Most vertebrates reproduce sexually; only for lizards and fishes normally reproduce parthenogenetically.
Sexual reproduction evolved among aquatic animals and then spread to land animals.
Fishes:
They reproduce in aquatic environment. In bony fishes fertilization is usually external and eggs contain only enough yolk to sustain the developing fish for short time and the growing fish must seek food. Thousands of eggs are produced and fertilized but few survive and grow to maturity. Some succumb to fungal and bacterial infections others to siltation and still others to predation. Thus to assure reproductive success the fertilized egg develops rapidly and the young achieve maturity within a short time.
Amphibians:
These vertebrates on land face the danger of drying out and tiny gametes were especially vulnerable. The amphibians were the first land vertebrates. Their life cycle is still linked to water. Fertilization is still external usually. Among frogs and toads, male grasps the female and discharges fluid containing sperm into the eggs as she releases them into water.
The developmental period is much longer in amphibians than in fishes although the eggs do not contain more yolk. An evolutionary adaptation in the life cycle is egg-larva-adult stages. Aquatic larval stage develops rapidly and the animals spend much time in eating and growing. After reaching sufficient size the larva metamorphoses into the adult (often terrestrial) form.
Reptiles:
Reptiles were the first successful land vertebrates. Evolutionary adaptations found in reptiles are:
(1) Internal fertilization that protects the gametes from drying out, freeing the animals from returning to the water to breed.
(2) Many reptiles are oviparous, others are ovoviparous. They lay eggs that hatch in the body of the female and the young are born alive.
(3) The shelled egg and extra embryonic membranes are also first seen in reptiles.
These adaptations allowed reptiles to lay eggs in dry places without danger of desiccation.
As the embryo develops, extra embryonic chorion and amnion protect it; the amnion creates a fluid filled sac for the embryo. The allantois permits gas exchange and stores excretory products. Complete development can occur within the egg shell. When the animals hatches it has developed to the point that it can survive on its own with some parental care.
Birds:
Birds have retained the adaptations for life on land that evolved in the early reptiles. With the exception of most waterfowl, birds lack a penis. Males simply deposit semen against the cloaca for internal fertilization. Sperms then migrate up the cloaca and fertilize the eggs before hard shells form. This method of mating occurs more quickly than internal fertilization that reptiles practice. All birds are oviparous and the egg shells are much thicker than those of reptiles. Thicker shells permit birds to sit on their eggs and warm them. This brooding or incubating hastens embryo development. When many young birds hatch, they require extensive parental care and feeding.
Mammals:
Most primitive mammals the monotremes (e.g. duck billed platypus and spiny anteater) lay eggs. All other mammals are viviparous and it is evolutionary adaptation and it has taken two forms.
(1) Marsupials developed the ability to nourish their young in a pouch after short gestation inside the female.
(2) The placentals retain the young inside the female, where the mother nourishes them by means of placenta. Mammary glands are unique mammalian adaptation that permit the female to nourish the young with milk that she produces. After the birth some mammals nurture their young until adulthood, when they are able to mate and fend for themselves.
Mammals reproduce, behaviour also contributes to the transmission and evolution of culture that is the key to the evolution of human species.
.jpg)