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Summary chapter 9
Summary study book Ecology of Robert E. Ricklefs, Rick Relyea (chapter 9 ) - ISBN: 9781319187729, Edition: 8th ed. 2018, Year of publication: -
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Reproductive strategiesThe evolution of reproductive strategies involves a large number of different factors, many
of which are influenced by ecological conditions. For example, organisms can evolve to
reproduce sexually or asexually; each strategy has unique costs and benefits, particularly
when species interact with parasites and pathogens. Organisms can evolve to reproduce as
separate sexes or as hermaphrodites. In many species, the sex ratio of offspring can be
altered to respond to changing ecological conditions. Finally, we see many different mating
strategies to improve fitness. This chapter explores how ecological conditions affect the
evolution of sex and the strategies that organisms have evolved to increase their fitness.
Sexual reproduction
Haploid germ cells, called gametes, are produced through meiosis within sex organs called
gonads. Each gamete contains a single full set of chromosomes. In animals, these haploid
cells can immediately act as gametes. In plants and many protists, the haploid cells develop
into multicellular, haploid stages of the life cycle that eventually produce gametes. When
two gametes combine to form an offspring, each parent contributes one set of
chromosomes. During meiosis, the distribution of the chromosomes into the gametes is
generally random and the subsequent mixing of chromosomes from the two parents results
in new combinations of genes in the offspring. Ultimately, two gametes join together in an
act of fertilization to produce a diploid zygote.
Sexual reproduction
This can be accomplished through either vegetative reproduction or parthenogenesis
Vegetative reproduction
Many plants can reproduce by developing new shoots that sprout from leaves, roots, or
rhizomes. Bacteria and some species of protists reproduce by binary fission.
Parthenogenesis
In contrast to vegetative reproduction, some organisms reproduce asexually by
parthenogenesis. In most cases, parthenogenetically produced offspring arise from diploid
eggs, which do not require any genetic contribution from sperm. Parthenogenesis has
evolved in plants and several groups of invertebrates. Animal species that reproduce only by
parthenogenesis are typically composed entirely of females. Parthenogenesis is relatively
rare in vertebrates. Parthenogenesis can produce offspring that are either clones of the
parent or offspring that are genetically variable. Clones are produced when germ cells
develop directly into egg cells without going through meiosis. In contrast, genetically
variable offspring are produced when germ cells proceed partially or entirely through
meiosis. In partial meiosis, germ cells pass through the first meiotic division, but suppression
of the second meiotic division results in diploid egg cells. Although a sexual union is not
involved, these eggs differ from one another genetically because of recombination between
pairs of homologous chromosomes and the independent assortment of chromosomes
during the first meiotic division. When the germ cells experience complete meiosis, the
gamete-forming cells of the female are haploid and then fuse to form a diploid embryo.
Cost of sexual reproduction
Sexual and asexual reproduction are both viable strategies, but sexual reproduction comes
with a number of costs to that organism. For example, sexual organs require considerable
energy and use resources that could be devoted to other purposes. In addition, mating itself
can be a substantial task. Many plants must produce floral displays to attract pollinators and
most animals conduct elaborate courtship rituals to attract mates. These activities require
time and resources. They can also elevate the risk of herbivory, predation and parasitism.
, Benefits of sexual reproduction
These benefits include purging harmful mutations and creating genetic variation that helps
offspring deal with future environmental variation, including the existence of rapidly
evolving parasites and pathogens.
Purging mutations
Mutations occur in all organisms, and some are harmful. In asexually reproducing
organisms, mutations are transmitted from one generation to the next, and so accumulate
over generations, especially if the asexual parents produce clonal offspring. In contrast,
during the random assortment of genes during meiosis in sexually reproducing organisms,
deleterious mutations may not be transmitted to gametes. Further, of all the gametes that
are produced, those that form zygotes may not contain the mutation. Alternatively, if both
parents carry a copy of a gene with a harmful mutation, and some of the gametes produced
do contain the mutation, then a union of two such gametes will be homozygous for the
harmful mutation. An offspring that is homozygous for the harmful mutation is likely not
viable and will fail to breed and pass the mutation to the next generation. Because species
that reproduce only asexually do you not have any means of purging mutations which would
lead to an eventual extinction. One way that such species could avoid extinction is by
producing offspring more rapidly the new deleterious mutations arise. In this case, some
individuals would always retain the non-mutated parental genotype and produce the Next
Generation, a process known as clonal selection.
Genetic variation and future environmental variation
The second benefit of sexual reproduction is the production of offspring with greater
genetic variation. Because environmental conditions vary, offspring with genetic variation
having increased probability of possessing gene combinations that will help them adapt to
different environmental conditions.
Genetic variation and evolving parasites and pathogens
To understand why sexual reproduction provides an evolutionary benefit when species
experience variation in both types and abundance of pathogens, we must first realise that
pathogens have much shorter generation times and much larger population sizes than most
of the host species they infect. Because pathogens have the potential to evolve at a much
faster rate than their hosts, they can involve ways to counteract host defences. Without
rapid host evolution, pathogens could drive their host to low numbers or even to extinction.
As we have seen, sexual reproduction produces offspring with a greater range of genetic
combinations, and some of these combinations might be better suited to come back
pathogen. In short, there is an evolutionary race between host, which are trying to evolve
adaptations rapidly enough to combat the pathogen, and pathogens, which are trying to
evolve adaptations rapidly enough to counter host defences. This is known as the red Queen
hypothesis.
Organisms can evolve as separate sexes or as hermaphrodites
Species in nature have about an incredible array of sexual strategies for male and female
function. Most vertebrates have a separate sexes, where as most plants are
hermaphrodites. Some plants produce separate male and female flowers. Although perfect
flowered hermaphrodites account for more than two thirds of flowering plant species,
nearly all imaginable sexual patterns are known. Population of some plants species can be
composed of a complex mixture of hermaphrodites, males, females, and monoecious
individuals. In other species, individual plants produce both perfect flowers and flowers only
male or female.
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