Download 1. Natural selection can only occur if there is variation among

1. Natural selection can only occur if there is
variation among members of the same species.
WHY?
Variation in a population results from mutation and the
recombination of alleles during meiosis and fertilization.
Mutation
• New alleles arise
from the mutation
of existing alleles,
usually by changing
one or a few base
pairs
Meiosis:
Crossing over of homologous
chromosomes during prophase I results
in a recombination of maternal and
paternal alleles within chromosomes
Independent assortment as
homologous chromosomes randomly
orient at metaphase I causing a
randomized inheritance of maternal
and paternal chromosomes within
gametes
Fertilization:
• New combinations of alleles appear
during fertilization
• As the unique set of haploid alleles in
the egg
• Combine with the unique set of
haploid alleles in the sperm
2. Species tend to produce more offspring than the environment can support.
· observation 1: All species have such great potential fertility that their population size
would increase exponentially if all individuals that are born reproduced successfully.
· observation 2: Populations tend to remain stable in size except for seasonal fluctuations.
3. Individuals that are better adapted tend to survive and produce more offspring
while the less well adapted tend to die or produce fewer offspring.
• 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 generation
(= struggle for existence).
4. Individuals that reproduce pass on
characteristics to their offspring.
• Survival in the struggle for existence is
not random, but depends in part on
the hereditary constitution of the
surviving individuals. Those
individuals whose inherited
characteristics best fit them to their
environment are likely to leave more
offspring than less-fit individuals (=
natural selection).
• this unequal ability of individuals to
survive and reproduce will lead to a
gradual change in a population, with
the inheritance of favorable
characteristics accumulating over the
generations.
In other words:
Natural selection is differential success in reproduction ( unequal ability of
individuals to survive and reproduce).
5. Natural selection increases the frequency of characteristics that
make individuals better adapted and decreases the frequency of
other characteristics leading to changes within the species.
• evolution is the cumulative change in
the heritable characteristics of a
population, or
• the changes in allelic frequencies in
the gene pool of a population over
time, as a result of natural selection,
genetic drift, gene flow, and mutation
pressure.
6. Adaptations are characteristics that make an individual suited to
its environment and way of life.
• Natural selection occurs through an interaction between the environment and
the variability inherent among the individual organisms making up a population.
• The product of natural selection is the adaptation of populations of organisms to
their environment.
7. Application: Changes in beaks of finches on Daphne Major.
• yearly variation in rainfall in the Galapagos Islands can be extreme due to
the weather cycles known as El Nino
• in wetter years, all sizes of seeds are available in large numbers
• smaller birds with smaller beak sizes eat more efficiently and have
higher rates of surviving and reproducing
• smaller beaked birds pass on the trait of smaller beaks to their
offspring
• as a result, in the following year the average beak size decreases
• that is, the frequency of the smaller beak alleles increases over time
• in dryer years, smaller seeds are quickly consumed, leaving only larger,
harder seeds
• larger birds with larger beak sizes eat more efficiently and have
higher rates of surviving and reproducing
• larger beaked birds pass on the trait of larger beaks to their offspring
• as a result, in the following year the average beak size increases
• that is, the frequency of the larger beak alleles increases over time
• As predicted by evolutionary theory, natural selection produces gradual
changes in traits in response to changes in the environment
8. Application: Evolution of antibiotic resistance in bacteria.
• shortly after development of antibiotics (e.g.
penicillin) nearly all bacteria were killed
during an application of antibiotics
• some variants of bacteria had a heritable
trait that gives them resistance to antibiotics
• the resistant bacteria have higher rates of
surviving and reproducing
• resistant bacteria pass on the trait of antibiotic
resistance to their offspring
• in each following generation the percentage of
antibiotic resistant bacteria increases
• that is, the frequency of the antibiotic
resistance alleles increases over time
• As predicted by evolutionary theory, natural
selection produces gradual changes in traits
in response to changes in the environment