species are more distantly related. The composition of specific proteins can also be compared by comparing the amino acid sequence between different species. More differences in the amino acid sequence indicate that the species are more distantly related.

Is Evolution Occurring?

Although populations in real life almost never enter a state known as Hardy Weinberg equilibrium, it provides a useful null hypothesis or baseline of evolution of a population.

3. Artificial Selection

Artificial selection is the intentional reproduction of organisms in a population that has a heritable trait that is deemed valuable by humans. Human desire in traits act as the selective pressure to determine how the population will evolve over time.

A current challenge for doctors involves the bacterial strain Clostridioides difficile, which no longer responds to traditional antibiotic treatments. Which of the following best explains why this particular strain of bacteria is resistant to antibiotic treatment? A. Mutations creating new alleles occur more frequently in this species of bacteria than in other species. B. The bacteria have high genetic variability and high reproductive rates. C. Crossing over during meiosis increases genetic variability in the bacteria. D. The bacteria are able to recognize and destroy the antibiotics by breaking them down extracellularly.

B

Practice Question: Because cheetahs, Acinonyx jubatus, experienced a bottleneck event about ten thousand years ago, living cheetahs exhibit very little genetic variation compared to other living species of large cats. Which of the following principles best justifies a claim that cheetahs, compared to other large cats, have a relatively low resilience to environmental perturbations? A. A captive population of cheetahs kept in a confined area will have a greater risk of contracting and passing on a virus. B. When a population is reduced, new niches become available in the environment, which can trigger adaptive radiation. C. Populations with little genetic diversity are less likely to contain individuals that can withstand different selective pressures. D. Populations with low genetic diversity are more likely to contain individuals that can withstand different selective pressures.

C

Allele Frequency Example: A A a a A A a a A A a a A A a a A A a a A A a a A A a a A A a a AA

Calculating Allele Frequency A allele frequency 🡪 10/18 = 0.55 = p a allele frequency 🡪 8 /18 = 0.45 = q 0.55+0.45=1

Evidence #4: Comparative Embryology

Comparative embryology shows that different species develop in a similar fashion during the development of the embryo.

Convergent Evolution

Convergent evolution is the process when different species that are relatively not closely related have evolved similar traits due to shared environmental habitats/selective pressures.

Ellis-van Creveld syndrome is a recessive genetic disorder that includes the characteristics of short stature and extra fingers or toes. In the general population, this syndrome occurs in approximately 1 in 150,000 live births. In a particular isolated population, however, the incidence of this syndrome among live births is 1 in 500. Assume that both the isolated population and the general population are in Hardy-Weinberg equilibrium with respect to this syndrome. Which of the following best describes the difference between the frequency of the allele that causes the syndrome in the general population and the frequency of the allele in the isolated population? A. The frequency of the Ellis-van Creveld allele is 0.002 in the isolated population and 0.0000066 in the general population, which suggests that selection for this trait is occurring in both populations. B. The frequency of the Ellis-van Creveld allele is 0.0447 in the isolated population and 0.0026 in the general population, showing that the rate of genetic mutation is highest among individuals in the isolated population. C. The frequency of the Ellis-van Creveld allele is 0.002 in the isolated population and 0.0000066 in the general population, which demonstrates gametic incompatibility between the populations. D. The frequency of the Ellis-van Creveld allele is 0.0447 in the isolated population and 0.0026 in the general population, which suggests that genetic drift has occurred in the isolated population.

D

Practice Question: Thoroughbred race horses are subject to intense artificial selection to produce faster-running horses. The results of breeding programs can be seen in the change in winning times of the Kentucky Derby from 1880 to the present, as shown in Figure 1. Which of the following best explains the trend shown in Figure 1? The distance the horses ran became shorter between 1880 and 1940. A. The distance the horses ran became shorter between 1880 and 1940. B. All of the selective breeding programs ended in 1940. C. After 1940, natural selection prevented any additional speed increases. D. By 1940, most genetic variation for speed had been lost from the gene pool.

D

Evolution

Descent with Modification, Evolution is the change in the frequency of traits (coded by genes) in a population of organisms across multiple generations.

Phylogenetic Trees & Cladograms

Diagram used to represent a hypothesis of evolutionary relationships between species that can be tested and are based on shared traits.

Divergent Evolution

Divergent evolution is the process where species sharing the same common ancestor (shown at the nodes) accumulate enough differences over time which eventually results in the formation of two new species.

Galapagos Finches

Drought in 1977 caused the finch population with smaller beaks to decrease because they couldn't eat the bigger seeds, causing the surviving finches to be the ones with bigger beaks and that trait to pass on over next generations. There was then storm where it rained for days causing finches with bigger beaks to decrease because they couldn't eat the smaller seeds, so finches with smaller or medium beaks were the ones that survived and passed on the genes. Overall, finches with medium beaks had the favorable trait and became more pronounced.

Bottleneck Events

During bottleneck events, an event occurs that causes many individuals in a population to be wiped out quickly. This causes genetic variation in the population to be greatly reduced and different compared to the original population. -Specific traits can be lost do to these kind of events AND some traits can become more prominent than before. NOTE: A genetic bottleneck creates random genetic changes without regard to adaptation

Evolution: Descent with Modification

Evolution is the change of traits in a population of organisms across multiple generations (over time).

2. Natural Selection

For natural selection to occur the following must be met. -Variation of traits -Reproduction of offspring -Selective pressure that causes differential survival and reproduction.

Directional Selection

Form of natural selection in which the entire curve moves; occurs when individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve

6. Genetic Drift

Genetic drift is random events that drastically alters the genetic diversity of the population. These change events are especially pronounced in smaller populations of organisms. There are two main types of genetic drift: -Bottleneck effect -Founder effect

Smaller vs. Large Populations

Genetic drift reduces the genetic variation in a population by reducing the population size. -Smaller populations are more vulnerable to going extinct because the population is less able to "adapt" in the face of changing environmental conditions due to the population having less genetic variation compared to a larger population.

The four ecomorphs are...

Grass Bush Trunk-Ground Twig Trunk-Crown Each ecomorph is comprised of different species that live on different islands.

Evidence #3: Comparative Structures

Homologous structures are similar characteristics observed across species due to shared common ancestry. Vestigial structures are remnants of organs or structures that had a function in an earlier ancestor, however it is usually no longer used in today's current organism. Similarities observed across eukaryotic cells such as organelles and linear chromosomes are additional evidence of common ancestry.

Postzygotic Barriers: Hybrid Infertility

Hybrid offspring are viable, but are infertile and cannot produce their own offspring.

Biological Species Concept

In this species concept, a species is defined as a population of organisms that are able to reproduce a healthy offspring, and this offspring can have more healthy offspring of their own.

Postzygotic Barriers: Hybrid Breakdown

Mating is possible but successive generations are unable to form successful offspring thus cutting off the "lineage".

Measuring Allele Frequency

Measuring the allele frequency helps determine the frequency of the specific allele (A or a) that makes up the gene pool/population. p = Frequency of dominant allele q = Frequency of recessive allele p + q = 1

Mitochondrial DNA and Inheritance

Mitochondria and chloroplasts contain their own DNA (endosymbiotic theory). -Mitochondrial DNA is a small circular chromosome which contains multiple genes that determines multiple traits. -Regarding inheritance, only the mother passes the mitochondria to all their offspring, and not the father.

1. Mutations

Mutations can occur due to errors in DNA replication, or outside factors such as radiation or chemicals, which causes changes in the DNA sequence of an organism's genome. This can result in the formation of new alleles, and if an organism inherits the mutation, they have the potential to pass this allele to subsequent generations.

5. Gene Flow

Organisms immigrate into or emigrate away from a population of organisms. When organisms carry their alleles with them when they move. Movement can cause changes in the allele frequency: -New alleles in a population -Removal of alleles in a population

What are the Phylogenetic Trees and Cladograms based off of?

Phylogenetic trees are based on evidence that reveals evolutionary relationships b/w species. -Ex: Anatomy, molecular biology, fossils, embryology, and more Because phylogenetic trees are hypotheses that can be tested, they are can change with new research!

All of these factors that cause evolution leads to...

SPECIES

Evidence of Evolution includes

Scientists overtime have accumulated a mountain of evidence that supports how life is all related and have evolved over time. Different fields of evidence are: 1. Biogeography 2. Fossils and Transitional Forms 3. Comparative Structures 4. Comparative Embryology 5. Molecular Biology 6. Mitochondrial Inheritance

4. Sexual Selection

Sexual selection is where reproduction of organisms is dependent on who can successfully court the opposite biological sex. Specific traits (such as coloration) enables an organism to potentially court the opposite sex to reproduce.

Phylogenetic Trees & Cladograms Components

Shared ancestral traits represents a characteristic that originated in an ancestor of the group species. This trait is shared among the whole group of interest -Ex: The trait of having a backbone (vertebrae) is a shared trait for all the organisms shown. Derived traits are "new" characteristics observed that differentiates one group compared to the rest. -Ex: Eggs with shells is a derived trait for crocodiles and birds. It's not observed in the other species present on the cladogram. Nodes represent a speciation event AND a shared common ancestral species for certain species shown. -Nodes that are closer to the root of the cladogram are older common ancestors shared across all the species of interest. Some phylogenetic trees will provide a timeline that provides a rough estimate when certain speciation events had likely occurred.

Founder Events

The founder effect occurs when a small group migrates away from the original population. The new group's gene pool is different than the original ancestral population. -Genetic variation is decreased when comparing the ancestral population to the now smaller founding population.

Postzygotic Barriers: Hybrid Inviability

The offspring is unable to survive gestation or live long enough to reproduce.

What is happening to the genetic variation in the population moving away?

There is less genetic variation because there is a smaller population, and the population that has moved will have to learn to adapt to its environment

Why is it bad if there is only a small amount of parent organisms?

There is little genetic variation since there are only alleles and genes from specific parents, and the offspring of the parents or just as susceptible to the same diseases as their parents because they have the same genes, so there will be little evolution. Ex: Purebred dogs- The process of artificial selection makes purebred dogs susceptible to diseases. This occurs because genetic variation has been reduced, causing certain conditions to become more frequent in a population.

What can we measure from the gene pool?

We can measure allele frequency

What are ways evolution could be measured in a population?

We can measure changes in DNA or genotypes of the population

Hardy Weinberg Equilibrium

When a population is not evolving, it is said to be experiencing Hardy Weinberg equilibrium. This means that the allele frequency of the population does not change over generations.

What does the bottleneck and founder effect show as a key difference between large and small populations?

When there is a larger population there is more genetic variation, and when there is a smaller population there is less genetic variation

Speciation

an event when a population of organisms become two new distinct species. Gene flow stops between these two populations, becoming two new species.

Determining Relative Age of Fossils

can be determined by comparing the depth of where the fossils were found and then comparing this to one another. Older fossils are found in deeper rock layers, while younger fossils are found closer to the surface.

Determining Absolute Age of Fossils

can be determined through radioactive dating. This is done by measuring radioactive elements and their "decay" into a more stable form (parent to daughter isotope). For example, scientists measure carbon-14's decay into nitrogen-14. Carbon-14's half life is 5730 years

Sympatric Speciation

ccurs without physical separation between two populations. The two populations diverge for various of reasons, one of which may be organism's preference to use specific ecological niches. An example is with apple maggot flies. One group prefers to lay eggs on hawthorne fruits, the other group lays eggs on apples. They are partially reproductively isolated.

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

Stabilizing Selection

form of natural selection in which individuals near the center of a distribution curve have higher fitness than individuals at either end of the curve

Homeobox Genes (Hox Genes)

genes that are highly conserved across species (gene is found across species). They are found in all animals because they control the development of the body plan of animals. The Hox genes that controls the body development plan of flies are observed in human beings, as well as many other species!

Trunk-Ground Species

long legs for speed found in: Hispaniola, Cuba, Puerto Rico, Jamaica

Grass Bush Species

long tails for stability on leaves found in: Cuba, Hispaniola, and Puerto Rico

What do you think will happen to the allele frequency in a population that is unaffected by any mechanisms of evolution?

nothing, all the alleles will stay the same

Prezygotic Barriers: Temporal Isolation

occurs when breeding occurs at different times, thus preventing different species from producing offspring.

Prezygotic Barriers: Mechanical Isolation

occurs when differences in the reproductive anatomy prevent members of different species from mating and producing offspring.

Prezygotic Barriers: Behavioral Isolation

occurs when individuals reject or fail to recognize mating behaviors of other species, thus preventing different species from producing offspring

Prezygotic Barriers: Habitat Isolation

occurs when there is physical separation between two populations, thus preventing different species from producing offspring

Allopatric Speciation

occurs when two populations become physically separated. The populations evolve to different environment pressures until eventually they become so different that they are no longer able to reproduce with one another.

Evidence #2: Fossils

remnants of animals that have been preserved in various methods. They provides evidence that life changes over time, some fossils are especially important because they reveal a transitional form The fossil record is the remnants of organisms captured in sedimentary rock and it provides us insight into how life changed over time. It also provides insight into the patterns of evolution (did it occur slowly or quickly). Punctuated equilibrium and Gradualism

Twig Species

shorter legs which allows for them to balance on smaller branches found in: Hispaniola, Cuba, Puerto Rico, Jamaica

Evidence #1: Biogeography

the study of where organisms live today and in the past. This helps us observe how evolution occurred in ancestral species, leading to descendant species found today.

Trunk- Crown

tiny hairs on feet; stick to leaves better found in: Hispaniola, Cuba, Puerto Rico, Jamaica

Gradualism

when evolution occurs evolution occurs slowly over hundreds of thousands or millions of years. A gradual change in life is observed in the fossil record

Punctuated equilibrium

when evolution occurs rapidly after a long period of stasis (stasis = no change). Sudden changes in life is observed in the fossil record