55-100 Lecture Notes - Homologous Chromosome, Sexual Selection, Directional Selection
5 pages72 viewsFall 2012
Chapter 22: The Mechanisms of Evolution
22.1 What Facts Form the Base of Our Understanding of Evolution?
•Darwin developed the major features of an explanatory theory for
evolutionary change based on two major propositions:
oSpecies are not immutable; they change over time.
oThe process that produces these changes is natural selection.
•Darwin also observed that, although offspring ten to resemble their parents,
the offspring of most organisms are not identical to one another or to their
•He suggests that slight variations among individuals affect the chances that a
given individual will survive and reproduce natural selection: differential
contribution of offspring to the next generation by various genetic types
belonging to the same population.
•Individuals do no evolve; populations do.
•A population is a group of individuals of a single species that live and
interbreed in a particular geographic area at the same time.
Adaptation has two meanings
•Refers both to the processes by which characteristic that appear to be useful
to their bearers evolve and to the characteristics themselves.
•With respect to characteristics, an adaptation is a phenotypic characteristic
that has help an organism adjust to conditions in its environment.
Population genetics provides an underpinning for Darwin’s Theory
•We cannot directly observe the genetic composition of organisms; what we
do see in nature are phenotypes, the physical expression of organism’ genes.
•The features of a genotype are its characters (e.g. eye colour).
•The specific form of a character (e.g. brown eyes) is a trait.
•A heritable trait is a characteristic of an organism that is at least partly
determined by its genes.
•A population evolves when its individuals with different genotypes survive or
reproduce at different rates.
•Population genetics has three main goals:
oTo explain the origin and maintenance of genetic variation
oTo explain the patterns and organization of genetic variation
oTo understand the mechanism that cause changes in allele frequencies
•Different forms of a gene, alleles, may exist at a particular locus.
•At any particular locus, a single individual has only some of the alleles found
in the population to which it belongs.
•The sum of all copies of all alleles at all loci found in a population constitutes
its gene pool.
•The gene pool contains the genetic variations that produce the phenotypic
traits on which natural selection acts.
Most populations are genetically variable
•Nearly all populations have genetic variation for many characters.
Evolutionary change can be measure by allele and genotype frequencies
•Allele frequencies are usually estimated in locally interbreeding groups,
Mendelian populations, within a geographic population of a species.
•Allele frequency = p = (number of copies of the allele in the pop. / sum of
alleles in the pop.)
•If only two alleles for a given locus are found among the members of a diploid
population, they may combine to form three different genotypes
polymorphic (more than one allele).
•The frequencies of different alleles at each locus and the frequencies of
different genotypes in a Mendelian population describe that population’s
The genetic structure of a population does not change over time if certain
•If an allele is not advantageous, its frequency remains constant from
generation to generation.
•The Hardy-Weinberg equilibrium describes a model situation in which
allele frequencies do not change across generations and genotype
frequencies can be predicted from allele frequencies (must apply to sexually
reproducing organisms). Several conditions must exist for a population to be
at Hardy-Weinberg equilibrium:
oMating is random.
oPopulation size is infinite.
oThere is no gene flow.
oThere is no mutation.
oNatural selection does not affect the survival of particular genotypes.
•The Hardy-Weinberg equation: p2 + 2pq + q2 = 1.
Deviations from Hardy-Weinberg equilibrium show that evolution is
•The patterns of deviation from Hardy-Weinberg equilibrium can help us
identify specific mechanisms of evolutionary change.
22.2 What are the Mechanisms of Evolutionary Change?
Mutations generate genetic variation
•Origin of genetic variation is mutation: any change in an organism’s DNA.
•Mutations appear to be random with respect to the adaptive needs of
•Most are harmful to their bearers or neutral, but if environmental conditions
change, previously harmful or neutral alleles may become advantageous.
•Can restore to populations alleles that other evolutionary processes have
•Create and help maintain genetic variation within populations.
•Mutation rates are very low for most loci: one in a million is a typical chance.
Gene flow may change allele frequencies
•Migration of individuals and movements of gametes between populations,
gene flow, are common.
•If the arriving individuals or gametes survive and reproduce in their new
location, they may add new alleles to the gene pool of the population, or they
may change the frequencies all alleles already present if they come from a
population with different allele frequencies.
Genetic drift may cause large changes in small populations
•Genetic drift—random changes in allele frequencies—may produce large
changes in allele frequencies from one generation to the next.
•Populations that are normally large may pass through occasional periods
when only a small number of individuals survive population bottlenecks;
genetic variation can be reduced by genetic drift.
•Genetic drift can have similar effects when a few pioneering individuals
colonize a new region.
oB/c of its small size, the colonizing population is unlikely to have all the
alleles found among members of its source population.
oThe resulting change in genetic variation, founder effect, is
equivalent to that in a large population reduced by a bottleneck.
Non-random mating changes genotype frequencies
•Mating patterns may alter genotype frequencies if individuals in a population
choose other individuals of particular genotypes as mates (non-random
oFor example, if females mate with males of the same genotype, then
homozygous genotypes will be overrepresented and heterozygous
•Self-fertilization (selfing), another form of non-random mating.
•Sexual selection is a particularly important form of non-random mating that
DOES change allele frequencies and often results in adaptations.
22.3 What Evolutionary Mechanisms Result in Adaptation?
•Adaptation occurs when some individuals in a population contribute more
offspring to the next generation that others allele frequencies in the
population change in a way that adapts individuals to the environment that
influenced such reproductive success (natural selection).
•Natural selection acts on the phenotype.
•The reproductive contribution of a phenotype to subsequent generations
relative to the contributions of other phenotypes is called its fitness.
•Changes in absolute numbers of offspring are responsible for increases and
decreases in the size of a population, but only changes in the relative success
of different phenotypes within a population lead to changes in allele
frequencies from one generation to another.
•The fitness of a phenotype is determined by the average rates of survival and
reproduction of individuals with that phenotype.
Natural selection produces variable results
•Natural selection can act on characters with quantitative variation in any one
several different ways:
oStabilizing selection—favouring average individuals
oDirectional selection –favouring individuals that vary in one direction
from the mean pop.
oDisruptive selection—favouring individuals that vary in opposite
directions from the mean of the population.
Sexual selection influences reproductive success
•Sexual selection is a special type of natural selection that acts on
characteristics that determine the reproductive success.
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