4 Contributors to Genetic Diversity:
1) Genetic Recombination:
- Homologous chromosomes (although having the same genes) will carry different alleles
(versions of those genes)
- These homologous chromosomes cross over to exchange alleles, increasing variaition
2) Random Segregation
- the maternal and paternal members of each homologous pair are different in that they
typically carry different alleles of many of the genes on that chromosome.
- When the chromosomes are pulled apart in meiosis, there is a random combination of
maternal and paternal chromosomes being pulled to either side of the cell
3) Alternative combinations at Meiosis II
- The resulting daughter cells will be genetically different, depending on how the
chromosomes align in metaphase II.
4) Random Fertilization
- The haploid products of meiosis are genetically diverse. The rather random combination
of these cells (or their descendants) during fertilization is a matter of chance that
amplifies the variability of sexual reproduction.
the masking of recessive alleles in heterozygotes makes it almost impossible to eliminate them
completely through selective breeding.
balanced polymorphism is one in which two or more phenotypes are maintained in fairly stable
proportions over many generations. Natural selection preserves balanced polymorphisms when
heterozygotes have higher relative fitness,
- Heterozygote Advantage. A balanced polymorphism can be maintained by heterozygote
advantage, when heterozygotes for a particular locus have higher relative fitness than
either homozygote. Example: in Africa people that are heterozygous for sickle cell
anemia have a higher survival rate.
Allele/Genes Allele - One of two or more versions of a gene. Alternate forms that a gene can take
One allele per gene is found in a gamete
Allele frequencies - The abundance of one allele relative to others at the same gene locus in
individuals of a population. The allele frequency remains the same within a population, no matter
how alleles are mixed. However, once a change in allele frequency occurs, evolution results.
Dominant allele - The allele expressed when more than one allele is present.
Recessive allele - An allele whose phenotypic effect is not expressed in a heterozygote. It is only
expressed when homozygous
Fixation - the change in a gene pool from a situation where there exist at least two variants of a
particular gene (allele) to a situation where only one of the alleles remains. Result of heavy
Gene duplication - any duplication of a region of DNA that contains a gene; it may occur as an error
in homologous recombination
Gene flow - The transfer of genes from one population to another through the movement of
individuals or their gametes.
Gene pool - The sum of all alleles at all gene loci in all individuals in a population.
Genetic drift - Random fluctuations in allele frequencies as a result of chance events; usually
reduces genetic variation in a population. Genetic drift isn’t as likely in a large population
- Bottleneck effect
- Founder effect
Genetic equilibrium - The point at which neither the allele frequencies nor the geno type
frequencies in a population change in succeeding generations.
Genotype frequencies - The percentage of individuals in a population possessing a particular
Heterozygote advantage - An evolutionary circumstance in which individuals that are heterozygous
at a particular locus have higher relative fitness than either homozygote.
Heterozygous - The state of possessing two different alleles of a gene.
Homozygous - State of possessing two copies of the same allele.
Crossing over - The recombination, in which chromatids exchange segments of DNA. Sister
chromatids exchange DNA
Diploid- cells have two homologous copies of each chromosome, usually one from the mother and
one from the father.
Haploid - An organism or cell with only one copy of each type of chromosome in its nuclei. Polyploids – organisms that have more than two copies of a chromosome. In these divisions, the
spindle fails to separate the duplicated chromosomes, which are incorporated into a single nucleus
with twice the usual number of chromosomes
- In plants, polyploids are often hardier and more successful in growth and reproduction
than the diploid plants from which they were derived. As a result, polyploidy is common
and has been an important source of variability in plant evolution.
- By contrast, in animals it is deadly to have polyploidy
Autopolyploidy - a diploid (2n) individual may produce tetraploid (4n) offspring, each with four
complete chromosome sets. Autopolyploidy occurs through an error in either mitosis or meiosis, so
that gametes spontaneously receive the same number of chromosomes as a somatic cell.
Polymorphy - The existence of discrete variants of a character is called a polymorphism. These
traits are described as polymorphic
- nearly half the loci surveyed in many populations of plants and invertebrates are
Genotype – all individuals have different genotypes. In diploids, each gene locus has two alleles.
The sum of all alleles at all gene loci in all individuals is called the population's gene pool.
genotype frequencies - the percentages of individuals possessing each genotype. Knowing that
each diploid organism has two alleles (either two copies of the same allele or two different alleles)
at each gene locus, a scientist can then calculate allele frequencies, the relative abundances of
the different alleles. For a locus with two alleles, scientists use the symbol p to identify the
frequency of one allele, and q to identify the frequency of the other allele.
Phenotype – outward expression of a gene. On a graph the range of variation for a phenotype
can be represented by a bell curve
phenotypic variation - differences in appearance or function that are passed from generation to
generation. This can be represented by a bell curve.
- Phenotypic variation within populations may be caused by genetic differences between
individuals, by differences in the environmental factors that individuals experience, or by
an interaction between genetics and the environment. As a result, genetic and
phenotypic variations may not be perfectly correlated.
- Organisms can have different genotypes and portray the same phenotype, or same
genotype and different phenotype.
Early in the twentieth century, geneticists were puzzled by the persistence of recessive traits
because they assumed that natural selection replaced recessive or rare alleles with dominant or
common ones. - the Hardy–Weinberg principle, specifies the conditions under which a population of
diploid organisms achieves genetic equilibrium, the point at which neither allele
frequencies nor genotype frequencies change in succeeding generations.
- Their work also showed that revessive alleles were not replaced by dominant ones, and
that the shuffling of genes in sexual reproduction does not in itself cause the gene pool
- genetic equilibrium is possible only if all of the following conditions are met:
1. No mutations are occurring.
2. The population is closed to migration from other populations.
3. The population is infinite in size.
4. All genotypes in the population survive and reproduce equally well.
5. Individuals in the population mate randomly with respect to genotypes
- If the genotype frequency changes = microevolution
- Easiest condition to meet for a biologist studying a small population is no gene flow
Incomplete dominance – in some genetic crosses (RRxrr) the ratio of visible traits is 1:2:1 in
the F1 generation with two seemingly intermediate conditions.
Speciation - the evolutionary process by which new biological species arise.
1. Allopatric speciation - The evolution of reproductive isolating mechanisms between two
populations that are geographically separated.
2. Parapatric speciation - Speciation between populations with adjacent geographic
3. Sympatric speciation – when reproductive isolation evolves between distinct subgroups
that arise within one population. Models of sympatric speciation do not require that the
populations be either geographically or environmentally separated as their gene pools
diverge. Changes in diet, behaviour, or chromosomes could effect reproductive isolation.
4. Some members of each population may mate with individuals from the other, producing
viable, fertile offspring in areas called hybrid zones.
Types of species
5. Biological Species Concept defines a species as a group of organisms that can
successfully interbreed and produce fertile offspring. 6. Phylogenetic Species Concept (which itself has multiple versions) defines a species as a
group of organisms bound by a unique ancestry.
7. Ecological Species Concept defines a species as a group of organisms that share a
distinct ecological niche.
8. Morphological Species Concept, the idea that all individuals of a species share
measurable traits that distinguish them from individuals of other species.
When geographically separated populations of a species exhibit dramatic, easily recognized
phenotypic variation, biologists may identify them as different subspecies
Some plant and animal species have a ring-shaped geographic distribution that surrounds
uninhabitable terrain. Adjacent populations of these so-called ring species can exchange genetic
material directly, but gene flow between distant populations occurs only through intermediary
populations. Example is the salamander
Polyploidy - The condition of having one or more extra copies of the entire haploid complement of
chromosomes. Polyploidy often originates from failure of the spindle to function normally during
mitosis in cell lines leading to germ-line cells. In these divisions, the spindle fails to separate the
duplicated chromosomes, which are incorporated into a single nucleus with twice the usual number
of chromosomes. The effects of polyploidy vary widely between plants and animals. In plants,
polyploids are often hardier and more successful in growth and reproduction than the diploid plants
from which they were derived. As a result, polyploidy is common and has been an important source
of variability in plant evolution.
Allopolyploidy - polyploids with chromosomes derived from different species. Precisely it is the
result of doubling of chromosome number in an F1 hybrid.
Autopolypoloidy - polyploids with multiple chromosome sets derived from a single species.
Autopolyploids can arise from a spontaneous, naturally occurring genome doubling Isolation
Reproductive isolation is fundamental to the Biological Species Concept. A reproductive isolating
mechanism is a biological characteristic that prevents the gene pools of two species from mixing
even when they are sympatric (occupying the same spaces at the same time)
- Ecological isolation – species that live in the same area but different parts of it.
Example is tigers live in dense forests, lions in grasslands; but both live in india. - temporal isolation if they mate at different times of the day or different times of the
- Behavioural isolation results when the signals used by one species are not recognized
by another. Female songbird