Unit 3 Learning Goals and Outcomes

Differentiate between different sources of diversity among individuals in a population, including heritable variation and environmentally-induced variation due to phenotypic plasticity

Heritable variation: based on organism's genes

Heritability: Proportion of observed differences in a trait among individuals of a population due to genetic differences

Environmentally-induced variation: variation that results from environmental influences

Identify variation that is of evolutionary significance

Variation provides options for natural selection to "choose" that is best for the population as a whole. That is why genetic variation is essential to natural selection.

Define biological evolution with respect to allele frequencies

Evolution is defined as any change in the frequency of alleles within a gene pool of a population from one generation to the next

Calculate allele frequencies given genotype frequencies or number of individuals with each genotype

p= freq of dominant allele

q= freq of recessive allele

p^2= % of homozygous dominant individuals

q^2= % of homozygous recessive individuals

2pq= % of heterozygous individuals

Explain (in your own words) the predictions of the Hardy-Weinberg Principle

This predicts how gene frequencies will be transmitted from generation to generation given a specific set of assumptions; if an infinitely large, random mating population is free from outside evolutionary forces, then the gene frequencies will not change over time and the frequencies in the next generation will be p^2 for the AA genotype, 2pq for the Aa genotype and q^2 for the aa genotype. In other words, a population is not evolving if in HW equilibrium.

List and restate (in your own words) the five assumptions/conditions of the Hardy-Weinberg principle.

1. Large population

2. No mutations

3. Random mating

4. No immigration/emigration

5. No natural selection

Predict allele and genotype frequencies of rare genetic disorders in a population from phenotypic data alone, ASSUMING that the population is in Hardy- Weinberg Equilibrium, and understand the limitations of your estimates

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Calculate the expected frequencies of offspring of particular genotypes or phenotypes expected in the next generation if the population is in hardy weinberg equilibrium given allele or genotype frequencies in the current generation

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Understand in what sense the Hardy-Weinberg equation represents the prediction of the null hypothesis of biological evolution.

The null hypothesis that is being tested is whether the observed and expected values are not significantly different from one another. Reject null hypothesis if chi square in <0.05

List the four processes that change allele frequencies and the five that change genotype frequencies in populations through time.

Change genotype Frequencies:

Mutation

Genetic Drift

Gene Flow

Natural Selection

Assortative (Nonrandom) mating

Process that does not alter allele frequencies

Assortative mating

Restate (in your own words) what it means for an allele to be fixed in a population or lost from a population

Allele Fixation: Before an event, there are at least two variants of an allele. After an event, such as genetic drift, only one of those variants remain as they had been favored.

Relate allele fixation to genetic diversity (e.g., what is the effect of fixation on genetic diversity?).

No genetic diversity

When an allele becomes fixed, genetic diversity decreases because only one of the many variants remain in the end.

Identify processes that can cause alleles to be fixed or lost and re-introduced.

Genetic Drift- allele frequencies shift randomly and sometimes may disappear/reappear in populations, so variatiants of a certain allele may disappear completely, leaving at the most one variant in the population. The lost alleles can always come back. Genetic drift is simply due to chance

Describe the concept of "random sampling of alleles" in genetic drift making specific reference to the parental gene pool and offspring genotypes.

When two individuals of particular genotypes mate and produce offspring, which alleles the offspring inherit are simply due to chance, thus being a process of "random sampling of alleles" since that random chance determines the next generation's gene pool

Understand how genetic drift can cause alleles to become more or less common or fixed in populations

Individuals with a certain allele leave/enter a population take/bring certain alleles with them, changing allele frequencies. That said, they can become more or less common, depending on how great of an impact genetic drift has on a population (whether it be small/large population size of how many individuals are migrating). With the help of genetic drift, allele fixation may occur as well because variants may leave/enter.

Compare and contrast the causes and consequences of the "founder effect" and population bottlenecks.

Founder Effect: Only a small numbers of individuals from the original large population made it to a new habitat. Whatever alleles these individuals were carrying, by chance, will be the ones constituting the gene pool of the new population.

Bottleneck Effect: A sudden reduction in population size that throws genetic diversity out of whack

Define gene flow and relate it to migration between populations

Gene flow: the process of individuals, of the same species, coming from different populations into new ones. When they mate with members of that population, they present new alleles, altering allele frequencies

Explain how gene flow influences effective population size, allele frequencies, and genetic divergence between populations living in different regions

Gene flow determines gene pool (the stock of different genes in an interbreeding population)

Effective Population Size: The number of individuals in a population that contribute offspring to the next generation. When gene flow is present, more individuals with new alleles become part of that effective population size, contributing their own alleles to the offspring

Gene flow can introduce new alleles to a population

In order for genetic divergence to occur, where a population speciates (separates into distinct species), gene flow must be absent. To diverge, populations must be isolated from one another, evolve independently, and then become reproductively isolated to be truly diverged

Understand how gene flow influences effective population size, allele frequencies, and genetic divergence between populations living in different regions

Non-random mating (positive and assortative mating) changes genotype frequencies because homozygotes become much more common than heterozygotes. But, because heterozygotes still contribute to allele frequencies, allele frequencies do not change due to non-random mating

Predict how inbreeding will change genotype frequencies, and be able to graphically illustrate why non-random mating will not by itself change allele frequencies

Inbreeding, in a way, is non-random mating. The difference between the two is that inbreeding will most likely change genotype frequencies because, for example, if individuals of a family are predisposed to be carriers of a certain gene, and members of those families breed with each other, they are likely to contribute affected offspring. The affected offspring change genotypic frequencies in a population because then homozygous recessive individuals become more and more common as individuals continue to inbreed.

Justify why inbreeding does not cause evolution directly, yet can speed the rate of evolutionary change

Inbreeding can cause the expression of deleterious alleles, leading to very affected individuals. This greatly lowers the fitness of the individual and its ability to survive. But, inbreeding does not directly cause evolution because only a proportion of inbreds are affected and selected against. It is in the long run that we see evolutionary change. More inbreeding leads to more affected individuals, leading to natural selection selecting against the affected phenotypes, lowering those deleterious allele frequencies.

Justify why ALL populations will evolve, making reference to assumptions made under the HW principle

ALL natural populations will evolve because...

For reference, here are the 5 conditions required to be in HW equilibrium:

Large breeding population

Random mating

No change in allele frequency due to mutation

No immigration/emigration

No natural selection

#1: some populations are large, some are small. Hardy-Weinberg assumes that an interbreeding population is indefinite because it is not evolving. But, even evolution occurs in large breeding populations because...

#2: assortative mating is much more likely than random mating. Individuals usually choose their mates, not just spontaneously breed (ex. female choice and male-male competition)

#3: Changes in allele frequency is inevitable. Genetic drift is the result of random changes in allele frequency. Mutations are random and spontaneous that cannot be prevented

#4: It is possible to isolate a population, which then leads to speciation, but, sometimes, that is not the case

#5: Environmental pressures WILL favor certain genotypes over the other.

List the four postulates of natural selection

1. Preexisting phenotype variation in the population

2. Variation is heritable (has genetic basis)

3. Differences in reproductive success among individuals

4. Reproductive success depends on phenotypic trait

Discuss the consequences of differential survival and reproduction for variation in a population. (Why is survival of the fittest not capturing the whole story?)

You have to be able to survive to produce offspring

You have to be able to acquire a mate and mate with that organism

Fitness

The reproductive success of an individual with respect to other individuals in the same population

While it is beneficial to be strong, fast, and healthy in order to survive for the longest amount of time, what really matters is one's ability to reproduce. A person with more offspring will have a higher fitness than that of someone with no offspring.

Compare and relate the roles of reproduction and survival in natural selection

Natural selection is the process by which traits that confer higher reproductive success in a particular environmental setting become more common in the next generation

If an offspring receives a trait that is environmentally favorable, they are more likely to survive and reproduce in that same environment than an organism who did not receive said trait.

Identify sexual selection as a sub-category of natural selection that increases reproductive success through mate acquisition

Female choice

Traits that may lower survival rate, but increases the chances of attracting a mate, can be evolved by natural selection

Male-to-male competition

Males compete for the opportunity to mate

Define fitness in the context of natural selection

The reproductive success of an individual with respect to other individuals in the same population

Biological fitness does not mean...

How well an organism fits its environment

Aggression/toughness

Physical fitness

Identify that evolution by natural selection results directly from intraspecific competition between individuals of different genotypes

Intraspecific competition: within the species there is competition.

Cheetahs: faster ones will survive more because they can get more food than the slower ones

Explain why natural selection does not result in evolution of a trait because a population "needed it", but can only operate on pre-existing variation in a population

Mutations themselves are random, and natural selection does not cause them.

Natural selection operates where there is variation in a population. There is no natural selection simply because an environment needed it, it occurs when there is variation that is favorable over other sorts of variation.

Defend the statement that selection is reactive, and not a directed process with foresight

Natural selection only works on the immediate, with individuals sexually selecting for and natural selection favoring individuals with certain phenotypes, without regard to possible future events. For example, in times of drought, a drought-resistant strain of a plant may become dominant, without regards to the possibility of rainy times coming.

Justify why traits/behaviors for the "good of the species" (but at the cost of an individual's fitness) would not be favored by natural selection

Because "survival of the fittest" takes precedence over all. Natural selection favors traits that allow for greater differential reproductive success in a population, and this comes at the cost of traits that could benefit the species

Predict how biotic and abiotic selection pressures result in changes of allele frequencies in a genetically diverse population

Abiotic: Non-living parts of an ecosystem (ex. mountains, rocks, air, dirt) Biotic: Living parts of the ecosystem (ex. plants, animals, bacteria, fungi)

Discuss the causes of heritable variation and the consequences of differential survival and reproduction for variation in a population

Heritable variation allows for possibly beneficial, negative, or even neutral mutations to get filtered during each generation by natural selection--weeding out negative mutations, passing on positive and neutral ones. Without heritable variation, species would quickly fall victim to parasites who take advantage of identical genetic material in a population and evolution would not occur.

Justify why mutation is a random process to introduce alleles, but evolution by natural selection is a nonrandom process that can alter allele frequencies in a population

Mutation happens spontaneously, but evolution is based off phenotype; a healthier phenotype that contributes to survival and higher fitness would be selected for.

Compare and contrast expect changes in allele frequency in a population depending on if that allele is under selection vs. experiencing drift

Selection: based on the phenotype's effect on the organism's fitness/survival; This is not random

Drift: completely random and due to chance

Compare and contrast different modes of natural selection and relate them to differences in fitness of phenotypes and resulting changes in allele frequencies: directional, stabilizing, disruptive

Directional: an increase in a type of phenotype is always beneficial. Not found too often in populations. Happens in smaller populations before stopping.

Stabilizing: an increase in a type of phenotype is only beneficial for a certain amount (ex: beak size) but any bigger/"better" will lead to a lower fitness and so it goes back down b.i.Happens with bigger populations.

Disruptive selection: Individuals of extreme phenotypes are favored, and any intermediate phenotypes are selected against (U-shaped curve)

Explain multiple ways in which a deleterious allele can persist in a population

the carrier does not show deleterious mutations so the carrier still survives -the carrier can then pass the mutation down to the offspring which allows for the mutation to be maintained for long periods of time in a population -almost impossible to eliminate recessive (deleterious) alleles from a population, since they can remain hidden in heterozygous individuals -something really detrimental must occur to the heterozygous state in order to fully eliminate all heterozygotes (and hence any traces of the gene), which is very rare. Inbreeding also allows deleterious alleles to persists in a population

Define a biological species

Populations of interbreeding organisms, reproductively isolated from other such groups

Define reproductive isolation and relate it to gene flow among population

Reproductive Isolation: when members of different populations are unable to mate and reproduce successfully

Isolation occurs when a population has separated long enough, with the absence of gene flow, so that the original population has diverged to the point where descendents cannot reproduce successfully

Explain why gene flow makes speciation by reproductive isolation less likely

If a population were to split into two, but each new population still interacts with each other, it is less likely for those populations to truly speciate since they are not reproductively isolated (the defining characteristic of speciation)

Compare and contrast forms of pre-zygotic and post-zygotic reproductive isolation and be able to give examples of each

Prezygotic isolation - prevents individuals of different species from conceiving an offspring successfully

-Habitat (Wrong Place)

-Temporal (Wrong Time)

-Behavioral (Wrong Pick up line)

-Mechanical (Doesn't fit)

-Gametic Barrier (Gametes can't combine)

Postzygotic Isolation - zygotes (offspring) form, but do not either survive or else have low fitness

-Hybrid Inviability: offspring has low fitness

-Hybrid Infertility: infertile offspring

Contrast allopatric and sympatric speciation

Allopatric Speciation - species that live in different geographic areas (mating is impossible)

Parapatry Isolation: Populations that live in adjoining geographic areas (mating is possible in overlapping areas)

Sympatric Speciation - populations that live in the same geographic area (live in same area, but do not mate)

Define the concept of divergence with respect to two recently isolated populations

Dispersal

Geographic barrier between populations was already there

Vicariance

Geographic barrier splits a population that was already there

Island-Mainland (genetic isolation)

The populations are free to genetically diverge, as there is no gene flow to "standardize" their allele frequencies

Be able to identify how genetic drift and different modes of natural selection can enhance divergence between recently isolated populations

Genetic Drift

-Allele frequencies changing due to chance alone

-The larger the population, the smaller the genetic drift (the smaller the fluctuations of allele frequency)

Modes of Natural Selection

-Directional

New direction in which selection wants to favor

individuals with a particular heritable trait

Decreases variance

-Stabilizing

Most of the individuals already have the

phenotype that maximize fitness for the

particular selective pressures in the

environment

Decreases variance

-Disruptive

Bimodal distribution

Natural selection with two fitness optima

Extremes of the range of variation

Low fitness of individuals in between

Increases variance

Identify why disruptive selection is a conducive mechanism to result in sympatric speciation

Disruptive selection is the result of when two extreme ends of a phenotype spectrum are favored by natural selection. With assortative mating, individual of the same extreme phenotype tend to mate with each other, eventually leading to divergence between both extreme phenotypes.

Explain how secondary traits (such as sexually selected traits) that lead to increased reproductive isolation can increase fitness of individuals among sympatrically diverging populations

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Define nodes and branches

Nodes: refers to a common ancestor

Branches: represents the divergence of species

Explain how we can use traits/characters to group related organisms

The closer the relativity of DNA sequences between two species, the more likely they are related. This also applies to traits and characters. The process of grouping organisms based on these traits is called cladistics

Define a clade and know that clades are nested groupings of organisms, clade within clade, that group organisms by ever more distant common ancestors

Clade: A group of species with the same common ancestor that have diverged via different environmental pressures

Compare and contrast shared derived traits and shared ancestral traits, and know which is used to define a clade

Shared Derived Traits: A character that defines a clade (based on the most recent common ancestor)

Shared Ancestral Character: A character, based on a common ancestor that brings all clades together (the beginning point of a phylogenetic tree)

Understand that any character that is a shared derived character for one clade, can be a shared ancestral character for another clade

Some clades consist of many branches, some may only consist of two. The more branches a clade consists of, the more and more recent common ancestors there are. So, for clades like that, a less recent ancestor (shared ancestral character) can be the most recent ancestor (shared derived character) for clades with only a few branches.

Contrast Monophyletic, Paraphyletic, Polyphyletic groupings

Monophyletic grouping: groupings group all descendants of a common ancestor into a group. (just right)

Polyphyletic groupings: grouping group distantly related species into a unnatural grouping (doesn't belong)

Paraphyletic: groupings missed some species that belong within a clade. (missing some)

Be able to use a set of characters for different species to create a cladogram, using principle of maximum parsimony

Maximum Parsimony means that the phylogenetic tree that minimizes the total number of character-state changes is preferred (the most simplest form possible)

Contrast homologous versus analogous characters, be able to give examples

Analogous Characters: A trait within species that the same function/form, but has originated and evolved independently from the other species

(ex. bat wing v. bird wing)

Homologous Character: A shared derived/ancestral character between species (how clades/phylogenetic trees are made)

Be able to identify a character as homologous versus analogous when presented with a cladogram of a lineage that displays these characters

Homology: characters of the same clade

Analogy: characters of different clades

Explain how convergent evolution can result in analogous traits

When two species of different ancestry undergo similar environmental pressures, the species will evolve in a way that they develop similar traits

Understand how DNA sequences can be used as characters in cladistics analysis

The more similar DNA sequences are (base to base) the closer related the species are

Explain the basic assumptions made in cladistic analyses, what errors can occur, what causes these errors in inferring evolutionary relationships to occur, and how to guard against errors in constructing phylogenies/cladograms

It is difficult, at first, to distinguish the differences between homologous and analogous characters. For example, the bat wing v. the bird wing. At first, some would assume that because they are both flying animals of similar size, they must have a common ancestor. But if one were to look at the x rays of both species's wings, the bone structure is very different, implying that they are, in fact, of no relation and have developed these characters via convergent evolution.

Ways to prevent this error

1. looking at DNA sequences

2. looking closely at anatomy of species