Download Evolution as Genetic Change

16-2 Evolution as Genetic Change
 Natural selection NEVER acts directly on genes
 An entire organism, survives, reproduces
 Natural selection determines which organisms survive
to reproduce
 Organisms dies before reproduction it does not pass its
alleles onto subsequent generations
 Organisms reproduce – alleles pass on to subsequent
generations
 EVOLUTION = change over time in the relative
frequencies of alleles in a POPULATION
 Populations evolve; not organisms
Natural Selection on Single Gene Traits
 Natural selection on single gene traits can lead to
changes in allele frequency which leads to evolution
 Example: brown lizards with red and black forms(due
to mutations)
 Red lizards more visible to predation – die allele for
red less common
 Black lizards – more radiation – faster, avoid
predation better than brown – allele becomes more
frequent
 If allele has no effect on “fitness” allele is not under
pressure from natural selection
Natural Selection on polygenic traits
 Natural selection can affect the distributions of
phenotypes in any of three ways: directional selection,
stabilizing selection, disruptive selection
 Directional selection: individuals on one end of curve
have higher fitness than opposite end or middle of
curve – big beak birds able to crack large seeds – if
food shortage causes only large size seeds to be
available – big beak birds survive – population favors
big beak
 Stabilizing selection: individuals in center of curve
have higher fitness – change of curve doesn’t change
much – human infants – small babies, low survival,
big babies low survival (difficulty being born) average
size babies – higher fitness
 Disruptive selection – opposite ends of curve favor
selection – results in 2 sub group populations
Genetic drift
 In small populations the expected results of genetic
crosses is not what is expected – random change in
allele frequency = genetic drift
 In small populations, individuals that carry a particular
allele may leave more descendants than other
individuals, just BY CHANCE. Over time these
CHANCE occurances cause an allele to become more
common
 Founder effect: small group of individuals from
original population colonize a new habitat. – these
carry different alleles from original population –
create new populations of organism – members of
population of different habitat have allele frequencies
different from parent population
Evolution versus genetic equilibrium
 To understand how evolutionary change operates look at what happens when no change takes place
 Identify conditions underwhich NO EVOLUTION
will occur
 Hardy-Weinberg principle – allele frequencies in a
population will remain constant unless one or more
factors cause the frequencies to change
 Genetic equilibrium – when no change occurs in the
allele frequencies
 5 conditions to maintain genetic equilibrium
o Must be random mating
o Population must be very large
o no immigration or emigration
o no mutations
o no natural selection
Hardy Weinberg equation
o p2 + 2pq + q2 = 1
o p = frequency of dominant allele
o q = frequency of recessive allele
o pq = hybrid
o
Process of speciation:
o how does natural selection or change in allele
frequencies result in speciation?
o Species – a group of organisms that breed with one
another and produce fertile offspring – share a
common gene pool
o Gene pools of two populations must become separated
for them to become new species
o REPRODUCTIVE ISOLATION – as new species
evolve, populations become reproductively isolated –
cannot interbreed successfully
REPRODUCTIVE ISOLATION – due to:
1. behavioral isolation (different mating songs)
2. geographic location – break up of Pangea (ostrich,
rhea, emu)
3. temporal isolation – reproduction is at different times
– different species of orchids release pollen only on a
single day – thus the pollen cannot pollinate another
species
Speciation in Darwin’s Finches
o founders arrive (from mainland – Ecuador)
o geographic isolation – members of founders
move to another island
o changes in gene pool – each island has specific
environmental pressurs – favoring certain
characteristics – changes in allele frequencies
o reproductive isolation – beak “fetishes” among
finches
o ecological competition – competition for
available resources “selects” traits
o continued evolution – over many generations
until 13 different species established themselves
Understanding evolution is important because evolution
drives changes in the world of drug resistance in bacteria
and viruses, and pesticide resistance in plants
Evolution of disease resistant bacteria
A gene on bacteria mutated – causing bacteria to 1) destroy
antibiotic or 2) hinder effectiveness of antibiotic
o result – a class of antibiotic resistant bacteria – named
“R plasmids” R = resistant
o should use of antibiotics be restricted???