Allele Frequencies and Evolution

Unit 5 Bio1107 Dees (FINAL)

Hardy - Weinberg Principal

theory that suggests a populations allele and genotype frequencies will remain constant from generation to generation, limited to very specific conditions

• can be used to tell if a population is evolving, if expected rates are different then presented ones

Conditions for Hardy - Weinberg Principal

1. must be autosomal

2. must sexually reproduce

3. only 2 alleles for the gene

4. must be a diploid organism

Conditions for Hardy - Weinberg Equilibrium

1. no mutations

2. no random mating (pair by chance, not choice)

3. no trait advantages

4. large population

5. no migration

NEVER ASSUME A POPULATION IS IN EQUILIBRIUM

Hardy - Weinberg Equation for Allele Frequency

p + q = 1

Hardy - Weinberg Equation for Genotype Frequency

p^² + 2pq + q² = 1

Genotype

The genetic combination of alleles within an individual expressing a trait

Phenotype

The physical manifestation of the allele combinations, usually influenced by environmental factors

Evolution

defined by the changes in allele frequencies in a population over time

When a mechanism of Hardy - Weinberg Equation is violated, it is stated a population is evolving

true

Processes that influence evolution

1. mutation

2. non-random mating

3. selection for trait advantages

4. genetic drift

5. gene flow

mutation in evolution

Hardy - Weinberg Assumption : no new alleles appear

Violated : mutations occur in alleles

Why evolution: new alleles → new genetic variation spreading

nonrandom mating in evolution

Hardy - Weinberg Assumption : individuals randomly mate

Violated : mates are chosen due to traits or proximity

Why evolution: allele combinations change because some individuals mate more than others

selection in evolution

Hardy - Weinberg Assumption : all individuals survive/reproduce equal

Violated : some traits help survival or reproduction (natural selection)

Why evolution: alleles with advantages increase over generations

genetic drift in mutation

Hardy - Weinberg Assumption : population is indefinitely large Violated : small populations experience random changes in allele frequencies

Why evolution: chance events make some alleles more or less common

gene flow in mutation

Hardy - Weinberg Assumption : no one enters or leaves pop.

Violated : immigration/migration occurs

Why evolution: new alleles enter or leave pop.

original source of genetic variation

mutation

the impact of evolution is generally small unless its couples with another genetic evolution mechanism

true

gene flow

the transfer of genetic mutation from one population to another through movement of individuals or gametes (pollen movement, migration), interbreeding with new populations

genetic drift

any changes in allele frequency due to a random event (gamete sampling, natural disasters)

types of genetic drift

1. founder effect

2. bottleneck effect

the founder effect

occurs when a small group of individuals becomes isolated from a larger population (immigration) , leading to reduced genetic diversity and different allele frequencies in the new population b/c of founders.

examples

1. South Africa having higher Huntingtons disease frequency b/c of founders

the bottleneck effect

a large reduction in population due to a catastrophic event that leads to a significant loss of genetic variation and alters allele frequencies in the surviving population. Only the allele frequency of the surviving population will be passed onto next generation.

selection

certain traits that are more likely to survive are favored and passed on to future generations

ex. individuals that choose mates with preferred traits

types of selection

1. sexual selection

2. artificial selection

3. natural selection

sexual selection

individual chooses a mate based on traits such as size, feather color, or song

ex. female humans choose mate based on height, birds choose mate based on bright feather color

artificial selection

selection caused by humans that is done by selective breeding

• beneficial to humans b/c used for crop production

• humans intently breed organisms to encourage the heritability of desired traits

• ex. corn, bananas, size of farmed chickens

natural selection

traits will naturally disappear when they are less advantageous because the more likely you are to survive the more likely you are to reproduce

• Charles Darwin and his birds

biological fitness

the ability for an individual to have a high number of fertile offspring

• can have large amount of offspring, but only thing that matters is how many are able to reproduce

gene pool

all of the individual alleles for all the genes of a species within a population

gene flow vs genetic drift

gene flow: movement of alleles between populations via migration

genetic drift: random change in allele frequencies due to chance events in small populations

mutated alleles with higher rates of survival are more frequently seen in populations

true

p

frequency of dominant allele

q

frequency of recessive allele

p²

frequency of homozygous dominant genotype

2pq

frequency of heterozygous genotype

q²

frequency of homozygous recessive genotype

in natural selection, the environment determines the best trait

true

beneficial traits lead to higher fitness

true

conditions needed for natural selection

1. trait variation exists in the population

2. at least some of the genetic variation is heritable

3. resources are limited which fuels competition

4. individuals with beneficial traits survive/reproduce more

trait variation in natural selection

not every individual in a population is identical, differences in size, speed, color, behavior, etc.

heritable variation in natural selection

difference in genetic makeup can be passed from parent to offspring, ex. a bodybuilders muscles are not passed down generations, but a turtles strong jaw is

competition in relation to natural selection

limited food, mates, and shelter lead to not all individuals surviving or reproducing

biological fitness in natural selection

organisms with higher fitness are more likely to survive and reproduce, passing on their traits to the next generation.

directional selection (natural selection)

pattern in natural selection in which one trait provides greater fitness

• population traits move in one direction over time, leading to an increase in that trait within the population.

• ex. cliff swallows that are fatter when going through a food drought are more likely to survive, increasing this trait in future populations

stabilizing selection (natural selection)

a trait that is intermediate will provide a greater fitness for a organism

• population traits move towards the average phenotype, leading to a reduction in the spread of the distribution

• ex. large birth size = more complication, smaller birth size = more complications, so the average birth weight is centered closer to mean

disruptive selection

a pattern where extreme traits provide higher biological fitness to organisms

• create a split, (bimodal) in the distrobution

• ex. in a population of birds, those with very light or very dark plumage have a higher survival rate than those with intermediate plumage.

sexual selection

form of selection that favors organisms with traits that increase their ability to obtain mates, and can sometimes reduce an organism's ability to survive

• elks with bigger antlers are more attractive and more likely to survive/reproduce, beetles with a long horn use as weapon and also attracts mates

asymmetry of sex

the ideas that female invest more energy in offspring that males in many species

• leads females to choose most genetically attractive males because they produce limited offspring (lots of effort put into raising)

• males produce unlimited offspring and mate with any available female

• results in commotion between males

leads to competitions between males to mate

asymmetry of sex

what evolution mechanism increases the number of alleles in a population?

Mutation

inbreeding

an additional form on non-random mating that occurs when closely related individuals breed, leading to increased homozygosity ( decreased heterozygotes) and potential inbreeding depression.

inbreeding depression

the reduced survival and fertility of offspring due to the mating of closely related individuals

• close relatives are more likely to share recessive alleles that can become harmful when they are homozygous

• related individuals are more Lilley to share the same alleles, increasing the probability that offspring will inherit identical copies

does inbreeding itself cause evolution

never

which evolution mechanism leads to adaptations (traits that increase fitness of individuals in specific enviroments)

selection

intrasexual selection

competition within a sex for access to mates, increase traits that enhance strength and body size

• to find a mate, you eliminate the competition by defeating others of the same sex

• ex. male seals fight each other for mates

intersexual selection

choice of mates by one sex based on the other sex, increase traits that enhance attractiveness

• to find a mate, you make yourself prettier

• ex. male peacocks with the prettiest tails are more like to get mates, breed out the “ugly” traits

non-random mating

where mates are selected based on specific traits

• sexual selection

• inbreeding

• assertive mating (positive = similar traits, negative = dissimular traits)

a species with a lower population will be largly impacted by genetic drift

true; because random events can have a greater effect on the allele frequencies in smaller populations, leading to reduced genetic variation.

morphological species concept

A way of defining species based on morphological characteristics, such as shape and size, to differentiate one species from another.

• known to be subjective and unreliable

• developed by Carl Linnaeus in the 1700’s

basically defines a species based on similar traits

how new alleles are introduced into a population

mutation

mechanisms in which new alleles are introduced into a population

gene flow, genetic drift, natural selection

effect of change in allele frequencies in larger populations

different parts of the population change in different ways , more likely to occur if alleles are unevenly distributes (some Elles found more often in certain regions)

traditional definition of species

a group of organisms that can interbreed and produce fertile offspring under natural conditions

biological species concept

defines a species as a group of organisms that interbreed and produce viable and fertile offspring, and are reproductively isolated from other groups

exceptions to biological species concept

asexual organisms (prokaryotes), extinct organisms, and species which can interbreed and produce viable offspring but don’t because of location (polar bear and brown bear considered different).

reproductive isolation

a set of mechanisms, behaviors, or physiological processes that prevent different species from inbreeding and producing fertile offspring. as a result, the groups stop exchanging allies with each other and begin to diverge

• key factor is speciation: the production of new species

mechanisms that result in reproductive isolation

reproductive barriers

pre-zygotic barriers

barriers that occur before fertilization, preventing mating or fertilization between species

types of pre-zygotic barriers

geographic isolation, temporal isolation, ecological isolation, behavioral isolation, mechanical isolation, and gametic isolation

geographic isolation

organisms from different populations are separated from each other by distance or physical barrier

ex. finches on the Galapagos island are separated by too much water to fly over

temporal isolation

different populations are unable to interbreed because they reproduce at different times

ex. some cicadas emerge every 17 years to mate, while others emerge after 13 years so their reproductive season overlaps every 221 years

ex. certain fruit flies mate in the morning and some only mate at night

ecological isolation

organisms are isolated due to the environments in which they prefer to live in,

ex. some squirrels prefer open grasslands and others prefer dense forests, resulting in limited interactions.

behavioral isolation

when the presence or bases of specific behavior prevents reproduction

ex. different firefly species use different light patterns to attract mates, so if they will not breed if they don’t recognize the light pattern of their species.

ex. bird species are attracted to the songs used in mating rituals, but only for their own species

mechanical isolation

occurs when different populations are physically unable to interbreed with each other.

ex. most common setback it incompatible reproductive organs in animals, but plant populations can use a different pollinator to transfer pollen

gametic isolation

happens when gametes from different populations are incompatible.

ex. many marine organisms release their gametes into the water for fertilization → eggs and sperm are recognized by surface proteins → if not present, fertilization cannot occur between certain egg and sperm.

postzyotic reproductive barriers

are mechanisms that prevent the development of viable, fertile offspring after fertilization occurs.

types of postzyotic reproductive barriers

1. hybrid inviability

2. hybrid sterility

hybrid inviability

different populations can mate with successful fertilization, but the embryos are unable to develop or do not survive.

hybrid sterility

inbreeding between different populations produces viable offspring, but they are unable to reproduce

critics of biological species concept suggest criteria such as .. should be used to determine species

• ecological (group of organisms adapted due to certain environmental resources)

• phylogenetic (group of organisms that share a history due to a decent from a common ancestor)

Speciation

2 or more species forming from a single ancestral species during a long period of reproductive isolation

hypothesized evolutionary relationships between species are displayed in diagrams known as…

phylogenetic trees

systematics

the investigation of evolutionary relationships and the classification based on those relationships

Phylogenetic Tree

bracing diagram that describes evolutionary relationship between a select group of organisms called taxa (singular taxon)

phylogenetic trees contain only hypotheses

true

the first phylogenetic trees were contrasted using observable traits, while more recent ones use

genetic information

nodes

branching points on phylogenetic trees that represent the most common ancestor

branches

lines on a phylogenetic trees that represent evolving lineages

synapomorphies

traits that are present in lineages and decent from a node or a trait, shared and derived characteristics that are unique to a group of organisms and their common ancestor

monophyletic groups

consists of a common ancestor and all of the lineages decent from that ancestor, single common ancestor for all its descendants that form a distinctive evolutionary unit.

non-monophyletic groups

do not include all of the linages defended from a common ancestor in a phylogenetic tree

• Many of the ways in which organisms have traditionally been categorized are not monophyletic groups and do not reflect evolutionary history

• ex. warm-blooded animals are grouped together, but only due to their shared trait of regulating body temperature

monophyletic groups are frequently associated with

synapomorphies (traits present in lineages that depend from a node), providing evidence for evolutionary relationships.

relatedness

how recently a species split from a common ancestor

more related : more recent common ancestor

less related : less recent common ancestor

parsimony

thee concept that the most simplest explanations are most likely to be true

parsimony in phylogenetics

evolutionary relationships are determined by generating many possible trees based on DNA sequences/traits and choosing the simplest one that requires the fewest evolutionary changes.

today, parsimony is the most commonly used construction methods for phylogenetic trees

false

homologous traits

traits that are all inherited by a group with a COMMON ANCESTOR

analogous traits

different organisms independently evolve to have similar traits that have the same function instead of from a common ancestor

• common ancestor that they share DOESN’T have the trait

ex. ducks and platypus both have webbed feet and lay eggs, but they are traits that they evolved to have over time

convergent evolution

when different organisms independently evolve similar traits that have the same function rather than inherit those traits from a common ancestor. traits are said to be analogous'.

where does speciation occur

at nodes, they show divergence from a common ancestor in a phylogenetic tree.