BIOLOGY UNIT 8 AND 9

THE BEGINNING OF LIFE

Theory of spontaneous generation: the idea that life can appear from inanimate objects. In ancient Egypt, they believed that the Sun’s heat on the sediment of the Nile produced snakes and crocodiles, while in the Middle Ages there were recopies of how to create living things from clay, animal faeces and food remains.

If a piece of meat is left for several days in a warm, damp place, flies and larvae appear. In the 17th century, controlled experiments were first carried out. Francesco Redi demonstrated that larvae found in rotting meat came from fly eggs (1668), and Louis Pasteur proved wrong the theory of spontaneous generation. He sterilized meat soup by heating it and observed that it didn’t decompose. One of the flasks had a bent neck while the other not. The bent neck avoids microorganisms to enter, avoiding decomposition.

Modern theories maintain that life began from molecules which existed on early stage of the Earth’s past. At the beginning of the 20th century, Oparin developed a theory which is still the basis for the hypotheses accepted by most scientists today. According to him, life arose from physical and chemical processes which occurred in the atmosphere of primitive Earth.

  1. Series of spontaneous chemical reactions between the atmosphere components (methane, ammonia, hydrogen and water vapor) 4000 million years ago. No O2.
  2. Electric charges provided the energy needed to start the reactions, from storms, intense ultraviolet radiation from the Sun and continuous volcanic eruptions. The synthesis of simple organic molecules began.
  3. Temperature fell, and water vapor in the atmosphere condensed leading to intense continuous rains that produced primitive seas, warmer and less deep than nowadays (named primordial soup by Oparin). Recently formed organic molecules accumulated there.
  4. Organic-molecules joined together and formed bigger molecules (proteins, polysaccharides).
  5. Newly formed biological molecules were insulated from water by inside structures called coacervates. However, there was still an exchange of molecules between water and coacervates. The more stable coacervates survived, while the less stable ones disappeared.
  6. Some of the stable ones divided and multiplied by synthesizing molecules (nucleic acids) that could replicate. This was the origin of cells. Life onwards will be considered an organism.

Present-day hypothesis: Oparin’s theory has been modified and improved. Stanley Miller (mid-20th cent.) tested part of the Oparin’s hypothesis by putting a mixture of gases (primitive atmosphere) in a container and subjected it to high voltage electric discharges. It produced various organic compounds.

Modifications and improvements: small organic molecules will join to form bigger ones only when certain minerals are present (clay or muddy soil) not in water. RNA would be the first nucleic acid to appear (DNA is too stable and complicated).

EVIDENCE OF EVOLUTION

  • Anatomical and morphological evidence: there are three types of organs:

Homologous organs: organs with the same structural pattern but with different functions. They prove divergent evolution (away from each other) or adaptive radiation (diversification of species into several forms), (E.g.: limbs in mammals).

Analogous organs: organs that carry out the same function but have different evolutionary origin, proving convergent evolution (similarities between species which have evolved separated from each other). E.g.: wings in insects and birds.

Vestigial organs: evolutionary remains of organs which tend to disappear. E.g.: wings in kiwi bird.

  • Fossil evidence: show how species change over time, coinciding with the theory of evolution.

Phylogenetics is the study of the evolution of related groups of organisms, whose fossils may have been found in different areas and which have slow but clear changes.

When fossils are compared to the ones today, normally there has been an increase in diversity and biological complexity.

Some organisms have had intermediate characteristics and then have evolved into different groups. These characteristics evolved or disappeared as the difference between species became greater.

  • Embryonic evidence: The similarities between embryos between different species show that there is a clear evolutionary between them. These similarities are greater and last for more time in groups of organisms that are closely related to each other. Ontogeny recapitulates phylogeny.
  • Biogeographical evidence: the gegraphicl distribution of animal and plant species can be explained according to theories of evolution. Over time, groups of organisms that are isolated geographically evolve differently to form new species.
  • Molecular evidence: the greater the molecular similarity between two groups of organisms, the closer their evolutionary relationship.
  • In all living organisms there are proteins and DNA molecules, which are made up of smaller molecules (amino acids and nucleotides) that follow specific sequences and characteristics for each organism. By comparing the sequences in different species and groups of organisms, the evolutionary relationship can be establishes.
  • Other evidence: Parasites: live off other species and have modified their organs to adapt to the host´s way of life. Tapeworms and leeches have different taxonomic phyla and do not share common ancestor, but use similar organs to attach to the host (convergent evolution).

Behavior: some instinctive behavior of animals is based on genetic inheritance.

PRESENT-DAY THEORIES OF EVOLUTION:

Darwin’s theory (which didn’t answer how differences between species came about) has been revised, developed and improved with the advance of scientific knowledge like the development of genetics at the beginning if the 20th century.

  • Neo-Darwinism considers that natural selection does not act on the individual in isolation but in the population it belongs to. Discoveries: Mendel’s law (hereditary traits and their transmission), sexual reproduction (gene combinations different from parental ones), genetic recombination (meiosis) and mutations (rapid changes in genes).

The synthetic theory of evolution combined the principles of Darwin’s theory with genetics and gene frequencies in populations. (Dobzhansky)

The theory of selfish gene is similar to neo-Darwinism: the unit of evolution is the gene, not the population. Competition occurs between genes, so individuals act as receptors and transmitters of genes. The final aim of the evolutionary process is to increase the frequency of some genes over others in a population (Dawkins)

Endosymbiotic theory: eukaryotic cells originated from the fusion of two types of bacteria. They acquired the capacity to phagocytose (consume or incorporate microorganisms) other cells, becoming cellular organelles like mitochondria. The interdependence (or symbiotic relationship) between these cells caused them to behave as a single organism. (Lynn Margulis)

  • Theory of punctuated equilibrium: the process of evolution has not always been slow and gradual but in many cases, the appearance of new species has happened quickly. It is based on sudden discoveries of fossils without any links to earlier forms, which are “macromutations” which affected the genes that regulated other genes. (Niles Eldredge)
  • Neutral theory of molecular evolution: the majority of mutations originate from genes neither advantageous nor disadvantageous for the individual, so natural selection does not apply.

THE ORIGIN OF NEW SPECIES

A species is a set of of individuals that have similar morphology, anatomy and physiology, can reproduce and create a fertile offspring. New species form due to changes produced in living organism. The stages in the formation of a new species are the following:

  • Production of evolutionary changes in populations: this happens due to natural selection, which favors ones and prejudices others. At the end, the new population is different from the original one but still belongs to the same species.
  • Genetic isolation of the new population: so that new species can evolve, they shouldn’t reproduce with the original one. Genetic isolation occurs thanks to different types of barriers: Geographical (prevents the physical contact between populations), ethological (new types behavior may appear that reject some individuals by others), sexual (anatomical difference that prevent mating or lack of synchronization between fertile periods), physiological (incompatibility of gametes which prevent fertilization), chromosomal (changes in the structure of chromosomes prevent the affected individuals from having an offspring with the rest of the population.
  • Gradual differentiation: after isolation, new species become more different because of the accumulation of changes due to mutations.
  • Speciation: populations are considered to be different species when they cannot reproduce with each other despite the isolation barriers had disappeared.

The process of evolution that causes the appearance of new species is called microevolution. The type of evolution that resulted in the appearance of large groups of species due to a drastic event is called macroevolution. It is not as easy to understand as microevolution. Some scientists believe that macroevolution is just the accumulation of microevolutionary changes over time. Although Darwin started to study both types, his reasoning of macroevolution is not convincing due to paleontological studies, which support that they are produced quickly.

ENVIRONMENTAL FACTORS

The characteristics of an environment that affect living organisms are called ecological factors. They can be biotic, which share their natural environment, or abiotic, which are physical or chemical conditions. In order to a species to develop normally, it needs a certain ecological factors to stand a certain limit, called limiting factors. If they are above or below an organism may not develop properly. Some examples are temperature, sunlight, water, oxygen content or concentration of salts and minerals.

ADAPTATION OF ORGANISMS TO THE ENVIRONMENT

Adaptation is the evolution of organisms to survive in a specific environment. They are anatomical, physiological or behavioral. Generalist organisms can survive in a great variety of environmental conditions like in temperate forests, while specialistorganisms cannot tolerate changes to their environment. For example, tropical forests, where the climate is stable.

  • Adaptations to water scarcity: It can take place in arid zones where there is little rainfall and moisture in the ground or in cold zones where water is so frozen that living things cannot access it easily. Adaptations are developed to obtain water and reduce its loss. In arid areas, plants have special tissues to store water, drought resistant seeds, deep and long root systems, small number of stomata that open at night to reduce water loss, hard small leaves and they complete their life cycle in the season that favors their development.
  • Adaptations to temperature changes: Biological processes of organisms are favored depending on temperature. If temperatures are too cold or hot, they cannot be carried out properly and can even stop. Bears have skin insulation (fat and hair), less blood flow to avoid heat loss and slow metabolic rate in winter (hibernation). Lizards search for warmer zones while bees come together to increase the temperature. Plants grow close to the ground to reduce heat loss, important organs grow underground and leaves fall in autumn.
  • Adaptations to sunlight:There are plants that grow better in shady places and other that prefer sunny areas: Green algae need to live near the water surface because it has only chlorophyll. Brown algae have chlorophyll and other pigment that allow them to live at greater depths. Red algae have red pigment that allows them to live in deeper zones.
  • Adaptations to varying salt concentration: It controls the amount of water that goes in and out of an organism’s cells in osmosis. If the environment has a great concentration of salt, water leaves an organism, making it die, while if concentration of salt is too low, water enters the organism making it die too. The fish who live in salty water can be bony (where the fish excretes salt through their gills and produce high concentration urine in low quantity) or cartilaginous (where the concentration inside and outside the fish is similar). Freshwater fish avoid the concentration of water inside them by stopping drinking, and eliminate water by expelling large amounts of low salt content urine).
  • Adaptations to lack of oxygen: There is less oxygen in high altitudes. Animals increase red blood cells and haemoglobin and their lung capacity.
  • Adaptations to lack of food: animals store energy reserves in their body, store food in hidden places or migrate to find food.

HOW LIVING THINGS AND THEIR ACTIVITIES CHANGE THEIR ENVIRONMENT:

  • The oxygen produced by autotrophic organisms increase the concentration of oxygen
  • Some animals like the earthwork make the soil more fertile by turning and airing it.
  • The roots of trees and action of animals break rocks and turn the soil, making erosion to occur easier.
  • The accumulation of large coral skeletons can create islands.
  • Sedimentaryrocks can be formed by layers of microorganisms.
  • Human beings are the ones that change the environment more abruptly, sometimes in a devastating and irreversible way.
  • Trees prevent sunlight to reach the ground, creating humidity. When water evaporates from a lake, it creates a humid microclimate. Roots of plants prevent erosion by holding the soil.

POPULATIONS: a population is set of organisms of the same species that inhabit the same territory and can reproduce together. They can be distributed randomly, in groups (clumped) or uniformly.

  • Types of populations:

Invertebrate colonies: individuals of the colony are connected to each other and are produced by asexual reproduction from the same parent.

Family groups: The individuals that make up a family come from a pair of animals whose offspring stay together with the family unit.

Caste system of social insects: These individuals come from one mother. They have anatomical and physiological differences which dictate their role in the community.

Social groups: These individual tend to live together and support each other mutually. Sometimes these groups are transitory such as during migrations.

  • Population dynamics: Populations increase, decrease and can even disappear. We can make predictions about its evolution over time if we study its dynamics. Evolution depends mostly on its biotic potential(r), the maximum reproductive potential of a population (the difference between the birth rate and the death rate). When the population arises, there is an initial slow growth (Latent phase), then a rapid growth (Exponential phase) and at the end growth slows and fluctuates around the carrying capacity (maximum number of individuals in a population that a specific environment can support). When a population becomes too numerous, reproduction is limited by lack of resources, competition and diseases.

  • Population growth: it depends on the number of successful reproduction from members at the reproductive age. The age structure diagram is a graphic representation of the distribution of individuals in a population according to age. They are useful for predicting the growth patterns of a population. In a decreasing population, individuals are mostly post-age, in a increasing population individuals are mostly reproductive age and in a population that will increase, individuals are mostly pre-reproductive age.
  • Population growth strategies:

R-strategies: produce numerous populations by using efficiently environmental resources; until they run out and have to emigrate or move into a lethargic state while waiting for better environmental conditions. E.g.: desert plants: quick flowering.

K-strategies: these species are well adapted to the environment and maintain a stable population with controlled growth. They care of the few offspring they have by investing large amount of resources, improving their chances of survival. They are common in ecosystems with few environmental variations.

COMMUNITIES: a set of populations of different organisms living in the same geographical are within a particular habitat. There is usually one species that is more important than others because of number of individuals or because of the species’ function in the community. Biodiversity is the biological variability among species in a community and the diversity between communities and ecosystems.

  • Dynamics of communities: Ecological succession is the sequential order in which plant communities and animal species associated to them change over time.

Primary succession: 1. an inhabited zone is colonized by pioneer organisms with few nutritional requirements like lichen and moss and then herbaceous plants. 2. The soil enriches gradually as biodiversity increases (shrubs). 3. Climax: Progressive evolution leads to a stable community, considering it to be established (pine and later oak and chestnut tree).

Secondary succession: When a population loses an important part of its populations due to climate change, natural disaster, erosion or reduction of water availability is called regression. A secondary succession occurs, resulting in a new community after a new evolutionary process

  • Communities and the soil:Soil is made up of mineral components that interact with organic components. It is created through biological and geological processes like weathering. It takes a long time to be formed and can be destructed by the lack of rain, fires, logging (cutting down trees) and the construction of buildings. The destruction of soil is a major loss of communities and can cause regression. The type of plants and animals that develop in and on the soil depend on the soil’s composition, structure and thickness. In poor soil quality only simple communities will colonize.