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Two different theories proposed by scientists to address the rate of evolution:
1. Gradualism - proposes that evolutionary change is slow, gradual, and continuous.
2. Punctuated Equilibrium - proposes that species have long periods of stability (several million years) interrupted by geologically brief periods of significant change during which a new species may evolve.
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.
Ex: A breed of lizards which are usually brown, give rise to a black lizard. However, the black variation is better camouflaged and so predators miss it. As time goes on the original pop n make decrease in size while the black variation increases.
When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. This situation keeps the center of the curve at its current position, but it narrows the overall graph
Evolutionary changes doesn’t always occur by natural selection.
In small populations, an allele can become more or less common simply by chance.
In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance . Over time, a series of chance occurrences of this type can cause an allele to become common in a population.
Ex . Genetic Drift can occur when small populations inhabit a new area.
These individuals may carry alleles in different relative frequencies than did the larger population from which they came. If so, the population that they found will be genetically different from the parent population
This is called the “ Founder Effect ”
This sometimes can lead to rare traits to be expressed.
To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place. So biologists ask: Are there any conditions under which evolution will not occur? Is there any way to recognize when that is the case? The answers to those questions are provided by the Hardy-Weinberg principle , named after two researchers who independently proposed it in 1908.
The Hardy-Weinberg principle states that a sexually reproducing population will have stable allelic frequencies (no genetic change) and therefore will not undergo evolution, given the following five conditions
If genes mutate from one form into another, new alleles may be introduced into the population, and allele frequencies will change.
No Natural Selection:
All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another. In other words, there can be no natural selection operating on the population.
Ex: If, random mating occurs, then the allele frequency will be the same in the next generation Total Parental Population = 500 Allele Frequency
The total number of genes in a population is its gene pool .
Let ‘p’ represent the frequency of one gene in the pool and ‘q’ the frequency of its single allele.
3. Geographic Isolation : With geographic isolation , two populations are separated by geographic barriers such as rivers, mountains, bodies of water, or even roads
1 2 3 4 1 2 3 4 1 2 A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat. Later in time, a few individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence. Speciation may also follow colonization of islands 3 and 4. And it may follow invasion of island a by genetically different descendants of the ancestral species.