Ch 41: Animal reproduction
Asexual reproduction is efficient but limits genetic variability
asexual reproduction is efficient:
o time and energy aren’t wasted on mating
o every member of the population can reproduce
o successful genotypes can be preserved
o mostly invertebrates; tend to live in stable environments
disadvantage of asexual reproduction:
o offspring are genetically identical to one another and to their parents
o genetic diversity is the raw material that enables natural selection to shape
adaptations in response to environmental change
aphids have both sexual and asexual reproductive periods
o in spring, females hatch from overwintered eggs and reproduce asexually
o in the fall, some offspring develop into males and reproduce sexually
budding – produces new individuals that grow from the bodies of older animals
o buds grow by mitotic cell division – cells differentiate before the bud breaks away
o bud is genetically identical to the parent
regeneration – can replace damaged tissues or lost limbs / form a complete individual
(sea stars)
o fission – see anemones can split in half; each half grows into a new animal
o some segmented marine worms develop segments with rudimentary heads; the
segments break apart and each one forms a new worm
parthenogenesis – offspring develop from unfertilized eggs: arthropods, some fish,
amphibians, reptiles
o most species also engage in sexual reproduction at other times
o parthenogenesis may determine the sex of the offspring (honeybees)
o in some species, it requires sexual behavior
for whiptail lizards – sexual activity triggers release of eggs from the
ovaries of the acting female
Sexual reproduction involves the union of haploid egg and sperm
sexual reproduction – only half the population bears young (a loss of efficiency)
o mating behavior involves costs and risks; time and energy, exposure to predation,
and physical damage
most eukaryotes reproduce sexually; genetic diversity is an evolutionary advantage that
outweighs the costs
in sexual reproduction, 2 haploid cells (gametes) form a diploid individual
gametes are produced by meiosis, which produces genetic diversity through crossing
over and independent assortment
gametogenesis occurs in the gonads
o males – gonads are testes; gametes are small, motile sperm
o spermatogenesis creates 4 sperm haploid cells from 1 diploid cell
, o females – gonads are ovaries; gametes are nonmotile ova, or eggs
o polar bodies are from first meiotic division; they allow the process to occur, then
they degrade – so one diploid cell results in 1 haploid egg (and 3 polar bodies
that cannot be fertilized) = this all occurs in ovaries
gametes are produced from germ cells – present early in development and distinct from
the rest of the cells of the body (somatic cells)
fertilization – union of sperm and egg:
o sperm and egg recognize each other
o activation of sperm
o plasma membranes fuse
o egg blocks entry of additional sperm
o egg is activated metabolically
o egg and sperm nuclei fuse
sperm and egg interactions are controlled by specific recognition molecules
o ensures that sperm interact only with eggs and prevents fertilization by the
wrong species; separation of species (species classified by whether they can
reproduce with each other)
especially important in aquatic species that release eggs and sperm into
the water
species with internal fertilization have egg-sperm recognition mechanisms
Asexual reproduction is efficient but limits genetic variability
asexual reproduction is efficient:
o time and energy aren’t wasted on mating
o every member of the population can reproduce
o successful genotypes can be preserved
o mostly invertebrates; tend to live in stable environments
disadvantage of asexual reproduction:
o offspring are genetically identical to one another and to their parents
o genetic diversity is the raw material that enables natural selection to shape
adaptations in response to environmental change
aphids have both sexual and asexual reproductive periods
o in spring, females hatch from overwintered eggs and reproduce asexually
o in the fall, some offspring develop into males and reproduce sexually
budding – produces new individuals that grow from the bodies of older animals
o buds grow by mitotic cell division – cells differentiate before the bud breaks away
o bud is genetically identical to the parent
regeneration – can replace damaged tissues or lost limbs / form a complete individual
(sea stars)
o fission – see anemones can split in half; each half grows into a new animal
o some segmented marine worms develop segments with rudimentary heads; the
segments break apart and each one forms a new worm
parthenogenesis – offspring develop from unfertilized eggs: arthropods, some fish,
amphibians, reptiles
o most species also engage in sexual reproduction at other times
o parthenogenesis may determine the sex of the offspring (honeybees)
o in some species, it requires sexual behavior
for whiptail lizards – sexual activity triggers release of eggs from the
ovaries of the acting female
Sexual reproduction involves the union of haploid egg and sperm
sexual reproduction – only half the population bears young (a loss of efficiency)
o mating behavior involves costs and risks; time and energy, exposure to predation,
and physical damage
most eukaryotes reproduce sexually; genetic diversity is an evolutionary advantage that
outweighs the costs
in sexual reproduction, 2 haploid cells (gametes) form a diploid individual
gametes are produced by meiosis, which produces genetic diversity through crossing
over and independent assortment
gametogenesis occurs in the gonads
o males – gonads are testes; gametes are small, motile sperm
o spermatogenesis creates 4 sperm haploid cells from 1 diploid cell
, o females – gonads are ovaries; gametes are nonmotile ova, or eggs
o polar bodies are from first meiotic division; they allow the process to occur, then
they degrade – so one diploid cell results in 1 haploid egg (and 3 polar bodies
that cannot be fertilized) = this all occurs in ovaries
gametes are produced from germ cells – present early in development and distinct from
the rest of the cells of the body (somatic cells)
fertilization – union of sperm and egg:
o sperm and egg recognize each other
o activation of sperm
o plasma membranes fuse
o egg blocks entry of additional sperm
o egg is activated metabolically
o egg and sperm nuclei fuse
sperm and egg interactions are controlled by specific recognition molecules
o ensures that sperm interact only with eggs and prevents fertilization by the
wrong species; separation of species (species classified by whether they can
reproduce with each other)
especially important in aquatic species that release eggs and sperm into
the water
species with internal fertilization have egg-sperm recognition mechanisms
