Lecture Six: Causes of Evolution
The Five Criteria for a Population to NOT EVOLVE
A population will NOT evolve if: it is infinitely large
there are no mutations
mating in the population is completely random
there is no immigration into the population or emigration out of it
there is no natural selection
Criterion 1. INFINITELY LARGE POPULATION SIZE
The larger the population, the larger the number of genetically different gametes
possible.
But in any population, the babies actually born are not numerous enough to carry
EVERY SINGLE GENE of the preceding population.
The smaller the population, the smaller the number of genetically different gametes
possible.
In very small population, this can cause a change in genes of the population just due to
random chance (random sampling error).
When a population evolves only because of small size resulting in random changes in
the populations genes, GENETIC DRIFT is said to be occurring.
Two basic "flavors" of genetic drift:
Founder Effect: a small sample of breeding individuals from a large population
colonizes a new area and stops breeding with the original population.
Bottleneck Effect: a large population is almost wiped out (due to something like a
hurricane, volcanic eruption, pathogen invasion or other catastrophe) except for a few
lucky individual survivors. They form the (non-representative) new population.
Criterion 2. NO MUTATION
Mutation is the raw material of evolution. Without genetic variation, no evolution.
The phenotype (for a particular trait) that is most common in a particular wild
population is known as the wild type.
Any allele other than the wild type is said to be mutant.
Examples of wild type traits in various species:
* agouti fur in wild rodent populations
* red eyes in wild Drosophila (fruit fly) populations
* black and white feathers in penguins
* orange and black fur in wild tigers
Mutant forms of each of these wild types exist, and may or may not confer a
selective advantage in wild populations.
* mutation is the only way new genetic material can arise in a population
* the larger the population, the more likely mutations will occur
* mutations may result in phenotypic traits that may be
* adaptive
* maladaptive
* neutral
...in the particular environment in which they occur.
Heterozygote advantage
This happens when the heterozygous genotype (Aa) of a particular gene is more
adaptive than either homozygous form (AA or aa). example: Sickle Cell Anemia
Somewhere in ancient West Africa, a mutation of the gene coding for normal
hemoglobin (in humans) occurred. It was recessive. We'll call it h, and the normal
version of the gene is H.
If two copies of the mutant gene are inherited, the person getting those copies will
have a disease called Sickle Cell Anemia. It is ultimately fatal.
In the area where the mutation occurred, a parasitic disease known as malaria took
many lives with a terrible fever. People with the genotype HH are highly susceptible to
infection with the parasite.
But a person with the genotype Hh is immune to malaria, and does not have
symptoms of sickle cell anemia!
The recessive allele, maladaptive in homozygous condition, is not removed from the
population because when it occurs in heterozygous condition, it is adaptive.
Criterion 3. RANDOM MATING
If you're considering a particular gene in a population of wild mice (let's say gene A,
with a recessive allele a; A codes for agouti fur and a codes for black fur), then if mating
is random, the two alleles will combine (in zygotes) at random.
However...
# If agouti mice are more likely to mate with agouti mice than with black mice, we say
positive assortative mating is taking place. (Like mates with like.)
# If agouti mice are more likely to mate with black mice than with agouti mice, we say
negative assortative mating is taking place.
# INBREEDING occurs when matings occur between related individuals significantly
more often than they occur between unrelated individuals chosen at random from within
the population. (This is a form of positive assortative mating)
# OUTBREEDING is the opposite: individuals mate with non-relatives more often than
would be predicted by random chance.
Inbreeding greatly increases the chances of recessive alleles being inherited in
homozygous form, and hence, showing up in the phenotype.
# If the recessive allele is deleterious (harmful), then the disadvantage is obvious.
Note that small, isolated populations (such as island populations separated from the
mainland) will eventually consist of members who are related to one another. This leads
to inbreeding. A SMALL, ISOLATED POPULATION WILL GENERALLY EVOLVE
MUCH MORE RAPIDLY THAN A LARGE POPULATION.
Criterion 4. NO MIGRATION:
Loss or addition of alleles from immigration or emigration will change the allele
frequency in the population .
Gene Flow is the process by which genes move between populations.
* gene flow spreads novel alleles that have arisen via mutation
* gene flow has a homogenizing (you know what "homogenized" means, right?) effect
if a recipient population is small relative to a donor population.
Lack of gene flow may eventually lead to speciation, but the rate at which this occurs
depends on the species
A hybrid zone is an area of secondary contact, where there may be limited
hybridization between two separate species that have come into contact after having
been separated and been subject to some degree of reproductive isolation.
# Introgression is the introduction of alleles from one species' population into another
(closely related) species, due to limited hybridization.
The Tale of Bambi and That Other Guy.
Why do some species that share many genes remain distinct in appearance, behavior
and reproduction, whereas others that have been separate for millions of years are still
able to hybridize? WE DON'T KNOW. It's one of life's little mysteries.
Criterion 5. NO NATURAL SELECTION
No genotype confers a reproductive advantage over another genotype.
Recall the tenets of Evolution by Natural Selection:
1. Organisms are capable of producing huge numbers of offspring
2. Those offspring are physically variable, and the variation is heritable
3. Those offspring must compete for limited resources
4. Those whose phenotypic characters allow them to best exploit those limited
resources will leave the most genes to succeeding populations.
An individual's DARWINIAN (EVOLUTIONARY) FITNESS is a measure of the
proportion of genes it contributes to succeeding generations.
Evolutionary fitness is defined by the environment. A phenotype which confers great
fitness today in a particular environment could be a liability if the environment changes
tomorrow!
An Interesting Type of Natural Selection: Sexual Selection
Sexual selection - This is a special case of natural selection based upon an individual's
relative ability to attract and mate with members of the opposite sex.
Sexual selection can operate in two ways:
1. Members of one sex compete against each other for mates. The ones that have
traits that make them better able to compete will leave more genes.
Example: Larger size of males in many mammal species.
Thick manes in male lions.
Antlers in male deer.
2. Members of one sex prefer a particular trait in the members of the opposite sex. The
individuals with that trait get more mating opportunities, and leave more genes.
This can result in SEXUAL DIMORPHISM
Examples: Bright colors in male birds
Different shapes of male and female humans.
Summing up...
Organic Evolution is change in the genetic composition of a population due to
# mutation
# genetic drift
# non-random mating
# gene flow
# natural selection.
MICROEVOLUTION: genetic change in a species over time without speciation
MACROEVOLUTION: the genesis of two reproductively isolated taxa from a single
ancestral taxon.
Some economically important examples of microevolution (many due to anthropogenic
factors) are occurring all around us, as we speak
* antibiotic resistant bacteria
* pesticide resistant insects (and other competitors of Homo sapiens)
So...you might ask, why can't larger organisms evolve resistance to poisons and other
selective factors. (Why couldn't birds, say, "evolve" an immunity to DDT, which causes
them to lay dangerously thin-shelled eggs?
1. GENERATION TIME
The shorter the cycle time between generations, the more opportunities there are for
genetic change and mutations to be incorporated into a population. (Vertebrates have
very long generation time in comparison to bacteria.)
2. EXAPTATION
Organisms cannot simply evolve a character because they "need" it. The genetic
machinery to create a phenotypically beneficial trait (in a particular environment) must
already exist in the gene pool in order to be selected. Such a pre-existing trait which
may confer a selective advantage under certain circumstances is known as an
EXAPTATION.
It all depends on whether mutations potentially conferring a selective advantage ever
occur in the genome. Sometimes (perhaps most of the time!), they simply do not.