Chapter 21 - Evolution

What are the five conditions required for a population to remain in Hardy-Weinberg equilibrium?

1. No mutations 2. Random Mating 3. No natural selection 4. Extremely large population size 5. No gene flow

Summarize the section listing four reasons why natural selection cannot fashion perfect organisms. (at least 4 sentences)

1. Selection can only act on existing variations. 2. Evolution is limited by historical constraints, like the basic body shape of the animal. 3. Adaptions are often compromises, beneficial adaptions often cause issues in other areas. 4. Chance, natural selection, and the environment interact. Basically genetic drift can screw a population over.

As a preview of what you'll learn in this reading, what are the three causes of microevolution?

1. natural selection, 2. genetic drift 3. gene flow

Why is nucleotide variation not necessarily translatable into phenotypic variation?

A lot of nucleotide variation happens in introns.

Describe some of the chromosomal changes (various types of polyploidy) that can cause sympatric speciation in plants.

Autopolyploidy is when an individual has more than two chromosome sets that are all derived from a single species, cause cell division error in gametes. Animals would usually die or be left sterile, but plants can just self pollenize, creating a new species. Allopolyploidy is when two different plant species produce a hybrid. Usually they are sterile because the set of chromosomes from one species cannot pair during meiosis with the set of chromosomes from the other species. However, many plants can reproduce asexually, which allows for a few generations of them to survive until mutations cause a fertile hybrid to form.

What is microevolution?

Change in allele frequencies in a population over generations.

What is genetic variation?

Differences among individuals in the composition of their genes or other DNA segments

Explain how each of the following maintain genetic variation in populations: Diploidy Balancing selection Heterozygote advantage Frequency-dependent selection

Diploidy: Having chromosomes in pairs Balancing selection: It's when natural selection maintains two or more forms of an allele in a population. Heterozygote advantage: It's when heterozygotes have a selective advantage. Frequency-dependent selection: It's when the fitness of a phenotype depends on how common it is in the population.

What's the difference between genetic variation that's "either-or", and variation that's gradual (or graded)? Think about human beings, and list a few examples of each.

Either-or is when the traits are like mutually exclusive, usually they are only determined by one gene locus, an example is the color of Mendel's pea plants, or eye color. Gradual traits are usually controlled by a few genes, and they are a whole continuum, like height.

What's the evidence for the allopatric speciation model?

Genetic studies of similar organisms across barriers often show them to be related.

Write a few sentences clarifying the differences between directional, disruptive, and stabilizing selection.

Imagine a range of phenotypes, like white fur gradually becoming darker until you get black fur. Directional selection is when one phenotype at the extreme end of the range is selected for. Stabilizing selection is when phenotypes in the middle of the range are selected for. Disruptive selection is when both extremes are selected for, but never the middle ground.

Describe natural selection in terms of allele frequencies.

It increases the frequency of alleles that confer reproductive advantage.

With reference to figure 21.17, explain why sickle cell disease is in such high frequency in certain African populations. Reference Genetics here!

It's a simply case, malaria is super deadly, and having heterozygosity for sickle provides a nice balance between the malaria resistance and the shortness of breath.

What's the Hardy-Weinberg equation? Write and annotate both equations!

It's an equation that describes a non-evolving population.

What is gene flow, and explain its possible effects upon a population's adaptiveness. Incorporate the study of Paris major into your answer.

It's the movement of alleles from one population to another, cause migration. With Paris major, two populations of the birds on an island were getting gene flow from mainland birds that would sometimes fly in. The mainland genes were actually pretty bad on the island, but the central population of birds kept getting so much mainland gene flow, that the genes kept on existing, and lowering the poor birds survival rates. In the eastern population their was less gene flow from the mainland, so their survival rates were much higher.

How can natural selection affect allele frequencies?

It's very nature is such that it can easily encourage an allele becoming more or less common, depending on what effect it has on survival.

How does allopatric speciation work?

It's when a population is split up geographically, thus interrupting gene flow, leading to the two populations diverging.

Define genetic drift, and use the case study of the greater prairie chicken to describe why it's usually bad for a population's long term survival prospects.

It's when chance events cause allele frequencies to fluctuate unpredictably from one generation to the next, like if all non red heads just got hit by lightning randomly, so now all humans are red heads. The greater prairie chicken in Illinois started to die out due to prairie being turned into farmland, the survivors existed basically by chance of being in the right spot. This population was now far more genetically homogenous, just by chance, and the inbreeding caused fertility to drop wildly.

What is sympatric speciation?

It's when speciation occurs without a physical barrier. It can occur due to things like polyploidy or sexual selection.

Compare microevolution to macroevolution? What different topics do they cover? How are they related?

Microevolution is just change in allele frequency. Macroevolution is evolution on the species level. Eventually microevolution will lead to macroevolution.

List some examples of variation that aren't genetic.

Musculature, weight, etc.

Describe the following sources of genetic variation Mutation at the molecular level Chromosomal mutations Rapid reproduction Sexual reproduction

Mutation at the molecular level: A change in nucleotide sequence. Chromosomal mutations: Changes in chunks of chromosomes, like duplication of chromosome segments. Rapid reproduction: Viruses and prokaryotes reproduce stupid fast, so mutations happen fast. Sexual reproduction: Any sexual reproduction creates an organism with a novel set of genes.

How can one species change over time into another species?

Natural selection leading to a species's whole population changing over time, until it has become a new species.

Summarize the section about "the key role of natural selection in adaptive evolution."

Natural selection leads directly to adaptive evolution, populations become more adapted to their environment because certain traits are selected for and certain traits are selected against.

Define Population Gene pool Fixed allele Allele frequency

Population: A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring. Gene pool: All copies of every type of allele at every locus in all members of the population. Fixed allele: It's when only one allele exists for a particular locus in a population, and all individuals are homozygous for it. Allele frequency: The proportion of how much an allele exists in a population.

What keeps closely related species (like mallard ducks and pintail ducks) from breeding with one another?

Prezygotic barriers and postzygotic barriers.

Use the text and figure 22.3 to compare and contrast prezygotic and postzygotic isolating mechanisms (going into some depth on the various types of reproductive barriers associated with each one). You should be able to list five prezygotic barriers and three post zygotic barriers from memory.

Prezygotic barriers happen before mating. 1. Habitat isolation 2. Temporal isolation, Ei, one is nocturnal on is diurnal. 3. Behavioral isolation, like different mating dances, 4. Mechanical isolation, different body shapes making sex physically hard 5. Genetic isolation, sperm and eggs cross species not being able to fertilize. Postzygotic barriers happen after mating 1. Reduced hybrid viability 2. Reduced hybrid fertility 3. Hybrid breakdown, where the first generation is alright, but later generations of hybrids are weak or sterile.

Contrast gradualism with punctuated equilibrium. What's the evidence for each model?

Punctuated equilibrium: This is when new species seem to branch off rapidly, and then change little until another speciation. The fossil record shows a bunch of species that seem to change little for millions of years, before suddenly undergoing extinction. Gradualism: This is just the gradual change you would expect, species slowly diverge from each other over time. Trilobites for example seem to have changed gradually over time.

What are hybrid zones?

Regions in which members of different species lacking complete barriers to reproduction meet and mate, producing hybrid offspring

What are the three possible outcomes of hybrid zones?

Reinforcement: This is when the hybrids have lower fitness, leading to the strengthening of reproductive barriers between species. Fusion: This is when gene flow continues to occur through the hybrid zone leading to reverse speciation as the two population's gene pools become more similar, and they become one species again. Stability: This is when the zones are stable, not much gene flow occurs between species, but the reproductive barriers don't strengthen.

How can gene variation be quantified.

The average percentage of loci that are heterogynous.

Describe the bottleneck effect and the founder effect.

The bottleneck effect happens when some event drastically reduces population, like natural disasters, leaving and by chance the survivors are similar genetically. Even when the population rebounds the genetic diversity will stay low. The founder effect is when a small group becomes isolated from a large group, the small group often has smaller genetic diversity.

What is relative fitness?

The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

Describe some of the genetic changes that underlie speciation?

The genetic changes can be quite varied, one gene or many. For example, certain Japanese snails have one gene that controls how their genes spiral, and different spiral directions prevent mating. Sometimes many genes cause speciation, such several gene loci changing that lead to both pre and post zygotic isolation. It all depends, but what it boils down to is that the genetic changes must cause reproductive isolation in some capacity.

Use the description of the Grants' study of the medium ground finch to explain why evolution is a population (as opposed to an individual) phenomenon.

The medium ground finches had to feed on hard seeds during a drought. The next generation of finches had more individuals with larger beaks, to eat the seeds better. But the individuals of the older generation never changed beak size, its just that large beaked individuals were more likely to reproduce.

What are some of the limits of the biological species concept? Describe why the morphological, ecological, and phylogenetic species concepts are useful additions to the biological species concepts.

The obvious limitation is that sometimes two species can produce viable hybrids. Species are messy. morphological species are just species grouped by physical traits. Ecological species re species grouped by niche. Phylogenetic species are defined as the smallest group of individuals that share a common ancestor.

What are reproductive barriers? Describe some cases where these barriers break down.

They are barriers that prevent hybrids from happening, like different mating dances, or different amounts of chromosomes. Polar bears and Grizzly bears are breeding more as their habitats begin to overlap, this is an example of barriers breaking down.

How does one species split into two descendant species?

Usually, because a species gets geographically separated somehow, or factors like disruptive selection.

How can habitat differentiation and sexual selection lead to sympatric speciation?

With habitat differentiation, sometimes a subpopulation ends up finding a new niche to be exploited. As they become more adapted to the new niche reproductive barriers start to form between them and the old population, such as genes that are beneficial in the new niche but harmful in the old one reducing hybrid fitness. Thus, speciation happens. Sexual selection can lead to speciation because it effectively acts as a prezygotic barrier. If some female fish only mate with red individuals, and some only mate with blue individuals, then speciation is gonna end up happening. Two populations will effectively form with little gene flow occurring between them.

What is a species?

a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.

What is the biological species concept?

species are groups of populations that can only breed with themselves.

What is speciation?

the process by which one species splits into two or more species