Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 13.4 Genetic Variation Produced in Sexual Life Cycles Contributes to Evolution
Sexual reproduction introduces genetic variation, which is essential for evolution. This variation arises through independent assortment of chromosomes, crossing over, and random fertilization.
Origins of Genetic Variation
Mutations are the original source of new alleles
Sexual reproduction reshuffles alleles into new combinations
Three main mechanisms during meiosis and fertilization generate variation:
Independent assortment of chromosomes
Crossing over
Random fertilization
Independent Assortment of Chromosomes
At metaphase I, homologous pairs align randomly
Each gamete receives a random mix of maternal and paternal chromosomes
The number of possible combinations is 2ⁿ, where n = haploid number
In humans (n = 23), this means about 8.4 million combinations per gamete
Crossing Over
Occurs during prophase I of meiosis
Nonsister chromatids exchange DNA segments at chiasmata
Produces recombinant chromosomes with genes from both parents
Each chromosome pair typically undergoes 1–3 crossover events
Increases genetic diversity by mixing alleles on the same chromosome
Random Fertilization
Any sperm can fuse with any egg
In humans, ~8.4 million possible gametes from each parent
Fusion results in ~70 trillion possible diploid combinations
Crossing over further expands the number of possible outcomes
Evolutionary Significance
Genetic variation allows populations to adapt to changing environments
Natural selection favors individuals with traits that increase reproductive success
Over time, favorable combinations accumulate
Even asexually reproducing organisms like bdelloid rotifers have alternative ways to generate genetic diversity (e.g., DNA uptake from the environment)
The near-universal use of sexual reproduction in animals highlights the evolutionary value of genetic variation
In a Nutshell
Sexual reproduction generates genetic diversity through independent assortment, crossing over, and random fertilization. This variation fuels evolution by enabling populations to adapt and persist through natural selection.