BIO2242 Lecture Notes - Lecture 20: Yolk, Viviparity, Oviparity
Lecture 20 – Vertebrate Life History I
Vertebrate Life History
• Great diversity
o 50,000+ species
o Phylogenetic history
• Habits/environments
• Morphology
• Body size
o Changes
Evolution of Vertebrates
• Transition into land
o External → internal fertilisation
o Amniotic egg
▪ More K direction
▪ Limitation on number eggs can have → internal fertilisation:
larger eggs
o Life cycles: complex → direct developers (except amphibians)
• Ectothermic → endothermic
o Land: much higher fluctuations in temperature
o Water: much more stable temperature
Vertebrates: Transition to Land
• Tetrapods – 4 legs
o Amniotic eggs that allow it to live on land
• Endothermic
o Birds and mammals
o Massive impact on life history: energetically
expensive to maintain body temperature
Amniotic Egg
• Amphibians
Problem
Solution
Examples
1. Protect the gametes
• Return to water
• Internal insemination
• Amphibians
• Amniote vertebrates
2. Protect the young
• Shelled yolky eggs
• Viviparity
• Postnatal parental care
(e.g. nest)
• Amphibians, reptiles,
birds, monotreme
mammals
• Some reptiles, most
mammals
• Some amphibians, some
reptiles, most birds and
mammals
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o No shell → prone to desiccation
o Only in aquatic environment so doesn’t dry out
o Dense protective jelly → prevent from drying out
o Yolk nourish embryo
o Waste permeates through jelly to
external environment
• Amniotes
o Facilitates life on land (no larval stage)
▪ Embryo in contained aquatic
environment
o Protective calcaerious shell (permeable)
o Yolk nourish embryo
o Three embryonic membranes
▪ Allontois: store embryo waste
▪ Chorion: blood vessels, gas
exchange
▪ Amnion: Fluid-filled, protects embryo
Patterns of Vertebrate Life History
• Vertebrate Life Cycles
o Fishes
▪ Complex life cycle
▪ Oviparous → viviparous
o Amphibians
▪ Complex life cycle
▪ Oviparous → viviparous
• Egg → tadpole → metamorph → frog
o Reptile
▪ Direct devlopers (no larvae stage)
▪ Oviparous → viviparous
▪ Lizards and snakes – live bearing
▪ Majority egg laying
o Birds
▪ Direct developers
• All egg laying
▪ Oviparous
o Mammals
▪ All direct developers
▪ Oviparous (apart from monotremes) → viviparous
▪ Extended gestation
▪ Delayed maturity → longer life span
Vertebrate Reproduction
• Asexual reproduction is rare
o Parthenogenesis: some fish, amphibians and reptiles
o Absent in birds and mammals
• Sexual reproduction
o Most species are Gonochoristic (separate females and males)
o Some hermaphroditic species
▪ E.g. sequential hermaphroditism in fishes
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