Download Mechanisms of Evolution

Mechanisms of Evolution
The Gene Pool
 The gene pool can be though of as a genetic sum of a
population. It is the sum total of all of the genes
possessed by the individuals in a population.
Remember:
 a population may contain many different alleles
 An individual can only contain two alleles
 evolution is the change in the genetic makeup of a
population
Therefore, a change in the gene frequency
Therefore, a change in the gene pool
The Hardy-Weinberg Principle (12.2) –
a mathematical basis
for evolution
 under certain conditions of stability, both gene
frequencies and genotype ratio’s remain constant from
generation to generation in sexually reproducing
populations
Conditions of Genetic Equilibrium:
1. the population must be large enough to make it highly
likely that chance alone could significantly alter gene
frequencies
2. mutation does not occur
3. no immigration or emigration
4. reproduction must be totally random
1. and 3. are sometimes met but 2. and 4. are NEVER met!
 if a population is not in equilibrium  it will undergo
natural selection…. microevoloution
The Hardy-Weinburg Theorem
 Allows us to predict what to expect for a non-evolving
population
p = frequency of dominant allele
q = frequency of recessive allele
p + q=1
based on % adding up to 100% or 1

(p + q)2 = 12
p2 + 2pq + q2 = 1
AA
Aa
aa
Ex. if p = 0.9 (A) and q = 0.1 (a), what are the genotype
frequencies?
p2 + 2pq + q2 = 1
(0.9)2+2(0.9x0.1) + (0.1)2 = 1
AA
Aa
aa
81%
18%
1%
pg. 549 1&2
But for all this to work, the population must remain in
equilibrium so…
These calculations are not always what we see in nature
 in fact, populations are rarely in perfect equilibrium as
frequencies change from generation to generation
Therefore, HW is not maintained  evolution is
occurring!!!!!
(mutation, migration, genetic drift, non-random
mating,natural selection)
Sample of Allele Frequencies in a Population of Moths:
Random Change (12.3) –
chance happenings that lead to BIG
changes over time
HW gives us insight into what causes evolution:
 small population  chance fluctuations can cause changes in
allele frequencies
 Non-random mating  preferred mates pass on their genes more
frequently
 Mutation  new allele or change in allele will change frequencies
of new and original alleles
 Migration  removal of alleles
 Natural Selection  certain alleles will out survive others therefor
increasing their frequencies
When a population is not in equlibrium many things can happen:
1. Genetic Drift: changes in the gene pool of a small
population due to chance. (Pg. 550)
a) Bottleneck Effect
Decrease in population because of some disaster. The
population is no longer ‘representative’ as certain alleles
are lost – elephant seal
(population was down to 20 in 1890s)
b) Founder Effect
Genetic drift in a new colony. Usually occurs because of
the small number of pioneers in an isolated area.
2. Gene Flow: A population may gain or lose alleles by
immigration or emigration.
3. Mutation:

changes to the gene pool of a population

these are always occurring (most genes
undergo mutation once every 50 000 to 100 000
duplications)

this is the original source of all variation!!!

can be neutral, harmful or beneficial to the
organism

it is those beneficial ones that usually lead to
increased reproductive success = adaptation =
evolution!
4. Non-Random Mating: an organism’s genotype
influences its selection of a mate
The physical efficiency and frequency of its mating, its
fertility, total # of zygotes, % of successful births etc.
Also:
1. Mating with nearby individuals promotes inbreeding
2. Tendency to mate with partners like oneself in some
ways: Assortative mating
3. Sexual Selection
The brutal truth eliminates all but the most
favorable gene combinations. This is why Darwin
referred to nature as:
“Red in Tooth and Claw”
(A bit chilling …)
Patterns of Selection (12.4) –
the ‘not-so-chance’ changes in
populations … there are selection pressures
 mutations provide a continuous supply of new genetic
variation
 natural selection leads to a variety of outcomes when
this genetic variation occurs within competitive
populations
 Ex. Sickle-Cell Anemia
3 selection pressures:
homozygous
s-cv:
strong negative against (severe affliction)
heterozygous: postive/negative dependent on area (anemic
but resistant to malaria)
homozygous
(normal Hb): positive/negative dependent on area (no
malaria resistance, but all functional Hb)
 these pressures lead to differential success in
reproduction (heterozygous is most favored in areas where
malaria is common)
 So… Although genes provide the source of variation,
natural selection acts on individuals and their
phenotypes.
Types of Selection
1. Stabilizing Selection pg. 557
 once a species has become well adapted to their
environment, selection pressures will tend to prevent them
from changing (= unseen evolution)
 the most common phenotypes are most favoured by the
environment
 natural selection eliminates extreme variations of a
particular trait
 most common selection pressure
2. Directional Selection
 environment favours individuals with more extreme
variations of a trait
 can result in observable change in a population
especially in large population with short generation times
3. Disruptive Selection
 favours individuals with variations at opposite extremes
of a trait over individuals with intermediate variations
 environment may favour more than one phenotype
 this is a significant evolutionary mechanism for the
formation of distinctive forms within a populations
 can lead to isolated breeding populations which leads to
speciation (creation of new species)
4. Sexual Selection
 favours the selection of any trait that influences the
mating success of the individual
ex.: sexual dimorphism  physical appearance of males
and females is very different and behavior is different
between the sexes
 female selection of males based on:
 appearance
 behavior
 male vs. male competition
 not environmental or else both sexes would have the
trait
 can result in detrimental traits – ex. body parts that are
obvious to predators
The bottom line of all of this is that natural selection is the
mechanism that drives the evolution of a species.
Cumulative Selection (12.5)
 Did the eye just appear suddenly in one generation? OR
 Did it evolve piece by piece over millions of generations?
(um … probably #2)
 the complete eye we know today evolved gradually
through a series of accumulated beneficial mutations
 Picture here
 the environment would have favoured the individual
capable of sensing light  get out of the sun or away
from predators
 evolution of a complex structure is a cumulative process
Evolution of Insect Pollination
1. Wind pollination =
2. gain slight
lots of pollen
stickiness = less pollen attract insects = even
has to be produced
3. make structures to
less pollen to produce
Altruism
 one organism benefits much more from the behaviour of
another organism than that organism itself
 Darwin’s “Fatal” part of his theory
 ex. wasp colonies of females helping only 1 female to
reproduce (sisters)
 May not pass on their genetics but at least passing on
the genetics of the family
Speciation: The Formation of a New Species (12.6)
 the formation of an entirely new species
 evolution at the species level is called microevolution
Species: members of a population that have the ability to
breed with each other under natural conditions
 often we can differentiate between species solely on
physical or morphological differences but sometimes
behavior or biological methods are needed
Ex. reproductive isolating mechanisms: any behavioural,
structural or biochemical traits that prevent individuals
of different species from reproducing successfully with
each other
Reproductive Isolating Mechanisms
Prezygotic Mechanisms  prevent fertilization
1. Ecological isolation – occupy different habitats
prevent
mating
2. Temporal isolation – diff. plants bloom at diff. times
of day
3. Behavioural isolation – wrong signals won’t attract a
mate
prevent
fertilization
4. Mechanical isolation – parts won’t fit
5. Gametic isolation – ex. biological markers, releasing
gametes into water
Some reproduction between species produces hybrids
which are often sterile ex. mule – mare for a mommy and
an ass for a daddy and all mules are female
Postzygotic Mechanisms
1. zygotic mortality – the zygote dies
2. hybrid inviability – it doesn’t live long
3. hybrid infertility – ie. mule
Modes of Speciation
 within isolated gene pools, any mutations and
subsequent selection processes that occur in one
population can no longer be shared with others – most
commonly result from geographical isolation
Allopatric Speciation
 the evolution of populations into separate species as a
result of a geographical isolation ex. a canyon, or
a river
Sympatric Speciation
 the evolution of populations within the same
geographical area into separate species – weird but does
happen (fluke – but isn’t all evolution this???…)
(pg. 575)