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Ch 23


Joshua Farr

on 22 February 2014

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Transcript of Ch 23

How do populations change?
To start:
Populations change...
individuals don't!
Over their lifetime, individuals
do not change in a Darwinian sense.
Selection acts on traits. Genomics
makes it easier to see this occur.
Natural selection acts on individuals...
but only populations evolve
Variation comes from two things
1. Mutations
2. Sexual reproduction
3. Selective editing
These concepts influence evolution!
So where do we see
examples of changes
Think Geographically!
Geographic distribution is the separation of gene pools
based on populations being in different areas,
experiencing different environ. pressures
and evolving in a corresponding fashion
Mutations occur 1:100,000 genes per generation

As we know, mutations Can be...?

Check out this link:
Sexual Reproduction
Shuffles existing alleles
For sexually reproducing species, gaining
diversity via recomb. more influential than mutation
A few terms and definitions before continuing

A population is a localized group of individuals capable
of interbreeding and producing fertile offspring

A gene pool consists of all the alleles for all loci in a population

A locus is fixed if all individuals in a population are homozygous
for the same allele
Variation comes
Types of variation?
Hardy Weinberg Equilibrium

If there are two alleles at any
particular locus, we can set
p and q as the variables for
those alleles (dom or rec)

The frequency of those alleles will
add up to 1

p + q = 1

The equilibrium follows this equation:
p^2 + 2pq + q^2 = 1

if a gene is 80% common at a loci, then the next generation will by 64 % homozygous dominant, 32% heterozygous, and 4% homozygous recessive.

It's important to know that the HW Eq. deals
with populations that are NOT changing! HW Eq. has a couple conditions that need to be met in order to occur.
- No natural selection
- No mutations
- Large population size
- No gene flow
If these are not met, then the population will evolve.

This is hypothetical and in real populations changes do
occur and allele frequencies change over time leading
to different genotypes and phenotypes.

So what's the purpose??? HW demonstrates that if nothing affects
populations, that their allele and genotypic frequencies will remain
the same...however, life gets in the way and makes things messy!

Alleles: A, a
p + q = 1
p = frequency of dominant allele (A)
q = frequency of the recessive allele (a)

p2 + 2pq + q2 = 1
p2 = AA frequency
2pq = Aa frequency
q2 = aa

Sexual Reproduction
Evolution is an overall process. Individuals experience random mutations, and if the mutation is beneficial, the organism will be more likely to survive, and reproduce, creating more individuals with that mutation.
Only the organisms better adapted to the current living situation will go on to reproduce and "have babies" .
First Evolutionary tree constructed in 1837: 1 year
after Darwin completed the voyage on the Beagle.

2000: Fly genome, 15,000 genes
2001: Human, 22,333 genes
2004: Chicken, 16,736 genes
Grapes: 30,434 genes ???
How do we figure out what these genes do?
Full transcript