Evolutionary Processes

soft inheritance

environmental forces can cause organisms to develop traits within their lifetime that are passed to offspring, Jean-Baptiste Lamarck

Struggle for existence

fate of most individuals is to die without reproducing

evolution

change in genetic composition of a population over time

natural selection

the evolutionary process by which individuals best adapted to their environment survive and reproduce, differential fitness of individual phenotypes within a population

Artificial selection

selective breeding of organisms by humans

homology

similar features from shared ancestry (humans, cats, whale, and bats have similar features in their “arms”)

vestigial

structures that have lost their original function (coccyx in humans)

evidence of evolution

fossils, molecular biology, direct observation, experimental evidence

Biological fitness

relative survival and reproduction success of an individual compared to other populations

consequence of evolution

favored phenotype becomes more common in future generations

adaptation

heritable trait that evolved in a population through natural selection. Refers to both the trait and the process

Gene flow

transfer of alleles between populations. Typically through migration of individuals or movements of gametes

genetic drift

random change in allele frequencies over time, largest effect on small populations

population bottleneck

large decrease in population size where only a few individuals survive, results in a random loss of genetic diversity

founder effect

small group of individuals establishes a new population, resulting in a random sample of the original population’s alleles

non-random mating

occurs when individuals choose mates with particular phenotypes

sexual selection

occurs when individuals of one sex mate preferentially with particular individuals of another sex rather than at random

Hardy-Weinberg equilibrium

Shows that allele frequencies in a population do not change when no evolution is occurring

stabilizing selection

occurs when the mean phenotype has higher fitness than the extreme phenotypes. Mean trait value unchanged, but genetic variation reduced.

Directional selection

Occurs when an extreme phenotype has higher fitness than the average phenotype. Mean trait value shifts, and genetic variation can be reduced

Disruptive selection

occurs when both extreme phenotypes have higher fitness than the mean phenotype. Mean trait value unchanged, but genetic variation increases

paradox of selection and genetic variation

genetic variation is “raw material;” for evolution, natural selection often eliminates genetic variation

diploidy

hides recessive alleles in heterozygotes, preserving genetic variation

heterozygote advantage

occurs when heterozygotes have higher fitness than homozygous, maintaining multiple alleles in a population

temporally variable selection

occurs when changing environments favor different phenotypes at different times, preventing allele fixation (fluctuating selection that ensures alleles aren’t lost)

spatially variable selection

occurs when different local environments favor different phenotypes (climate, not based on changing conditions)

evolutionary history

new traits must evolve from preexisting structures, limits the possible evolutionary outcomes

evolutionary trade-offs

trait that improves fitness in one context reduces fitness in another