KEY WORDS evolution reproductive barrier vestigial post-zygotic reproductive barrier reproductive isolation pre-zygotic reproductive barrier mechanical isolation temporal isolation behavioral isolation gametic isolation geographic isolation habitat isolation speciation allopatric speciation sympatric speciation reduced hybrid viability reduced hybrid fertility natural selection selective breeding selective force (pressure) sexual selection directional selection stabilizing selection disruptive selection subspecies species mutation Hardy-Weinberg Equilibrium allele frequency extinction coevolution convergent evolution genetic drift bottleneck effect founder effect adaptive radiation Bergman's Rule Trends in Evolution
In biology, evolution can be defined as "descent with modification." This means that evolution CANNOT occur at the individual level. Rather, evolution is a change in the gene pool over time.
All living things (bacteria, animals, plants, etc.) are designed to reproduce. This is because all of their ancestors successfully reproduced. Those species who fail to reproduce will die out, or go extinct.
All living things on earth are related to one another and have a common ancestor that lived about 4 billion years ago.
Mechanisms of Evolution
a) Natural Selection - Natural selection is a mechanism of evolution that occurs when the natural environment selects for or against a particular trait. This selective pressure (or selective force) causes certain alleles to become more common in the population.
Natural selection occurs to organisms when the environment around them changes. Those individuals best suited to the new environment survive and reproduce (more often) and pass their characteristics on to the next generation. Those individuals less suited to the new environment die and/or fail to reproduce (or they do it less often); these "poor" characteristics get weeded out of the population. Their allele frequency decreases.
An adaptation is a characteristic that arises due to natural selection that allows an organism to better live, survive, and reproduce in a new environment. (A preadaptation is a feature that an organism has evolved that accidentally becomes useful at doing something else.)
a) Speciation (the creation of a new species) usually occurs when a small group of individuals becomes geographically isolated from the main population, and inbreeding occurs.
b) A BIT MORE DETAIL
If two populations of a given species are separated for a long enough period of time (often this means millions of years), the two separate gene pools will slowly accumulate mutations that they don’t share with each other because these two populations are not breeding with each other. Mutations change not just the sequence of nitrogen bases within the DNA (genes), but also the sequence of amino acids within the polypeptides that those genes code for, which, in turn, alter the shape of the proteins that those polypeptides are contained within. Proteins must be shaped just right in order to work correctly, because all proteins grab onto something.
If an organism’s proteins aren’t shaped right and can’t grab onto their specific molecules when they’re supposed to, then all manner of structural and chemical problems will arise within the cells of the embryo and it may die. If these two populations are reproductively isolated for long enough, they will evolve into different species, because their proteins are no longer compatible with each other. This sequence of events is called speciation.
The mutations that spread through each gene pool by the reproduction of successive generations often aren’t random; natural selection favors the mutations that better allow an individual organism to reproduce and leave behind more children; these mutations spread rapidly through the gene pool over the course of a few generations. Because separated populations are often exposed to different environmental conditions (habitats), they are also exposed to different selective pressures, and natural selection will act on each population differently. This encourages genetic divergence and speciation.
c) Allopatric speciation - occurs when a population becomes geographically divided.
d) Sympatric speciation - occurs within the geographical range occupied by both the parent species and the new species. This usually occurs in animals due to a niche shift: a few members of a species start exploiting a new habitat or resource, or are active at a different time of day (i.e., becoming nocturnal instead of diurnal).
e) A species may be divided into two or more subspecies. These are populations that are on their way to becoming two different species. At the moment, though, members of both populations can still produce viable offspring together - though they don't usually mate with each other due to some reproductive barrier. Two different subspecies often look similar to one another in body form, but their size and coloration is often different. Two subspecies tend to live in two different geographical ranges, which may or may not overlap.
Two Categories of Reproductive Barriers
a) Behavioral isolation - Two subspecies no longer find each other sexually attractive and therefore hardly ever mate with each other anymore.
b) Mechanical Isolation - The two subspecies' genitals have changed shape enough that they don't physically fit together anymore.
c) Habitat Isolation - For example, this occurs when one subspecies of bird has begun foraging on the ground, while the other continues to forage at the tops of trees.
d) Gametic Isolation - The two subspecies' sperm and egg can't fuse together anymore, because their surface proteins have changed shape enough that they can no longer bind together; the sperm cell can't "grab" the egg cell.
e) Temporal Isolation - One subspecies is more active during the day, the other at night; since they're not active at the same time of day together, they never mate (it's hard to mate when one of the two mating partners is asleep).
f) Geographical Isolation - The two subspecies don't live in the same geographical region; they never meet, so they never mate.
a) Reduced Hybrid Variability - Hybrids have detrimental physical characteristics that make them more susceptible to predators, parasites, or diseases and/or makes them less capable of finding food or shelter.
b) Reduced Hybrid Fertility - Hybrids between the two species/subspecies are sterile or are completely sexually unattractive to members of either species/subspecies.
Coevolution and Convergent Evolution
a) Coevolution occurs when the evolution of one species affects the evolution of another.
b) Convergent Evolution occurs when two similar (yet independent) environments select for similar characteristics in two unrelated species.
Types of Selection
a) Males of a species are polygamous when they are not needed to help raise the children. Ex: dogs, most insects, etc.
b) Males of a species are monogamous when they are needed to help raise the children. Ex: humans, seahorses, songbirds, etc.
Sexual Selection -- males and females of a species evolve somewhat independently of each other. Selective forces favor the individuals of both sexes who leave behind the most offspring. Oftentimes, this means that the two sexes will have different reproductive strategies.
In this type of natural selection, individuals with extreme or unusual traits are selected against. For example, in humans, alleles for extremely tall or short height are selected against.
In this type of natural selection, individuals at one end of a range are selected for. For example, dark-colored moths are selected for, because they are more easily camouflaged, survive longer, and leave behind more offspring.
In this type of natural selection, extreme and/or unusual traits are selected for. Common traits are selected against.
Selective Breeding (Artificial Selection)
This is NOT a type of natural selection, because it is done by humans. In this type of selection, humans select for or against the most desirable traits in plants and animals. For example, they may breed the least aggressive dogs to produce fewer aggressive dogs in the next generation.