Evolution
Unit 5 B
The
evidence for evolution
“There is grandeur in this view of life”,
Evolution
may be broken down into two fields of study.
Microevolution studies the change in allele
frequencies in a population over time. That certain species of bacteria are
becoming more resistant to antibiotics is a good example of microevolution.
Macroevolution is the study of the
changes in species over time (speciation).
The definition of a species is not without
controversy. There are at least six concepts of species, but for our purposes,
the biological concept of species will suffice. This concept of species states
that a species consists of a population or group of populations whose members
have the potential to interbreed with one another in nature to produce viable,
fertile offspring.
A species
is the largest biological unit in which genetic exchange can occur and in that
is reproductively isolated from other populations. A population is the smallest biological unit that can evolve.
Individuals
are selected, but populations evolve.
Evidence for evolution pervades biology.
1.
Biogeography
(geographical distribution of species)
- island populations
- South American tropical animals
-
2. Fossil
record
- transitional fossils
have been found
- there is a compelling
order to chronological appearance of fossils
3.
Comparative anatomy
“Ontogeny recapitulates phylogeny” - comparative
embryology
Homologous structures – derived from a common
ancestral structure (vertebrate forelimbs)
Analogous structures – derived from different
ancestors but fulfill the same ecological/environmental need.
fins and wings
Vestigial organs – remnants of structures that
serve little or no purpose to the organism, ex. remnants of pelvic girdles and
legs in snakes; the human appendix
4. Molecular
biology
- protein comparison, DNA
analysis, DNA-DNA hybridization, fossilized DNA, molecular clocks
Evolution predicts that related species should
have a greater similarity in DNA sequences than unrelated species. This is
evident in DNA analysis. Molecular
biology has unveiled the unity and diversity of life.
5.
Industrial melanism – The English peppered moth, Biston betularia,
experienced natural selection due to changes in the environment brought about
by pollution.
6.
Antibiotic resistance in bacteria – resistant bacteria become more prevalent due to
selection pressures
7.
Hybridized plants
– the tolerance of many plants to polyploidy and hybridization has led to the
commercial development of new species, including bread wheat (from three
ancestors) and the wild mustard family (cabbage, cauliflower, broccoli, brussel sprouts, kale and kohlrabi).
8. New
investigations
– ongoing research is gathering evidence of speciation events in Drosophila, Rhagoletis,
and Nereis, all animal species.
Hardy-Weinberg
Equilibrium
To observe microevolution, one must be able to
compare an evolving population to a non-evolving population.
This baseline was established by Hardy and
Weinberg in the early 20th century. This began the field of study
known as population genetics.
The gene
pool of a population represents the total aggregate of genes in a population
at any given time. In other words, the gene pool consists of all the alleles of
all genes in all individuals in the population. These alleles will combine to
form the next generation.
A fixed
allele is an allele that is present at a frequency of 100% in a population,
i.e. all individuals are homozygous for that allele. Allele frequency may be
determined when there are two or more alleles present in the population.
The Hardy-Weinberg
Theorem states that allele frequency in a population will remain unchanged
from generation to generation unless acted upon by other agents. Their model, known as H-W equilibrium, describes the genetics of a non-evolving
population. This model serves as the baseline for determining whether
microevolution is occurring.
To maintain equilibrium, five conditions must be
met (memorize these!)
1. large population
2. no migration
3. no mutations
4. no natural selection
5. random mating.
These conditions do not exist in Nature, they are conditions of an ideal, non-evolving
population.
Causes
of microevolution
When one or more of these conditions are not met,
the population may experience microevolutionary
change.
1. Genetic drift – random changes in allele frequency in small
populations due to chance.
Why do
surveys usually ask 2 000 people for their opinion? Why not
20?
In a small population, chance events may cause the
allele frequency to change from generation to generation.
This is especially apparent in populations whose
numbers are less than 100.
Bottleneck
effect:
natural disasters may decimate a population. The small surviving population is
unlikely to be representative of the original population. Some alleles may be
overrepresented, some underrepresented, or some may be extinct.
Ex. elephant seals. The population was
reduced to 20 individuals through overhunting in the
late 19th century. Alleles are fixed at 24 different genes; there is
no variation at those loci. The surviving cheetah population in
Founder effect. When a
small subset of population migrates to a new area, the individuals form a
founding colony. The smaller the founding population, the less likely it is to
be representative of the parent population.
The disease porphyria variegata is much more common than in either the Dutch
population from whom the Boers are descended or the surrounding African
populations. This interesting disease is familiar to those of us who have seen
the film 'The Madness of King George'. In
2. Gene flow - migration in and out of areas may
cause the loss or gain of alleles. Gene
flow tends to reduce variations between neighbouring populations.
Gene flow is one of the concerns over genetically
modified food crops. If pollen from the GMO passes into wild populations, then
we may lose that natural diversity.
3. Mutation – a mutation by its
definition may change an allele. This may cause the loss or gain of an allele
in the gene pool. This in turn affects the frequency of other alleles at those
loci. Mutations are so rare that it cannot be considered to be the driving
force of evolution. Mutation does however introduce variation.
4. Natural selection – If phenotypes show differences in their ability
to produce viable, fertile offspring, then some alleles may be passed on in a
disproportionate manner to the next generation.
The peppered moth experience is a good example of
natural selection affecting allele frequency.
Variation in a population allows natural selection
to occur.
Environmental changes or pressures will tend to favour those phenotypes that can survive the changes.
Therefore, natural selection is considered to be the only agent of
microevolution that is adaptive.
5.
Non-random mating
– Non-random mating by itself will not change allele frequencies, but it has
the potential to increase the proportion of homozygotes
in the population.
I’ll explain in class.
There are three main categories of non-random
mating:
1. Disassortative:
opposites mate (like with unlike)
2. Assortative: (like
with like). In snow geese, it is common to see blue mate with blue, and white mate with white. Mating preference is also seen with body
size.
3. In-breeding: (I’ll tell you in class) mating
with a close relative
4. Self-breeding: the most extreme case of
non-random mating. Mendel’s pea plants commonly self-breed, that’s
why he had to set up his experiments so carefully.