Variation and sexual reproduction

Sexual reproduction

reproduction involving fusion of gametes from two parents

Costs of sexual reproduction

males do not produce offspring and only half of each parent’s genome is passed on

Genome disruption

successful parental genomes are broken up during sexual reproduction

Benefits of sexual reproduction

increased genetic variation in populations

Genetic variation advantage

provides raw material for natural selection and adaptation

Red Queen hypothesis explanation

sexual reproduction persists due to host

Host fitness

ability to resist or tolerate parasitism

Parasite fitness

ability to feed reproduce and locate new hosts

Sexual reproduction in hosts

genetic variability reduces susceptibility of offspring to parasites

Asexual reproduction

reproduction involving one parent with no fusion of gametes

Asexual genome transfer

entire parental genome passed to offspring

Asexual reproductive rate

offspring produced more rapidly and in larger numbers

Colonisation advantage

single individual can establish large populations

Genome maintenance

advantageous in stable or narrow niches

Vegetative cloning

asexual reproduction in plants

Parthenogenesis

reproduction from unfertilised female gametes

Parthenogenesis distribution

more common in cooler climates or low parasite density regions

Asexual limitations

reduced ability to adapt to environmental change

Mutation in asexual populations

provides limited variation enabling some evolution

Horizontal gene transfer

exchange of genetic material between individuals

Prokaryotic gene transfer

allows rapid evolutionary change

Meiosis

nuclear division producing haploid gametes from diploid cells

Homologous chromosomes

chromosomes with same genes at same loci

Sister chromatids

identical chromatids joined at the centromere

Crossing over

exchange of DNA between non-sister chromatids

Recombination

production of genetically different chromosomes

Independent assortment

random orientation of homologous chromosome pairs

Meiosis I

separation of homologous chromosomes

Meiosis II

separation of sister chromatids

Meiosis outcome

four haploid genetically different cells

Sex chromosomes

chromosomes determining sex

SRY gene

gene on Y chromosome encoding testes-determining factor

Heterogametic males

XY males lacking many X-linked alleles

Sex-linked inheritance

inheritance patterns due to X and Y chromosome differences

X chromosome inactivation

random inactivation of one X chromosome in females

Dosage compensation

prevention of double gene expression from X chromosomes

Carrier females

reduced expression of deleterious X-linked alleles

Hermaphrodites

individuals with both male and female reproductive organs

Hermaphrodite advantage

ability to mate with any encountered individual

Environmental sex determination

sex determined by environmental factors

Temperature-dependent sex

reptile sex determined by incubation temperature

Sequential hermaphroditism

sex change due to size competition or parasitism

Adaptive sex ratios

offspring sex ratio adjusted to resource availability