Lec #10: Adaptation and Variation

Evolutionary Process

Variation acted upon by Natural Selection, leads to Genetic Divergence, supported by Reproductive Isolation, can lead to Speciation

Populations evolve (not individuals)

Microevolution: The change in allele frequencies in a population over generations.

Two types of traits

Discrete characters: either or basis (2 or more distinct forms; colors, spots)

Quantitative characters: vary along a continuum (range of values; height)

Gene Pool

All the genes in a population (pool of genetic resources)

Genome

All the genes in a species

Hardy - Weinberg Principle

Mathematical formulation to describe how to maintain the frequencies of alleles of a population over time (genetic equilibrium)

- dominant allele will not drive out recessive allele

- genotype frequencies can be predicted & will not change (conditional)

- considers all alleles in the gene pool

- Consider the combination of alleles in all of the crosses in a population

- Select gametes at random (random mating)

Conditions for equilibrium, allele frequencies will be stable through successive generations if: (rare)

1. there has been no gene mutations (usually will in a long time)

2. the population is very large (usually small)

3. the population is isolated (usually have immigration and emmigration)

4. the gene has no effect on survival/reproduction (no natural selection) (neutral, but usually has adaptive traits)

5. mating is random

Hardy-Weinberg equilibrium is useful, allows us to make a prediction if various evolutionary forces are not operating

3 processes drive the population away from equilibrium:

1. Natural selection: select best adaptive trait for any given environ (beats the neutral assumption)

2. Gene flow: movement of genes in and out of populations as individuals travel between them (breaks assumption of isolated environ)

3. Genetic drift: random chance/event (breaks assumption that population is large)

Results in changes to a populations gene pool from generation to generation

Sources of Variation (to see if trait is evolving thru space and time)

1. Mutation: heritable changes in DNA (genes) that give rise to altered gene products

• The only source of new alleles (increases allelic variation)

• New alleles may alter the amino sequence of the encoded protein altering the function of the protein

• Wild type alleles produce expected phenotypes, mutant alleles may produce an unexpected phenotype

• Mutations give rise to structural, functional or behavioral modifications that can increase, decrease or are neutral to an individual’s survival and reproduction (can change the freq of traits)

2. Recombination: taking what you have and reshuffling

In multicellular organisms only mutations in cell lines (gametes) that produce gametes can be passed to offspring, not somatic cells

Chromosomal mutation

Big deletions, duplications, inversions, translocations of DNA/chromosomes

- not common, usually negative, may be lethal, occasionally positive

Plants are better at handling than animals

Point mutation

more common, but still rare addition, subtraction, substitution of nucleotide base(s)

- due to errors in DNA replication or environmental mutagens (radiation or chem exposure)

- DNA mutations can result in changes in mRNA, amino acids, proteins

- Silent (no effect on protein function), Missense (alters protein) & Nonsense (incomplete or non-functional protein; stop codon) mutations

- can be negative, positive, or neutral (most never expressed because of coding regions) based on environ

Types of Mutations

deleterious mutations

beneficial mutations

neutral mutations: build up because they don’t change in freq, could be advantageous tmr, the reason why we have sexual reproduction, more genetic variation, arsenal of neutrality is an advantage if environ shifts

Most mutations are negative or neutral but even if advantage is small, natural selection or chance events may preserve or enhance the frequency of a beneficial mutant gene. (representation in the next generation)

asexual organism mutations spread faster (antibiotics), while sexual organism has a lot of steps and checks (this isolated spontaneous event is why there are rare mutations)

There are more genetic variation if there is a big population

Building blocks of evolution (the raw material for evolutionary change)

- Leads to the large biological diversity past and present

- Provides the genetic diversity that natural selection acts on

- 1 zygote has 3 new mutations

Recombination

Variation: Individuals inherit different combinations of alleles (genotype) which leads to variation in phenotype

2. Crossing over at meiosis I (puts novel combinations of alleles in chromosomes)

3. Independent assortment at meiosis I (puts mixes of maternal and paternal chromosomes into gametes)

4. Fertilization (combines alleles from two parents)

5. Change in chromosome number or structure (the loss, duplication, or repositioning of genes)

Non-Random Mating

Common over random mating

Max heterozygous is 50%

Positive assortative mating: (like phenotypes mate)

- if no selective advantage it will not decrease allele frequencies

- but decreases heterozygosity (out of hardy-Weinberg equilibrium)

Negative assortative mating: (disassortative - like phenotypestend not to mate)

- if no selective advantage it will not decrease allele frequencies

- but increases heterozygosity (out of hardy-Weinberg equilibrium)

Non-random mating: Inbreeding

Inbreeding: Sexual reproduction between 2 genetically related individuals of the same species; has no one to breed with; typically recessive homozygous, which will increase and be expressed (negative fitness and lose ability to change)

- Increase the genetic load (# of potential harmful negative genes)

- decrease the amount of genetic variation in a population

- decreases heterozygosity

Exception: self-fertilization/breeding in plants (selfing)

- better than not breeding at all

- have mechanisms to purge deleterious alleles (lowers risk but still happens though) (checks and balances)

(alternation of generations – multicellular haploid phase)

Non-random mating: Crossbreeding

Crossbreeding: Sexual reproduction between 2 genetically dissimilar individuals of the same species; good thing; increase homozygous dominant and heterozygous

- decrease the genetic load (# of potential harmful negative genes) by covering it up with dominant traits (more beneficial typically)

- increase the amount of variation in a population (increase heterozygous) incase environ changes

- increases heterozygosity

Hybridization: (Form of crossbreeding); negative

- Sexual reproduction between 2 different species

- Usually results in infertility (low sperm or egg count) or sterility (no viable sperm or egg)

Ex: Mule (male = sterile, female = infertile), Liger

Types of variation

1. Normal variation: (quantitative - continuous) Ex: height, weight, etc. bell curve

2. Polymorphic variation (discreet) Ex: eye color, blood groups

3. Ecogeographic variation (variation along an ecological or geographic gradient) – can occur as a cline Ex: plant morphology along H2O gradient or bird wings along latitudes