Download Gene Frequency and Speciation

BIO 120
Field Natural History
Spring
1
LECTURE 7
Gene Frequency & Speciation
I. Concept of species.
A. Biological Species Concept
1. Species are groups of a) actually or potentially interbreeding natural populations
b) that are reproductively isolated from other such groups and
c) that are capable of producing fertile offspring (offspring that can reproduce).
2. The definition of a population is a group of interbreeding organisms that occupy a given
geographic area.
3. Biological species concept helps eliminate the problems of environmental effects on
genotype expression.
B. 3 key concepts that support the biological species concept
1. The geographic area of the population can be world-wide.
2. Fertile offspring is one key component of the concept of species.
a. If a female horse and a male donkey produce an offspring, it is called a mule.
b. If the cross is between a female donkey and a male horse, it is called a hinny.
c. Both are sterile.
d. A cross between to species is called a hybrid.
3. The mating must occur naturally.
II. A basic source of variation among individuals – one step up from base pairs.
A. Concept of genes.
1. The field of genetics is defined as the study of factors that determine the characteristics of
an organism and how they are inherited.
2. The biological unit that geneticists are interested in is the gene.
a. Recall, that a gene is a series of bases along a strand of DNA that code for a particular
protein or proteins.
B. Concept of alleles.
1. Different instructions for the same trait (gene) are called alleles.
2. But, not all alleles are expressed, even though there are two. In fact, one allele is sometimes
expressed while the other is not.
a. A dominant allele expresses itself and hides the activity of the other allele.
b. A recessive allele, when present with a dominant allele does not express itself.
c. A recessive allele will only be expressed when there are two copies of the recessive
allele.
3. For example, the trait for a widow’s peak is governed by a dominant allele (W).
a. If you inherited a dominant allele (W) and a recessive allele (w), you would have a
widow’s peak (Ww).
b. But, if you had two recessive alleles (ww) for hairline, your hairline would be straight.
4. Not all traits are based on two alleles. There may be more than two.
a. For example, blood type is based upon 3 alleles that produce 4 different blood types; A,
B, AB and O.
BIO 120
Field Natural History
Spring
2
5. Gene pool - all of the alleles of all of the organisms in a population.
a. Example of gum ball machine.
b. Each gum ball color represents a separate allele.
c. In this example, there are only four colors, or four alleles.
(Red, yellow, green and blue]
d. Inserting a quarter gives you only 2 gumballs, i.e. 2 alleles – one from the sperm and
one from the egg.
e. Given that there are only 4 colors, there are 10 possible color combinations. Each time
you insert a quarter, you can only receive one of the 10 possible combinations.
6. Allele frequency is the frequency with which an allele shows up in the population.
7. Just because a trait is dominant, does not mean it is the most prevalent.
a. Its frequency in the population may be low. e.g dwarfism, six fingers.
III. Speciation – the process of generating a new species.
1. In nature, a species may be widely distributed with many local populations.
a. Each of these local populations may have a slightly different allele frequency.
b. These differences in allele frequency may confer slightly different advantages,
depending upon the local environment.
2. Example with Sugar Maple- i) northern ecotype ii) central ecotype, iii) southern ecotype
a. Ecotype – a genetic variety within a species that arises as a result of adaptations to the
local environment. Usually inter-fertile (can interbreed) but generally prevented from
doing so by ecological barriers.
3. The differences in these three ecotypes suggests that a different set of alleles is better suited
in one type of environment than another.
4. If a barrier to interbreeding is introduced among the 3 ecotypes, alleles that are not suited for
that particular environment will be lost from the local population.
5. If the new, isolated population remains isolated long enough, it will not be able to produce
fertile offspring if reintroduced to the original population.
6. Speciation occurs when an isolated population becomes genetically separate from
another population.
7. Genetic isolation is maintained by:
a. Ecological isolation. They are physically isolated from one another.
b. Seasonal isolation –Their breeding seasons or reproductive cycles differ.
• This can also be called temporal isolation. E.g. Nocturnal vs. diurnal habits.
c. Behavior isolation –The mating behaviors differ, individuals cannot recognize potential
mates.
8. Post-zygotic isolation –species can mate, but there will not be the successful transfer of
genetic material to subsequent generations. E.g. mules.