Summary
Summary chapter 8 ecology:economy of nature
- Module
- Zoology BSc
- Institution
- University Of Reading (UoR)
Summary of 5 pages for the course Zoology BSc at UoR
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The structure of DNAGenetic info is contained in the molecule deoxyribonucleic acid, also known as DNA. Each
strand is composed of subunits called nucleotides and each nucleotide is composed of a
sugar, a phosphate group, and one of four different nitrogenous bases: adenine, thymine,
cytosine, and guanine. Genetic info is coded in the particular order of the different
nitrogenous bases.
Genes and alleles
Genes are regions of DNA that code for particular proteins, which in turn affect particular
traits. In diploid organisms – those having two sets of chromosomes – one allele comes from
the mother’s gamete and the other comes from the father’s gamete. As you may recall,
each gamete is haploid, meaning that it has just one set of chromosomes. In many cases, a
change in alleles can create differences in an organism’s phenotype. Many phenotypes in a
population can span a range of values because they are polygenic traits.
Dominant and recessive alleles
Every individual has two copies of each gene, one inherited from its mother and one from
its father. Exceptions to this rule include genes located on sex chromosomes, the gene of
organisms that reproduce by self-fertilisation, haploid organisms, and organisms such as
plants that alternate between haploid and diploid generations. Fortunately, most harmful
alleles are recessive and are therefore not expressed in a heterozygous individual. Any
dominant harmful alleles that might arise are expressed when homozygous or
heterozygous. Because they reduce fitness, dominant harmful alleles are strongly selected
against and removed from the population over time. In contrast, recessive harmful alleles
are expressed when homozygous but not when heterozygous, so they can persist in a
population because they are not selected against when they occur in heterozygous
individuals.
Sources of genetic variation
One of the most common ways to generate variation is through sexual reproduction. By
combining a haploid sex cell of one parent with a haploid sex cell of another parent, new
combinations of alleles can be produced in the offspring across many different
chromosomes. Ways of genetic variation include mutation, recombination and random
assortment. Mutations can occur anywhere along the chromosomes, although some regions
of the chromosome can experience higher frequencies of mutation than others. Many
mutations have no detectable effect and are referred to as silent, or synonymous,
mutations. Other mutations may simply alter the appearance, physiology, or behaviour of
the individual. When phenotypic changes are better suited to the environment, these
phenotypes will be favoured by natural selection. Some mutations, however, can cause
drastic, often lethal, changes in the phenotype. During meiosis, pairs of homologous
chromosomes- one member of which is inherited from each parent – line up next to each
other. When the two chromosomes in the pair do not exchange any DNA, we end up with
haploid cells that contain unaltered chromosomes. However, sometimes the two
chromosomes in the pair do exchange DNA, in a process called crossing over. In some cases,
crossing over can also occur between nonhomologous chromosomes. In either case, new
genes are not being created, but new combination of alleles are produced that have the
potential to produce new phenotypes.
evolution through random processes
random processes can facilitate evolutionary change in a population. In addition to
mutation, random processes include genetic drift, bottleneck effects and founder effects.
, Mutation
We have seen the mutation is an important way in which genetic variation can arise in a
population. Because genes often code for functions that are vital to performance and
fitness, mutations that negatively impact these functions are not favoured by selection.
However, a small fraction of mutations can be beneficial. The more genes that a species
carries, the higher the probability that at least one gene will experience a mutation.
Similarly, the larger the size of the population, the higher the probability that an individual
in the population will carry a mutation.
Genetic drift
Genetic drift is more common in small populations because random events can have a
disproportionately large effect on the frequencies of genes in the population.
Bottleneck effects
When a population experiences a large reduction in the number of individuals, the survivors
carry only a fraction of the genetic diversity that was present in the original, larger
population. Moreover, after being reduced to a small population by the bottleneck effect,
the small population can then experience genetic drift. Population reductions can result
from natural causes, for example, weather or anthropogenic causes. The bottleneck effect is
of particular interest because the subsequent reduction in genetic diversity may prevent the
population from adapting to future environmental changes.
Founder effects
Following the founding by this small population, genetic drift can cause additional
reductions in genetic variation
Evolution through selection, a non-random process
The non-random process of selection also plays a substantial role in evolution. Selection can
change the phenotypes (and therefore the gene frequencies) of a population. Depending on
how the environment varies over time and space, selection can influence the distribution of
traits in a population in three ways: stabilizing selection, directional selection, and disruptive
selection.
Stabilizing selection
After stabilising selects for parents who possess intermediate phenotypes, their progeny
have a more narrow distribution of phenotypes.
Microevolution operates at the population level
The random and non-random processes that cause evolution can operate at a variety of
levels. Microevolution is the process responsible for producing different breeds.
Microevolution is affected by random process and selection. Selection at the microevolution
level can be further divided into artificial selection and natural selection.
Natural selection
Artificial and natural selection both operate by favouring certain traits over others. Both
select on traits that are heritable; the difference lies in how traits are selected. There can be
multiple ways to improve an individual’s fitness, and all of them are favoured by natural
selection, regardless of the resulting phenotype. Most evolutionary biologists agree that the
diversification of organisms over the long history of life on Earth has occurred primarily by
natural selection. Natural selection is an ecological process: it occurs because of differences
in reproductive success among individuals endows with different form or function in a
particular environment. That is, as individuals interact with their environment – including
physical conditions, food resources, predators, other individuals of the same species, and so
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