Evolution and Natural Selection

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Biological Change
Natural Selection and the
Evidence for Evolution
What is evolution?
• The modern theory of evolution is a
fundamental concept in biology
• Evolution – change in populations over
time
• Charles Darwin was the first to publish
his ideas of how species evolve
Common descent
• The scientific theory that all living
organisms on Earth descended from a
common ancestor.
– The structures and functions of all living organisms are
encoded in the same basic nucleic molecules, DNA and
RNA.
– Similarities in amino acid sequences between various
organisms also suggest common descent
– The fossil record also shows cases in which one plant or
animal type evolved into different types over time.
Charles Darwin
Began
his work in 1831 (age 21) as the
naturalist on the HMS Beagle
As
the ship’s naturalist, it was his job to
study and collect biological specimens at
each port along the rout
On
the Galapagos Islands, Darwin
studied many species that were unique to
the islands, but similar to species
elsewhere. These observations led him to
consider the possibility that species can
change over time.
It
took him 22 years to find an
explanation for how species change over
time
Charles Darwin
• 1859 published On the Origin of
Species
– his theory of natural selection to explain
how organisms evolve
– Darwin developed his ideas while sailing
on the Beagle
Adaptations: Evidence for
Evolution
• Structural adaptations arise over time
– Mimicry – where one species resembles
another species
– Camouflage – where a species blends
with their surroundings
• Physiological adaptations arise over
time
– Antibiotic resistance of bacteria
Other Evidence for
Evolution
• Fossils
• Anatomy
– Homologous structures – structures that are
similar in anatomy but have different function
• Common evolutionary origin
– Analogous structures – body parts similar in
function but have different structure
• No common evolutionary origin
– Vestigial structures – body structure that has
no function in present-day organisms but was
probably useful to an ancestor
• Biochemistry
– DNA, RNA
More Evidence for Evolution
• Embryology
As development
continues from
embryo to a
more mature
organism, the
differences
increase,
however, in the
earliest stages of
growth and
development,
many vertebrate
embryos are
remarkably similar
Biological Change
Species Change
Principles of Darwin’s Theory
of Natural Selection
• Variation
• Heritability
• Overproduction
• Reproductive Advantage
Remember…
The Principles of Darwin’s Theory
• Overproduction of
offspring
• Variation
• Heritabiltiy
• Reproductive
Advantage
(preferential selection
of traits)
Species Change
• Organisms that are best matched to
their environment are more likely to
survive and reproduce
• Adaptation – a trait that improves an
organism’s change for survival and
reproduction
Adaptation
• Individual members of a single species
exhibit differences in their appearance
and function
• Difference result from random
changes in genetic material from
sexual reproduction and mutations
Natural Selection
• Organisms with traits that help them
survive and reproduce pass their
characteristics to their offspring.
• Helpful traits survive and spread
through the population
• Harmful traits disappear over time
• As a result, a population may evolve
into a new species
Natural Selection
reproduction
Inherited traits
Random mutations
Variation
Adaptations
Survival
Mismatch with
environment
Death; no offspring
Populations evolve, not
individuals!
• Variation exists among individuals within a
population
• An individual with variations that make them
poorly adapted to the environment will not
survive and reproduce
– Remember genotypes (genes) and phenotypes
(expression of genes)
• Evolution occurs as a population’s genes
and their frequencies change over time
Natural Selection Acts on
Variation
• Some variations increase or decrease
an organism’s chance of survival in a
particular environment
– Three types
• Stabilizing selection
• Directional selection
• Disruptive selection
Types of Natural Selection
• Directional selection
– Occurs when individuals at one end on the
frequency distribution are better adapted to the
environment than those in the middle
• Disruptive selection
– Occurs when individuals near the upper and
lower ends of the distribution are better adapted
than those in the middle
• Stabilizing selection
– Occurs when individuals near the center of the
distribution are more fit than individuals at either
end
Disruptive Selection
Stabilizing Selection
Directional Selection
Determine the type of natural selection
indicated by the following examples.
• Members of a population of Amazon tree frogs hop
from tree to tree searching for food in the rain
forest. They vary in leg length. Events result in
massive destruction of the forest’s trees. After
several generations, only long-legged tree frogs
remain alive.
• Different grass plants in a population range in length
from 8cm to 28 cm. The 8-10 cm grass blades
receive little sunlight, and the 25-28 cm grass blades
are eaten quickly by grazing animals.
• The spines of sea urchin population’s members vary
in length. The short-spined sea urchins are
camouflaged easily on the seafloor. However,
long-spined sea urchins are well defended against
predators.
Question
• A population of woodpeckers have beak lengths
ranging from 2cm-4cm. The woodpeckers with 3
cm beaks are able to reach more insects in the
trees in which they feed.
Explain the following:
A.Is longer beak length an advantageous
adaptation?
B. How would you expect the population of
woodpeckers to evolve after a very long period of
time?
C. What type of selection would this be an example
of? Explain your answer.
Classification of organisms
• Organisms are classified based on
internal and external characteristics
• Species – most specific unit of
classification
The Evolution of Species
• Speciation – the process of the
evolution of a new species
– Occurs when members of similar
populations no longer interbreed and
produce fertile offspring
Evolutionary History
• Phylogeny describes the evolutionary history
of a related group of species
• All organisms on Earth evolved from a single
ancestor
• Life on earth began about 3.5 billion years
ago; since that time, new species have
emerged, lived and died out
• New species evolve from pre-existing
species
Fossil Evidence
• Through fossil evidence, physical and
molecular similarities between ancient
species and modern species have
been found.
• Physical and molecular similarities
between diverse species has also
been found.
Phylogeny of Humans
Phylogeny of Humans
Extinction
• Organisms try to survive in their
environments through adaptation,
however many species become
extinct
• Extinction can occur because of
environmental changes, human
interference or as a result of failure to
adapt to new conditions
Extinction Cont.
• Extinction is a natural and important part of
evolution
• It is estimated that 999 of every 1,000 species
that have ever lived on Earth have become
extinct
• The average species survives between 2
and 10 million years
• Even the most highly adapted species
become extinct.
Mass Extinctions
• There have been many mass
extinctions during Earth’s history
• Mass extinction = when more than 50%
of species were wiped out
• Mass extinctions make it possible for
new species to develop
• Surviving species are able to diversify
Biological Change
Identifying change in species
Changes in genetic equilibrium
• Mutations cause genetic change
– Caused by environmental factors such as
radiation, chemicals, or can simply occur
by change
• If a mutation is useful, it persists and
becomes part of the gene pool
• Lethal mutations cause death and are
quickly eliminated from the gene pool
Mutations
• Natural Mutations occur at a regular
rate.
• The number of differences between
the genetic material and different
species estimates how long ago two
species share a common ancestry
Types of Evolution
• Macroevolution: The generation of
major change in the assemblage of
organisms: speciation
• Microevolution: Changes in the
gene pool of a population that
result in changes in allele
frequencies; they arise without the
influence of selection pressure
Mechanisms of Evolution
Populations evolve, not individuals
Population Genetics
• When Charles Darwin developed his theory
of natural selection in the 1800’s, he did so
without knowing about genes
• The principles of today’s modern theory of
evolution are rooted in population genetics.
• Today we refer to the Synthetic Theory of
Evolution, which included the principled of
genetics
Genes and Populations
• Gene pool: The collection of genes in a population
– Because diploids have only two versions of each gene,
each has only a small fraction of possible alleles in a
population
• Genotype: The genetic makeup of an individual at a given
locus, taking into account the two possible alleles
– Genotype frequency is the proportion of a given
genotype in the population
– Allele frequency refers to the proportion of a particular
allele, such as A or a
• Phenotype: the traits of an individual
– Phenotype frequency is the proportion of a given
phenotype in the population
– Phenotype frequency is influenced by the dominance
characteristic of an allele
Disruption to Genetic Equilibrium
• Genetic Drift – alteration of allelic
frequencies by chance events
• Can greatly affect small populations
– Ex. Amish of Lancaster County
Pennsylvania – 6 fingers and toes
• Individuals in this community have a 1 in 14
chance of having this mutation
• Individuals in the larger population of the
United States only have a 1 in 1,000 chance of
having the mutation
Disruption to Genetic Equilibrium
• Gene flow – the transport of genes by
migrating individuals
– When an individual leaves a population, it
takes its genes with it
– When an individual enters a population, it
introduces new genes
Alleles and Population
Genetics
• Although individuals are affected by the process of
natural selection, it is the makeup of the population
that is critical for determining the subsequent
generations
• Changes in the gene pool refer to changes in the
frequency of the alleles
• If the allele frequencies in a population do not
undergo change over time, we say that the
population is in genetic equilibrium, the population
is not evolving.
Population Stability
• The conventional view might be that
dominant alleles would eventually come to
dominate the gene pool, and the recessives
disappear
• They do not necessarily do so; in fact, allele
frequencies change only when influenced
by other factors.
• The stability of populations over time is
explained by the Hardy-Weinberg
Equilibrium.
Hardy Weinberg Principle
• This condition can be modeled as the
Hardy-Weinberg Equilibrium, which requires:
–
–
–
–
–
Large population size
No migration
Random mating
No net mutations
All genotypes have similar selective value
• This is idealized and rarely actually occurs,
but is a useful tool
The Ps and Qs of H&W
• Imagine 2 alleles, A and a
– p is the frequency of A
– q the frequency of a
• So, p + q = 1
• The mathematical equivalent of a random
mating can be given by multiplying this
relationship by itself
• Therefore, (p + q)2 = 1 = p2 + 2pq + q2
– p2 = frequency of AA
– 2pq = frequency of Aa
– q2 = frequency of aa
• Given this condition, we can always work out the
frequencies of each allele in a sexual population
that is not evolving.
H-W: Example
• Remember, since (p + q)2 = 1
• p2 + 2pq + q2 = 1
• Let’s say that a population has the
following genotypic and allelic
frequencies
• Note how all frequencies add up to
1.0
A Fun Experiment in Class
• Tongue rolling is described by a simple dominant character, T
and we can study the HW equilibrium using this trait, in this
class
1. Find the frequency of homozygous recessives (q2)in the class
– Can you roll your tongue? If so you are either TT or Tt
• Note how many can roll tongues ________
– If not, you are tt
• Note how many cannot roll tongues ________
• Take this number and divide by the class total: ______;
this is the frequency of homozygous recessives (q2).
2. What is the frequency of p?
– Since p + q = 1, then 1 - q = p
– Take the root of q2 from above. ___________
– We can now calculate p. p = 1 - q
5 Factors Upset Genetic
Equilibrium
• Mutation
•
•
•
•
•
Nonrandom mating
Genetic Drift
Gene Flow
Natural Selection
All of these are conditions that were required by the
H-W equilibrium to NOT occur
• They cause changes in allelic frequency, and result
in microevolution
• They all occur routinely
Population Genetics
• Populations evolve not individuals!
– All changes in a population occur at the
gene level
• Variations lead to adaptations
– Adaptations that are beneficial become
more common in the population
• Genotype frequency is the proportion
of a given genotype in the population
Things that lead to change in
populations
• Reproductive Barriers
– Sexual selection - male/female choice in
mates
– Reproductive isolation
• Prezygotic isolation
– Geographic Isolation – when a physical barrier
divides a population
– Ecological and behavioral barriers
• Postzygotic isolation
– Hybrid offspring cannot develop or reproduce
Speciation
• Speciation occurs when a population
diverges, and can no longer breed
and produce viable offspring, often
due to reproductive isolation
– Allopatric speciation – when a physical barrier
divides one population into two or more
populations. The separate populations will
eventually contain organisms that, over time will
no longer be able to breed successfully
– Sympatric speciation – when a species evolves
into a new species without a physical barrier.
Sometimes based on resource utilization,
sometimes caused by polyploidy in plants.
Patterns of Evolution
• Adaptive radiation (aka: divergent
evolution)
– results when populations adapting to different
environmental conditions change , becoming
less alike as they adapt, resulting in new species.
– In other words, when an ancestral species
evolves into an array of species to fit a number of
diverse habitats
• Ex. Hawaiian Honeycreepers, Galapagos finches,
African cichlids
• Convergent Evolution – when distantly
related organisms evolve similar traits
– Occurs when unrelated species occupy similar
environments
– Similar environmental pressure  Similar pressures
of natural selection
Patterns of Evolution
• Coevolution – When the evolution of
one species affects the evolution of
another species.
– Mutualism is a form of coevolution
– Hummingbirds and flowers
– Poisonous newts and the predators that
can eat them
Speciation can occur quickly or
slowly…
• Gradualism – the idea that species originate
through a gradual change of adaptations
• Punctuated Equilibrium – hypothesis that
speciation occurs relatively quickly, in rapid
bursts, with long periods of genetic
equilibrium in between
– Environmental changes, or introduction of
competitive species can lead to rapid changes
– Happens quickly – in about 10,000 years or less
• Both gradualism and punctuated
equilibrium are supported by fossil evidence
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