Evolution Flashcards
HOMOLOGOUS ANATOMICAL STRUCTURES
Organs, found on different organisms, having a similar basic structure, similar topographic appearance and relatively similar functions, are called HOMOLOGOUS. They have changed, however over time to serve different functions.
The best example which illustrates this, is the existence of the limb-bone pattern called the pentadactyl limb.
The limbs of many animals show adaptations in their function in different environmental conditions, however all have a similar general pattern implying a COMMON ANCESTOR to all these animals. This type of evolution is called adaptive radiation (or divergent evolution).
NATURAL SELECTION AND THE THEORY OF EVOLUTION
Natural Selection is the mechanism by which new species arise from pre-existing species. This was based on the following observations and deductions:
• Individuals within a population increase in exponential numbers
• Due to lack of food the numbers of individuals in a population remain
approximately constant.
• Many individuals fail to survive and reproduce. There is a “struggle for existence” within a population.
• Variation exists within all populations
• In the “struggle for existence” those individuals showing variation best adapted to their environment have a “reproductive advantage” or are “selected for” and produce more offspring than less well-adapted organisms which are “selected against”. Every organism is therefore subjected to a process of selection, based upon the suitability for survival given the conditions which exist at the time. The organism’s environment exerts a selection pressure. Selection pressure determines the spread of any allele within the gene pool.
• Individuals lacking favourable characteristics are less likely to survive long enough to breed. Over many generations their numbers will decline. The individuals with favourable characteristics will breed, with consequent increase in their numbers. The inheritance of one small variation will not, by itself, produce a new species. However, the development of a number of variations in a particular direction over many generations will gradually lead to the evolution of a new species.
SOURCES OF VARIATION
Mutation-is the original source of new genes or alleles
• Sexual reproduction through gamete formation. Male and female gametes fuse to form a new organism. These two gametes carry different characteristics which cause variation to the new organism.
- Meiosis through two ways:
- random distribution of homologous chromosomes on the spindle during
metaphase I.
- crossing over which occurs during prophase I between non-sister chromatids of homologous chromosomes and through fertilization
II. ANTIBIOTIC RESISTANCE IN BACTERIA
The discovery of antibiotics in the middle of the 20th century was proven one of the great triumphs in medicine. The introduction of antibiotics proved, however, a strong selection pressure for strains of bacteria that have the genetic capability of being resistant to the effects of antibiotics.
Genes that give resistance to an antibiotic can be found in that microorganisms that naturally makes the antibiotic. So a gene for resistance can be transferred to a bacterium that has it to one that doesn’t have it (horizontal gene transfer).
Another way that a bacterium might obtain a gene for resistance is by random
mutation.
Thereafter in the population, the resistant strains, in the absence of competition, will spread (vertical gene transfer) and have an advantage over bacteria which will be eliminated by the antibiotic. As a result, new antibiotics have to be developed to eliminate the resistant bacteria.
There are cases where bacteria show multiple resistance to several antibiotics.
SPECIATION AND POPULATION DIVERGENCE
separate (by natural phenomena such as a fire, a volcanic eruption etc OR by other mechanisms such as migration in an isolated area) so that they do not interbreed, they will evolve in different ways. The characteristics of the two populations will gradually change since they will become adapted to the new environment. If the two populations, after a period of time, will merge again but they fail to interbreed this means that they have become TWO different species. This is because there is continuous variation in the differences between populations. Populations gradually diverge by evolution to become different species. This idea comes into contrast with the idea that when distinct species were created have neither changed nor evolved!
THE USE OF A BINOMIAL SUSTEM TO NAME ORGANISMS
Classification is the attempt to arrange living organisms into groups. In the classification of living organisms the basic group in the species. Every species is classified into a genus. A name is given to each species called the binomial nomenclature (a “name” and a “surname”). The first name is applied to the genus of each organism and is written with a capital letter and the second name is applied to the species of the organism and is written with a small letter. Examples: Humans scientific name is Homo sapiens. The most common pine tree found in Greece is Pinus halepensis (note that scientific names were written in italics, or otherwise are underlined).
The binomial naming of all organisms is universal among biologists and it has been agreed and developed by a series of international congresses. By a universal naming confusion can be avoided.
THE USE OF DICHOTOMOUS KEYS
In ecological studies (field work) it is important to be able to identify the organisms found on the field- this is called species identification, and is usually done using keys. A key is a numbered list of external characteristics which DO NOT CHANGE (sizes are not used) where it leads to identification of a species.
NATURAL CLASSIFICATION
Early classification by taxonomists was based on MORPHOLOGY alone, but now take an EVOLUTIONARY bias, representing PHYLOGENETIC relationships. This means that
the species in a group have evolved from one common ancestral species. Advantages of classification:
1. Species identification- it is easier to find out to which species an organism belongs.
2. Predictive value- if sever members of a group have a given characteristic, then another member in this group will have the characteristic.
3. Evolutionary links- classification of species in one group can be used to predict how they evolved.
Many times reclassification is necessary when new evidence shows that the previous taxon (group) contains species that have evolved from different ancestral species.
For example taxonomists in the late 20th century have agreed that all organisms should be grouped into 5 KINGDOMS: one kingdom was the prokaryotes and 4 kingdoms were including all eukaryotes. The kingdom therefore was the highest group in taxonomy. Late studies however on ribosomal RNA have shown that among prokaryotes there are many large differences so that these can no longer be included in the same Kingdom. They have introduced the term DOMAIN to create an even higher taxonomic group were organisms would be classified as following:
THREE DOMAINS- Archaea, Eubacteria (bacteria) and Eukaryota (eukaryotes).
Viruses are not included into any domain since they are not considered as living.
CLASSIFICATION FROM KINGDOM TO SPECIES
Every group of classification used is called a taxon. Species is a taxon while kingdom in another. Species are classified in a series of taxa called the hierarchy of taxa. The taxa are the following from these including more organisms with only a few common characteristics to those with many common characteristics.
Kingdom Phylum Class Order Family Genus Species
Each Kingdom includes several Phyla, each Phylum includes several Classes etc
I. CHARACTERISTICS OF PLANTS
• Bryophytes (mosses and liverworts)
A division of nonflowering plants characterized by rhizoids rather than true roots and
having little or no organized vascular tissue. Most liverworts grow prostrate and consist
of a flattened, branching (but undifferentiated) green structure, the thallus. They
reproduce by spores found in a capsule. The capsule develops at the end of a stalk.
• Filicinophytes (ferns)
Non-flowering plants with roots, leaves and short non-woody stems. Leafs are divided into pairs of leaflets (pinnate). They reproduce by spores found in sporangia in the underside of the leaves.
Coniferophytes (conifers such as pines, firs etc)
Shrubs or trees with roots, woody stems and narrow leaves resembling needles.
Reproductions with seeds found in cones.
Angiospermophytes (flowering plants)
Very variable but usually have roots, woody stems and leaves. Reproduction by seeds which develop in flowers. The ovaries become fruit which ensures seed dispersal.
CHARACTERISTICS OF ANIMALS
Overview of Plant Phyla
. These are the characteristics of 7:
Porifera (sponges)
Bodies with no clear symmetry, usually attached to a surface, pores through the body, no mouth or anus.
• Cnidaria (jellyfish and sea anemones)
Radial symmetry, may have tentacles with stinging cells (knidocytes), mouth but no anus.
• Platyhelminths (flat worms, tapeworms)
Bilateral symmetry, flat softened body, mouth but no anus
Annelida (earthworms, leeches)
Bilateral symmetry, segmented body, mouth and anus.
• Mollusca (mussels, snails, squids, octopuses)
Bilateral symmetry, muscular foot and mandle (producing a shell in some of them), may
have a shell, mouth and anus.
• Arthropoda (insects, crabs, spiders)
Bilateral symmetry, exoskeleton, jointed appendages, mouth and anus.
• Reptiles (lizards, snakes, crocodiles, turtles)
Dry, scaly, impermeable skin, lungs with extensive folding, internal fertilisation, soft
shell around eggs, one type of teeth.
• Birds
Feathers growing from skin, lungs with parabronchial tubes, wings instead of front legs, hard shelled eggs, beak but no teeth.
• Mammals
Hairs growing from the skin, lungs with alveoli, give birth to live young, mammary
glands secrete milk, teeth of different types.
CLADISTICS
Clade: A group of organisms consisting of a single common ancestor and all descendants of that ancestor. The tree diagrams showing clades are called cladograms. The branching points on cladograms are called nodes.
Evolutionary relationships are now taken from polypeptide sequence or DNA sequence comparisons (see genetics notes) and not from anatomical features. Sequence differences accumulate gradually, so there is positive correlation between the number of differences between two species and the time they diverged from a common ancestor.
Cladistics: A method of classification of living organisms based on the construction and analysis of cladograms.
Although cladograms can provide strong evidence for the evolutionary history of a group, they cannot be regarded as proof. Cladograms are constructed on the assumption that the smallest possible number of mutations occurred to account for current base or amino acid sequence differences. Sometimes this assumption is not correct and evolutionary pathways were more complex. The best way to overcome this problem is to compare several versions of cladograms produced with the use of different genes.
