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Apr 29, 2024 · It occurs because meiosis separates the two alleles of each heterozygous parent so that 50% of the gametes will carry one allele and 50% the other and when the gametes are brought together at random, each B (or b )-carrying egg will have a 1 in 2 probability of being fertilized by a sperm carrying B (or b ).
- Overview
- Key points:
- Introduction
- Hardy-Weinberg equilibrium
- But is that realistic?
- Hardy-Weinberg assumptions and evolution
- Some genes may satisfy Hardy-Weinberg, while others do not
- Mechanisms of evolution
When a population is in Hardy-Weinberg equilibrium, it is not evolving. Learn how violations of Hardy-Weinberg assumptions lead to evolution.
•When a population is in Hardy-Weinberg equilibrium for a gene, it is not evolving, and allele frequencies will stay the same across generations.
•There are five basic Hardy-Weinberg assumptions: no mutation, random mating, no gene flow, infinite population size, and no selection.
•If the assumptions are not met for a gene, the population may evolve for that gene (the gene's allele frequencies may change).
•Mechanisms of evolution correspond to violations of different Hardy-Weinberg assumptions. They are: mutation, non-random mating, gene flow, finite population size (genetic drift), and natural selection.
In nature, populations are usually evolving. The grass in an open meadow, the wolves in a forest, and even the bacteria in a person's body are all natural populations. And all of these populations are likely to be evolving for at least some of their genes. Evolution is happening right here, right now!
To be clear, that doesn't mean these populations are marching towards some final state of perfection. All evolution means is that a population is changing in its genetic makeup over generations. And the changes may be subtle—for instance, in a wolf population, there might be a shift in the frequency of a gene variant for black rather than gray fur. Sometimes, this type of change is due to natural selection. Other times, it comes from migration of new organisms into the population, or from random events—the evolutionary "luck of the draw."
In this article, we'll examine what it means for a population evolve, see the (rarely met) set of conditions required for a population not to evolve, and explore how failure to meet these conditions does in fact lead to evolution.
[I'm totally new to population genetics! Where should I start?]
First, let's see what it looks like when a population is not evolving. If a population is in a state called Hardy-Weinberg equilibrium, the frequencies of alleles, or gene versions, and genotypes, or sets of alleles, in that population will stay the same over generations (and will also satisfy the Hardy-Weinberg equation). Formally, evolution is a change in allele frequencies in a population over time, so a population in Hardy-Weinberg equilibrium is not evolving.
That's a little bit abstract, so let's break it down using an example. Imagine we have a large population of beetles. In fact, just for the heck of it, let's say this population is infinitely large. The beetles of our infinitely large population come in two colors, dark gray and light gray, and their color is determined by the A gene. AA and Aa beetles are dark gray, and aa beetles are light gray.
In our population, let's say that the A allele has a frequency of 0.3 , while the a allele has a frequency of 0.7 . If a population is in Hardy-Weinberg equilibrium, allele frequencies will be related to genotype frequencies by a specific mathematical relationship, the Hardy-Weinberg equation. So, we can predict the genotype frequencies we'd expect to see (if the population is in Hardy-Weinberg equilibrium) by plugging in allele frequencies as shown below:
[What is the difference between allele and genotype frequency?]
Let's imagine that these are, in fact, the genotype frequencies we see in our beetle population (9% AA, 42% Aa, 49% aa). Excellent—our beetles appear to be in Hardy-Weinberg equilibrium! Now, let's imagine that the beetles reproduce to make a next generation. What will the allele and genotype frequencies will be in that generation?
To predict this, we need to make a few assumptions:
As we mentioned at the beginning of the article, populations are usually not in Hardy-Weinberg equilibrium (at least, not for all of the genes in their genome). Instead, populations tend to evolve: the allele frequencies of at least some of their genes change from one generation to the next.
In fact, population geneticists often check to see if a population is in Hardy-Weinberg equilibrium because they suspect other forces may be at work. If the population’s allele and genotype frequencies are changing over generations (or if the allele and genotype frequencies don't match the predictions of the Hardy-Weinberg equation), the race is on to find out why.
What causes populations to evolve? In order for a population to be in Hardy-Weinberg equilibrium, or a non-evolving state, it must meet five major assumptions:
1.No mutation. No new alleles are generated by mutation, nor are genes duplicated or deleted.
2.Random mating. Organisms mate randomly with each other, with no preference for particular genotypes.
3.No gene flow. Neither individuals nor their gametes (e.g., windborne pollen) enter or exit the population.
4.Very large population size. The population should be effectively infinite in size.
5.No natural selection. All alleles confer equal fitness (make organisms equally likely to survive and reproduce).
Note that we can think about Hardy-Weinberg equilibrium in two ways: for just one gene, or for all the genes in the genome.
•If we look at just one gene, we check whether the above criteria are true for that one gene. For example, we would ask if there were mutations in that gene, or if organisms mated randomly with regards to their genotype for that gene.
•If we look at all the genes in the genome, the conditions have to be met for every single gene.
While it’s possible that the conditions will be more or less met for a single gene under certain circumstances, it’s very unlikely that they would be met for all the genes in the genome. So, while a population may be in Hardy-Weinberg equilibrium for some genes (not evolving for those genes), it’s unlikely to be in Hardy-Weinberg equilibrium for all of its genes (not evolving at all).
Different Hardy-Weinberg assumptions, when violated, correspond to different mechanisms of evolution.
•Mutation. Although mutation is the original source of all genetic variation, mutation rate for most organisms is pretty low. So, the impact of brand-new mutations on allele frequencies from one generation to the next is usually not large. (However, natural selection acting on the results of a mutation can be a powerful mechanism of evolution!)
•Non-random mating. In non-random mating, organisms may prefer to mate with others of the same genotype or of different genotypes. Non-random mating won't make allele frequencies in the population change by itself, though it can alter genotype frequencies. This keeps the population from being in Hardy-Weinberg equilibrium, but it’s debatable whether it counts as evolution, since the allele frequencies are staying the same.
•Gene flow. Gene flow involves the movement of genes into or out of a population, due to either the movement of individual organisms or their gametes (eggs and sperm, e.g., through pollen dispersal by a plant). Organisms and gametes that enter a population may have new alleles, or may bring in existing alleles but in different proportions than those already in the population. Gene flow can be a strong agent of evolution.
•Non-infinite population size (genetic drift). Genetic drift involves changes in allele frequency due to chance events – literally, "sampling error" in selecting alleles for the next generation. Drift can occur in any population of non-infinite size, but it has a stronger effect on small populations. We will look in detail at genetic drift and the effects of population size.
•Natural selection. Finally, the most famous mechanism of evolution! Natural selection occurs when one allele (or combination of alleles of different genes) makes an organism more or less fit, that is, able to survive and reproduce in a given environment. If an allele reduces fitness, its frequency will tend to drop from one generation to the next. We will look in detail at different forms of natural selection that occur in populations.
Oct 31, 2023 · Page ID. Boundless. Learning Objectives. Use the Hardy Weinberg equation to calculate allelic and genotypic frequencies in a population. The Hardy-Weinberg principle states that a population’s allele and genotype frequencies will remain constant in the absence of evolutionary mechanisms.
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It makes sense that the Hardy-Weinberg equilibrium is not stable, since a change from the equilibrium genotype frequencies will generally be associated with a change in allele frequencies (p and q ...
Learning Objectives. Know and recognize the five assumptions of the Hardy-Weinberg principle. Use the gene pool concept and the Hardy-Weinberg principle to determine whether a population is evolving at a locus of interest. Measuring Evolutionary Change: the Hardy-Weinberg Equilibrium Principle.
Hardy-Weinberg law, an algebraic equation that describes the genetic equilibrium within a population. It was discovered independently in 1908 by Wilhelm Weinberg, a German physician, and Godfrey Harold Hardy, a British mathematician. The science of population genetics is based on this principle,
The Hardy-Weinberg equilibrium is a principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors.
