2.3 Variation and sexual reproduction

What are the costs of sexual reproduction

males are unable to produce offspring; only half of each parent's genome is passed onto offspring, disrupting successful parental genomes

Why do benefits outweigh costs in sexual reproduction

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

What co-evolutionary interactions may select for sexually reproducing hosts

Parasites and hosts

What hosts have greater fitness

Hosts better able to resist and tolerate parasitism

What parasites have greater fitness

Parasites better able to feed, reproduce and find new hosts

What happens if hosts reproduce sexually

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

Why can asexual reproduction be a successful reproductive strategy

as whole genomes are passed on from parent to offspring

What happens in asexual reproduction

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

When is maintaining the genome of the parent an advantage

In very narrow, stable niches or when re-colonising disturbed habitats

What is parthenogenesis

reproduction from a female gamete without fertilisation

What are examples of asexual reproduction in eukaryotes

vegetative cloning in plants and parthenogenesis in lower plants and animals that lack fertilisation

What can happen in asexual reproduction

Offspring can be reproduced more often and in larger numbers

Where is parthenogenesis more common

in cooler climates, which are disadvantageous to parasites, or regions of low parasite density or diversity

How can asexually reproducing populations adapt with ease to changes in thier environment

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

What do organisms that reproduce principally by asexual reproduction have

Mechanisms for horizontal gene transfer between individuals to increase variation, for example the plasmids of bacteria and yeasts

What can prokaryotes do

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

homologous pairs

What are homologous chromosomes

chromosomes of the same size, same centromere position and with the same sequence of genes at the same loci

What are linked genes

genes on the same chromosome, crossing over can result in new combinations of the alleles of these genes

What are the stages of meiosis 1

The chromosomes, which have replicated prior to meiosis 1 each consist of two genetically identical chromatids attached to the centromere

The chromosomes condense and the homologous chromosomes pair up

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

This crossing over of DNA is random and produces genetically different recombinant chromosomes

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

The orientation of the pairs of the homologous chromosomes at the equator is random

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

Cytokinesis occurs and two daughter cells from

What is independant assortment

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

What happens in meiosis 2

Each of the two cells produced in meiosis 1 undergoes a further division during which the sister chromatids of each chromatid are separated

A total of four haploid cells are produced

What is determined by the presence of sex chromosomes

The sex of birds, mammals and some insects

In most mammals what determines the development of male characteristics

SRY gene on Y chromosome

What do heterogametic (XY) males lack most of

The corresponding homologous alleles on the shorter (Y) chromosome

What happens to homogametic females (XX) in the early stages of development

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

How does deleterious mutations on these X chromosomes affect carriers

They are less likely to be affected

As the X chromosome inactivated in each cell is random what happens

Half of the cells in any tissue will have a working copy of the gene in question

What a hermaphrodites

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

What do hermaphrodites do

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

What is a benefit 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

What determines sex and sex ratio

Environmental rather than genetic factors

What is environmental sex determination in reptiles controlled by

Environmental temperature of egg incubation

Why can sex change within individuals of some species

as a result of size, competition, or parasitic infection

Why in some species can the sex ratio of offspring be adjusted

In response to resource availability