How Populations Evolve II
Individuals can adapt to their environment but they cannot evolve
Although natural selection acts on individuals, it is the population that evolves over time
as adaptive traits become more common in the group and other traits change or disappear
Thus, evolution refers to generation-to-generation changes in populations
Natural selection is more an editing process than a creative mechanism
Natural selection can only amplify or diminish heritable traits
Natural selection is not goal-directed; it does not lead to perfectly adapted organisms …
because things change
A trait that is favorable in one situation may be useless - or even detrimental - in
different circumstances
Darwin did not know about Mendel’s findings
He did not know how the variations that are the raw material for natural selection
arise in a population or how they are passed from parents to offspring
Mutation
New alleles originate by mutation providing the genetic variation that serves as raw
material for evolution
Sometimes, duplication of a gene or small pieces of DNA through errors in meiosis
can provide an important source of genetic variation and eventually lead to new
genes with novel functions
Populations Evolve
At the population level, biologists focus on the gene pool, which consists of all the
alleles, for all the genes, for all the members of the population
Gene Pools
Consider a wildflower population with two varieties of different colored
blooms
The allele for red flowers (R) is dominant to the allele for white flowers (r)
If these are the only two alleles affecting flower color and this population is
stable (not evolving) for flower color, then genotype frequencies can be
calculated from the allele frequencies
Let p = ƒ (R) and q = ƒ (r)
Then p + q = 1
The Hardy-Weinberg formula, p2 + 2pq + q2 = 1, can be used to calculate
the frequencies of genotypes in a gene pool from the frequencies of the
alleles
Public health scientists use the Hardy-Weinberg equation to calculate the carrier
frequency for certain inherited diseases
PKU
is a recessive disorder that prevents the breakdown of the amino acid
phenylalanine and
occurs in about one out of every 10,000 babies born in the United States
Microevolution
A nonevolving population is in genetic equilibrium, also known as Hardy-Weinberg
equilibrium, in which the gene pool remains constant from generation to generation
and the frequencies of alleles (p and q) and genotypes (p2, 2pq, and q2) are
unchanged
Evolution requires a change in allele frequency within a population
That change in allele frequency is evolution at its smallest scale microevolution
So processes that affect the allele frequencies in a gene pool are agents of
microevolution
Genetic drift
Genetic drift is the chance alteration of allele frequencies in a population
It has its greatest effects on small populations
Bottleneck effect
Certain catastrophic events can reduce the size of a population drastically
The small surviving population may not be representative of the original
population’s gene pool
The reduced gene pool is now the starting material as the population once
again increases
The bottleneck effect will usually reduce the genetic variability in a population
The reduction in individual variation will reduce adaptability and can
threaten the viability of the species
The founder effect
Genetic drift in a new colony can lead to allele frequencies that are drastically
different from the population that the founders left
Gene flow
Gene flow occurs from the movement of genes (alleles) from one population to
another
This is due to migration
The Hawaiian islands were formed through volcanic activity
Yet we find amazing examples of biodiversity
Like silverswords
Natural selection
Only natural selection consistently leads to adaptive evolution
Those individuals in a population that are better suited to the environment will be
more likely to survive and reproduce
Their contributions to the gene pool in subsequent generations will be greatly than
those that are less well suited
Evolutionary fitness is determined by the number of offspring an individual produces
The more offspring surviving and reproducing, the greater the impact on the gene
pool
Fitness only applies to a specific environment at a specific time
If the environmental conditions change, the relative fitness of an individual can
change as well
So “survival of the fittest” is misleading
It should really be “survival of the fit-enough”
So changes in allele frequencies can occur from generation to generation
Can we see phenotypic changes as well?
Peter and Rosemary Grant have studied the finches on the tiny Galapagos island of
Daphne Major
Every year, they capture each bird, take various physical measurements, and then
release the bird
In 1976, there were many more birds than there were in 1978
There were also some profound differences between the birds in 1976 and the
ones in 1978
That small difference allowed the finches to eat the tougher seeds that were
present after the drought
Natural selection does not create advantageous traits
It can only act upon the existing genetic variation
But it’s all about timing
Three General Outcomes of Natural Selection
When natural selection occurs, what are the possible results?
Results of natural selection
Directional selection
Disruptive selection
Stabilizing selection
Sexual selection
Sexual selection is a special form of natural selection
It occurs when differences in reproductive success are tied to differential success in
mating
Sexual dimorphism is a distinction in appearance between males and females not
directly associated with reproduction or survival
Does sexual selection always select for those that are the most fit?
Sometimes, it’s an evolutionary trade-off