Bio Module 5 - Population Genetics

Gregory Mendel

• monk who did research at monastery

• foundation of modern genetics 

• his work went unnoticed until well after it was published 

plant mendel worked with 

pea plants 

continuous variation 

results from the action of many genes to determine a characteristic (like human height)

Blending Model

characteristics of your parents just mix together

• breaks down in the F2 generation 

Lamarckian Inheritance of Acquired Traits

offspring inherit characteristics that were acquired during their lifetime

• people with big muscles will have kids with big muscles

• #wrong

epigenetics

intergenerational transmission where the environment determines the genes we turn on and off

mendel’s experiment

did breedings of pea plants over and over again

• he bred two pure bred plants

• every time he saw the 3:1 phenotype ratio as the outcome of the second cross

alleles 

versions of a gene 

number of alleles per individual

2

dominant allele

• represented by a capital letter

• masks the recessive allele in a heterozygous genotype

recessive allele

• represented by a lowercase letter 

• can be masked by the dominant allele in a heterozygous genotype

phenotype

physical characteristic

genotype

genetic expression

gene 

smaller locations on the genome 

homozygous

two dominant or two recessive alleles (AA, aa)

heterozygous

one dominant and one recessive allele (Aa)

job of the punnet square 

estimate allele frequencies 

discontinuous variation

traits inherited in distinct classes - they keep their distinctness

why mendel chose pea plants 

• several generations can be evaluated in a short period of time

• large quantities could be gathered

hybridizations 

mating two true-breeding individuals that have different traits 

F1 generation 

first generation of offspring 

F2 generation 

offspring of a cross of F1 generation 

trait

a variation in the physical appearance of heritable characteristic

what each single letter represents on the outside of a punnet square

possible haploid condition

what each double represents on the outside of a punnet square

possible diploid condition

punnet square rules

• male alleles on the top

• female alleles on the left

• dominant allele written before recessive

chromosome

genes grouped together

what the Hardy-Weinberg Equilibrium assumes

a completely stable (non-evolving) population

“funny stuff” 

• natural selection

• reproductive skew/sexual selection 

• migration  

• genetic drift (sometimes)

• mutations (rare tho)

equation for allele frequencies 

p + q = 1

Hardy-Weinberg Equation

• sum of all the genotypes in the population 

p² + 2pq + q² = 1

p² 

homozygous dominant

2pq

heterozygous pairs 

q²

homozygous recessive 

adaptive 

the alleles that become more common are the ones that best fit individuals to the current local environment

fisher

The Genetical Theory of Natural Selection

modern synthesis 

the coherent understanding of the relationship between natural selection and genetics 

• combined math and bio 

3 modern synthesis scientists we learned about 

• Haldane - wrote about popular science and spirituality 

• Huxley - Aldous Huxley (Brave New World) brother 

• Fisher - math nerd but a jerk. Tried to use science to justify racism  

microevolution

population change overtime

• “small-scale changes within a population over relatively short periods”

macroevolution

processes that gave rise to new species and higher taxonomic groups with widely divergent characters

• “large-scale, long-term changes leading to the divergence of new species and higher taxonomic groups” 

population genetics

how selective forces change a population through changes in allele and genotypic frequencies

allele frequency

rate at which a specific allele occurs in a population

what a change in allele frequency represents 

evolution 

gene pool

sum of alleles in a population

genetic drift

allele frequencies can change randomly

• simply by chance events 

increases the rate of genetic drift 

low population size 

3 circumstances where the loss of genetic diversity is a serious concern 

small populations, after a population bottleneck, inbred populations 

founder effect

an event that initiates an allele frequency change in an isolated part of the population, which is not typical

• few members of a large population migrate to a new location and establish a new population

Godfrey Hardy and Wilhelm Weinberg

stated that a population’s allele and genotype frequencies would are inheritantly stable and the frequencies would not change unless acted on by an evolutionary force

fixed allele

when an allele becomes ubiquitous in the population (100% have it)

purged allele

when an allele is no longer found in the population (0% have it)

mutations

• creates new alleles from already-existing alleles by altering them slightly or greatly

• rare, so not an important cause of evolution

bottleneck effect

large populations suddenly become small due to an environmental disaster or newly arrived predator or competitor greatly reducing numbers

inbreeding

mating between closely related individuals

consequences of inbreeding

leads to the association of harmful recessive genetic mutations that can lead to disabilities, disease, or death

assisted gene flow

scientists often increase genetic diversity by bringing additional members of the species into captivity to simulate gene flow, or the movement of new genes into the population

gene flow 

introducing new alleles to the population or, at minimum, increase the frequency of alleles already found in the population

effects of gene flow

• can bring new alleles into a population, on which selection or drift might then act, but it will not, by itself, significantly alter allele frequencies in the population (if population is small and sporadic)

• can be powerful enough, by itself, to significantly change allele frequencies in the population that receives them. And if the immigrants are coming from a population that experiences a different environment, the alleles they bring with them could be maladapted to their new environment. (for large populations)

• between two populations can prevent them from evolving into distinct species.

• from a large population (which contains a lot of genetic diversity) to a small population (which could lose or has lost genetic diversity by genetic drift) can maintain or restore genetic diversity in the latter.

population genetic structure

When scientists compare populations within a species and see differences in the frequencies of genotypes

acquired traits 

traits that cannot be passed on 

genetic variability 

diversity of alleles and genotypes in a population 

geographical variation

differences in phenotype variation due to geographic variation

cline

species’ populations vary gradually across an ecological gradient

cause of reproductive isolation 

geographic isolation by selection 

microevolution

change within species

macroevolution

grand trends of evolution over geological time

reproductive isolation 

the inability (or greatly reduced ability) to interbreed successfully with members of other species even when living side by side 

species

population (or set of populations) with a distinct constellation of traits that fit to a particular ecological niche

• reproductively isolated from other population

character displacement

when two populations come back into contact after being separated, but their niches still overlap considerably, this is the process that allows the new species to evolve further ecological differences

two things that strengthen reproductive barriers 

• hybrids between two populations are at a disadvantage 

• some members of the two populations are more inclined to pick members of their own species as mates 

reinforcement

the process by which selection can strengthen reproductive barriers

• lack of compatibility between the two species split from isolation 

allopatric speciation

when all or much of the differences between the two species evolved while in allopatry

allopatry 

occurring in separate, non-overlapping geographic areas 

sympatry

occurring in the same geographic area

speciation

when one species splits into two

reasons for sympatric speciation 

• individuals on opposite sides of population 

• competition amongst population 

• preference 

• behavioral isolation 

• small event happens that only impacts part of the population 

• part of the group doesn’t migrate 

be familiar with scientific paper about dinos…

Dr. O’Connor

• originally interested in geology rather than paleontology

• studied in both the US and China to study dinosaurs and heritage reasons

• specifically studies transition period from dinos to birds

• prioritizes soft tissue membrane

• “punk rock paleontologist”