Biology Chapter 46: Animal Reproduction

Animal Reproduction

Types of Reproduction

• Sexual reproduction: creation of offspring by fusion of haploid gametes (egg and sperm)
• Asexual reproduction: creation of offspring without fusion of egg and sperm
• Advantages of sexual reproduction: increased genetic diversity, adaptation to environment
• Advantages of asexual reproduction: rapid reproduction, conservation of energy
• Disadvantages of asexual reproduction: limited genetic diversity, susceptibility to disease

Examples of Animals

• Asexual reproduction: budding of sponges and hydra, more primitive forms of animals
• Sexual reproduction: most vertebrate animals, including humans

Fertilization

• Internal fertilization: occurs within an organism, enables sperm to reach an egg in a dry environment
• External fertilization: occurs outside an organism, requires a moist habitat to prevent gametes from drying out
• Fertilization: union of egg and sperm, resulting in the formation of a zygote

Reproductive Organs

• Gonad: organs that produce gametes (testes in males, ovaries in females)
• Male reproductive organs: seminiferous tubules, epididymis, vas deferens, urethra
• Female reproductive organs: ovaries, oviduct (fallopian tube), uterus (womb)

Gametogenesis

• Gametogenesis: production of gametes from germ cells
• Spermatogenesis: formation of sperm, resulting in 4 sperm
• Oogenesis: development of a mature egg, resulting in 1 egg, prolonged process

Hormones in Reproduction

• Hypothalamus: secrets GnRH, directing release of FSH and LH from the anterior pituitary
• FSH and LH: stimulate sex hormone production by gonads (testosterone, estrogens: estradiol and progesterone)
• Oxytocin: stimulates the uterus to contract (positive feedback)

Menstrual and Ovarian Cycles

• Regulated by sex hormones
• Menopause: end of ovarian and menstrual cycles in females

Conception

• Fertilization of an egg by a sperm, occurring in the oviduct (fallopian tube)

Neurons

• Neurons are nerve cells that transfer information within the body.
• A neuron consists of a cell body, dendrites, and an axon.

Structure of a Neuron

• Cell body: contains most of the neuron's organelles.
• Dendrites: highly branched extensions that receive signals from other neurons.
• Axon: longer extension that transmits signals to other cells at synapses.

Synapses and Neurotransmitters

• Synapse: a junction between an axon and another cell.
• Neurotransmitters: chemical messengers that pass information from the transmitting neuron to the receiving cell at synapses.

Types of Neurons

• Sensory neurons: transmit information about external stimuli such as light, touch, or smell.
• Interneurons: integrate (analyze and interpret) the information.
• Motor neurons: transmit signals to muscle cells, causing them to contract.

Glial Cells

• Glial cells (Glia): supporting cells that aid neurons in their functions.

Mechanisms of Signal Transmission

• Resting potential: the membrane potential of a neuron that is not sending signals, characterized by:
  • Many open K+ channels and fewer open Na+ channels.
  • K+ diffuses out of the cell, resulting in a buildup of negative charge within the neuron.
• Action potential: changes in membrane potential, signals conducted by axons, involves:
  • Opening other types of ion channels, triggering a depolarization.
  • A reduction in the magnitude of the membrane potential.
  • If a depolarization shifts the membrane potential sufficiently, it results in an action potential (all or none process, requiring a threshold).

Nervous Systems

• A nerve is a bundle of axons from multiple neurons.
• The nervous system has two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
• CNS consists of the brain and spinal cord, while PNS consists of neurons that carry information to and from the CNS.

Glial Cells

• Glial cells are supporting cells of neurons that provide nourishment, support, and regulation.

Brain Structure

• The forebrain consists of the cerebrum, thalamus, hypothalamus, and amygdala.
• The cerebrum is the largest structure in the brain and is essential for language, cognition, memory, and consciousness.
• The hypothalamus is a control center that regulates body temperature and biological clock.
• The amygdala is responsible for storing and recalling emotional memory.

Brain Regions

• The midbrain is a region of the brain.
• The hindbrain consists of the cerebellum, medulla, and pons.
• The cerebellum coordinates movement and balance, and helps in learning and remembering motor skills.

Cephalization

• Cephalization is the clustering of sensory organs at the front end of the body, characteristic of bilaterally symmetrical animals.

Neuronal Plasticity

• Neuronal plasticity refers to the ability of the nervous system to be remodeled or reorganized after birth.

Ecology and the Biosphere

• Ecology is the scientific study of the interactions between organisms and the living and nonliving components of their environment.
• There are four main branches of ecology: population ecology, community ecology, ecosystem ecology, and biosphere ecology.

Climate

• Climate refers to the long-term prevailing weather conditions in an area.
• The four major components of climate are temperature, precipitation, sunlight, and wind.

Biomes

• Biomes are major life zones characterized by vegetation type or physical environment.
• There are two main categories of biomes: terrestrial biomes and aquatic biomes.

Terrestrial Biomes

• Tropical rain forests: dominated by broadleaf evergreen trees, vertically layered, and intense competition for light.
• Deserts: occur in bands near 30° north and south of the equator, with low and highly variable precipitation.
• Savannas: warm year-round, with dominant plants being grasses and forbs.
• Chaparral: occurs in mid-latitude coastal regions, with highly seasonal precipitation and dominated by shrubs.
• Temperate grasslands: precipitation is highly seasonal, with dry winters and wet summers.
• Coniferous forests: the largest terrestrial biome, with annual precipitation of 30-70cm and dominant vegetation being evergreen conifers.
• Temperate forests: significant amount of precipitation, with dominant plants being deciduous trees.
• Tundra: cold winters, with permafrost restricting the growth of plant roots.

Aquatic Biomes

• Oceans: cover 75% of the Earth's surface, with a salt concentration of 3%.
• Two types of aquatic biomes: marine biomes and freshwater biomes.
• Marine biomes:
  • Estuaries: nutrient-rich and productive transition zones between a river and the sea.
  • Intertidal zones: periodically submerged and exposed by the tides.
  • Ocean pelagic zones: expanses of open water.
  • Coral reefs: formed from the calcium carbonate skeletons of corals, highly diverse species.
  • Marine benthic zones: consist of the seafloor below the surface waters of the coastal, deep sea hydrothermal vents.
• Freshwater biomes:
  • Lakes: undergo a mixing of their waters called turnover in the spring and autumn.
  • Wetlands: inundated by water at least some of the time, supporting plants adapted to water-saturated soil.
  • Streams and rivers: current (headwater vs. downstream).

Convergent Evolution

• Convergent evolution: two evolutionarily unrelated species may live within the same type of biome but on different continents.

Population Ecology

• A population is a group of individuals of the same species living in an area.

Population Density

• Population density is the number of individuals per unit area.
• It can be examined by counting, sampling method, checking animal droppings, and mark-recapture method.

Population Dispersion

• Population dispersion refers to the pattern of spacing among individuals within the boundaries of the population.
• There are three types of population dispersion: clumped, uniform, and random.
• Clumped: individuals aggregate in patches.
• Uniform: individuals are evenly spaced.
• Random: the position of each individual is independent of others.

Survivorship Curves

• Survivorship curves are a plot of the proportion of numbers in a cohort still alive at each age.
• There are three types of survivorship curves: Type I, Type II, and Type III.
• Type I: Low death rates during early and middle life and a sharp increase in death rates later in life, found in large mammals.
• Type II: Constant death rate over the life span, found in some rodents, invertebrates, lizards, and annual plants.
• Type III: High death rates for the young; death rate steeply declines for survivors of early period die-off, found in organisms that produce very large numbers of offspring but provide little or no care.

Population Growth

• Population growth can be exponential or logistic.
• Change in population size = births – death.
• Carrying capacity (K) is the maximum population size that a particular environment can sustain.
• When N (population size) is small compared to K, the population grows rapidly.
• When N is large and resources are limiting, the population grows slowly.
• When N equals K, the population stops growing.

Factors Limiting Population Size

• There are two types of factors that limit population size: density-dependent and density-independent.
• Density-dependent factors: when a death rate increases or a birth rate decreases with increasing density (e.g. disease, competition).
• Density-independent factors: A birth rate or death rate that does not change with population density (e.g. natural disaster).

Population Dynamics

• Population dynamics refer to the fluctuations in population size over time.
• Some populations fluctuate at unpredictable intervals, while others undergo regular boom-and-bust cycles (e.g. snowshoe hares and lynx).

Ecological Footprint

• An ecological footprint summarizes the aggregate land and water area needed to sustain a person, city, or nation.

Interspecific Interactions

• Interspecific interactions occur between individuals of different species
• Competition: when individuals of different species use a limited resource, affecting survival and reproduction of both
• Competitive exclusion: local elimination of the inferior competitor due to shared resource use
• Resource partitioning: differentiation of niches enabling coexistence of similar species, possible in time and space

Types of Interspecific Interactions

• Exploitation: one species benefits, the other is harmed (e.g., predation, herbivory, parasitism)
• Predation: a predator kills and eats a prey species
• Herbivory: an herbivore eats parts of a plant or alga
• Parasitism: a parasite derives nourishment from a host, harming it
• Mutualism: both species benefit
• Commensalism: one species benefits, the other is neither harmed nor helped

Defense Mechanisms of Prey

• Aposematic coloration: bright warning coloration
• Cryptic coloration: camouflage
• Batesian mimicry: a harmless species mimics a harmful model
• Mullerian mimicry: multiple unpalatable species resemble each other

Community Ecology

• Species diversity: composed of species richness and relative abundance
• Trophic structure: feeding relationships between organisms in a community
• Energy transfer: from autotrophs to herbivores to carnivores, with decomposers as the final link
• Energetic hypothesis: food chain length is limited by inefficient energy transfer
• Foundation species: have strong effects due to their size or abundance, providing habitat or food
• Keystone species: exert strong control through pivotal ecological roles, despite not being abundant
• Ecosystem engineers: create or alter their physical environment

Ecological Succession

• Pattern of colonization and species replacement following a severe disturbance
• Primary succession: occurs in areas with no previous vegetation
• Secondary succession: occurs in areas with previous vegetation, but was disturbed or destroyed

Ecosystems

• An ecosystem is composed of all living organisms in an area and the abiotic factors with which they interact.
• Ecosystem dynamics involve two main processes: energy flow and chemical cycling.

Energy Flow

• Energy enters most ecosystems as sunlight.
• Autotrophs convert sunlight into chemical energy through photosynthesis.
• Chemical energy is passed to heterotrophs as food.
• Energy is eventually dissipated as heat.

Trophic Levels

• Primary producers: autotrophs that build organic molecules using sunlight or inorganic compounds as energy sources.
• Examples of primary producers include photosynthetic plants, algae, and prokaryotes.
• Primary consumers: herbivores that eat primary producers.
• Secondary consumers: carnivores that eat herbivores.
• Tertiary consumers: carnivores that eat other carnivores.
• Decomposers: heterotrophs that obtain energy from detritus, including prokaryotes and fungi.

Trophic Efficiency

• Trophic efficiency is the percentage of production transferred from one trophic level to the next.
• Average trophic efficiency is about 10%.

The Carbon Cycle

• Photosynthetic organisms convert CO2 into organic molecules.
• Carbon reservoirs include:
  • Fossil fuels
  • Soils and sediments
  • Dissolved compounds in oceans
  • Living biomass
  • The atmosphere
  • Sedimentary rocks
• CO2 is taken up through photosynthesis and released through cellular respiration.
• Volcanoes and the burning of fossil fuels and wood contribute significant amounts of CO2 to the atmosphere.

Biodiversity and Threats

• Biodiversity encompasses genetic diversity, species diversity, and ecosystem diversity
• Habitat loss has caused the greatest number of extinctions on record
• Introduced species compete with native species and displace them, leading to extinctions
• Overharvesting occurs when organisms are harvested at rates exceeding the ability of their populations to rebound
• Global change refers to alterations in climate, atmospheric chemistry, and broad ecological systems that reduce Earth's capacity to support life

Biodiversity Hotspots and Ecosystem Disruptions

• Biodiversity hotspots are relatively small areas with numerous endemic and endangered species
• Eutrophication occurs when an ecosystem changes from nutrient-poor to nutrient-rich, leading to algal blooms and "dead zones"
• Biological magnification occurs when toxins are passed up the food chain and concentrated in organisms at higher trophic levels

Plastics and Climate Change

• Plastics are the most common type of marine waste, with microplastics being a significant concern
• Human activities are driving climate change through the release of gaseous waste into the atmosphere, leading to a steady increase in atmospheric CO2 over the past 170 years
• Burning fossil fuels and deforestation are the major factors driving climate change

Mass Extinctions

• Previous mass extinctions have been caused by rise of sea levels, continental drift, intense volcanic activity, climate change, and meteor impact
• The current potential 6th mass extinction, known as the Holocene, is driven by human activities
• Mass extinction can pave the way for adaptive radiation by eliminating many species and allowing new ones to arise and adapt to different ecological niches
• Pathogens can evolve more efficient forms of reproduction in new environments due to the lack of resistance in hosts through natural selection.