6. Variation & Sexual Reproduction

what are the 2 main strategies that organisms use for reproduction

sexually

asexually

what are the costs of sexual reproduction

males are unable to produce offspring

only half of each parent’s genome passed onto offspring

what could having only half of each parents genome lead to

disruption of 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 in sexual reproduction lead to

maintenance of a large gene pool

ability to cope with changing environmental situations

organisms to participate in evolutionary arms races (parasite vs host)

what do hosts with greater fitness do

be better able to resist and tolerate parasitism

what do parasites with greater fitness do

be better able to feed, reproduce and find new hosts

what do hosts that reproduce sexually lead to in terms of infection by parasites

the genetic variability in their offspring reduces the chances that all will be susceptible to infection by parasites

why could asexual reproduction be successful

whole genomes are passed on from parent to offspring

what are the main advantages of asexual reproduction

rapid colonisation

maintaining the genome

what happens in rapid colonisation and what could it lead to

just 1 parent can produce daughter cells and establish a colony of virtually unlimited size over time

when is maintaining a genome most advantageous

in very narrow, stable niches or when recolonising disturbed habitats

what are the ways of asexual reproduction in eukaryotes

vegetative cloning

budding

parthenogenesis

what is parthenogenesis

reproduction from a female gamete without fertilisation

how often does asexual reproduction occur compared to sexual

more often

how many offspring does asexual reproduction produce compared to sexual

larger number

when is parthenogenesis more common

cooler climates

why does cooler climates make parthenogenesis more common

its disadvantageous to parasites, or regions of low parasite density or diversity

what are disadvantages of asexual reproduction

populations arent able to adapt easily to changes in the environment

how do mutations help asexually reproducing populations

provide some degree of variation and enable some natural selection and evolution to occur

what mechanism do organisms that asexually reproduce have

horizontal gene transfer between individuals to increase variation

what is horizontal gene transfer

transfer of genetic material within same generation

what are examples of mechanisms that allow for gene transfer

plasmids of bacteria and yeast

what type of organism can transfer genes horizontally

prokaryotes

what is the advantage of transferring genes horizontally

leads to faster evolutionary change than 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

what do chromosomes appear as in diploid cells

homologous pairs

what features do homologous chromosomes have

same size

same centromere position

same sequence of genes at the same loci

what are the stages of meiosis 1

interphase & prophase 1

crossing over

metaphase 1

independant assortment

anaphase & telophase 1

what is the process of interphase & prophase 1

chromosomes which have replicated prior to meiosis I, each consist of 2 genetically identical chromatids attached at the centromere

the chromosomes condense and the homologous chromosomes pair up

whats the process of crossing over

chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged

what does crossing over lead to

new combinations of alleles of the genes

what are linked genes

genes that are on the same chromosome

is the crossing over of DNA random or defined

random

what does the random crossing over of DNA lead to

producing genetically different recominant chromosomes

what is metaphase 1

spindle fibres attach to the homologous pairs and line them up at the equator of the spindle

what is the process of independent assortment

each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin

is the orientation of the pairs of homologous chromosomes at the equator random or defined

random

what is the process of anaphase & telophase 1

chromosomes of each homologous pair are separated and move towards opposite poles

cytokinesis occurs and two daughter cells form

what are the stages of meiosis 2

metaphase 2

anaphase & telophase 2

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

how many haploid cells are produced in meiosis

4

what is the sex of birds, mammals, and some insects dependant on

the presence of sex chromosomes

what gene determines the development of male characteristics in males

SRY gene on the Y chromosome

what does heterogametic mean

only 1 X chromosome

what does homogametic mean

2 X chromosomes

what is the significance of the difference in chromosomes between males and females

males lack most of the corresponding homologous alleles on the shorter Y chromosome

what can the lack of corresponding homologous alleles lead to

sex linked patterns with carrier females and affected males

what happens between the 2 X chromosomes in females

1 of the 2 chromosomes in each cell is randomly inactivated at an early stage of development

what is X chromosome inactivation

a process by which most of 1 X chromosome is inactivated

what does X chromosome inactivation prevent

a double dose of gene products which could be harmful to cells

why are carriers less likely to be affected by any deleterious mutations on the X chromosomes

since the X chromosome inactivated in each cell is random, half of the cells in any tissue will have a working copy of the gene

what are hermaphrodites

species that have functioning male and female reproductive organs in each individual

how do hermaphrodites work

they produce male and female gametes and usually have a partner with which to exchange gametes

what is the advantage of being a hermaphrodite

the chance of encountering a partner is an uncommon event, there is no requirement for that partner to be of the opposite sex

what other factors can affect sex and sex ratio in other species

temperature

size

competition

parasite infection

how does environmental sex determination in reptiles work

environmental temperature of egg incubation

how does the sex ratio change in some species

in response to resource availability