9. Populations

What are the fundamental units of biological heredity?

DNA molecules, Chromosomes, Genes.

Population

A group of organisms living in proximity of each other. Members of a population are more likely to mate with another member rather than an outsider. Similar to the definition of society.

Biological population

A group within which the body of genes is transmitted (passed down) through generations. It retains genetic identity. They reproduce in geographical or cultural isolation from other groups of the same species.

Geographical Isolation

Geographical causes leading to isolation, like population living in remote areas like islands, arctic, and desert areas.

Cultural isolation

Cultural causes leading to isolation, like religious groups, different languages, different classes of society, and racism/racial discrimination.

Gene pool

The sum total of all of the genes within a population, frequencies are different from genotypes. “How many brown eyes do we have?”

Species

The largest mendelian population can be subdivided into smaller biological populations.

What does an individual need to have to be considered part of a population/species?

They should be able to reproduced and produce fertile offspring that can also reproduce. For example, donkeys and horses can reproduce, but their offspring is not fertile. So they’re not the same species.

Phenotypic Variation within populations

Comes from two sources, variation in genotypes in the population and different environmental pressures.

Continuous variation

Variation expressed as a smooth range, gradient between two extremes. These have a normal bell-shaped curve. Examples include height, weight, skin color. Involves polygenic traits.

Discontinuous variation

When a population is divided into several, discrete and obvious groups. Such as blood types, male or female. Racists consider skin colour as discontinuous variation. Caused by monogenic traits.

Evolution

In simple terms, the relatively minor changes in the gene pool of a population from one generation to the next one. There are thousands of genes in a population all subject to change. New genes are created by mutation, genes are recombined creating new phenotypes, acted upon by natural selection.

Biological evolution

The change in gene frequencies.

Forces of Evolutionary Change

Includes Mutation, Natural Selection, Genetic Drift, Gene Flow

Directional selection

A type of natural selection that favors phenotypic traits at one extreme of the range of variation within a population. The average value of a trait shifts in a consistent direction over time.

Normalizing selection

Also known as stabilizing selection, a type of natural selection that favors intermediate phenotypic traits (not extreme). Acting against individuals with extreme phenotypes. Reducing phenotypic variation and keeps the average/normal phenotype. Ex. Birth weight

Balancing selection

A type of natural selection that favors genetic diversity within a population.

Darwinian Evolution

An evolution that is a directional process. Coming from pressures of the environment and adaptation.

Non-Darwinian Evolution

An evolution that is largely random, and cannot predict the direction of change. While not being necessarily adaptive. leading to a loss in genotypic variability. There are two types, Genetic drift and Gene flow.

Genetic Drift

A collection of random processes that affect gene frequencies. Occuring in a small isolated population. It acts along with genetic variability and produces changes in proportions of gene combinations. Decreases genetic variability. Increases homozygosity

Founder Effect

An example of genetic drift. Is when a new population is established, founded by settlers, colonists or survivors of a catastrophe. Occurs in small populations. The gene frequency will be from the founders, which differs significantly from the parent/ancestral population. The founders represent a small and biased sample of the original parent population.

Gene flow

Transfers or sends genes from one population of a species to another. (Aka migration or interbreeding) It promotes heterozygosity and keeps species together. Increases genetic variability. Increases heterozygosity

Punctuated equilibrium

Stephen Jay Gould believed evolution happened slowly and gradually, but there are periods of rapid change. Domestication of animals, beginning of agriculture in human societies are examples of this process.

Specialist

An adaptive mode that has a slow rate of change. Does things better than anyone else. But has a narrow niche.

Generalist

An adaptive mode that does everything sort of well. It has a wide niche and much more stable than specialists. They have a fast rate of change.