Biol 1020 - Lectures 34,35,36: Population Genetics and Speciation

Population

• a group of the same species living in a specific environment/area

• each individual in a population has different combinations of alleles for each gene

  • some genes will only have one allele in the population

  • some genes will have a few alleles in the populatioon

  • some genes will have many alleles in the population

Microevolution

• the change in allelic frequencies in a population over time

• some genes will only have one allele in the population

• some genes will have a few alleles in the population

• some genes will have many alleles in the population

Natural selection works on ______, evolution works on ______

• individuals: beneficial alleles lead too survival and reproduction, passing those alleles to progeny.

• populations: as beneficial alleles get passed on and poor alleles dont, over generations the ratio of beneficial to detrimental alleles shift in the population

What does genetic variation cause?

• changes in phenotypic variation based on changes in genes/DNA sequences

  • nucleotide variability results in a small fraction of phenotypic changes due to the preseence of introns presence between the coding regions of DNA exons

  • point mutations in introns result in neutral variation

Sources of genetic variation

• new alleles arising through mutations

  • can be harmful or masked in the heterozygote

• heritable changes in germline cells, non-heritable in somatic cells

  • errors in meiosis during gametogenesis

• spontaneous mutations through errors in DNA replication

• Induced mutations through exposure to mutagens

• sexual reproduction during crossing over, independent assortment, and random fertilization

How did human globin genes evolve

• duplication events

• a single globin gene underwent duplication and divergeencee

• mutations in the diverged globin genes resulted in two different functioning gens on two different chromosomes

Gene pool

consists of all copies of every type of allele at every locus in all members of a population

fixed alleles

• occur when there is only one allele for a particular locus in a population (ie., all individuals are homozygous for that trait)

Hardy-Weinberg Equilibrium conditions

1. no mutations

2. must have random mating

3. no natural selection

4. population size must be large

5. no gene flow (iee. no migration of individuals in or out of the population)

HW: allelic frequencies

p+q = 1

• determine alleles: a or A

• p = proportion of dominant alleles

• q = proportion of recessive alleles

HW: genotypic frequencies

1 = p² + 2pq + q²

• determine geneotype: AA, Aa, or aa

• p²= proportion of homozygous dominant (AA)

• q²= proportion of homozygous recessive

• 2pq = proportion of heterozygous (Aa)

Genetic drift

the change in frequency of an existing gene variant in the population due to random chance.

Think of a coin flip x10: more likely to get deviation away from 50% heads:tails, ie. 7:3 wouldn’t be surprising

The same thing happens in populations with small numbers of individuals. Just through chance alone the frequency of alleles can shift around. Alleles can even be lost in the population! This is NOT natural selection.

Bottle Neck Effect

• an example of genetic drift

• a sudden change in thee environment that randomly kills a large number of individuals (and the alleles they carry)

  • it is random which individuals die and which survive (has nothing to do with survivability)

• this can drastically alter the population’s allele frequency

• fires, floods, volcanic eruptions, deep freezes, etc, can all be bottlenecking events

Founder effect

• example of genetic drift

• when a group of individuals and the alleles they carry move to a new ara and form a new population

  • original population where they came from still exists

Summary of genetic drift

1. genetic drift is significant in small populations

• chance eventss can cause an allele to be disproportionately over - or underrepresented in the next generation

• smaller population = bigger effect

2. genetic drift can cause allelic frequencies to change at random

• an allelee can increase one year due to drift, then decreasee the next year

• the change year to year is not predictable

3. genetic drift can lead to a loss of genetic variation within populations

• fluctuations over time can cause alleles to be eliminated in populations

• less genetic variation → less evolution (bc natural selection can only act on variation

4. genetic drift can cause harmful alleles to be fixed

• can be fixed at random due to drift

Gene Flow

• Transfer of alleles into (immigration) or out of (emigration) population due to the movement of fertile individuals or their gametes (i.e., plant pollen)

• Reduces genetic differences between populations because alleles are exchanged

Natural Selection

If a specific allele is beneficial, the frequency of that allele goes up in a population over generations

• If a specific allele is harmful, the frequency of that allele goes down in a population over generations

• Let’s pretend a new insect species arrives in the area where these plants grow, and it prefers to eat plants with red flowers

  • Over generations, the CR allele is selected against (because the insects eat those plants before they can make seeds)

  • The CW allele is selected for (because the insects ignore them, so they can produce seeds by crossing with another white flower or a pink flower)

  • What kind of selection is this (HINT think back to the beginning of the course)

Relative fitness

• the contribution an individual maks to the gene pool of the next genereation relative to the contributions of other individuals

Directional selection

• shifts the overall makeup of the population by favouring variants that are at one extreme of the phenotypic distribution

• eg. peppered mothss

Disruptive selection

• favours variants at both ends of the phenotypic extreme

• eg. coho salmon: big and small good for fertilization, medium bad

Stabilizing selection

• removes extreme phenotypic variations from the population and preserves the intermediate types

• eg. birth weight

Sexual selection

• individuals with certain inherited traits are more likely than others to obtain mates

• can result in sexual dimorphism; differences in secondary sexual characteristics between males and females of the same species

Intrasexual selectioon

• selection within the same sex where individuals compete directly for mates of the opposite sex

Intersexual selection

• “mate choice”, individuals in one sex are choosy in selecting mates

Balancing selection

• maintains two or more phenotypic forms in a population

Frequency dependent selection

• the fitness of a phenotype depends on how common it is in the population

Heterozygous advantage

• heterozygous individuals have great relative fitness than either of the homozygous individuals

• natural selection will maintain two or more alleles at that locus

Species definition 1: Biological specieis concept

• a species is a group of populations whos members have the potential to interbreed in nature and produce fertile offspring

  • organisms must reproduce sexually for this definition

  • must be able to see successful reproductiion (ie. doesn’t work with fossiil record)

  • formation of new species depends on reproductive isolation

• can be a bit fuzzy since hybrids can occur

  • usually infertile or weak, and dont surivive except when they do (mules, grolar bears)

Species definition 2: Morphological species concept

• classification of species based upon physical traits such as size, shape, or other features of morphology, etc

  • no observation of breeding needed

  • can use organisms with observation data

  • can be used for the fossil reecord

  • can lead to false classifications

• some species have a lot of morphological variation (ie. dogs)

Species definition 3: Ecological species concept

• views a species in terms of its ecological niche, the sum of how members of the species interact with nonliving and living things within its environment

  • can accomodate asexual and sexual species

  • emphasizes disruptive selection as organisms adapt to differeeent environments

What keeps species separate

• for two species to be separate, a mechanism must be in place that keeps them from breeding and mixing their genes via gene flow

Prezygotic barriers

• prevent mating or fertilization between species

Postzygotic barriers

• operate after hybrid zygotees are formed

Habitat isolation

• two species that occupy different habitats within thee same area may encounter each other rarely, even though they are not separated by obvious physical barriers (eg. mountains)

Prezygotic barrier

example:

the apple maggot fly feeds on and mates on hawthorns and apples

the blueberry maggot fly feeds and mates on blueberries, even if in the same geographic range

Temporal isolation

• species that breed at different times in the day, different seasons, or different years cannot mix their gametes

Prezygotic barrier

eg.

Behavioural isolation

• species have different behaviours that are not preferred by individuals of other species

  • enables mate recognition

  • Prezygotic barrier

Mechanical isolation

• physical incompatibility of reproductive parts; mating is attempted, but morphological differences prevent its successful completion

Gametic isolation

• molecular incompatibility of eggs and sperm; mating is attempted, but molecular differences prevent its successful completion

  • sperm of one species might not be able to fertilize the egg of another

Reduced hybrid viability, development or survival

• Postzygotic barrier

• hybrids do not develop fully or are unable to reproduce due to interactiion of parental genes

Reduced hybrid fertility

• Postzygotic barrier

• hybriids, even if vigorous (healthy) cannot produce offspring

Hybrid breakdown

• Postzygotic barrier

• some first generation hybrids are fertilee and viablee, but when they breed with one another or with the parental species, the F2 generation are feeable or sterile

Allopatric speciation

• when a population is divided into geographically isolated populatiions due to changes in geography or water bodies

  • a geographic barrier separates two populations of the same species, restricting gene flow

• often, one population is small, leading to:

  • founder effect

  • genetic drift in common

• differential environmental pressures between the two locations can trigger natural selection

  • allele frequencies shift, mutations can form new alleles, new genes through duplication, and new functions

Sympatric speciation

• when a population is divided without geographic isolation

  • can arise through polyploidy, habitat differentiation, or sexual selection

polyploidy

cells/organisms contain more than two paired homologous sets of chromosomes

autoploidy

doubliing of chromosomes in the same species (most common in plants)

allopolyploidy

hybridization of two different species, followed by cell division error

habitat differentiaiion

• when a subpopulation of a speciess start to exploit a habitat or resourcee not used by the parent population

In a population of peacocks, a peaheen selectively chooses a peacock bassed on how shiny and large his tail feathers are. Which assumption of Hardy Weinbeerg doees this violate?
a. random mating

b. large populatioon

c. no mutations

d. no gene flow

a. random mating

In canada, 1 in 12000 births result in a child with PKU. PKU iis a recessive inherited disease in which a person can’t metabolizee phenylalanine propeerly

What percentage of the population are heterozygote carriers of the diseasee causing recessive PKU allele

Which of the following describee the effect oon the gene pool following a catastrophic reduction in population size

a. founder effect

b. disruptive selection

c. bottleneck effect

d. mutation

e. gene flow

c. bottleneck effect

Which of thee following is not like the other

a. genetic drift

b. founder effect

c. bottleneck effect

d. gene flow

d. gene flow

A new insect arrivees in thee arae where snapdragon plants grow, and it prefeers to eat plants with red flowers, but not the pink or white oness. Over generations, the Cr allele is selecteed against (because the insects eeat thosee plants beforee they can make seeds). The Cw allele is selected for (because the insects ignore them, so they can produce seedds by crossing with a white or pink flower). What kind of selection is this?

a. disruptive

b. directional

c. balancing

d. stabilizing

b. directional

All the deviations from Hardy Weinberg Equilibrium we discussed causee thee alleliic frequencies in a population to changee, however all are not equal in their effect. Which of the following is the greatest driveer for microevoolutioon

a. genetic drift

b. gene flow

c. bottleneck effect

d. natural selection

e. mutatiioons

d. natural selection

Which species concept does not require evidence of reproduction

a. biological species

b. ecological species

c. morphological species

c. morphological species

Which is a way we can define a species

a. biological species

b. morphological speecies

c. chemiospeciees

d. ecological species

e. all of the above

f. none of the above

e. all of the above

sockeye salmon return to their spawning grounds with members of their own cohhort; those fish who hatched in the same year. The prezygotic barrier preventing salmon from different cohorts matiing with each other is called

a. temporal isolation

b. habitat isolatiion

c. behavioural isolation

d. mechanical isolation

e. gamete isolation

a. temporal isolation

Which kind of speciation does not require geographic isolation

a. allopatriic

b. sympatric

b. sympatric

Which kind of speciation would genetic drift have a more pronouncd effeect on thee population

a. sympatric

b. allopatric

b. allopatric

Which example can leead to both allopatric and sympatric speciiation

a. habitat differnetiation

b. sexual selection

c. polyploidy

d. geographic barrieres

b. sexual selection