Download The Means of Evolution Microevolution What Is It that Evolves? What

The Means of Evolution
Microevolution
What Is It that Evolves?
• Evolution occurs at the level of populations,
populations not
individual organisms—population consists of members
of species that occupy the same geographical region.
• Populations that become reproductively isolated (no
interbreeding, no gene flow) can evolve independently
and in divergent ways.
What Is It that Evolves?
• Mutations create new alleles. (Reminder: Alleles are
variant forms of a gene; each individual can have a
maximum of two alleles for each gene.) All alleles in a
population collectively constitute the gene pool;
genetic variation produced by gene mutation is the
raw material for evolution.
Evolution as a Change in the Frequency of
Alleles
• Microevolution (evolution at the smallest scale) refers to
change in allele frequency in population over time
(macroevolution refers to large-scale patterns in evolution, like
species formation; discussed later).
• Illustrate evolutionary divergence in population with example
(e.g., allele conferring camouflage coloration increases in
population of frogs in presence of predators, because animals
bearing that allele survive and transmit their genes to next
generation at higher frequency than animals lacking cryptic
color).
Essay: Detecting Evolution: The Hardy-Weinberg
Principle
• British mathematician, Godfrey Hardy, and a German
physician, Wilhelm Weinberg, working independently
early in the 20th century, arrived at an insight that in a
sufficiently large population, allele frequencies will
not change unless one of the five agents of
microevolution is at work.
Essay: Detecting Evolution: The Hardy-Weinberg
Principle
• Species of flower, two colors, red (R, dominant) and white (r,
recessive). Determine that 78% of alleles are R, 22% r. In the
next generation, we have three genotypes, RR, Rr, and rr.
Hardy-Weinberg helps us determine frequency of genotypes if
we know the allele frequencies for R and r.
• Conventional symbols in the equation: p will stand for the
frequency of the more common R allele and q will stand for the
frequency of the less common r allele.
Essay: Detecting Evolution: The Hardy-Weinberg
Principle
• The Hardy-Weinberg Equation . . . p2 + 2pq + q2.
• . Applied to this example means . . . R2 + 2Rr + r2.
– And this yields (0.78 x 0.78) = 0.6084 the frequency of the RR
genotype.
– Frequency of the Rr genotype = 2(0.78 x 0.22) = 0.3432.
– Frequency of the rr genotype = (0.22 x 0.22) = 0.0484.
Five Agents of Microevolution
• Allele frequency in population can change in response
to five evolutionary forces.
Five Agents of Microevolution
• Mutations.
– Create new alleles; rate is low; most mutations are neutral,
some deleterious, very few beneficial.
Five Agents of Microevolution
• Gene flow.
– Movement of alleles from one population to another due to
migration; common in nature.
Five Agents of Microevolution
• Genetic drift.
– Random change in allele frequency with the greatest impact on
smaller populations. Population crash can create genetic
bottleneck with loss of some alleles from population entirely
due to chance (e.g., Northern elephant seals; cheetahs in wild
face grim future due to loss of genetic variability). Also,
population can become limited in genetic variability if it’s
founded by a small number of individuals (founder effect; total
color blindness in 10 percent of population of Pingelap,
Micronesian island; polydactyly in Amish).
Five Agents of Microevolution
• Nonrandom mating
– Refers to situation in which some individuals reproduce
more than others, as when females prefer, and mate with,
males with certain traits (sexual selection). Alleles carried
by frequently reproducing members of population will
become more common. Assortative mating occurs when
males and females display distinct mating preferences (e.g.,
if short individuals mate with other short individuals), but
this does not change overall allele frequencies in the gene
pool.
Five Agents of Microevolution
• Natural selection.
– Acts by favoring adaptation; adaptation is modification in
structure or behavior of organisms over generations in
response to environmental change. It promotes survival, and
ultimately reproductive success, in that particular
environment. Alleles conferring adaptive traits increase in
frequency (e.g., zebra stripes have adaptive value—colorblind lions cannot readily distinguish zebras in savanna
grass that appears as a pattern of black and white to them).
Essay: Lessons from the Cocker Spaniel: The
Price of Inbreeding
• Idea of inbreeding and genetic drift in example of Cocker
Spaniels – estimated that 25% of “purebred” dogs have some
serious genetic defect.
What Is Evolutionary Fitness?
• Concept of fitness refers to an individual’s success in
passing on genes to offspring relative to other
members of that population, in a specific
environment.
What Is Evolutionary Fitness?
• Studies of Galapagos finches by Peter and Rosemary Grant
illustrate change in allele frequency, adaptation, fitness, and
natural selection, taking place “before our very eyes.”
Survivors of drought on the island had beaks 6 percent larger
than those of pre-drought population of birds; large bills
allowed them to gather the large, tough seed that was
prevalent under dry conditions; offspring of drought survivors
had beaks 4–5 percent larger than pre-drought population, so
allele frequencies are changed in next generation.
What Is Evolutionary Fitness?
• There’s no overall “fittest” phenotype for any
organism. A trait is favored in specific
environment if it favors survival and
reproductive success under those conditions.
Three Modes of Natural Selection
• When natural selection acts on polygenic traits (that show a
normal distribution), it can follow one of three trends.
Three Modes of Natural Selection
• Stabilizing selection.
– Intermediate characteristics are favored (e.g.,
human babies with very high or very low birth
weights have lower survival than babies with
intermediate birth weights; most babies have
intermediate birth weights because of natural
selection against the extreme phenotypes).
Three Modes of Natural Selection
• Directional selection.
– One of the two possible extremes is favored (e.g.,
dark-colored peppered moths in regions of England
with industrial pollution; smoothness of brachiopod
shells, emerging over evolutionary time).
Three Modes of Natural Selection
• Disruptive selection.
– Natural selection moves characteristics toward
both extremes, and intermediate phenotypes
become rarest (e.g., populations of West African
birds with either large or small, but not intermediate
size, beaks).