Transcript Microevolution: How Does a Population Evolve?

Microevolution: How Does a
Population Evolve?
Chapter 16
The Evolution of Evolution.
Blending inheritance
Inheritance of Acquired Characteristics
Genetics alone causes evolution
Modern synthesis – evolution is due to
natural selection working on inherited traits
• Population genetics
• Microevolution – changes in the
frequency of the alleles of genes in a
population.
– Industrial melanism
• Macroevolution - the process by which
species of organisms originate, change
and go extinct.
• What is the source of variation within a
population?
– Either point mutations of genes or
chromosomal mutations
• If there is only one choice (allele) for a
gene, the population is homozygous for
that gene.
• If there are two or more choices (alleles)
for a gene, the population is polymorphic
for that gene.
• If the members of a population come in
two or more forms, the population is
polymorphic.
• Most human traits are polygenic –
controlled by many genes
– These traits vary smoothly and
continuously within a population.
– The graph of these traits is a bell curve.
•
In a changing environment, highly variable
populations evolve more rapidly than less
variable populations.
• The factors that determine the genetic
variability of a population are:
1. The rate at which mutations accumulate in
the DNA.
2. The rate at which changes spread through a
population (reproductive rate).
3. The rate at which deleterious mutations are
eliminated from a population by natural
selection.
How much genetic variation is
there?
• In humans about 25% of all proteins have
an alternate form which is present in at
least 5% of the population.
• In humans about 7 % of our genes are
heterozygous.
– Invertebrates -13%
– Plants -17%
– Drosophila – 25%
• Remember: Natural selection works only
on the Phenotype which is an interaction
of the genotype and the influences of the
environment (basically what the individual
looks like).
• Genetic variation is fuel for evolution
• Yet, natural selection favors those traits
best suited to the environment and weeds
out the rest.
• All of the genes of all the individuals in a
population is called the gene pool.
• Hardy-Weinberg principle: sexual
reproduction by itself does not change the
frequencies of alleles within a population.
Genotype frequencies stay the same from
generation to generation as long as
certain conditions are met.
• Hardy-Weinberg equilibrium:
• p + q = 1 and
p2 + 2pq + q2 = 1
Conditions:
1.
2.
3.
4.
There was random mating
There is a large population size
There are no mutations
There is no breeding with other
populations
5. There is no selection, either natural or
artificial
• In reality, these conditions are hardly ever
met, but it gives us a standard against
which to measure evolution.
• Of all the conditions mentioned, only
natural selection leads to adaptive change.
• The rest cause changes in gene frequency
which may or may not be adaptive.
1) Random mating
• Only practiced by organism which release
gametes on the wind or in the water.
• Assortative mating – based on choice
– May be without regard to one’s phenotype
– Positive assortative mating – choose
individuals like ourselves
• Inbreeding – increases the incidence of
recessive disorders, leading to a less
healthy population
– Negative assortative mating - outbreeding
2) Large population size
• Random drift or genetic drift is a change
in the allele frequency due to random
events. This is more likely in a small pop.
• Founder effect –a small subset of a
population founds a new population.
• Bottleneck effect – the population is
reduced to a few individuals by some
random disaster or harsh selection pressure
(such as over hunting).
• Causes new mutations to spread or be
removed.
3) No Mutations
• Does not happen in nature.
• The DNA copying mechanism is nearly
perfect, but mistakes are made.
• These mistakes result in new phenotypes
which are subjected to selection and the
basis for adaptive change,
4) No interbreeding between
populations
• Gene flow occurs as the result of
interbreeding between two populations.
• Individuals immigrate and bring new
alleles into the population.
• It increases the variation within a
population.
• It makes adjacent populations more alike.
4) No selection
• Natural selection
– Harmful genes are selected against
– Useful genes accumulate
Types of Natural Selection
• Directional selection –
selects for one end of the
bell curve
Types of Natural Selection
• Stabilizing selection
– the extremes of a
population are
selected against and
the average is
favored.
Types of Natural Selection
• Disruptive selection
– selects for extremes
and against the
average.
Disruptive selection
Taste good
Taste bad
Sexual selection
• Male competition
– Male competes against other males for territory, or
access to females
– Anything that gives him an advantage makes him
more likely to pass on his genes
• Female selection ( or male selection)
– Leads to sexual dimorphism
– Male must prove he is genetically good enough
– Plumage, gifts, nesting site or mating rituals
• Natural selection can also encourage
genetic variation when different alleles of a
gene are equally useful.
– Different local environment
– One allele is better at a certain time of year
– Balanced polymorphism
– Sometimes the superiority of the heterozygote
may maintain a high incidence of an allele
which is harmful to the homozygote
• Sickle cell anemia and malaria
Blue = malaria
Red = sickle cell anemia
Purple = overlap