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evolution (chapter 23 the evolution of populations or microevolution…
evolution
chapter 23
the evolution of populations or microevolution
hardy wienburg equation
gene pools and alleles
gene pool- all copies of every allele at every locus in all members of a population
if only one allele exists in a population for a particular trait, all of the population is homozygous and the allele is said to be fixed
each allele has a proportion in the population
hardy wienburg equation
hardy wienburg equilibrium
p squared (homozygous dominant) + 2pq (heterozygous) + q squared (homozygous recessive) =1
p+q=1
applying
testing whether evolution is occurring in a population
seeing what percent of the population carries harmful diseases
doesn't yield an exact number, but an approximation
conditions for
no natural selection
very large population size
allele frequencies fluctuate in small populations
random mating
no gene flow
no mutations
things that alter genetics of population
natural selection
individuals with traits better suited to the environment tend to produce more offspring
genetic drift -more likely in small populations
bottleneck effect -a severe drop in population size that might by chance cause certain alleles to be underrepresented or missing
such as the decrease in population of the greater prairie chickens in illinois
they ended up with a harmful condition that's normally rare. it was fixed introducing chickens from different states
founder affect -when a few individuals are isolated from a larger population
gene flow
if the population has gene flow with another population there can be more variation
such as the unbanded island snakes that still can occasionally be banded because of immigrants from the banded mainland population
genetic varition
sources of genetic variation
altering gene number or position
large scale changes in genes are usually harmful, but minor ones may be neutral
gene errors without severe effects could be passed on for generations
rapid reproduction
more mutations can be made in organisms that reproduce rapidly, such as bacteria
viruses can quickly form a resistance to drugs, which is why AIDS often has to be treated with a combination of several drugs
formation of new alleles
a mutation is a change in an organisms dna, even a mutation as small as a single base can have a significant impact, such as in sickle cell disease
although this is very insignificant because a gene has thousands of dna molecules and a lot of dna molecules are on introns
mutations that alter phenotype are almost always harmful
usually these harmful alleles are destroyed by natural selection, but they can be carried on by heterozygous individuals
but some mutations do not harm or benefit the organism
only mutations in gametes can be passed on
sexual reproduction
in species that reproduce sexually, genetic variation can be caused by
independent assortment of chromosomes
random fertilization
crossing over
genetic variation
organisms can have different genes for different tratis
they can also have different dna molecules
natural selection is the only mechanism of evolution
natural selection
directions of selection
disruptive
favors variants at both extremes but not intermediate
stabilizing
favors intermediate types and removes extremes
directional
shifts overall makeup of population by favoring variants at one extreme
natural selection in adaptive evolution
natural selection matches an organism to its environment
since the environment is constantly changing, natural selection is not headed toward any goal, but just responding to the changes
genetic drift and gene flow can be helpful to survival, but it is just chance whether they do or not
sexual selection -when individuals with certain characteristics are more likely than individuals of the same gender to obtain mates
females are thought to have evolved a preference for males with good genes if the traits preferred by females indicate the males overall genetic qualities
sexual dimorphism -secondary differences between genders
one gender competing with itself for mates
one gender being choosy about selecting their mates
balancing selection
frequency dependent selection
when the fitness of a phenotype depends on how common it is in the population
like with the scale eating fish, it is more fit to have a left mouth if the right mouth is more common and vice versa
heterozygote advantage
when it is more advantageous to be heterozygous for a particular gene
for example people who are heterozygous for sickle cell anemia are less likely to get malaria,
natural selection does not lead to perfection
evolution can only act on existing structures
adaptations are often compromises
for example a seal cannot walk well on rocks, but it is efficient at swimming
selection can only act on existing variations
chance, natural selection, and the environment interact
random chance and a constantly changing environment do not work toward a goal of perfection
chapter 24 the origin of species
hybrids and hybrid zones
a gene could be passed from one population to another with a hybrid
if the hybrid gene was beneficial and if the hybrid survived
the shape of hybrid zones is based on the ranges of parent species and the survival rate of hybrids
a change in the environment can change where species interbreed
for example the carolina ad black capped chikadees
and the northern and southern flying squirrels
reproductive barriers
fusion -if reproductive barriers are weak and there's a potential for gene flow
like in the murk of lake Victoria, where the female's of different species can no longer tell the males apart
stability -if hybrids continue to be produced either because the hybrids survive or because the parent species continue to mate
reinforcement -natural selection strengthening prezygotic barriers, reducing the formation of hybrids
what causes "speciation"
allopatric speciation
caused by geographic isolation
such as the snapping shrimp on either side of the isthmus of panama
sympatric speciation -if a separate species is made without geographic barriers
polyploidy -when an organism has more than two sets of chromosomes
autopolyploid -more than two sets of chromosomes from the same species, could be caused by mistakes in mitosis or meiosis
a tetraploid (one with four sets of chromosomes) is reproductivly isolated from its original species
a polyploid can be formed when two different species interbreed
allopolyploid -a fertile polyploid hybrid
Tragopogon miscellus
is a hybrid of two european species. it can only reproduce asexually or through more hybridization of its parent species
many crops are polyploid such as oats, cotton, potatoes, tobacco, wheat
researchers can generate new polyploids with desirable qualities
sexual selection -supposedly a contributor to sympatric speciation
such as in the cichlid fish in lake Victoria
habitat differentiation -if different members of a species chose to use different parts of a habitat, they may end up never meeting each other
details of speciation
speciation rates
once speciation begins, it can be completed very rapidly
such as the
H. anomolus
sunflower, which is thought to come from a hybrid of
H. annus
and
H. petiolaris
speciation begins after gene flow in the populated is inturrupted by the environment
and is completed when the populations diverge genetically enough that there is reproductive isolation
genetics of speciation
reproductive isolation can result from a single gene
such as with the japanese snails with the shells that spiral in opposite directions
or the different colored flowers that are visited by different polinators
or it can be caused by many genes
fossil record
the fossil record is thought to show sudden dramatic increases in different species
punctuated equilibria -the conclusion is that the species evolved very quickly and then stayed the same for a long time
other species are thought to have evolved gradually
from speciation to macroevolution
the speciation process repeated over and over again is what eventually lead to different groups of organisms or macroevolution
separate species
biological species concept
members of a species have the potential to interbreed and produce viable, fertile offspring
reproductive isolation
prezygotic barriers
mechanical -sex organs are incompatable
behavioral -no attraction between species
temporal -species breed at different times
gamete isolation
sperm can't make it to egg
or sperm and egg are incompatable
habitat isolation -two species never meet
postzygotic barriers
reduced hybrid fertility -offspring can't reproduce
hybrid breakdown -hybrid's offspring can't survive
reduced hybrid viability -most offspring don't survive for some reason
limitations of biological species concept
it doesn't apply to asexual organisms
can't analyze the mating barriers of fossils
other definitions
ecological species concept -species determined by how they act in the environment
the problem with this is that different organisms could do the same thing in nature and the same organisms could do different things in nature
morphological species concept -shape and appearance determines species
chapter 22
Darwin and the idea of evolution in general
Darwin's history
the beagle
Darwin's professor recommended him to go on a survey ship with captain Robert Fitzroy
while he was on the voyage darwin observed plants, animals, and fossils
and read lyells
principles of geology
Darwin's focus
natural selection
he came up with the idea of natural selection: organisms best fit to their environment surviving and passing on their genes
publication
darwin hesitated for many years, but finally published all his ideas in a book so that someone else wouldn't publish it for him
adaptation
darwin theorized that the diversity of organiams that he saw could be because they had all adapted to their environment
education
his father sent him to medical school in edenburgh
he also studied to be a clergymen at cambridge university
darwin lived 1809-1882
origin of species
descent with modification
darwin attributed the unity of life to a common ancestor of all organisms
he saw all of life descending from that ancestor in the pattern of a branching tree
observations and inferences
observation 1- members of population have varied traits
observation 2 - species produce more offspring than the environment can support
inference 1 -individuals with better suited traits survive and pass their genes on
inference 2 -over generations, this will lead to an accumulation of favorable traits
key features of natural selection
populations evolve, not individuals
natural selection can only change traits that are different between organisms of the population
natural selection always depends on the environment
ideas before evolution
ideas about change over time
cuvier
noted that different species were in different fossil layers. thought that this was caused by various catastrophic events
hutton and lyell
hutton thought they were explained by gradual mechanisms
lyell thought that things were going the same speed they as are going today throughout all of history
paleontology
the study of fossils (remains of organisms from the past)
lamark
use and disuse
thought that an organism using a feature caused it to be larger and stronger and passed on to its offspring
drive to be complex
also thought that organisms desired to be more complex
ideas
proposed an idea for how evolutionary change occurred
scala natura and classification of species
creation
the belief that God created the earth and the species in it all at once(Darwin was the first to come up with an alternative idea to this)
linnaeus
developed a format for naming species and started to develop the classification system that grouped species according to similarities
linnaeus said the resemblences among species were because of the way they were created
aristotle
lined up organisms of a ladder of increasing complexity
scala natura
also said species could not change
(immutability of species)
evolution and evidence
natural selection in introduced species
drug resistant bacteria
and the
Stapholococcus aureus
that became resistant to penicillin and other antibiotics
as more antibiotics are invented, the S. aureus and other bacteria continue to become resistant to them
natural selection is not creative but merely favors organisms with an advantage
for example the soapberry bugs who lived on different species of plants developed different shaped mouth parts
homology -the study of similar features between species, thought to be the result of common ancestry
molecular
all dna is similar and all organisms use ribosomes
homologies and concept maps
organisms are arranged on an evolutionary tree that looks like a concept map, starting with the "simple" ones and branching off into more different complexities
anatomical
closely related species share features
their embryos look similar
vestigial structures -unnecessary structures thought to be remnants of evolutionary ancestors
convergent evolution
species with very similar features that are supposed to have evolved separately
such as the flying squirrel in america and the sugar glider in australia
called analogous structures
the fossil record
fossils of stickleback fish are thought to show the transformation between land mammals and ocean mammals
biogeography
the study of geographic distributions of species
pangea -original single continent of earth's land
endemic species are thought to be related to species on the closest mainland
evolution as a theory
natural selection can be observed and tested in nature
