through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram.
Natural selection, inheritance, and divergence of character are mechanisms used to avoid extinction of a species. Do all work together or are each individual mechanisms acting on their own?
Amongst many animals, sexual selection will give its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone, in their struggles with other males.
Sexual selection and natural selection work hand in hand because they assure that the male that carries the best traits to adapt/survive producing more offspring so that those traits are passed on. This can assure the survival of the species.
For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure.
competition for similar resources and space
Moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their common parent (A) more numerous
Descent! Species inherit traits that are favorable to survival of the individual and the species.
The truth of the principle, that the greatest amount of life can be supported by great diversification of structure, is seen under many natural circumstances. In an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be severe, we always find great diversity in its inhabitants.
greater life=greater diversification? Constant struggle that pushes species to deviate which creates more diverse/abundant life
As the differences slowly become greater, the inferior animals with intermediate characters, being neither very swift nor very strong, will have been neglected, and will have tended to disappear.
The inferior species (less variation) is unable to survive.
From these several considerations I think it inevitably follows, that as new species in the course of time are formed through natural selection, others will become rarer and rarer, and finally extinct.
Survival of the fittest. As adapted species grow and populate the world, there is less space for those that did not adapt and they go extinct because of lack of resources, space, and variation.
Furthermore, the species which are most numerous in individuals will have the best chance of producing within any given period favourable variations.
There is likely more reproducing that is occurring because there are more individuals that are able to reproduce. This allows variation to occur more quickly and more abundantly.
artificial selection
Humans identify desirable traits within a species and use those traits to develop desirable phenotypic traits by breeding. What are common examples of artificial selection? Is artificial selection deemed a positive or a negative thing?
That natural selection will always act with extreme slowness, I fully admit.
Natural selection is NOT an overnight process, it could take years, decades, centuries, or even more.
will give time for any new variety to be slowly improved
Concerns of predation will be minimized
If there exist organic beings which never intercross, uniformity of character can be retained amongst them, as long as their conditions of life remain the same, only through the principle of inheritance, and through natural selection destroying any which depart from the proper type; but if their conditions of life change and they undergo modification, uniformity of character can be given to their modified offspring, solely by natural selection preserving the same favourable variations.
Natural selection and intercrossing work together for the development/advancement of a species. Are these separate acts or is intercrossing and natural selection always acting together?
Finally then, we may conclude that in many organic beings, a cross between two individuals is an obvious necessity for each birth; in many others it occurs perhaps only at long intervals;
Crossing between two species is necessary for birth and therefore, population conservation or growth. For larger mammals that have long gestation periods, and whose numbers are rapidly depleting, it makes it harder to conserve their species or to grow their species because there are less opportunities for crossing.
hermaphrodites
a person or animal with both male and female sex organs or other sex characteristics
Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure.
Natural selection acts by preserving the modifications that are made in an organism so help them better survive
No naturalist doubts the advantage of what has been called the "physiological division of labour;"
the duties for maintaining the existence/survival of a species is distributed among their different tissues/organs
transmitted these advantages to their male offspring.
Descent of characteristics!
The males of carnivorous animals are already well armed; though to them and to others, special means of defence may be given through means of sexual selection, as the mane to the lion, the shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield may be as important for victory, as the sword or spear.
Males are given particular characteristics that equip them for competing or winning the affection of a female counterpart
Sexual Selection.
from of natural selection arising through preference of one sex for a certain characteristic in an individual of another sex
In social animals it will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change.
Would this be similar to kin selection?
when one part of the organisation is modified through variation, and the modifications are accumulated by natural selection for the good of the being, will cause other modifications, often of the most unexpected nature.
Is this saying that when a species is modified via variation, it typically lead to more modification, if the modifications are benefiting the being?
should succeed.
Species have an environment/conditions that are better suited for their survival. Many species are confined to a very particular environment.
forms of life are now different from what they formerly were.
Species constantly change- even if it is in the slightest way
The proportional numbers of its inhabitants would almost immediately undergo a change, and some species might become extinct.
Current problem plaguing the world. Some species that produce quickly can adapt and others cannot. The elimination of a particular species can have cascading effects on the surrounding species.
If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?
Survival of the fittest
We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature.
Human intervention
competition should be most severe between allied forms,
They have to "deal" with the survival of the fittest because they are in need of the same resources.
The dependency of one organic being on another, as of a parasite on its prey,
Parasitism- interaction between two species in which one benefits at the expense of the other
Hence it is quite credible that the presence of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district!
Food chain!
Here we see that cattle absolutely determine the existence of the Scotch fir; but in several parts of the world insects determine the existence of cattle.
This is like a food chain where one needs the other to be able to survive. If one were removed, it could affect the whole chain. What are common examples of a food chain?
heath
An area of open/uncultivated land
On the other hand, in some cases, as with the elephant and rhinoceros, none are destroyed by beasts of prey:
The primary predator of these animals would be humans. As human numbers continue to drastically rise, the number of large game animals continues to decline.
The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pair of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair.
Yet there is considerably less than half a million left in the world! Is this due to human interference or lack of breeding, or both?
As the missletoe is disseminated by birds, its existence depends on birds;
Would this be considered mutualism?
progeny
The descendant/s of a person, animal, plant, etc. (i.e. their offspring)
inherited by its offspring.
Descent of new/adapted characteristics that can preserve a species.
Natural Selection
Organisms better adapted to their environment can survive and produce more offspring than those that cannot.
In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round certain species. Species very closely allied to other species apparently have restricted ranges.
Wondering what is meant by the restricted ranges? The closely related clusters sounds like a tree of life where there are constant subsects of a species branching off that have similar traits- is this what it is referring to?
many species of a genus have been formed through variation,
Variation can form an entirely new species. A lot of species are currently having to change (produce variation) because of climate change, and the result are species that have totally different behaviors, genetic makeups, or habitats. The ability for a species to survive depends on its ability to produce variation.
it would then rank as the species, and the species as the variety;
It can change from a variety to a species? So time and amount of variation can likely affect its classification.
descend from common parents, and consequently must be ranked as varieties.
Does descending from common parents make it a variety? What if their commonality dates back to millions of years ago? Or what if there commonality is as simple as "green." Would these make it change from variety to a separate species?
variations in points of structure
Great example- points of structure can be a slight variation within one plant but what if the structure that is different is wood and/or bark versus no wood/bark? This makes the variation quite profound.
species present an inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which as varieties.
This is part of the trouble defining what a "species" is. There is so much variation within a species that it can be difficult to know where one species ends and another starts.