Microevolution

BIO1131 - Delcourt

Micro- vs Macro- evolution

Microevolution: a change in allele frequency in a population or species across generations

→ focus on variation within populations and evolution over short times

Macroevolution: evolution above the species level

→ focuses on variation among species, like origin of new species, over long times

→ result of microevolution at large

Microevolution is caused by 4 processes

1. Mutation

2. Migration (w gene flow)

3. Genetic Drift

4. Natural Selection

Mating is often non-random; why?

• Relatives may mate more or less often than expected (inbreeding or outbreeding)

• Individuals can self-fertilize more/less than expected

• Individuals may mate more often w others that are more or less similar to them (assortive mating)

Positive vs Negative Assortive Mating

+ve: mating between individuals w similar traits

-ve (disassortive mating): mating between individuals w dissimilar/different traits

non-random mating affects _________ frequencies because with positive assortive mating we see more __________

genotype, homozygotes

Non-random mating vs Sexual Selection

• Sexual selection is when mates are chosen based on traits that ⇑ reproductive success

• Non-random mating is where mates are chosen based similiarities/dissimilarities in traits or their genetic relatedness

Is random mating a mechanism of evolution? Why?

NO! Although genotype frequencies are affected, allele frequencies are not. The assumption is that all individuals within a pop. have equal chance to reproduce and contribute to the next generation, so there is no selection.

Inbreeding increases the frequency of _____________ and a decreases that of _____________ across the whole _________ (ie all ______). This is a deviation from:

homozygotes, heterozygotes, GENOME, loci

⇨ this is a deviation from HW expected genome frequencies

Inbreeding depression can worsen the loss of __________ ____________ (allele diversity) that happens due to _______ _______. It gets worse and worse because as the population shrinks, it results in even _____ inbreeding, creating an _______ _______.

genetic variation, genetic drift, more, extinction vortex

What causes inbreeding depression?

2 hypothesises:

• The DOMINANCE hypothesis

• HETEROZYGOTE advantage

Dominance Hypothesis

Selection cannot efficiently remove a harmful, recessive allele bc its effect is masked in heterozygotes. There is a tendency for deleterious alleles to be partially to completely recessive. If homozygosity increases (result of inbreeding), these alleles will be expressed in the phenotype and lower fitness.

Heterozygote Advantage

At some loci, heterozygotes have higher fitness than either homozygote. Natural selection will thus favour heterozygotes and keep both alleles in the population. If inbreeding occurs, heterozygosity will decrease and fitness will thus decrease too.

Outbreeding (aka inbreeding avoidance) + (7) examples

Many organisms have evolved traits that reduce the likelihood of inbreeding

Examples

• kin recognition

• dispersal

• delayed maturation

• extra-pair copulations

• self-incompatibility

• separation of reproductive organs

• having both sexual organs

Heterosis

the heterozygote advantage for more or less the entire genome — a phenomenon that occurs when outbreeding causes an ⇑ in fitness

Mutation are a change in the genetic info of an organism’s DNA; how do they arise?

ERRORS: during DNA replication, crossing over (we do this for crispr), repairing DNA damage

🩻 can be induced by environmental or chemical mutagens

Since mutations can create new alleles, they are the:

ultimate source of genetic variation!

The effect of a mutation is random — it can have 3 types of effects

1. Deleterious (frequent)

2. Neutral (relatively frequent)

3. Beneficial (rare)

Distribution of Fitness Effects

• most new neutral mutations are lost via drift, but a few can drift to fixation

• the combo of mutation + drift can maintain substantial genetic variation in a population

DONT ASSUME that deleterious = not in a population / or that beneficial = everywhere in the pop.

two types of mutation

• Small scale (point)

  • substitution: single nucleotide is replaced (can be synonymous/silent)

  • insertion/deletion: nucleotides are added/removed (alters the reading frame)

• Large scale

  • translocations, inversions, chromosomal loss, duplications…

  • these are mutations in the chromosome # or chromosomal structure

Mutation rate is the # of mutational events for a given target / per unit time, what are some units of time? (3)

• replication event

• per sexual generation

• per cell division

Migration is a mechanism of evolution via ______ ______

gene flow

Gene Flow

the movement of alleles between populations

⇨ occurs via migration of individuals or gametes

Gene flow is similar to mutations in that it:

introduces and removes alleles from a given population

Gene flow has a larger effect on allele frequency that mutation because:

migration rates are >> mutation rates

What does gene flow end up doing?

homogenizes populations, reducing genetic differences between populations. It impedes local adaptation

A lack of gene flow will allow populations to ________

diverge

Gene flow can be harmful for a population by constantly introducing ____________ alleles from other ___________. Or, it can be beneficial for a population by __________ _________ alleles

maladaptive, populations, spreading, beneficial

Explain the example of atlantic salmon and how gene flow affects them

🎣On aquaculture farms, farmed salmon have low genetic diversity (inbreeding, etc)… the environment on these farms is very different from the wild.

🐟 When farmed salmon escape, the fugitives breed with local salmon populations (gene flow) and introduce maladaptive alleles into these populations

Genetic Drift is the process where _________ changes occur in ________ frequencies across _____________

random, allele, generations

Genetic drift is caused by ________ _________, and its effect can overwhelm _________ in smaller populations.

sampling error, selection

Without _______ ______, genetic drift will cause populations to:

gene flow, diverge from one another!

Population Botllenecks

a severe and rapid decrease in population size that reduces genetic variation, increases genetic drift

ex: a founder event, human activities, disease

Natural selection arises from a variation in relative fitness — what the 4 components to fitness?

1. Viability Selection: arises probability of survival to reproduction

2. Sexual Selection: arises from variation in reproduct. success

3. Fecundity Selection: variation in gametes (ex: sperm)

4. Gametic Selection: variation in survival of haploid gametes (probability they can successfully fertilize/fuze)

Forms of Selection (3)

1) Directional (favours one extreme)

2) Stabilizing (favours the “medium”, disfavours extremes)

3) Disruptive (favours both extremes)

Correlated Traits

traits can be often correlated due to:

• pleiotropy (gene w an effect on 2 diff traits)

• physical linkage

• statistical linkage

Since traits (ex depth and length of beaks) can be correlated, this means:

indirect selection can occur — traits can evolve despite not being under direct selection

you CANNOT ASSUME that bc a trait changed it must have been a target of selection, or that selection favoured a change in that direction. Why?

traits can be correlated!! ex: the allele that causes sickle cell anemia is harmful when homozygous, but advantageous when heterozygous!

Frequency-Dependent Selection

a kind of selection where the fitness of a phenotype depends on said phenotype’s frequency in that population

Frequency-Dependent selection is considered __________ selection, and can be +ve (phenotype being _______ is advantageous) or -ve (phenotype being ______ is advantageous).

directional, common, rare

Balancing selection is a form of selection that occurs in __________ frequency-dependance, it actively:

NEGATIVE, maintains genetic variation