BIOL121 - Midterm 2

evidence that all species share a common ancestor (LUCA):

• All life is made of cells

• Universal flow of biological information - DNA and RNA to amino acids / proteins

• All cellular life uses DNA as a way of storing information

• All life uses ribosomes to make proteins

evidence of evolutionary relatedness (species are related / from a common ancestor):

Homologies - traits present in 2 or more organisms that were inherited from their common ancestor

evidence of evolutionary change through time:

• Extinctions - in the past (fossils) and present

• Transitional features

• Vestigial traits

• Real time evidence of change

transitional feature:

A trait in a fossil species that is intermediate between those of ancestral and derived species

vestigial structures:

Structure that was inherited from an ancestor that now has no function or reduced function

• Form when a lineage experiences a different set of selective pressures than its ancestors

what is a mutation?

Any change in the nucleotide sequence of the DNA

how are mutations caused?

• Mistakes made during DNA replication

• Environmental factors (chemical / radiation exposure)

why are mutations ESSENTIAL to evolution?

Mutations create new alleles which generates genetic / phenotypic variation

• but they are a weak evolutionary mechanism, and need other mechanisms to spread variation

what is fitness?

• A measure of the relative reproductive success of individuals

• Individuals who pass more genes to the next generation (produce more offspring) have a higher fitness

what is gene flow?

The movement of genes into or out of a population

• An organism leaves its home population and moves to a new population where it mates

• An organism does not move (ex. trees) but the gametes move between populations (ex. via wind, water, bees)

what are the ONLY evolutionary mechanisms that can introduce new alleles to a population?

Mutations and gene flow

important implications of gene flow for evolution:

• In the absence of other mechanisms, gene flow can lead to different populations becoming more similar

• If gene flow between populations is restricted or stops, other mechanisms can result in populations diverging from each other, which can lead to speciation

what is genetic drift?

changes in allele frequencies in a population over time due to random differences (chance) in survival and / or reproduction

populations affected by genetic drift?

occurs in all populations that are not infinite in size

• Has a stronger effect on smaller populations

what are the 2 opportunities (events) for genetic drift?

• Bottleneck event

• Founder event

bottleneck event:

Occurs when a population’s size is drastically reduced for at least 1 generation

• Can be due to a catastrophe (wildfire, flood, ect.)

• Can randomly reduce genetic variation in the remaining species

founder event:

Occurs when a new population is started by a few members of the original population in a new environment

• Can likely result in genetic variation in the newly founded population

genetic drift without a reduction in population size:

• Due to random events related to reproduction

• Individuals being in the right place at the right time for reproduction, resulting in more mating opportunities

what is natural selection?

A process that occurs when individuals with a certain phenotype (and genotype) are more likely to survive and reproduce, and therefore pass more alleles to the next generation

outcomes of natural selection:

• An increase in frequency of individuals with the advantageous phenotype / genotype over generations

• Population becomes more well-adapted to the environment over generation, but if the environment changes so can the direction of selection

3 criteria for natural selection to occur:

• There must be variation amongst individuals in the population for some trait

• Variation must be heritable (have a genetic basis)

• Variation in the trait must be associated with differences in fitness

what are the 3 types of natural selection:

• Directional

• Stabilizing

• Disruptive

directional selection:

Favours individuals with a phenotype at one end of the distribution

• Shifts in one direction

• Changes the average value for a trait in a population (increases or decreases)

• Phenotypic / genotypic variation is reduced

stabilizing selection:

Favours individuals with an intermediate phenotype

• Shifts to the middle, selects against extreme phenotypes at both ends of the distribution

• Average value of trait remains the same

• Phenotypic / genotypic variation in reduced

disruptive selection:

Favours individuals at both extremes of the distribution

• Selects against intermediate phenotype

• Average value of traits remains the same

• Phenotypic / genotypic variation is increased

3 requirements must be met for a trait to be an adaptation:

• Heritable

• Functional (has a purpose)

• Positive effect on fitness

3 forms of adaptations:

• Structural

• Physiological

• Behavioural

structural adaptations:

adaptations that change the physical, outward features of an organism / species to increase survival

• ex. gills on fish, webbed feet on ducks, strong legs on frogs

physiological adaptations:

adaptations that change an internal body process to regulate an maintain homeostasis

• ex. temperature regulation, release of toxins / poisons

behavioural adaptations:

adaptations that change the behaviour of an animal in response to an external stimulus

• ex. hibernating in the winter

3 requirements for evolution by sexual selection:

• Must be variation in phenotype amongst individuals in the population

• Variation must be heritable

• Must be differences in fitness associated with differences in phenotype

idea of sexual selection:

• sexually dimorphic traits could be explained by the struggle for mates / mating opportunities

• Certain conspicuous traits that potentially decrease survival could be selected for if they gave an advantage in the struggle reproduction

2 mechanisms of sexual selection:

• Intrasexual selection

• Intersexual selection

intrasexual selection:

• competition between individuals of the same sex for mates

• direct interactions - individuals may physically combat each other

• individuals may evolve ‘weapons’ that give them advantage in a fight

• indirect interactions - males evolve ornaments / behaviours that signal to other males their ‘fighting ability’

intersexual selection:

• Mate choice, what would make a male more enticing to a female than another male

• A male signals they were better at providing direct benefits to a female

• A male signals they would provide better indirect benefits (good genes)

monophyletic groups (clades):

Includes the most recent common ancestor of all the organisms, and all of the descendants of the common ancestor

paraphyletic groups:

includes the common ancestor but not all descendants

polyphyletic groups:

includes decendants but not the common ancestor

synapomorphy:

homologous traits that were inherited from the most recent common ancestor to the group (derived traits)

• Characterizes / distinguishes monophyletic groups

autapomorphy:

A trait unique to one lineage

• helps to distinguish taxa

symplesiomorphy:

A homologous trait inherited from a more distant ancestor

• Does not help distinguish taxa

principle of parsimony:

Parsimony is the assumption that the simplest explanation is most likely to be true

• ex. more parsimonious to assume 4 legs arose in the common ancestor to all tetrapods, and was lost in lineage leading to snakes, compared to arose independently 3 times

homologous traits:

Traits shared by 2 or more taxa because the trait was inherited from a common ancestor

• indicate relatedness

Structural homologies

Inherited from a common ancestor, same structure but could have different functions in different taxa

Developmental homologies

Developmental similarities between organisms during early embryonic stages

Genetic homologies

Genetic similarities between organisms due to common ancestry

analogous traits:

Traits that look the same / have the same function but do not indicate relatedness

• Analogous traits arise due to convergent evolution

• Similar in function, arose due to environmental demand, but arose independently in separate lineages

primitive:

‘more primitive’ implies that a species in its current form has existed for a longer period of time / is more evolved

• Species are all equally evolved

• Species have all been continuing to evolve since they diverged from a common ancestor

how to tell more closely related taxa:

More closely related taxa share a common ancestor more recently

what does it mean if a species is in HWE?

Means evolution is not acting on the gene

If a population is not in HWE, then at least of of these assumptions has been violated:

• The population is infinitely large

• No natural selection

• Random mating - no sexual selection

• No gene flow

• No mutations