BIO120H1 Chapter Notes - Chapter 5: Outcrossing, Asexuality, Zygosity
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Origin of sexual reproduction (textbook)
Key Concepts/ Terms:
•Sexual and asexual reproduction
•Self and cross-fertilization
1) Modes of reproduction: Asexual and sexual, Outbreeding and inbreeding
2)Evolution and adaptive significance of sex: Models, Environment heterogeneity
3)Outbreeding mechanisms: Inbreeding depression, Plant sexual systems
Reproduction serves as a mechanism of regeneration or multiplication. Can provide a means of dispersal
to new environments/ replacement of adult individuals in existing local environments. Genetic viewpoint-
reproduction can provide means by which genetic material in population is recombined and shuffled to
produce new combinations.
Two major modes of reproduction occur in living organism: asexual reproduction and sexual reproduction
Asexual reproduction: (often called cloning) involves replication of identical genetic entities. The
results of asexual reproduction is the multiplication of identical genotypes. In asexual
reproduction there is no alternation of haploid and diploid cells, no meiosis, no gametes. Instead,
differentiated tissue simply undergoes a new cycle of developmental events. Asexual reproduction occurs
in many unicellular organisms, eg. bacteria, believed to be primitive mode of reproduction. Many plant
species also reproduce by asexual methods in addition to sexual reproduction.
Sexual Reproduction: new individuals arise through fusion of male and female gametes. The gametes
arise from meiosis and possess half of the genetic material found in the somatic cells of an organism. The
reassortment of genetic material that takes place during meiosis is the principle factor that has made
possible the evolution of eukaryotic organism over the past 1.5 billion years.
Sexual reproduction acts to increase genetic variability in three ways:
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2)Crossing-over during meiosis
Organism with separate sexes (females and males) all zygotes arise from union of gametes from different
genetic individuals. Termed outbreeding. If individuals are related they will share many genes through
descent. Depending on degree of genetic relatedness among sexual partners, the resulting offspring may
be inbred to some extent. Outbreeding and inbreeding are relative terms that concern the degree
of genetic relatedness among mates. Inbreeding often leads to less vigorous offspring through the
phenomenon known as inbreeding depression.
Consequences of inbreeding in human populations could be:
1)Higher incident of mortality
3)Tay-Sachs disease (deterioration of brain and spinal cord in children)
4)Congenital colour blindness
5)Ichthyosis congenital (severe skin disease
6)Other mental and physical abnormalities
Since most plants are hermaphroditie, opportunities exist for the most extreme form of inbreeding: self-
fertilization. Self-fertilization causes levels of heterozygosity in a population to be reduced by one half at
each generation of mating. In species that regularly self-fertilize, levels of heterozygogosity are considered
lower than are found in outcrossing organisms, and populations may ultimately consist of near
homozygous genotypes with little opportunity for recombination.
Asexual genotypes transmit two copies of their genes to the next generation, whereas on average, sexual
genotypes transmit only one.
What advantages have sexually produced progeny to compensate for this two-fold disadvantage? The
answer is not yet known for certainty, but most evolutionist have sought explanations that operate at the
level of individual selection, and involve immediate advantages to sexually produced progeny in local
environments. The major general advantage to sex is thought to be that it can combine favourable
mutations together in one genome, and can eliminate harmful mutations.
A similar problem to that of the evolution of sex concerns the evolution of self-fertilization in
hermaphrodite organisms such as plants. Imagine that in a population of outbreeding, but self-fertile,
hermaphrodite plants a gene arises that causes all of a plant’s ovules to be self-fertilized, but that the
gene does not affect the plants ability to disperse pollen to other outcrossing genotypes in the population.
As R.A. Fisher first pointed out in 1941, such a gene would have an automatic selective advantage. This is
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