Unit 7: Natural Selection

LaMarck theory

If an organism changes during life in order to adapt to its environment, those changes are passed on to its offspring.

Evolution through use and disuse. Inheritance of acquired characteristics.

Fitness

Ability to contribute genes to the next generation (reproduction)

Mutation

A change in a DNA sequence. Happens spontaneously and unavoidably

Alleles

Different versions of the genes for a trait (dominant or recessive)

Grant and Grant

Studied the finch population on an isolated island in the Galapagos.

Measured the beak dimensions of all birds on the island every year for decades.

changes in population can happen very quickly in response to bottleneck

Artificial selection

When reproductive success is determined by human requirements

Biogeography

Fossils distributed in patterns that reflect the continents at one point in time being connected

Molecular Biology

- Molecular evidence comes from comparing DNA, RNA, and proteins across different organisms.

Genetic Similarities: All living organisms share a universal genetic code, and closely related species often have very similar DNA sequences.

Protein Homology: Many proteins (such as cytochrome c) are remarkably similar across different species, which supports the idea of common descent.

Molecular Clocks: By measuring the rate of genetic mutations, scientists can estimate the time since two species diverged, providing a timeline for evolutionary events.

Fossil Record

preserved remains or traces of organisms from the past that establishes history of life on Earth

Temporal Sequence

Fossils found in different rock layers show gradual changes in species over millions of years.

Transitional Fossils

Show evolutionary progression between groups

Vestigial structures

structures that have lost their primary adaptive purpose

Homologous Structures

Structures present in a common ancestor, which have diverged during evolution. (Divergent evolution)

Analogous Structures

Structures that have evolved multiple times in different lineages to fill similar adaptive needs. (Convergent evolution)

Population

localized group of interbreeding individuals

Gene pool

collection of alleles in the population

Allele frequency

how common that allele is in the population

Evolution

change of allele frequencies in a population

Genetic Drift

• Random, non-selective, changes in allele frequency due to chance. Can lead to loss of genetic diversity.

• Has a larger effect on smaller populations, since each individual is more of the total alleles.

Founder Effect

The descendants of a small, founding population have different allele percentages than the population the founders came from.

Bottleneck Effect

The survivors of a catastrophic decrease in a population may have a different allele frequency than the pre-______ population.

Ex: Earthquake, fire, or flood

Gene Flow

Movement of alleles due to immigration and emigration.

Sexual Selection

Persistence of traits that signify fitness and aid in reproduction.

Directional Selection

• Shifts the overall makeup of the population by favoring variants that are at one extreme of the distribution.

• Darker mice favored living among dark rocks—their darker fur conceals them from predators.

Disruptive Selection

• Favors variants at both ends of the distribution.

• In a patchy habit made up of light and dark rocks, mice of intermediate color are at a disadvantage.

Stabilizing Selection

• Removes extreme variants from the population and preserves intermediate types.

• If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against.

Non-evolving population

• Large size (no genetic drift)

• Random mating (no sexual selection)

• Stable environment (no natural selection)

• No immigration/emigration (no gene flow)

• No mutations.

No real population is in H.W. equilibrium.

Biological species

A group of organisms that are capable of successfully producing fertile, viable offspring.

Gradualism

species are the product of slowly accumulating, small evolutionary changes

Punctuated equilibrium

species undergo long periods of very little change, followed by rapid, large evolutionary changes

Adaptive Radiation

One species evolves into many species that occupy open niches

Reproductive Isolation

Speciation occurs when a population can no longer interbreed with any other population.

Allopatric Reproductive Isolation

ex) emergence of mountain range, formation of land bridge, evaporation of large lake producing several small lakes

reproductive isolation due to physical separation

Sympatric Reproductive Isolation

Examples: part of population switching to new habitat/food source, or polyploidy plants.

reproductive isolation occurring while occupying the same area.

Pre-zygotic Species Barriers

EX: (physical) - geographical

(temporal) - time of day/season

(behavioral) - mating rituals

(mechanical) - incompatibility of reproductive structures

(chemical) - incompatibility of proteins on gametes

Prevent gametes from combining into a fertilized zygote

Post-zygotic Species Barriers

EX: Reduced viability - Hybrid is frail

Reduced fertility - Hybrid is sterile

Hybrid breakdown - Fertile, viable hybrids mate, but their offspring are weak or sterile.

Prevent the hybrid zygote from becoming an organism capable of reproducing

Panspermia

One of two major hypotheses for life on Earth.

Life from Earth came from extraterrestrial life. (arriving on meteor or similar delivery system)

Abiogenesis

One of two major hypotheses for life on Earth.

Life from Earth came from non-life. Requires 4 major milestones to occur:

1. Development of biological molecules that living systems are made of

2. Development of Proto-cells: encapsulation of those chemicals into isolated systems.

3. Information Molecule Evolution: development of an information storage molecule that can be inherited.

4. Reproduction of living systems.

RNA World

The hypothesis that RNA preceded DNA. Due to RNA’s “double function” in living systems as an information storage molecule and a catalytic molecule.

Cleaves its own RNA or another RNA molecule. Ribosomal RNAs form the two subunits of ribosome, catalyzing peptide bond formation between two amino acids.

Evolution of Metabolism

The development of the major ways that living systems process matter and energy.

“Heterotroph Hypothesis”:

Glycolysis → Photosynthesis (oxygenation of atmosphere) → Aerobic Cellular Respiration (requires oxygen).

Endosymbiosis

The origin of eukaryotic cells from prokaryotic ancestors. Free-living ancestors of eukaryotic mitochondria and chloroplasts were engulfed by eukaryotic ancestors, and a symbiotic arrangement was established.

Miller-Urey Experiments

Simulated “Early Earth” conditions (no O2).

Starting with simple chemical building blocks and appropriate energy supply, simple organic molecules were produced.

Radiometric Dating

Used to date geological formations and fossils.

Establishes chronological history of Earth, and establishes Earth’s age at ~4.5 billion years.

*Radioactive decay occurs when the nucleus of a radioactive atom spontaneously transforms into an atomic nucleus of a different, more stable isotope.

Some isotopes are radioactively unstable and decay over a known period of time.

Half Life: the amount of time it takes for one-half of a radioactive isotope to decay.

We can use this to determine the age of a sample with radioactive isotopes, based on the ratio of how much of the parent and daughter isotopes are present.

Monophyletic

A group that consists of an ancestral species and all of its descendants

Paraphyletic

A group which consists of an ancestral species and some, but not all, of its descendants

Polyphyletic

A group which includes distantly related species but does not include their most recent common ancestor

Outgroup

Diverged from the lineage leading to the ingroup (before the common ancestor of the ingroup)

Identifying the ______ helps determine the direction of evolutionary change and which traits are ancestral or derived.

Ancestral Traits

inherited from a common ancestor and shared by multiple species descended from that ancestor.

Derived Traits

traits evolved recently and unique to a specific lineage.

A population in Hardy Weinberg equilibrium:

No mutations: The allele pool is not altered by genetic changes.

Random mating: Individuals pair by chance, not based on genotype or phenotype.

No natural selection: All genotypes have equal probabilities of survival and reproduction.

Large (effectively infinite) population size: This minimizes genetic drift.

No gene flow (migration): No new alleles are introduced or lost through movement in or out of the population.

Hardy Weinberg equation

p²+2pq+q²=1

• p² is the frequency of the homozygous dominant genotype,

• 2pq is the frequency of the heterozygous genotype, and

• q² is the frequency of the homozygous recessive genotype.

How do you calculate frequency of an allele in a population?

For a gene with two alleles (e.g., A and a):

• The frequency of allele A (p) is calculated by:
  p = (frequency of AA + (1/2)) x (frequency of Aa)

• The frequency of allele a (q) is:
  q = (frequency of aa + (1/2) x (frequency of Aa)

Note: p + q = 1