what is Natural Selection

Download Report

Transcript what is Natural Selection

What is Natural Selection and What conditions make
it possible?
From On The Origin of Species
“Can it, then, be thought improbable, seeing that variations
useful to man have undoubtedly occurred, that other variations
useful in some way to each being in the great and complex
battle for life, should sometimes occur in the course of
thousands of generations? If such do occur, can we doubt
(remembering that many more individuals are born than can
possibly survive) that individuals having any advantage,
however slight, over others, would have the best chance of
surviving and of procreating their kind? On the other hand, we
may feel sure that any variation in the least degree injurious
would be rigidly destroyed. This preservation of favourable
variations and the rejection of injurious variations, I call Natural
Selection (pp.80-81)
Major points
Organisms in a population have heritable variations
More organisms are born than resources or
environment can support
Organisms with traits that give a survival
advantage will be favored
Favored organisms will reproduce more
From National Science Education
Evolution is the consequence of the interaction of
The potential of species to increase in number
The genetic variation among offspring due to
mutation and recombination of genes
A finite supply of the resources and stable
environments required for life
The ensuing selection of those offspring better
able to survive limited resources and changing
environments and leave offspring
Let’s explore a scenario
A farmer sprayed his barn and cattle with a solution of
insecticide A. The insecticide killed nearly all the flies.
Sometime later, however, the number of flies was again
large. For a second time, the farmer sprayed with
insecticide A. The result was similar to that of the first
spraying. Most, but not all, of the flies were killed.
Again, within a short time, the population of flies
increased, and they were again sprayed with
Insecticide A. This sequence of events was repeated
five times; then it became apparent that insecticide A
was becoming less and less effective in killing the flies.
Pair up and discuss
Explain what happened
Darwins Evolution
Evolution by Natural Selection was first proposed by
Charles Darwin in 1859
Publication of The Origin of Species
Also proposed by Alfred Wallace who published a
paper at the same time
Figure 22.1 The historical context of Darwin’s life and ideas
Let’s follow a logical argument
Observation 1: All species have great potential
fertility & their population size would increase
exponentially if all individuals born, reproduced
Observation 2: Populations tend to remain stable in
size except for seasonal fluctuations
Observation 3: Environmental resources are limited
Inference 1: Production of more individuals than the
environment can support, leads to a struggle for
existence among individuals of a population with
only a fraction of offspring surviving each
Observation 4: Individuals of a population vary in
their characteristics. No two individuals are alike
Inference 2: Survival in the struggle for existence is
not random & depends in part on the heredity of
the individuals. Those individuals whose inherited
traits best fit them to their environment are likely to
leave more offspring than less fit individuals.
Inference 3: This unequal ability of individuals to
survive and reproduce will lead to a gradual
change in a population with favorable
characteristics accumulating over the generations
Dark moths on light colored bark are easy targets for
hungry birds but are hidden on pollution darkened trees.
Consequences of Overproduction of
Food might become scarce
Territories might be limiting for both mating and
Density might get so great that disease and parasites
would become epidemics
Predator populations will also grow because of the
increase in population size of prey, and begin to
whittle down the herd.
Why do organisms vary?
Living organisms vary as a result of sexual reproduction
 Meiosis allows a large variety of genetically different gametes to
be produced by each individual (2n) where n = haploid
chromosome #
23 = 8,000,000 different gametes
 E.g. Humans 2
without crossing over.
 This occurs through segregation of maternal and paternal
chromosomes and crossing over in prophase I of meiosis (exchange
of pieces of chromosomes).
 Fertilization allows alleles from 2 different individuals to be
brought together in one new individual (1/8,000,000 x
Evolution in Response to Environmental
Antibiotic resistance in Bacteria:
In 1950, Japanese physicians began to notice that some
patients suffering from bacterial dysentery, which
produces severe diarrhea, did not respond to antibiotics
which had been effective in treating this type of infection
Apparently resistance to the antibiotics had evolved in
certain strains of shigella the pathogen
Eventually, researchers identified the specific genes that
conferred antibiotic resistance
Antibiotic resistance II
Some of these genes coded for enzymes that destroyed
certain antibiotics such as tetracycline and ampicillin.
These genes turned out to be carried on plasmid, now
called R plasmids for resistance.
Exposure of a bacterial population to a specific antibiotic
will kill antibiotic sensitive bacteria, but not those that
have the R plasmid
The theory of Natural Selection predicts that “The fraction
of the population carrying the R plasmid will increase”
Antibiotic resistance III
Resistant strains have increased since that time
R plasmids can pass from one bacterial cell to another
through conjugation (bacterial sex)
Some R plasmids carry as many as 10 genes for
resistance to that many antibiotics
This can occur because of transposons, or transposable
elements or “jumping genes”
In bacterial cells transposons can move within
chromosomes, from the plasmid to the chromosome or
vice versa, and from plasmid to plasmid
Antibiotic resistance IV
Transposons bring multiple genes for antibiotic resistance
into a single R plasmid by moving the genes to that
location from other different plasmids.
A sort of bacterial recombination for favorable mutations
Without invoking transposons, when doctors encounter an
antibiotic resistant strain of bacteria they simply switch
Resistance to the new antibiotic soon develops so another
switch is made.
Resistance to this soon develops…
This is how multi drug resistant bacterial strains have
Other examples of Natural Selection
Evolution of Insecticide-resistant Insects
Occurred in 100’s of species of insects
Insecticides used to kill insect pests in farmlands,
swamps, backyards & homes
Some examples include DDT, now banned in many
countries & malathion
Early results were encouraging  small amounts dusted
on a crop may kill 99% of insects
Evolution of Insecticide – Resistant Insects
Subsequent spraying was less and less effective
Resistant animals weren’t killed and their offspring
inherited the genes for insecticide resistance
In each generation the proportion of resistant
insects increases.
The population has adapted to a change in its
Figure 22.12 Evolution of insecticide resistance in insect populations
Other examples:
Evolution of drug resistant HIV strains
Metal tolerance in plants – see ecology and
evolution review packet
Figure 22.13 Evolution of drug resistance in HIV
Survival of the Fittest?
From www.whimsical-art.com