How evolution in reproduction has taken place in vertebrates
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.
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