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Evolution - Coggle Diagram
Evolution
The Evolution
of Populations
Natural selection acts on individuals, but only
populations, not individuals, evolve
Ex: medium ground finches evolved in
response to seed shortage on Daphne Major
Microevolution
, the change in allele frequencies in
a population over generations, is evolution at its
smallest scale
Three mechanisms cause allele frequency change:
– Natural selection (adaptation to the environment)
– Genetic drift (chance events alter allele frequencies)
– Gene flow (transfer of alleles between populations)
Genetic variation makes
evolution possible
Genetic variation
refers to the differences in
genes or other DNA sequences among individuals
Genetic variation originates when new genes and
alleles arise by mutation, gene duplication, or other
processes
Point mutations in noncoding regions usually cause
neutral variation, variation with no selective
advantage or disadvantage
Most genetic variation in sexually reproducing
organisms results from recombination of alleles
New combinations of existing alleles occur through
three mechanisms:
– Crossing over (exchange of genetic material between homologous chromosomes during meiosis)
– Independent assortment (random distribution of chromosomes into gametes during meiosis)
– Fertilization (random combination of gametes)
The Hardy-Weinberg equation
can be used to test whether a population is
evolving
The
gene pool
consists of all copies of every allele
at every locus in all members of the population
The Hardy-Weinberg equation describes the
expected genetic makeup for a population that is
not evolving at a particular locus
A
population
is a group of individuals of the same
species that live in the same area and interbreed
If the observed genetic makeup of the population
differs from expectations under Hardy-Weinberg,
then the population may be evolving
Hardy-Weinberg
equilibrium
If a population is not evolving, genotype and allele
frequencies will be constant from generation to
generation
Natural selection, genetic drift,
and gene flow can alter allele frequencies in a
population
Three major factors alter allele frequencies directly and bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Natural selection can cause
adaptive evolution
, a
process in which traits that enhance survival or
reproduction increase in frequency over time
Genetic Drift
The
founder effect
occurs when a few individuals
become isolated from a larger population
Ex: genetic drift could occur if a few
individuals are indiscriminately blown to a new island
by a storm
The
bottleneck effect
occurs when there is a
drastic reduction in population size due to a sudden
change in the environment
Genetic drift
is a process in which chance events
cause allele frequencies to fluctuate unpredictably
from one generation to the next
Gene Flow
Gene flow
consists of the movement of alleles
among populations
Alleles can be transferred through the movement of
fertile individuals or gametes (for example, pollen)
Gene flow tends to reduce variation among
populations over time
Gene flow affects adaptation to local environments
Ex: mainland and island populations of
Lake Erie water snakes have different color
patterns
Ongoing migration of banded snakes from the
mainland population maintains disadvantageous
alleles for banding pattern on the islands
A strong banding pattern is favored on the
mainland; unbanded snakes are better
camouflaged on islands
Gene flow can also increase a population’s fitness
Ex: the spread of alleles for
resistance to insecticides
Alleles have evolved in some populations that confer
insecticide resistance to these mosquitoes
Natural selection is the only
mechanism that consistently causes adaptive
evolution
Relative fitness
is the contribution an individual
makes to the gene pool of the next generation
relative to the contributions of other individuals
There are three ways in which natural selection can
alter the frequency distribution of heritable traits:
–
Directional selection
favors individuals at one
extreme end of the phenotypic range
–
Disruptive selection
favors individuals at both
extremes of the phenotypic range
–
Stabilizing selection
favors intermediate variants
and acts against extreme phenotypes
Only natural selection consistently increases the
frequencies of alleles that provide reproductive
advantage
Sexual Selection
Sexual selection
is a process in which individuals
with certain heritable traits are more likely to obtain
mates than other individuals of the same sex
For example, males and females may differ in size,
color, ornamentation, and behavior
It can result in
sexual dimorphism
, a difference in
secondary sexual characteristics between the
sexes
Intrasexual selection
is direct competition among
individuals of one sex (often males) for mates of the
opposite sex
Ex: males may patrol a group of females
and prevent weaker males from mating by defeating
them in combat
Intersexual selection
(mate choice) occurs when
individuals of one sex (usually females) are choosy
in selecting their mates
Balancing Selection
In
frequency-dependent selection
, the fitness of
a phenotype depends on how common it is
Ex: frequency-dependent selection results
in approximately equal numbers of “right-mouthed”
and “left-mouthed” scale-eating fish
Heterozygote advantage
occurs when
heterozygotes have a higher fitness than both kinds
of homozygotes
Ex: the deleterious sickle-cell allele is
maintained at relatively high frequencies in some
regions due to heterozygote advantage
Balancing selection
preserves variation at some
loci by maintaining stable frequencies of two or
more phenotypes
Evolution is supported by scientific evidence
Four types of data document the pattern of evolution
–Direct observations
–Homology
–The fossil record
–Biogeography
Direct Observations of Evolutionary Change
Biologists have documented evolutionary change in thousands of scientific studies
2 Examples are natural selection in response to introduced species and the evolution of drug-resistant bacteria
Natural Selection in Response to Introduced Species
Herbivores often have adaptations to help feed efficiently on their primary food source
Ex: soapberry bugs use a long “beak” to feed on seeds embedded within the fruits of various plants
Feeding is most effective when beak length is closely matched to seed depth within the fruit
In southern Florida, soapberry bugs feed on native balloon vines with large fruit; they have long beaks
In central Florida, they feed on introduced golden rain trees with smaller fruit; they have shorter beaks
The Evolution of Drug-Resistant Bacteria
The bacterium Staphylococcus aureus occurs on the skin or nasal passages of about one in three people
Certain strains, called methicillin-resistant S. aureus (MRSA), are pathogens that can cause potentially fatal infections
Ex: clone USA300 can cause “flesh-eating disease”
Methicillin works by inhibiting an enzyme used by bacteria to produce cell walls
Resistance increases in the presence of methicillin because MRSA strains are more likely to survive and reproduce than nonresistant strains
In 1943 penicillin became the first widely used antibiotic to treat bacterial infections
Penicillin resistance evolved in S. aureus by 1945
A new antibiotic, methicillin, was introduced in 1959
Methicillin resistance evolved in S. aureus by 1961
Homology
Homology
, similarity resulting from common ancestry, is another type of evidence for evolution
Related species can have characteristics that have an underlying similarity yet function differently
Anatomical and Molecular Homologies
Homologous structure
s are anatomical resemblances that represent variations on a structural theme present in a common ancestor
Ex: the forelimbs of all mammals have the same arrangement of bones, but different functions
Vestigial structures
are remnants of features that served a function in the organism’s ancestors
Ex: snakes arose from ancestors with legs; the skeletons of some snakes retain vestiges of pelvis and leg bones
Homologies and “Tree Thinking”
Characteristics shared by many species date to a deep ancestral past; homologies that evolved more recently are shared only within smaller groups
Ex: tetrapods, like all vertebrates, have a backbone
Unlike other vertebrates, all tetrapods also have limbs with digits
Evolutionary trees
are diagrams that reflect hypotheses about the relationships among groups
Relatedness is determined by the recent common ancestor, not the proximity of groups on the tree
Evolutionary trees show relative timing of events, not actual dates
A Different Cause of Resemblance: Convergent Evolution
Convergent evolution
is the evolution of similar, or analogous, features in distantly related groups
Analogous traits arise not through common ancestry, but through independent adaptation to similar environments
Ex: the sugar glider is an Australian marsupial that superficially resembles the flying squirrel, a North American eutherian
The Fossil Record
The
fossil record
provides evidence of the extinction of species, the origin of new groups, and changes within groups over time
Ex: the fossil record supported the DNA-based hypothesis that cetaceans are close relatives of even-toed ungulates
Fossil evidence shows that living cetaceans and even-toed ungulates are more different from each other than were earlier members of these groups
Fossils can document important transitions, such as the transition from land to sea in the ancestors of cetaceans
Ex: Pakicetus, an early cetacean, closely resembles Diacodexis, an early even-toed ungulate
Biogeography
Evidence from
biogeography
, the scientific study of the geographic distribution of species, provides support for evolution
Species distributions are influenced by continental drift, the gradual movement of Earth’s landmasses
Ex: 250 million years ago, all landmasses formed a single large continent called Pangea
By 20 million years ago, they had drifted apart to form the continents near their present locations
Understanding continental drift and modern species distribution helps predict when and where different groups evolved
Ex: freshwater fish in the family Galaxiidae live in South America and Australia, separated by wide stretches of open ocean
All three species share an ancestor dating back to the time that these continents broke away from Pangea
Descent with modification
Darwin’s Focus on Adaptation
Adaptations
are inherited characteristics that enhance an organism’s survival and reproduction in specific environments
Darwin proposed natural selection as an explanation for adaptation
Natural selection
is a process in which individuals with certain inherited traits tend to survive and reproduce at higher rates because of those traits
Ideas from The Origin of Species
Descent with modification by natural selection explains three broad observations
–The unity of life
–The diversity of life
–The ways organisms are suited to life in their environments
Descent with Modification
Darwin used descent with modification to describe his view of life
By this view, all organisms are related by descent from a common ancestor that lived in the past
Darwin viewed the history of life as a tree, with multiple branchings from a common trunk
–Labeled branches represent groups of organisms living in the present day
–Unlabeled branches represent extinct groups
–A fork represents the most recent common ancestor of all lines of evolution branching from that point
Large morphological gaps between related groups are explained by branching and extinction events
Ex: living elephant species are similar because they split from a recent common ancestor
Artificial Selection, Natural Selection, and Adaptation
Humans modify species through
artificial selection
, breeding only individuals with desired traits
Darwin drew two inferences from two observations
–Observation #1: Members of a population often vary in their inherited traits
–Observation #2: All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce
Inference #1: Individuals with inherited traits that increase survival and reproduction in an environment tend to produce more offspring than other individuals
Inference #2: The unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations
Key Features of Natural Selection
Individuals with certain heritable traits survive and reproduce at a higher rate than other individuals
Natural selection increases the frequency of adaptations that are favorable in an environment
If the environment changes, natural selection may drive adaptation to new conditions, giving rise to new species
Darwin’s Research
Charles Darwin (1809–1882) had a consuming interest in nature throughout his life
After graduation, he took a position as naturalist on a five-year, worldwide voyage on the HMS Beagle
The Voyage of the Beagle
He noted that fossils resembled living species from the area in which they were found, and living species resembled other species from areas nearby
Darwin hypothesized that species from the mainland colonized and then diversified on the islands
On the Galápagos Islands, Darwin collected many similar, but different species of birds, some unique to individual islands, others found on multiple islands
Animals unique to the islands resembled species on the nearby mainland of South America