Chapter 23The Evolution of Populations

Population Genetics

Combines Darwinian selection and Mendelian inheritance

Population genetics - study of genetic variation within a population. 

Emphasis on quantitative characters. 1940s – comprehensive theory of evolution (the modern synthesis).

Until then, many did not accept that Darwin’s theory of natural selection could drive evolution.                                   

“The modern synthesis” combined discoveries from • paleontology, • taxonomy, • biogeography, and • population genetics.

• It emphasizes the importance of populations as units of evolution,

•natural selection as the most important mechanism of evolution,

•and gradualism.

Allele frequencies define gene pools

As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females):

320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r

500 flowering plants

480 red flowers 20 white flowers

320 RR 160 Rr 20 rr

Population - a localized group of individuals of the same species. 

Species - a group of populations whose individuals have the ability to breed and produce fertile offspring. 

Individuals near a population center are, on average, more closely related to one another than to members of other populations.

A population’s gene pool is the total of all genes in the population at any one time.

 

If all members of a population are homozygous for a particular allele, then the allele is fixed in the gene pool.

The Hardy-Weinberg Theorem                        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.

Assumptions of the H-W Theorem:

- 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.

Hardy-Weinberg Equilibrium                                   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. The Hardy-Weinberg Equation:                                     1.0 = p2 + 2pq + q2

                                                where p2 = frequency of AA genotype; 2pq = frequency of Aa plus aA genotype; q2 = frequency of aa genotype

But we know that evolution does occur within populations.

Evolution within a species/population = microevolution.

Microevolution refers to changes in allele frequencies in a gene pool from generation to generation. Represents a gradual change in a population.

 Causes of microevolution:                        1)  Genetic drift

2) Natural selection (1 & 2 are most important)

3) Gene flow

4) Mutation

1) Genetic drift

Genetic drift = the alteration of the gene pool of a small population due to chance.

Two factors may cause genetic drift:                                     a) Bottleneck effect 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.

b) Founder effect may lead to reduced variability when a few individuals from a large population colonize an isolated habitat.

*Yes, I realize that this is not really a cheetah.

2) Natural selection                                    As previously stated, differential success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance).

The only agent that results in adaptation to environment. 

3) Gene flow                                    -is genetic exchange due to the migration of fertile individuals or gametes between populations. 

4) Mutation                                    Mutation is a change in an organism’s DNA and is represented by changing alleles.   Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool.

The original source of variation.

Genetic Variation, the Substrate for Natural Selection            Genetic (heritable) variation within and between populations: exists both as what we can see (e.g., eye color) and what we cannot see (e.g., blood type). Not all variation is heritable.

Environment also can alter an individual’s phenotype [e.g., the hydrangea we saw before, and…

…Map butterflies (color changes are due to seasonal difference in hormones)].

Variation within populations

Most variations occur as quantitative characters (e.g., height); i.e., variation along a continuum, usually indicating polygenic inheritance.

Few variations are discrete (e.g., red vs. white flower color).

Polymorphism is the existence of two or more forms of a character, in high frequencies, within a population.  Applies only to discrete characters.

Variation between populations

Geographic variations are differences between gene pools due to differences in 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.

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.

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? Example: Height variation in yarrow along an altitudinal gradient. Can you think of a reason for the plants to evolve differently?

Mutation and sexual recombination generate genetic variation                        a.  New alleles originate only by mutations (heritable only in gametes; many kinds of mutations; mutations in functional gene products most important).                                    - In stable environments, mutations often result in little or no benefit to an organism, or are often harmful.                                    - Mutations are more beneficial (rare) 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.

Diploidy and balanced polymorphism preserve variation                        a.  Diploidy often hides genetic variation from selection in the form of recessive alleles.

Dominant alleles “hide” recessive alleles in heterozygotes. b.  Balanced polymorphism is the ability of natural selection to maintain stable frequencies of at least two phenotypes.                                    Heterozygote advantage is one example of a balanced polymorphism, where the heterozygote has greater survival and reproductive success than either homozygote (Example: Sickle cell anemia where heterozygotes are resistant to malaria).

Frequency-dependent selection = survival of one phenotype declines if that form becomes too common.

(Example:  Parasite-Host relationship. Co-evolution occurs, so that if the host becomes resistant, the parasite changes to infect the new host. Over the time, the resistant phenotype declines and a new resistant phenotype emerges.)

Neutral variation is genetic variation that results in no competitive advantage to any individual.                                    - Example:  human fingerprints.

A Closer Look: Natural Selection as the Mechanism of Adaptive Evolution            Evolutionary fitness - Not direct competition, but instead the difference in reproductive success that is due to many variables.

Natural Selection can be defined in two ways:                        a.  Darwinian fitness- Contribution of an individual to the gene pool, relative to the contributions of other individuals.

And,

b.  Relative fitness

- Contribution of a genotype to the next generation, compared to the contributions of alternative genotypes for the same locus.

- Survival doesn’t necessarily increase relative fitness; relative fitness is zero (0) for a sterile plant or animal.

Three ways (modes of selection) in which natural selection can affect the contribution that a genotype makes to the next generation.   a.  Directional selection favors individuals at one end of the phenotypic range. Most common during times of environmental change or when moving to new habitats.

Directional selection

Diversifying selection favors extreme over intermediate phenotypes.

 - Occurs when environmental change favors an extreme phenotype.

 

Stabilizing selection favors intermediate over extreme phenotypes.

 - Reduces variation and maintains the current average.

- Example = human birth weights.

Diversifying selection

Natural selection maintains sexual reproduction

-Sex generates genetic variation during meiosis and fertilization.

-Generation-to-generation variation may be of greatest importance to the continuation of sexual reproduction.

-Disadvantages to using sexual reproduction: Asexual reproduction produces many more offspring.

-The variation produced during meiosis greatly outweighs this disadvantage, so sexual reproduction is here to stay.           

All asexual individuals are female (blue). With sex, offspring = half female/half male. Because males don’t reproduce, the overall output is lower for sexual reproduction.

Sexual selection leads to differences between sexes

a.  Sexual dimorphism is the difference in appearance between males and females of a species.

-Intrasexual selection 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 intersexual selection (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.

Natural selection does not produce perfect organisms                       a.  Evolution is limited by historical constraints (e.g., humans have back problems because our ancestors were 4-legged).  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.