Organisms and Evolution - Variation and Sexual Reproduction Flashcards
What are the costs of sexual reproduction?
Males unable to produce offspring;
Only half of each parent’s genome passed onto offspring,
Disrupting successful parental genomes
Why do the benefits of sexual reproduction outweigh the costs?
Due to an increase in genetic variation in the population
What does genetic variation provide?
The raw material required for adaptation, giving sexually
reproducing organisms a better chance of survival under changing selection pressures
Why may co-evolutionary interactions between parasites and hosts select for sexually reproducing hosts?
Hosts better able to resist and tolerate parasitism have greater fitness. Parasites better able to feed, reproduce and find new hosts have greater fitness
What happens if hosts reproduce sexually?
The genetic variability in their offspring reduces the
chances that all will be susceptible to infection by parasites
How can asexual reproduction be a successful reproduction strategy?
Whole genomes are passed on from parent to offspring.
Just one parent can produce daughter cells and establish a colony of virtually unlimited size over time
When is maintaining the genome of a parent an advanatage?
Particularly in very narrow, stable niches or when re-colonising disturbed habitats
Give examples of asexual reproduction in eukaryotes?
Vegetative cloning in plants and parthenogenesis in lower plants and animals that lack fertilisation
What is parthenogenesis?
Reproduction from a female gamete without fertilisation
In what reproductive strategy can offspring be reproduced more often and in larger numbers?
Asexual reproduction
Where is parthenogenesis more common?
In cooler climates, which are disadvantageous to parasites, or regions of low parasite density or diversity
Describe asexually reproducing populations?
They are not able to adapt easily to changes in their
environment, but mutations can occur that provide some degree of variation and enable some natural selection and evolution to occur
What do organisms that reproduce principally by
asexual reproduction also often have?
Mechanisms for horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeasts
How can prokaryotes exchange genetic material?
Horizontally, resulting in faster evolutionary change than in organisms that only use vertical transfer
What is meiosis?
The division of the nucleus that results in the formation of haploid gametes from a diploid gametocyte
How do chromosomes typically appear in diploid cells?
As homologous chromosomes
What are homologous chromosomes?
Chromosomes of the same size, same centromere position and with the same sequence of genes at the same loci
What is the first step of meiosis 1?
The chromosomes, which have replicated prior to meiosis I, each consist of two genetically identical chromatids attached at the centromere
What is the second step of meiosis 1?
The chromosomes condense and the homologous chromosomes pair up
What is the third step of meiosis 1?
Chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged
What are linked genes?
Those on the same chromosome
What can crossing over result in?
New combinations of the alleles of these genes
What is the fourth step of meiosis 1?
This crossing over of DNA is random and produces genetically different recombinant chromosomes
What is the fifth step of meiosis 1?
Spindle fibres attach to the homologous pairs and line them up at the equator of the spindle
What is independent assortment?
Each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin
What is the sixth step of meiosis 1?
The chromosomes of each homologous pair are separated and move towards opposite poles
What is the seventh step of meiosis 1?
Cytokinesis occurs and two daughter cells form.
What happens in meiosis 2?
Each of the two cells produced in meiosis I undergoes a further division during which the sister chromatids of each chromosome are separated.
A total of 4 haploid cells are produced
What is the sex of birds, mammals and some insects determined by?
The presence of sex chromosomes
What determines the development of male characteristics in mammals?
The SRY gene on the Y chromosome
What do heterogametic (XY) males lack?
Most of the corresponding homologous alleles on the
shorter (Y) chromosome
Heterogametic (XY) males lack most of the corresponding homologous alleles on the shorter (Y) chromosome. What can this result in?
Sex-linked patterns of inheritance as seen with carrier females (XBXb) and affected males (XbY)
What happens to X chromosomes in homogametic (XX) females?
One of the two X chromosomes present in each cell is
randomly inactivated at an early stage of development
What is X chromosome inactivation?
A process by which most of one X chromosome is
inactivated
What does X chromosome inactivation prevent?
A double dose of gene products, which could be harmful to cells
Who are less likely to be affected by any deleterious mutations on these X chromosomes?
Carriers
What is the result of the X chromosome inactivation in each cell being random?
Half of the cells in any tissue will have a working copy of the gene in question
What are hermaphrodites?
Species that have functioning male and female reproductive organs in each individual
What type of gametes do hermaphrodites produce?
Both male and female gametes and usually have a partner with which to exchange gametes
What is the benefit to the individual of being a hermaphrodite?
If the chance of encountering a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex
How is sex and sex ratio determined in other species?
By environmental factors rather than genetic factors
How is environmental sex determination in reptiles
controlled?
By environmental temperature of egg incubation
Sex can change within individuals of some species as a result of what factors?
Size, competition, or parasitic infection
In some species the sex ratio of offspring can
be adjusted in response to what?
Resource availability
