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BIOA02H3 Lecture Notes - Infertility, Meiosis, Zygote

Biological Sciences
Course Code
Mary Olaveson

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Biology Chapter 23 Notes
The term species refers to different “kinds” of organisms, however it is difficult to determine boundaries
that separate one kind from another
It is possible for someone knowledgeable about a group of organisms to distinguish species based on
their looks because organisms of the same species change very little over geographic distances
Morphological Species Concept: The classification of organisms based on their
The originator of the binomial system of classification, Carlous Linnaeus, used this concept in his
Because sometimes members of the same species do not look alike, scientists need to look at other
factors to classify between them
Speciation: The process by which one species splits into two or more daughter species,
which thereafter evolve into separate lineages
This process is very gradual. Because of the slowness of this process, often two populations at various
stages in the process of becoming a new species will exist and it is important to classify them as either
being a member of species A or species B
Reproductive Isolation: When individuals of a population mate with each other, but not with
individuals of other population
If reproductive isolation occurs, the population constitute a distinct group within which genes
recombine and so they are an independent evolutionary unit.
Biological Species Concept: Definition of species proposed by Ernest Mayer; “Species are groups of
actually or potentially interbreeding natural populations which are
reproductively isolated from other such groups
This definition does not apply to organisms that reproduce asexually
Not all evolutionary changes result in a new species; speciation occurs only if those evolutionary
changes result in a split of one species into 2 or more daughter species
It is also possible that if two populations are isolated from each other, and sufficient differences in their
genetic structure accumulate, then when they come together again, they may not be able to exchange
For speciation to occur, it requires such interruptions in gene flow. This is possible in 2 ways:
1. Allopatric Speciation
Allopatric Speciation: Species that occur when a population is divided by a physical barrier.
Also known as Geographic Separation
It is the dominant cause of speciation in most organisms
The barriers can form by continental drift, rising sea levels, climate change, glaciers
advancement/retreatment etc.
Usually, the populations that are separated are fairly large and they evolve differently because
of gene drift, environmental change and other reasons
Allopatric Speciation may also occur if a new species crosses a pre-existing barrier and finds a
new isolated population
The effectiveness of the barrier separating the isolated populations depend on the size,
mobility, and dispersing abilities of the species
It is also possible for an interruption of gene flow even without the presence of a physical
barrier. I.e. if two populations are separated by a patch of land which is unsuitable for a species
2. Sympatric Speciation
Sympatric Speciation: A partition of a gene pool without any physical isolation
In this for a speciation, a physical barrier is not required, but rather some form of disruptive
selection within a population which favours individuals with a certain genotype
Polyploidy: Most common means of sympatric speciation; the production within an
individual of duplicate sets of chromosomes. Can occur from chromosome
duplication in a single species (autopolyploidy) or from the combination of the
chromosomes of 2 different species (allopolyploidy)
Autopolyploid individuals occur when cells that are normally diploid (2 sets of chromosomes)
accidentally duplicate therefore becoming tetraploid (4 sets of chromosomes). If a tetraploid
individual mates with a normal diploid individual, their offspring turn out triploid (3 sets of
chromosomes) because of meiosis and they end up being sterile.
Because their offspring are sterile, they fail to further produce offspring with diploid individuals;
however a tetraploid individual can produce offspring with other tetraploids, therefore
polyploidy result in complete reproductive isolation in two generations.
Allopolyploids may be produced if two individuals of different, but closely related species, mate
(hybridize). Unlike autoployploidy, they are fertile
Speciation through polyploidy is much more common in plants than in animals because plants
can reproduce through self fertilization
Once a barrier to gene flow is established, the two separate populations can further their difference in
genetic variation through different evolutionary mechanisms, which decrease the probability they can
mate together
However, geographic isolation does not necessarily lead to reproductive isolation. If two populations are
separated by a barrier but the conditions in which they evolve are similar, they will remain
morphologically similar and reproductively compatible.
Reproductive incompatibility may arise from 2 major mechanisms: prezygotic and postzygotic
reproductive barriers
1) Prezygotic Barriers
Prezygotic Reproductive Barriers: Mechanisms that prevent individuals of other species or
populations from interbreeding that operate before
Some of these barriers include:
Habitat Isolation: different populations may choose different habitats to live and mate
therefore they never come in contact during their respective mating
Temporal Isolation: If the mating periods of two populations do not match.
Mechanical Isolation: Differences in the size and shape of the reproductive organs of two
Gamete Isolation: Sperm of one population may not attach to the egg of the other
Behavioural Isolation: Individuals of one population may reject, or fail to recognize individuals
of another population as potential mates
2) Postzygotic Barriers
Postzygotic Reproductive Barriers: Genetic differences that accumulate while the
populations are isolated from one another
Some of these barriers include:
Low hybrid zygote variability: Hybrid zygotes may fail to mature normally causing
Low hybrid adult viability: Hybrid zygotes may survive less well than normal offspring
Hybrid Infertility: Hybrid may mature normally, but be sterile