Chapter 23 The Evolution of Populations. A. Population Genetics 1. Combines Darwinian selection and Mendelian inheritance a. Population genetics is the study of genetic variation within a population. Importance of quantitative characters.
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The Evolution of Populations
1. Combines Darwinian selection and Mendelian inheritance
a. Populationgenetics is the study of genetic variation within a population. Importance of quantitative characters.
b. In the 1940s, a comprehensive theory of evolution, called modern synthesis, was formed. Until then, many did not accept that Darwin’s theory of natural selection could drive evolution.
- Modernsynthesis combined discoveries from different fields including paleontology, taxonomy, biogeography, and population genetics.
It emphasizes the importance of populations as units of evolution, with natural selection as the most important mechanism of evolution, and backs up the idea of gradualism.
Say that we have 500 flowering plants 480 with red flowers, 20 with white flowers and that the alleles express themselves by pure Mendelian inheritance. We know:
Of the red, some will be RR and some Rr;Suppose 320 red are homozygous (RR) and 160 are heterozygous (Rr). The white will be only rr.
We know there are 1000 copies of the genes for color (we know this because the plants are diploid). Thus, the allele frequencies are (in both males and females):
320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r
a. Remember, a population is a localized group of individuals of the same species. A species is a group of populations whose individuals have the ability to breed and produce fertile offspring.
b. Individuals near a population center are, on average, more closely related to one another than to members of other populations.
c. A population’s genepool is the total of all genes in the population at any one time.
d. If all members of a population are homozygous for a particular allele, then the allele is fixed in the gene pool.
a. The Hardy-WeinbergTheorem is used to describe a non-evolving population* Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool.
* Natural populations are not expected to actually be in Hardy-Weinberg equilibrium. Deviation from H-W equilibrium usually results in evolution. Understanding a non-evolving population, helps us to understand how evolution occurs.
- Large population size: small populations can have chance fluctuations in allele frequencies (e.g. fire, storm)
- No migration: immigrants can change the frequency of an allele by bringing in new alleles to a population
- No net mutations: if alleles change from one to another, this will change the frequency of those alleles
- Random mating: if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles.
- No natural selection: If some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.
The gene pool of a non-evolving population remains constant over multiple generations; i.e., the allele frequency does not change over generations of time.
d. Hardy-Weinberg Equation
1.0 = p2 + 2pq + q2
where p2= frequency of RR genotype; 2pq = frequency of Rr plus rR genotype; q2 = frequency of rr genotype
Figure 23.5 (p. 457) – The Hardy-Weinberg theorem.
But we know that evolution does occur within populations. What causes it?
Microevolution refers to changes in allele frequencies in a gene pool from generation to generation. Represents a gradual change in a population.
1. Causes of microevolution
a. Genetic drift
- Geneticdrift is the alteration of the gene pool of a small population due to chance.
Figure 23.7 (p. 461) – Genetic drift.
- Bottleneckeffect may lead to reduced genetic variability following some large disturbance that removes a large portion of the population. The surviving population often does not represent the allele frequency in the original population.
Figure 23.8 (p. 461) – The bottleneck effect: an analogy.
- Foundereffect may lead to reduced variability when a few individuals from a large population colonize an isolated habitat (example, retinitis pigmentosa).
b. Gene flow
- Geneflow is genetic exchange due to the migration of fertile individuals or gametes between populations.
Gene flow and human evolution:
1. Genetic (heritable) variation exists within and between populations. Exists both as what we can see (e.g. eye color) and what we cannot see (e.g. blood type).
Remember, not everything that we see is due to the genotype, the environment can alter an individual’s phenotype (e.g. the hydrangea we saw before)
Fig. 23.9 – Map butterflies (color changes are due to seasonal difference in hormones).
Most variations occur as quantitative characters (e.g. height) that vary along a continuum usually indicating polygenic inheritance. Few variations are discrete (e.g. red versus white flower color).
- Polymorphism is the existence of two or more forms of a character, in high frequencies, within a population. This applies only to discrete characters. An example would be the red versus white flower color.
- Geographicvariations are differences between gene pools due to environmental factors. Natural selection may contribute to geographic variation. It often occurs when populations are located in different areas, but may also occur in populations with isolated individuals.
Figure 23.12 (p. 465) – Geographic variation between isolated populations of house mice. Normally house mice are 2n = 40. However, chromosomes fused in the mice in the example, so that the diploid number has gone down.
Note that populations are separated by mountains and that the populations evolved differently from each other!
- Cline, a type of geographic variation, is a graded variation in individuals that correspond to gradual changes in the environment. (Example: Body size of North American birds tends to increase with increasing latitude. Can you think of a reason for the birds to evolve differently?)
Figure 23.11 (p. 464) – Clinal variation in a plant. Growth height has some genetic basis. Can you think of a reason for the plants to evolve differently?
2. Mutation and sexual recombination generate genetic variation (p. 459)
a. New alleles originate only by mutations (changes in the nucleotide sequence of DNA).
- In stable environments, mutations often result in little or no benefit to an organism.
- Mutations are more beneficial in changing environments. (Example: HIV resistance to antiviral drugs)
b. Sexual recombination is the source of most genetic differences between individuals in a population.
- Vast numbers of recombination possibilities result in varying genetic make-up. FILM!
Frequency-dependentselection = survival of one phenotype declines if that form becomes too common
c. Neutralvariation is genetic variation that results in no competitive advantage to any individual
- Example: human fingerprints
D. A Closer Look at Natural Selection as the Mechanism of Adaptive Evolution
1. Natural Selection increases the frequencies of certain alleles over a period of time that includes many generations. The way that natural selection works is twofold:
a. Evolutionary (Darwinian) fitness Contribution of an individual to the gene pool, relative to the contributions of other individuals: the number of offspring may be greater or less than the number of offspring produced by others..
b. Diversifyingselection favors extreme over intermediate phenotypes.
- Occurs when environmental change favors an extreme phenotype.
c. Stabilizingselection favors intermediate over extreme phenotypes.
- Reduces variation and maintains the current average.
a. Sexual dimorphism is the difference in appearance between males and females of a species.
- Intrasexualselection is the direct competition between members of the same sex for mates of the opposite sex. This gives rise to males most often having secondary sexual equipment such as antlers that are used in competing for females.
- In intersexualselection (mate choice) one sex is choosy when selecting a mate of the opposite sex. This gives rise to often amazingly sophisticated secondary sexual characteristics, e.g. peacock feathers.
a. Evolution is limited by historical constraints (e.g. humans have back problems because our ancestors were 4-legged). The legacy of natural selection, our evolutionary history constrains what we can achieve.
b. Adaptations are compromises. Humans are athletic due to flexible limbs, which often dislocate or suffer torn ligaments.
c. Not all evolution is adaptive. Chance probably plays a huge role in evolution and not all changes are for the best.
d. Selection edits existing variations. New alleles cannot arise as needed, but most develop from what already is present. Evolution adaptation is random and not goal-oriented!