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Most other nutrients are too toxic to be stored in large quantities.
Summary of Section 2.6
The product of survival to reproductive maturity and reproduction (viability
and fecundity) is the measure of fitness of a phenotype.
Summary of Sections 3.2 and 3.3
Natural selection acts on the different phenotypes within a population.
It can only affect the course of evolution if these differences are heritable,
i.e. produced by different genes.
Summary of Sections 3.2 and 3.3
Random fluctuations in allele frequencies occur in populations and genetic drift is likely to have larger effects in small populations.
Summary of Sections 3.2 and 3.3
The founder effect describes the effect on genetic variability of the small size of a colonizing population, which might consist of one or a few individuals. Such a population can never contain more than a fraction of the total genetic variability of the parent population. An allele that is rare in the parent population has a chance of becoming common if it is present in a founder
population, provided that it is not eliminated by genetic drift when the population is small.
Summary of Sections 3.2 and 3.3
Independent assortment and recombination during meiosis, followed by fertilization, can generate huge genetic variability.
Summary of Section 3.4
The source of new alleles is through mutations that give rise to new DNA sequences arising as point mutations or as a result of rearrangements of larger segments of chromosomes.
Summary of Section 3.4
Chromosomal mutation may result in situations in which groups of genes may remain clustered together through meiosis as supergenes, or in polyploidy where the genetic changes may bring about reproductive isolation and speciation within a very small number of generations.
Summary of Section 3.4
Evolution may take place through changes in the gene pool as a result of mutation, genetic drift or natural selection, singly or in combination.
Summary of Section 3.5
Sexual reproduction has two essential features that distinguish it from asexual reproduction: meiosis and syngamy. Meiosis is the process of cell division that is intrinsic to the production of haploid gametes; syngamy is the fusion of two gametes to form a zygote.
Summary of Sections 4.2 and 4.3
In sexual reproduction, syngamy can occur between gametes derived from two different individuals that are unrelated, a process known as outbreeding, or between gametes derived from closely related individuals (inbreeding).
Summary of Sections 4.2 and 4.3
Asexual reproduction is reproduction without sex, and refers to the production by a single parent of diploid progeny that are exact genetic replicas of that parent.Wholly asexual organisms have no meiosis at all in their life cycles and reproduce by mitotically derived somatic tissues (vegetative reproduction) or mitotically derived single cells (apomixis).
Summary of Sections 4.2 and 4.3
Parthenogenesis is the development of a new individual, either male or female, from an unfertilized egg. In some parthenogenetic systems, diploid eggs are produced by one of several chromosome doubling devices.
Summary of Sections 4.2 and 4.3
Hybridogenesis has some of the features of sexual reproduction. In a diploid hybridogen, the genome derived from one parental species is transmitted to the egg without recombination, while the genome of the other parental species is discarded. The haploid egg is then fertilized by sperm of a second male of the same species, restoring the hybrid condition.
Summary of Sections 4.2 and 4.3
Gynogenetic fish result from crosses between two sexually reproducing species. The resultant hybrid female produces diploid eggs, via either premeiotic endomitosis or apomixis, and then mates with a related bisexual male. The sperm stimulates embryogenesis, but syngamy between egg and sperm does not occur and only the mother’s genome is passed on to the offspring.
Summary of Sections 4.2 and 4.3
Inbreeding depression usually results from the exposure of deleterious recessive alleles to selection and both plants and animals have evolved mechanisms to reduce its occurrence.
Summary of Sections 4.2 and 4.3
In spite of the disadvantages of inbreeding depression, both plants and animals may benefit from some degree of inbreeding.
Summary of Sections 4.2 and 4.3
Two costs of sexual reproduction are commonly recognized: the cost of recombination and the cost of producing sons, also referred to as the cost of meiosis. In addition, there are also the costs incurred in finding and fertilizing a mate.
Summary of Section 4.4
To offset these costs, sexual reproduction must also confer benefits if it is to persist. Two types of benefits have been discussed: long-term benefits associated with more rapid evolution of species or populations, and shortterm benefits to individuals.
Summary of Section 4.4
Rapid evolution of species or populations in the long term may be facilitated by the more rapid fixation of combinations of advantageous mutations in sexual populations or by the reassembly, by recombination, of mutation-free genotypes (a solution to Muller’s ratchet).
Summary of Section 4.4
There are several theories about why sex arises and persists in the short term. They rely primarily on the advantage of sex to individuals in a changing environment. Some theories focus on changes in the organism’s physical environment (sib-competition model, lottery model) whereas others focus on changes in the biological environment (Red Queen hypothesis).
Summary of Section 4.4
Male and female sexes are defined by the difference in the size of their gametes. The prevalent theory holds that distinct sexes evolved because it is advantageous, on the one hand, to produce numerous gametes (sperm) and,
proportion of males
on the other, to produce large gametes (eggs) because the zygote’s survival is
(d)
20 30
enhanced by large size.
Summary of Section 4.5
The sex ratio is usually 1 : 1 because the reproductive success of all males in a population must equal the reproductive success of all females. If the population sex ratio deviates from 1: 1, natural selection favours individuals that produce more offspring of the rarer sex.
Summary of Section 4.5
Biases away from a 1 : 1 sex ratio do occur in nature, either as a result of manipulation of primary sex ratios or because of unequal investment in the two sexes in offspring that require parental care.
Summary of Section 4.5
There are two major categories of sex determination: genetic and environmental.Within these categories, there are many contrasting mechanisms that differ even in quite closely related groups.
Summary of Section 4.5
There are two kinds of hermaphrodites, simultaneous and sequential. Simultaneous hermaphrodites may or may not be capable of self-fertilization.
Summary of Section 4.6
Some animals and plants switch sex once during their lifetime, from male to female, or vice versa.Whether it is advantageous to begin as one sex or the other depends on how reproductive success as a male or as a female changes with an individual’s size or age.
Summary of Section 4.6
Host species throughout the animal kingdom have evolved complex arrays of defences against pathogens, which include physical barriers, reflexes such as coughing and behaviours such as grooming. If these barriers are breached, then the first (and in invertebrates, the only) defence against pathogens is the molecular and cellular mechanisms known as innate immunity.
Summary of Section 5.2
Innate immune defences pre-exist in the host animal before it comes into contact with pathogens for the first time; constitutive defences are permanently activated whereas induced defences increase locally or exist in a precursor form and are activated by the presence of pathogens.
Summary of Section 5.2
Molecular defences include wound closure and coagulation, agglutination of pathogens, antimicrobial peptides and enzymes, oxidizing chemicals and other toxic molecules, pore-forming proteins that puncture cell membranes, and molecules that interfere with pathogen replication. Some of these molecules are generated in induced cascade reactions.
Summary of Section 5.2
Cellular defences involve different types of leukocytes: some destroy pathogens by phagocytosis, others engage in cell-mediated cytotoxicity, and collectively they synthesize and secrete many of the molecular defences listed above.
Summary of Section 5.2
Many of the genes for molecules involved in innate immunity are highly conserved between widely divergent animal phyla, which may indicate a common origin in shared ancestors.
Summary of Section 5.2
Innate immunity relies on broad-spectrum defensive mechanisms, which are targeted against unique pathogen carbohydrates commonly found in the surface structures of pathogens. Innate immune mechanisms are non-specifically effective against all pathogens that contain these molecules.
Summary of Section 5.2
The effectiveness of the innate immune response to invasion by a particular pathogen is more-or-less constant, no matter how many times the same pathogen is encountered (i.e. it does not adapt during an exposure).
Summary of Section 5.2
Innate defences are self-tolerant and are not directed against the host’s own cells unless they betray the presence of intracellular pathogens. Unique histocompatibility antigens on host cells are recognized as ‘non-self’ if grafted into another member of the same species, provoking graft rejection.
Summary of Section 5.2
Plants have evolved a wide range of physical and chemical defence mechanisms, in response to a variety of herbivores and pathogens. Interactions between some groups of herbivorous insect and plant species have occurred over tens of millions of years.
Summary of Section 5.3
The plant apparency theory and the resource availability hypothesis can explain the diversity of plant chemical defences, i.e. why certain plant species have different types and amounts of defence, leading to the idea of qualitative and quantitative defences.
Summary of Section 5.3
Alkaloids are important plant secondary compounds implicated in defence against a range of herbivores and pathogens. The plant defence theory was the first to suggest the defence role of secondary chemicals like alkaloids.
Summary of Section 5.3
Induced defence, an example of a qualitative defence, has been found in various plant species. The principles of the mechanisms, in particular the cascade response, are similar to those in animals.
Summary of Section 5.3
Adaptive immunity is found only in vertebrates and depends on small lymphocytes, which exist in clones initially of a few hundred cells each. All members of the same clone carry identical antigen receptors with binding sites that fit a unique cluster of amino acids (sometimes with sugar molecules) called an epitope.
Summary of Section 5.4
Molecules with epitopes in their structure are called antigens. The antigen receptors on small lymphocytes are highly specific and can distinguish between closely related pathogen strains on the basis of their unique epitopes.
Summary of Section 5.4
In most vertebrates, large antigen receptor repertoires are generated by random somatic gene recombination. Somatic genes are not passed on from one generation to the next, so the codes for receptors that bind to epitopes on pathogens encountered during an animal’s lifetime are not inherited by its offspring.
Summary of Section 5.4
Phagocytes also produce activating cytokines and select appropriate targets for adaptive immunity by presenting digested fragments of pathogens to helper T cells.
Summary of Section 5.4
Corticosteroids suppress immune responses and may be important in switching energy resources away from defence against pathogens to other functions such as reproduction, metamorphosis and hibernation.
Summary of Section 5.4
Some amphibian populations may be declining due to increased mortality from pathogens. Increased susceptibility to opportunistic infections may be due to reduced immune responsiveness caused by environmental stressors, or to the recent emergence of more virulent pathogen strains.
Summary of Section 5.4
There are several lines of evidence to show that organisms involved in attack and defence have coevolved and that trade-offs are occurring between devoting resources to defence or to other functions such as reproduction.
Summary of Section 5.5
A consequence of coevolution between hosts and their pathogens is that virulence tends towards an optimum level which ensures the survival of both the host and the pathogen species. Hosts may tolerate a greater pathogen load during periods of high reproductive effort.
Summary of Section 5.5
Comparison of the phylogenies of hosts and parasites provide further evidence for coevolution. Blood parasites in the order Haemosporina seem to have coevolved with their dipteran (fly) vectors rather than their vertebrate hosts.
Summary of Section 5.5
Coevolution of some plants and insect herbivores has led to the latter using the plants’ defence for themselves, which in turn has contributed to the evolution of warning coloration and mimicry.
Summary of Section 5.5
Experimental evidence for coevolution in plants and herbivores has come from a study in which the removal of herbivores was associated with increased fitness of plants, due to the reduced cost of defence.
Summary of Section 5.5
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